News

The work of our former master student Livio Redard-Jacot was awarded the “Best Master Thesis in Experimental Physics at UZH”, sponsored by Detrics Ltd. The award is given annually to the best thesis in experimental physics of the year and determined by a panel of internal and external experts to UZH. Livio worked on the electric field characterization in XENONnT and the search for radiation from wave function collapse under the supervision of Dr. Christian Wittweg. At the award ceremony, the panel of juries highlighted the “high impact of the work on the field” and “how well the thesis is written, explaining complex concepts in an comprehensible and accurate way”. Congratulations, Livio!

We are delighted to share our participation in the Symposium University of Tokyo – ETH Zurich – University of Zurich, Interdisciplinary Strategic Partnership Tokyo - Zurich, held on 16-17 October 2023 at ETH Zürich, Hönggerberg Siemens Auditorium. The event united groups from various fields, including Architecture, Education, Plant Biology, Particle Physics, Digital Economies, Physical Chemistry, and Biomedical Imaging/Chemical Biology. Notably, our colleagues Diego Ramirez Garcia and Marta Babicz presented their work. Marta's poster, 'From LEGEND to Reality: Cracking the Code of Creation,' received recognition as the first prize winner for the best flash talk and poster among 31 participants. This symposium provided a valuable opportunity for learning and networking with fellow researchers and industry professionals. We look forward to future events and extend our gratitude to all who contributed to its success.

The GERDA collaboration presented the measurement of the two-neutrino double-β (2νββ) decay rate of ⁷⁶Ge performed with the GERDA Phase 2 experiment, which was published in Phys. Rev. Lett. today as editor's suggestion. Results were drawn from data collected with nine high-purity Broad Energy Germanium detectors between December 2015 and April 2018 with a total exposure of 11.8 kg yr. These detectors were chosen over all the 37 detectors presented during the Phase 2 operation given that their active volumes could be determined to an accuracy of 1-1.5%, which ultimately determined the efficiency for detecting 2νββ decays. By operating  high-purity germanium detectors in combination with a liquid argon veto system featuring excellent background-rejection capabilities, we achieve a superior signal-to-background ratio of 22:1, reducing the background by more than tenfold in the energy region dominated by the 2νββ decay. Using a binned maximum likelihood to estimate the number of 2νββ decays within the energy window of 560 and 2000 keV, we obtain a decay half-life of T_1/2 = (2.022 ± 0.018 _stat ± 0.038 _sys)×10²¹ yr, which is the most precise 2νββ decay half-life measurement of ⁷⁶Ge. In addition, the effective nuclear matrix element extracted from T_1/2 is M_eff = (0.101 ± 0.001), aligning with the high precision reached by several experiments in the last years and presenting one of the most precise measurements of a double-beta decay process.

Congratulations to Dr. Yannick Mülller who successfully defended his Ph.D. thesis this September. During his thesis, Yannick worked on the GERDA experiment and on the LEGEND-200 experiment at LNGS.  In particular, he was involved in the design, construction and installation of the LEGEND calibration hardware,  in the characterisation of the neutron flux emitted by the custom-made sources, and in background predictions via Monte Carlo simulations. He also performed a search for BSM physics in the low-energy data of GERDA, and in particular he searched for ALPs, dark photons and other rare interactions. 

The students and postdocs from our group attended and contributed to several conferences this summer, both local and international. At EPS-HEP, in Hamburg, our postdoc Ricardo Peres gave a talk about DARWIN, its science, and associated R&D projects, and Yanina Biondi, a former PhD student and postdoc in our group, described the first results of the purity monitor phase of Xenoscope. At TAUP, in Vienna, Christian Wittweg, another postdoc in the group, gave a talk on the search of solar 8B neutrinos with the XENONnT detector. The group was also represented in this years' Swiss and Austrian Physical Society annual meeting, in Basel, by our PhD students Mariana Rajado (DARWIN) and Paloma Cimental (Xenoscope), and our postdocs Marta Babicz (LEGEND), Jose García Cuenca (Xenoscope), and Christian (XENON), who presented the recent work on the focus projects of the group. In Coimbra, at the IBER conference, Ricardo gave a talk on the status and results of XENONnT. And, to end the busy season, our group was present at the LIDINE conference, in Madrid, where our PhD student Alexander Bismark presented his work on Xenoscope and photosensor R&D, our postdoc Diego Ramírez García gave a talk covering the first science run results of XENONnT, and our postdoc Marta Babicz discussed the status and plans of the DARWIN observatory.

To study construction and operation challenges for the planned multi-tonne scale dark matter detector DARWIN, we designed and built Xenoscope, a vertical full-scale demonstrator aiming at investigating the electron transport and survival over a 2.6 m drift path and the high voltage distribution for the DARWIN time projection chamber (TPC), among others. The first results from a several-months run with 343 kg of xenon were today published in the European Physics Journal. We deployed a 53-cm tall purity monitor immersed in the cryogenic liquid to measure the electron drift lifetime as a function of the xenon purification flow. After 88 days of continuous purification, the electron lifetime reached a value of 664 ± 23 ms. We also measured the drift velocity of electrons for electric fields in the range (25-75) V/cm, and calculated the longitudinal diffusion constant of the electron cloud in the same field range. For the next phase of the experiment, a 2.6 m TPC has been installed in Xenoscope, which in the next months will be operated in dual (gas-liquid) xenon phase mode in order to expand on these results for the entire DARWIN vertical scale.

The XENON collaboration reported first results on a search for Weakly Interacting Massive Particles (WIMPs) with the XENONnT experiment. The results were published in Physical Review Letters this week. No significant excess of nuclear recoil events with energies between 3.3 and 60.5 keV was found in a blind analysis. With this, the collaboration placed a best upper limit of 2.58 × 10⁻⁴⁷ cm² on the spin-independent WIMP-nucleon cross-section for a 28 GeV / c² WIMP at 90 % confidence level. The data analysis used 95.1 days of data acquired between July and November 2021 in a sensitive (4.18 ± 0.13) tonne inner volume of the XENONnT dual-phase time projection chamber that holds a total of 5.9 tonnes of liquid xenon. Together with the LZ and PandaX-4t experiments, XENONnT places the most stringent constraints on WIMP interactions and significantly improves on the previously leading XENON1T results. This was enabled by reducing the intrinsic ⁸⁵Kr and ²²²Rn concentrations in the liquid target to unprecedented levels, giving an electronic recoil background rate of (15.8 ± 1.3) events per tonne, year and keV in the region of interest. Since the conclusion of its first science run, XENONnT has continued its multi-year measurement campaign in order to improve constraints on and ultimately detect WIMP dark matter interactions.

Congratulations to Dr. Ricardo Peres who successfully defended his Ph.D. thesis at the end of June. During his thesis, Ricardo worked on the XENONnT detector (data analysis and 3D electrostatic field simulations) and on the DARWIN demonstrator (Xenoscope) in our laboratory at UZH.  In particular, he was involved in the analysis of XENONnT's first science run and in the effort towards a supernova neutrino signal detection with XENONnT. He was responsible for the top SiPM array in the Xenoscope 2.6 m tall TPC, in particular also for characterising all the employed units at cryogenic temperatures.

 Our group members Dr. Christian Wittweg, Gabriela R. Araujo and Paloma Cimental, have received Postdoc and Candoc grants from the Forschungskredit UZH Founds. Christian and Paloma, members of the XENON collaboration, are currently searching for 124-Xe double weak decays with data from the XENONnT experiment. Gabriela, a member of the PALEOCCENE collaboration, is working towards the characterisation and establishment of a scalable read-out of colour centers for passive detectors for neutrinos and dark matter. We congratulate Christian, Gabriela, and Paloma for this achievement!

Congratulations to Dr. Giovanni Volta who successfully defended his Ph.D. thesis at the end of May. During his thesis, Giovanni worked on the PMT array of the XENONnT time projection chamber (from PMT tests in LXe in our lab, to design and assembly of PMT cabling and of the array), on TPC assembly and underground installation, commissioning, as well as on the data analysis of XENONnT's first science run. In particular, Giovanni performed the first search for nucleon disappearance with XENONnT data. He was also involved in the TPC response characterisation, the study of optical parameter, and in estimating the electronic recoild background from detector materials.

Congratulations to Dr. Frédéric Girard who successfully defended his Ph.D. thesis at the end of April. During his thesis, Frédéric worked on the DARWIN demonstrator (Xenoscope) in our laboratory at UZH. He was involved in designing, building and commissioning a large, full-scale vertical demonstrator for  DARWIN, which was operated successufully in 2022 and in the design of the 2.6 m tall TPC. Frédéric also developed a data-driven electron drift lifetime model for Xenoscope's first run and was responsible for testing various VUV-sensitive SiPMs at cryogenic conditions in our laboratory.

The Xenoscope vertical demonstrator for the DARWIN experiment utilizes an array of silicon photomultipliers within its 2.6 m long time projection chamber (TPC). The array consists of  48 VUV4 MMPCs from Hamamatsu (model S13371-6050CQ-02) with a 12-channel readout and is placed above the liquid xenon column. It is operated as a light readout for the secondary proportional scintillation signals coming from extracted electrons in the TPC. The design and development of the silicon photomultiplier array of Xenoscope, covering the structural and electronic design, sensor characterisation at cryogenic temperature and signal simulation was carried out by PhD student Ricardo Peres. This work was recently published in JINST. 

The description of the production process and of the characterisation of low-neutron emission 228-Th sources for the calibration of the neutrinloess double-beta decay experiment LEGEND-200 was published todayin JINST 18, 2023. We actively contributed to the measurement and analysis of the neutron flux emitted by the sources, revealing a negligible contribution to the background level in the region-of-interest. Using source-insertion systems developed by our group at UZH, these radioactive sources are regularly deployed in the vicinity of the high-purity Ge detectors in LEGEND-200, which started a first science run in late 2022. The goal is to calibrate the energy response of the Ge diodes, to measure their resolution, and to tailor and validate pulse shape analysis algorithms.

Thefirst results from the XENONnT experiment were recently published in Physical Review Letters as an editors' suggestion. The experiment is honing in on electronic recoil events in hopes of clarifying an intriguing excess that appeared two years ago in the predecessor experiment, XENON1T. A stunningly low background was achieved (see figure, right), with the first science run having an exposure of ~1 tonne-year. Although hopes for new physics disappeared with the absence of an excess this time around, the XENONnT background spectrum is prominently adorned by none other than second-order weak processes. New, world-leading limits were set for solar axions, an enhancement in the neutrino magnetic moment, and bosonic dark matter. The search for WIMP dark matter is well underway, with new results coming soon.

From September 21st to 23th 2022, our PhD students Ricardo Peres and Paloma Cimental, and the postdoc Marta Babicz participated in the annual LIDINE (Light Detection In Noble Elements) conference in Warsaw, Poland. Ricardo presented his work on the design and development of the SiPM top array of Xenoscope, with a focus on their performance after the installation and characterisation in the dual phase LXe TPC Xurich II (slides). Marta, who recently joined the group, presented the results of her PhD work on the light detection system of the ICARUS detector (slides). Paloma presented a poster on the Xenoscope facility at UZH (poster). In her poster, Paloma presented the results achieved during the first measurement campaign and some of the technological challenges that are being addressed towards the next phase of Xenoscope. Paloma’s poster was recognised by the conference participants as the best, congratulations!

Our group’s PhD students Giovanni Volta and Ricardo Peres participated in the Universitas 21 Three Minute Thesis (3MT) Competition of the University of Zürich. 3MT is an academic contest of skills and expertise in the arena of science communication where participants have three minutes to bring their research across to a general audience. The event took place on the 14th of September at UZH Zentrum with twelve competitors from throughout UZH.

Giovanni presented The dark side of the Universe, a light-hearted climb into the mysterious world of dark matter and dark matter detection with liquid xenon. Ricardo presented Never miss a light show, a journey through the Supernova phenomena and its detection through massive particles, which won the competition’s Audience Award!

Our group and the astroparticle physics group in Freiburg (Germany) have worked on the last five years on research and development challenges towards the DARWIN multi-ton scale xenon observatory, an effort funded by European Research Council (ERC) grants. Some of our Freiburg colleagues, including the group leader Prof. Dr. Marc Schumann (a former member of our group) visited us for the 10th FR-UZH ERC meeting.

The discussions focused on the status of the full-height (Xenoscope, UZH) and full-with (PANCAKE, Freiburg) DARWIN demonstrators, as well as photosensor characterization, properties of liquid xenon, detector technologies for background suppression, software developments for experiment monitoring, and sensitivity studies of the DARWIN physics reach.

New results on two-neutrino double-electron capture of ¹²⁴Xe and neutrinoless double-beta decay of ¹³⁶Xe in XENON1T and XENONnT have been published as an editors' suggestion in Physical Review C with group alumna Chiara Capelli and postdoc Christian Wittweg as corresponding authors.

Two-neutrino double-electron capture occurs in ¹²⁴Xe and first evidence was observed by XENON1T already in 2019. Using more data and an advanced analysis, the new result exceeds the “5 sigma” discovery significance threshold. The measured half-life of the process is 1.1 × 10²² years – the longest half-life of any radioactive decay observed to date. The hypothetical neutrinoless double-beta decay of ¹³⁶Xe has not been discovered in any isotope by any experiment to date. Its measurement would signify that neutrinos are their own anti-particles and could give us deeper insights on why matter kept existing after the big bang. The paper shows that XENON1T, even though it was built to search for dark matter, starts to be competitive with purpose-built experiments. XENONnT, which is currently taking data, will be even more sensitive.

Congratulations to Dr. Yanina Biondi who successfully defended her Ph.D. thesis at the end of August! During her thesis, Yanina worked on the DARWIN demonstrator (Xenoscope) in our laboratory at UZH and on physics studies with the DARWIN observatory. She was involved in designing, building and commissioning a large, full-scale vertical demonstrator for  DARWIN, in particular in the purity monitor, which was operated successufully in 2022 and in the design of the 2.6 m tall TPC. Yanina also had leading involvement in the sensitivity study of DARWIN to the neutrinoless double decay of 136-Xe and she carried out a study on DARWIN's sensitivity to solar axions via the axioelectric effect and inverse Primakoff scattering.

Our group operates the high-purity germanium counting facility Gator at the Gran Sasso Underground Laboratory in Italy, at an average depth of 3600 meter water equivalent. Gator is dedicated to low-background γ-ray spectrometry and is employed for material screening and selection in ultra-low background, rare-event search experiments in astroparticle physics. The detector is equipped with a passive shield made of layers of copper, lead and polyethylene, and the sample cavity is purged with gaseous nitrogen maintained at positive pressure for radon suppression.

In our recent publication we describe the latest upgrades and performance of Gator, which currently yields a background rate of (82.0±0.7) counts/(kg⋅day) in the energy region 100 keV to 2700 keV, a 20% reduction compared to the previously reported rate. We further show the stability of various operation parameters as a function of time, summarize the sample analysis procedure, and demonstrate Gator's sensitivity by examining one material sample, a candidate photosensor for the DARWIN experiment.

Christian Wittweg was appointed as analysis coordinator for the XENONnT dark matter search experiment. He is joining a three-person team that organizes the data analysis within the collaboration of 170 scientists from 27 institutions in 12 countries. The XENONnT experiment is currently taking data at Laboratori Nazionali del Gran Sasso in Italy. It succeeds XENON1T which set world leading limits on interactions of dark matter particles with xenon. XENONnT will improve on these results and the physics program will be be expanded to searches for rare nuclear physics processes and neutrino interactions. With Christian as analysis coordinator the group continues its strong involvement in the XENON project, which includes data analysis as well as experimental hardware.

Gabriela R. Araujo has recently participated in the Humboldt Conference "Clues to a mysterious Universe - exploring the interface of particle, gravity and quantum physics". There, she presented a poster on LEGEND-200 and its materials used to shift the short-wavelength UV scintillation from liquid argon towards the visible spectral region, such as blue and green. In the poster, she presented some of the actual wavelength-shifting materials illuminated by blinking UV LEDs. For the distinctive poster, she was awarded the conference poster prize (one of 3 equal winners). Congratulations!

Three world leading collaborations, XENON, LUX-ZEPLIN and DARWIN, are joining forces to build the next generation xenon-based dark matter experiment. The consortium, under the name XLZD (not to be confused with xkcd), had their first joint meeting after signing the MoU. The XLZD meeting took place 27-29 June 2022 at the Karlsruhe Institute of Technology. Discussions included topics such as the science reach of the next generation observatory , xenon procurement and budget, the timeline, major design questions and scaling, and siting. The meeting concluded with the launch of the official XLZD website.

A member of our group, Ricardo Peres, has been awarded the UZH Postdoc Grant. This Grant goes to promising young academics who are embarking on a Postdoc and our group congratulates Ricardo for having been awarded it! He will continue as a member of the Xenon and DARWIN collaborations and his project will focus on the first observation of Boron-8 solar neutrinos in LXe TPCs and on photosensor R&D for DARWIN. 

As part of the R&D of the liquid argon instrumentation of LEGEND-200 and LEGEND-1000, our group has been characterizing wavelength-shifting materials such as tetraphenyl butadiene (TPB) and polyethylene naphthalate (PEN). In this scope, we have recently published the paper "R&D of wavelength-shifting reflectors and characterization of the quantum efficiency of TPB and PEN in liquid argon". In this work, the absolute quantum efficiency of TPB and PEN in LAr (at 87K) were estimated for the first time. A summary of the R&D of the wavelength shifting reflectors for LEGEND-200 and the results from the paper were recently presented by Gabriela R. Araujo at the neutrino and dark matter (NDM 2022) conference this month.

Laura Baudis was awarded the Charpak-Ritz Prize, jointly given by the French (Société Française de Physique) and the Swiss Physical Societies, for her leadership in international astro-particle physics collaborations, outreach activities and seminal contributions to dark matter research. The joint prize was established in 2016 to highlight the links between the two societies. It is called the “Charpak-Ritz Prize” in memory of Georges Charpak and Walther Ritz, French and Swiss physicists who left their mark on physics. The prize recognises outstanding contributions to the field of physics or to its development. Laura Baudis will present her award talk at the next meetings of the French and Swiss physical societies. See here the news at the Faculty of Sciences, UZH.

Congratulations to Dr. Kevin Thieme who successfully defended his Ph.D. thesis at the end of March! During his thesis, Kevin worked on the DARWIN demonstrator (Xenoscope) in our laboratory at UZH and on R&D with a small, two-phase xenon TPC. He was involved in designing, building and commissioning a large, full-scale vertical demonstrator for the DARWIN Observatory. Kevin also had leading involvement in the first two-phase xenon TPC operated with a top SiPM array, Xürich, and he analysed the Xürich data with respect to the W-value in liquid xenon. He obtained a value which is lower than the older, standard value used in the literature, but consistent with a more recent value from the EXO-200 collaboration.

We congratulate Dr. Patricia Sanchez-Lucas who received a grant within the Spanish research program Juan de la Cierva - Incorporación. The program supports young researches to conduct their scientific careers, covering the grantee's salary and initial lab expenses. Patricia will move to the University of Granada to work together with the Astroparticle Physics Group of Prof. Antonio Bueno and conduct R&D on liquid argon TPCs towards the DUNE experiment. Patricia is a postdoc in our group since 2017, working mainly on the DARWIN project, and in Granada she will continue her work on DARWIN as well. We congratulate Patricia and look forward to continue working together with her!

Vera Hiu-Sze Wu was a master student in our group and worked on the low-energy calibration of the GERDA experiment, and on the characterisation of wavelength shifting and reflective materials for the LEGEND-200 experiment with the Liquid Argon Setup (LArS) in our laboratory.  Her thesis was awarded the Dectris Prize for the "Best Master Thesis in Experimental Physics at UZH". The Experiment Physics Prize is an annual award given to an author of an exceptionally good Master thesis in the field of experimental physics at the University of Zurich. The Prize is awarded by the high-tech company Dectris Ltd. We congratulate Vera for this achievement and wish her all the best for her PhD studies with the DARWIN group of Prof. Kathrin Valerius at KIT!

On the fifth of November 2021, Laura Baudis became a member of the Academy of Sciences and Literature Mainz. The Academy is an association of personalities from the fields of science, literature and music and serves to nurture the dialogue between these fields and interdisciplinary exchange. The Academy has three main classes and Laura Baudisis a member of the mathematics and natural science class. 

On September 17th 2021, our Ph.D. students Gabriela Araujo and Giovanni Volta gave talks at the fifth LIDINE (LIght Detection In Noble Elements) conference. Gabriela presented the R&D work and characterization of wavelength-shifting reflectors for the liquid argon (LAr) veto of LEGEND-200, in construction at LNGS, and for future LAr-based detectors (slides). Giovanni, on behalf of the XENON collaboration, presented the current status of the 494 VUV-sensitive photomultiplier tubes installed in the new XENONnT time projection chamber, currently in data taking mode at LNGS (slides).

On September 16th 2021, our measurement of the mean electronic excitation energy in liquid xenon, commonly known as the W-value, was posted on arXiv. This value relates the energy deposition of a particle interaction in liquid xenon with the number of released scintillation and ionisation quanta and thus, determines the absolute microscopic energy scale of a liquid xenon detector. We obtained a value of (11.5 +0.2 -0.3) eV in our Xurich II TPC at UZH based on keV-scale calibration data with ³⁷Ar and ^83mKr and single electron events. This value is well-compatible with a measurement in EXO-200 from 2020 but lower than the commonly used value of (13.7+-0.2) eV measured by E. Dahl in 2009. On the right we show a schematic of the so-called Doke plot that visualises the measurement principle.

UPDATE (December 2nd): The article is now published in the European Physical Journal C: Eur. Phys. J. C 81, 1060 (2021). We thank our referees for the great input and feedback.

Laura Baudis and the Phd students Alexander Bismark, Yannick Müller, Ricardo Peres, and Giovanni Volta participated in the joint annual meeting of ÖPG and SPS. The meeting was held with in-person attendance in Innsbruck. Laura introduced DARWIN as a next-generation observatory for dark matter and neutrino physics which is projected to pursue a broad physics program: from probing WIMPs down to the neutrino floor to solar neutrinos, supernova detection and neutrinoless double-beta decay. Alex talked about the upgrade of the germanium counting facility Gator. This facility is used to screen and select radiopure materials for low-background experiments. Yannick presented the planned LEGEND neutrinoless double-beta decay experiment, which will supersede the current generation of germanium neutrinoless double-beta decay experiments with more active mass and lowered backgrounds. Ricardo gave an overview of the recent progress of XENONnT. XENONnT has finished commissioning and started taking science data. Giovanni presented his study on nucleon disappearance in XENON1T. The observation of such a process would provide crucial hints for understanding the baryon-asymmetry of the Universe.

In parallel M. Sc. student Simon Buse and postdoc Christian Wittweg attended the online-only TAUP 2021 conference. Although the conference took place online, the organizers had prepared a digital venue resembling the conference’s original destination Valencia. Simon presented a poster on new constraints on strongly interacting sub-GeV dark matter via electron scattering. These limits were derived using the small-scale above-ground Xurich detector at UZH. Christian’s poster discussed the detection prospects for the second-order weak decays of Xe-124 with upcoming large-scale XENON detectors such as DARWIN. In case of a positive detection of neutrinoless double-beta decay, ¹²⁴Xe decays could help with understanding the mechanism mediating the process.

Yanina Biondi and Gabriela R. Araujo, members of the DARWIN and LEGEND groups respectively, have recently received Postdoc and Candoc grants from the 'Forschungskredit' UZH funds. These grants go towards promising young academics who are embarking on a Postdoc or working on a PhD project. Our group congratulates Yanina and Gabriela for this achievement!

After months of remote conferences the Swiss institute of particle physics (CHIPP) convened for an in-person meeting in Spiez. Taking special care against the spread of COVID-19, 100 senior as well as young scientists from all of Switzerland enjoyed two days of plenary talks, poster sessions and in-person discussions. PhD student Gabriela Rodrigues Araujo and postdoc Christian Wittweg participated for the astro-particle physics group at UZH. Gabriela gave a talk on the Search for neutrinoless double beta decay with GERDA and LEGEND-200 and presented a poster on the "Wavelength-shifting reflectors for LEGEND-200". Christian discussed Direct dark matter detection: Recent results from XENON in a plenary talk.

On May 25th 2021, our Ph.D. student Yanina Biondi gave a talk at the fifth TIPP (Technology and Instrumentation in Particle Physics) 2021 conference, titled Purity monitor and TPC design for Xenoscope, where she discussed the technical challenges and the current design of these instruments. She discussed in detail the in-house photocathode production and its optimization, giving promising results for its instrumentation in the near future inside our Xenoscope facility.

On May 31st 2021, the first technical paper describing our Xenoscope facility was posted on arXiv. Xenoscope is a full-scale vertical demonstration platform which will be used to develop and test key technologies necessary to the realisation of the future DARWIN observatory. Our main objective is to demonstrate the drift of electrons over a distance of 2.6 m by investigating HV distribution and xenon purification methods. In this paper, we describe the different systems built into Xenoscope, such as the cryogenic and purification systems, two recovery and storage systems, the slow control, the purity monitor and the TPC. We also report on the successful commissioning of the Xenoscope facility. The first full run was performed over 40 days of continuous operation with ~80 kg of LXe. Next, Xenoscope will be outfitted with the first phase of the experiment, the purity monitor, by the end of Q2 2021.

On April 30th 2021 a new report describing the quality test of R11410-21 photomultiplier tubes for the XENONnT experiment has been posted on the arXiv. In this work, a total of 368 PMTs were characterized in liquid/gas xenon environments, thanks to an international collaboration with 4 institutions joining the effort: Stockholm University, University of Zurich, Max-Planck-Institut für Kernphysik in Heidelberg, and Weizman Institute of Science in Rehovot. One current PhD student, Giovanni Volta, and two former students in our group, Dr. Adam Brown and Dr. Julien Wulf, tested a total of 105 PMTs in our laboratory with the MarmotX liquid xenon detector, which can house ten PMTs at a time.  The new tests were targeted at the detection of light emission and the degradation of the PMT vacuum through small leaks, which can lead to spurious signals known as after-pulses, both of which were observed in the XENON1T experiment. The results were used to select the photosensors which were installed in the XENONnT time projection chamber, which contains two arrays with 494 PMTs in total: 253 in the top array placed in the gas phase, and 241 the bottom array in the liquid. The XENONnT experiment is currently under commissioning at the Laboratori Nazionali del Gran Sasso in Italy, with a first science run to start in May 2021. Given the improved photosensors characterisation procedure compared to XENON1T, we expect significantly better PMT performance in the new detector.

Today we received the central component of our liquid recovery and storage system for the Demonstrator, the Ball of Xenon (BoX), a pressure vessel that will hold our entire xenon inventory at room temperature. It was manufactured by KASAG Swiss AG in Langnau i.E. based on our conceptual design and will increase our recuperation speed and hence, the future R&D work significantly. A vacuum-insulated liquid line connects the bottom of the Demonstrator cryostat to BoX via which the xenon can be drained in liquid form. BoX will be implemented into the Demonstrator system in the next weeks and then complement the gas recuperation system that is already in place, both projects of our PhD student Kevin Thieme.

Our DARWIN subgroup looks back to a very productive and successful first quarter of the year. After the final assembly steps and following a thorough test campaign of all the cryogenics and gas systems of our full-height DARWIN Demonstrator, the first (commissioning) run was successfully conducted. It started at the end of February with the fill of about 80 kg of xenon and lasted until the beginning of April with a successful gas recuperation. During that period, the recirculation speed of the gas purification loop and the heat exchange efficiency was benchmarked and all the subsystems were studied in operation. We learned about the behaviour of the setup under different thermodynamic conditions and in particular about the procedures for filling, recirculation and recuperation. For that reason, we are very happy that everything went well and the systems work safely and stably as designed and expected. On the right, you can see a picture of the copper cold head taken through a view port on the cryo-tower. Liquid xenon is condensing in a small stream and returning from the heat exchangers in a larger stream. We are looking forward to the upcoming upgrades before the next run: The installation of the first stage of the purity monitor as well as the add-on liquid recovery system. 

We are happy to announce that two members of our group have been honoured with the XENON Medal 2020: the former PhD student Chiara Capelli (currently a postdoctoral fellow at LBNL) and the current PhD student Giovanni Volta. The medals were awarded at the last XENON collaboration meeting for Chiara and Giovanni's  outstanding contributions to the assembly and commissioning of XENONnT time projection chamber at the Gran Sasso Laboratory during the coronavirus outbreak in early 2020.

At this year’s International Workshop on Neutrino Telescopes, our PhD student Ricardo Peres presented a XENON talk on “Recent Results from XENON1T and Multi-messenger Future of XENONnT”. The 19th edition of the renowned conference was held online from the 18th to the 26th of February, covering topics from Large Detectors for Neutrino Astrophysics to Neutrino Physics and Cosmology. Ricardo’s talk focused on the main results from XENON1T and the future of XENONnT as a neutrino observatory and potentially a multimessenger prototype, alongside its goal to search for particle dark matter. The talk can be found here (PDF, 6 MB)and a recording of the session here.

The wavelength shifting reflectors are a key component of the liquid argon veto of the new LEGEND-200 experiment. They will increase its background vetoing efficiency by both wavelength shifting VUV light into the visible region and by reflecting visible light. Our LEGEND team participated in the development of these reflectors as well as its evaporation with the wavelength shifting (blue-emitting) compound TPB. The entire hardware for moving the evaporator in vacuum for several meters inside the cryostat was developed at UZH by our LEGEND team along with the electronics and mechanical workshop. The hardware assembly and evaporation was undertaken underground at the Gran Sasso Laboratory at the end of 2020 and the reflectors were successfully coated in situ with vacuum evaporated TPB.

Congratulations to Dr. Chloe Ransom who successfully defended her Ph.D. thesis at the beginning of February! During her thesis, Chloe worked on the GERDA and LEGEND experiments to search for the neutrinoless double beta decay of ⁷⁶Ge. Her work was focussed on the energy calibration analysis, which is fundamental for the determination of the energy scale and resolution of the germanium detectors. Apart from increasing the ability to identify a sharp peak at the Q-value of the 0νββ-decay, an excellent resolution also minimises the irreducible background from 2νββ-decays. As an example, her results served as input to the final GERDA analysis, which achieved the world’s best lower limit on the half-life of the decay, 1.8 x 10²⁶ years (90% C.L.).

Our DARWIN team completed another important milestone in the construction of the full-length DARWIN demonstrator. Last week we installed the cryogenic tower, which houses the systems that allows us to liquefy the xenon and keep it cold in a controlled way. The construction of the cryogenic tower followed the meticulous design made by Frédéric Girard as a part of his PhD thesis work. With this step the entire cryogenic system is assembled and we are ready to start the commissioning of the full-length DARWIN demonstrator facility.

GERDA has reported its final results on the search for the neutrinoless double-beta (0νββ) decay of ⁷⁶Ge in the recent issue of Physical Review Letters. No signal has been observed, but all goals of the final phase of the experiment have been achieved.

The reported lower limit for the 0νββ half-life of 1.8x10²⁶ yr coincides with the expected value for the sensitivity of the experiment; a more stringent value for the decay of any 0νββ isotope has been measured never before. Similarly, the reported background rate of 5.2x10^-4 counts/(kg∙yr∙keV) in the signal region is second to none in the field, demonstrating not only the feasibility of a background-free experiment at high exposure but also providing the foundation for a next generation experiment with significantly higher sensitivity. More details can be found here.

Congratulations to Dr. Chiara Capelli who successfully defended her Ph.D. thesis at the beginning of December! During her thesis work, she performed a first search for the neutrinoless double beta decay of 136-Xe with XENON1T data. In particular, she worked on the signal reconstruction in the MeV energy range and achieved the best energy resolution to date in a two-phase xenon time projection chamber, namely 0.8% at 2.458 keV, the Q-value of the 136-Xe decay. This work was published and featured in EPJ-C 80 (2020) 8, 758. For XENONnT, she was responsible for the design, construction and installation of the light calibration system of the VUV-sensitive photosensors, currently used for calibrating the PMTs in the liquid xenon phase. Chiara Capelli also participated in the installation and initial commissioning of the XENONnT TPC: she remained at LNGS (together with another PhD student in our group, Giovanni Volta) during the start of the new pandemic and completed the installation of the TPC underground. 

Every year in November - during an event organised by our department - we present with posters and flyers our research topics to prospective bachelor and master students. In 2020, the organisation of such an event was not possible due to the corona pandemic. Our students and postdocs thus created three short videos to present our main research on dark matter, neutrinoless double beta decay and other topics in the framework of the DARWIN, GERDA/LEGEND and XENON projects. You can find these videos under this link. If you are a prospective bachelor and master student, and interested in a concrete project, please get in touch! We also offer small R&D projects in our laboratories in the Physics Department.

During this month, the DARWIN experiment has received considerable attention from the media. Two articles have appeared, one in the Nature News and the other in the German magazine Spektrum, describing the DARWIN project and its huge effort to give the WIMPs the last chance to be detected. We want to congratulate the whole DARWIN Collaboration and recommend to everybody the interesting reading.

In a search for solar axions, bosonic dark matter, and an enhanced neutrino magnetic moment, the XENON1T collaboration found an excess of events in the low-energy electronic recoil region. The collaboration is not making any claims about its nature, as the source of these events is not yet known. The signature of the excess is similar to what might result from a tiny residual amount of tritium in the detector, but could also be a sign of something more exciting—such as the existence of solar axions or the indication of previously unknown properties of neutrinos. The results are now published in Physical Review D and featured in a Viewpoint article in the journal entitled “Dark Matter Detector Delivers Enigmatic Signal”. The results also yielded a flurry of theoretical explanations of the excess, a synopsis of which is featured in the Physical Review, entitled "Theorists React to Potential Signal in Dark Matter Detector".

Congratulations to Dr. Adam Brown who successfully defended his Ph.D. thesis on October 1st! During his thesis work, he performed a study of inelastic dark matter interactions in XENON1T, tested photomultiplier tubes for the XENONnT experiment with our MarmotX liquid xenon detector and built the bases, connectors and cables necessary to power and readout the signals from the PMTs.  Adam Brown was also involved in the commissioning of the XENONnT time projection chamber, especially in the assembly of both photosensor arrays at LNGS. 

Our DARWIN team is making progress towards the commissioning of the full-length DARWIN demonstrator, under construction here at the University of Zurich. This week we installed the heat exchanger, a component that will increase the efficiency of our recirculation system by reducing the cooling power requirements. The successful installation follows the detailed study and design by PhD student Kevin Thieme. We are looking forward to the next step: the installation of the cryogenic tower.

The DARWIN Collaboration Meeting was held from September 9 to 11, 2020. This time the meeting had to be online since given the covid-19 situation we could not travel to LNGS as previously planned. During three days we discussed the status of the R&D projects towards DARWN as well as the sensitivity and background studies which are ongoing. Our group contributed with 6 presentations. Thanks to the effort of all the participants we had a very successful and fruitful virtual meeting; however we hope to meet in person next time.

The DARWIN Collaboration has recently published the projected sensitivity of the DARWIN observatory to the neutrinoless double beta decay of ¹³⁶Xe. The analysis, based on a detailed Monte Carlo simulation of the background, shows that the experiment can achieve a half-life sensitivity of 2.4 x 10²⁷ years for an exposure of 5 t (natural xenon) and 10 years.  Our group has significantly contributed to this study, which confirms that DARWIN, the ultimate  dark matter experiment, will be in addition comparable in its science reach to dedicated double beta decay experiments. The details of the analysis can be found in Eur. Phys. J. C 80. 808 (2020).

Our student Livio Redard-Jacot recently completed their Bachelor thesis as part of the group. During this unusual online semester, he successfully completed the simulated electron recoil background spectrum of the DARWIN experiment at the low energies (below 1.5 MeV). With these simulations he estimated the sensitivity of DARWIN to solar axions and axion-like particles, a topic receiving a lot of attention since the last results of the XENON Collaboration. The details of the analysis can be found in his thesis, titled "Sensitivity of the DARWIN experiment to axions and axion-like particled." (PDF, 3 MB)

The XXIX International Conference on Neutrino Physics and Astrophysics (Neutrino 2020) was organized by Fermilab and held online from June 22 to July 2, 2020. It is an important international conference series held biannually in the field of neutrino physics, where progresses on both theory and experiment are presented.

Our group contributed a total of 6 posters on XENON1T/nT, DARWIN, GERDA, and LEGEND-200 experiments:

1. Analysis of High Energy Events in XENON1T (Chiara Capelli)
2. The XENONnT direct Dark Matter detection experiment (Giovanni Volta)
3. Sensitivity of the DARWIN observatory to the neutrinoless double beta decay of 136-Xe (Yanina Biondi)
4. Solar Neutrino Detection Sensitivity in DARWIN via Electron Scattering (Shayne Reichard)
5. Energy Calibration for the GERDA Experiment (Junting Huang, Chloe Ransom)
6. The calibration system of the LEGEND-200 experiment (Yannick Müller)

We congratulate our group member Dr. Junting Huang for having been awarded the Forschungskredit Postdoc Fellowship 2020 for his project "Lepton and Baryon Number Violation Searches with High Purity Germanium Detectors". Junting is a member of the GERDA and LEGEND collaborations, which operate ultra-pure germanium crystals in a large liquid argon cryostat to search for neutrinoless double beta decay and other rare interactions.

Today we announced that data from our XENON1T detector, the world's most sensitive dark matter experiment, show a surprising excess of events. We do not claim to have found dark matter. Instead, we have observed an unexpected rate of events, the source of which is not yet fully understood. The signature of the excess is similar to what might result from a tiny residual amount of tritium, but could also be a sign of something more exciting—such as the existence of a new particle known as the solar axion or the indication of previously unknown properties of neutrinos. Read the press release issued by the university and faculty. The manuscript detailing the analysis is here: arXiv:2006.09721.

We have built the first dual-phase xenon TPC (Xurich II) with a SiPM top array. It provides a good spatial resolution of ~1.5 mm for the horizontal event position reconstruction. This novel detector concept was characterised down to energies of 2.82 keV with internal ³⁷Ar and ^83mKr sources. The figure on the right shows the anti-correlation of the obtained charge and light yields at varying drift fields which we also compared with predictions from NEST. Read the full story in Eur. Phys. J. C 80, 477 (2020).

Postdoc Neil McFadden was awarded the Physics Department Chair's Award for Best Dissertation from the University of New Mexico, where he defended his thesis. His thesis defense, titled "Studying Properties Of Xenon Doped Argon and Developing Optical Simulation Techniques for the LEGEND collaboration, a Neutrinoless Double Beta Decay Experiment ", granted him the honor of graduating with Distinction. 

We are very happy to announce that our former bachelor student (now master) Stefan Hochrein was honoured with the Semester Award Autumn-2019 for his outstanding scholarly work. He conducted his bachelor thesis on Kr-83m calibration of the first dual-phase xenon time projection chamber with silicon photomultiplier readout in our group and was supervised by our phd student Kevin Thieme with the help of our postdoc Patricia Sanchez. We wish him all the best for his promising future.

The installation of the XENONnT Time Projection Chamber (TPC) was successfully completed at the Gran Sasso Underground Laboratory (LNGS). Several PhD students and postdocs in our group participated in the assembly and installation on site. In particular, Adam Brown, Chiara Capelli and Giovanni Volta remained at LNGS and continued to work underground during the difficult time of the Covid-19 pandemic, with a few members from the collaboration. Together, they ensured that the photosensors and the light calibration system are operational, and that the TPC is in a safe mode in vacuum, inside the cryostat. The XENONnT experiment will start the commissioning phase later in 2020, once access to LNGS is restored.

Two new  members recently joined the group working on XENON, DARWIN and LEGEND. Alexander Bismark, from Freiburg University is a new PhD student working on XENONnT and the future DARWIN. Neil McFadden, a postdoc who completed his PhD at the University of New Mexico, Albuquerque, is working on the DARWIN demonstrator and on the LEGEND project. We are happy to welcome them in our group!

Congratulations to Dr. Michael Miloradovic, who very successfully defended his Ph.D. thesis on January 30. His thesis work deals with the characterisation of new, inverted coaxial detectors for GERDA, with a background model for the natural Ge detectors, with pulse shape simulations for the BEGe and inverted coaxial detectors, as well as with the production and characterisation of new, low-neutron emission calibration sources for the last phase of the GERDA project.

We would like to congratulate three students who successfully worked with us in the second half of 2019: Basak Çigdem Özcan worked on the characterisation of SiPMs as potential replacements for PMTs in xenon detectors. She is now finishing her studies in physics at the Bilkent University in Turkey. Nicole Schermer tested new SiPM pre-amplifier boards with low thermal output for use in liquid xenon. She is now continuing her master studies at the Technical University of Munich, in Germany. As part of her masters thesis project, Rachana Yajur worked on light collection efficiency simulations for SiPM rings in the 2.6 m tall DARWIN demonstrator TPC. She also tested a SiPM in a TPC mock-up in order to validate the simulation results. She is completing her masters studies at the Indian Institute of Technology in Madras. We thank all three students for the hard work and wish them much success for the future.

XENON1T has now published the world-leading constraints on light dark matter in Physical Review Letters as an Editor's Suggestion. XENON1T’s new letter details an “S2-only analysis”, where events without S1s are still considered. Advances in detector construction and analytic techniques have led to a background levels that is one thousand times lower than what was achieved in previous S2-only searches.

For example, the S2 electron cloud becomes broader as it drifts upward, like a drop of ink that spreads out in water. The deeper the event, the broader the cloud, and the longer the S2 signal lasts. Thus, XENON1T could reject most of the events at the top and bottom, even without the S1, by rejecting very short and very long S2 signals.

Theorists predict that dark matter would collide with the heavy xenon nuclei and produce nuclear recoils. In this case, the S2-only technique is sensitive to energies 2-3 times lower than traditional analyses. We consequently find improved constraints on light dark matter. On the other hand, dark matter might collide with electrons that surround the nucleus, producing electronic recoils. These interactions create much larger S2 signals than nuclear recoils of the same S1 size. S2-only searches thus improve the energy threshold for these models by as much as a factor of ten. Combined with the lower background, XENON1T’s S2-only results enhance the constraints on such models by several orders of magnitude.

This letter was also featured in a synopsis from APS Physics.

Three new members recently joined the group working on GERDA/LEGEND: Yannick Mueller from Karlsruhe Institute of Technology, Gabriela Rodrigues Araujo from the Technical University of Munich, and Junting Huang from the University of Texas at Austin. Yannick and Gabriela are Ph.D. students, and Junting is a postdoc. We are happy to welcome them in our group!

The latest results from the GERDA experiment were published in Science this week. GERDA reached the 1.1x10²⁶ y sensitivity to the half-life of the neutrinoless double beta decay of 76-Ge. Combined with other experimental results ussing 76-Ge detectors, the sensitivity to the effective Majorana neutrino mass is 0.07 - 0.16 eV. The leading performance of GERDA in terms of background suppression, energy resolution and sensitivity opens the way to LEGEND, a next-generation Ge experiment with sensitivity to half-lives of 10²⁷ y. A first phase, with 200 kg ⁷⁶Ge is under preparation at LNGS.:DOI: 10.1126/science.aav8613 and arXiv:1909.02726

Under this motto, the University of Zürich and the ETH organised the Zürich science days of this year. Scientifica 2019 took place from Friday, August 30 to Sunday, September 1 and it was a great success. During these three days, more than 30,000 visitors took the opportunity to discuss directly with the researchers about science facts that sound like science fiction today, but which might be commonplace tomorrow. Our group participated actively in the booth called “Dark Matter-Antimatter-Does it matter?” and in the opening ceremony with a talk called "Die dunkle Seite des Universums". Thanks to all the participants (Yanina Biondi, Chiara Capelli, Ricardo Peres, Giovanni Volta, Patricia Sanchez and Laura Baudis) for sharing their enthusiasm for our research with the numerous visitors.

Congratulations to Dr. Rizalina Mingazheva, who very successfully defended her Ph.D. thesis on August 29. Her thesis work deals with the energy calibration of the GERDA experiment, as well as with background studies and new physics searches. In particular, she performed the first search for bosonic super-WIMPs with masses up to 1 MeV with GERDA data. Rizalina will stay with our group for another four months, as a postdoc on GERDA and LEGEND.

Three students recently completed their Bachelor thesis as part of the group. Stefanie Javet analysed data from a local experiment to search for an annual modulation in the decay rate of a variety of beta-decaying isotopes, placing an upper limit on the size of any such modulation, in her thesis (PDF, 5 MB) titled "Data processing and analysis of the modulation experiment”. Simon Buse contributed to the ongoing effort searching for neutrinoless double-beta decay (0νββ) in XENON1T, performing the calibration of O(MeV) energy deposits and selecting the volume of XENON1T with the best sensitivity to 0νββ, in his thesis (PDF, 10 MB) titled “Energy Calibration and Fiducial Volume Selection for the XENON1T Experiment in the Neutrinoless Double Beta Decay Regime”. Stefan Hochrein demonstrated the potential advantages of silicon photomultipliers (SiPMs) for future large dual-phase xenon time projection chambers (TPCs) by performing a ^83mKr calibration of the local Xurich detector, a dual-phase TPC recently upgraded with SiPMs, in his thesis (PDF, 11 MB) titled "Calibration of the first dual-phase xenon time projection chamber with silicon photomultiplier readout”.

In the context of the ERC-funded Xenoscope project, our group is designing and building a full height DARWIN demonstrator. Last week, technicians of FMS-Technik AG together with our PhD students Frédéric and Kevin installed the 4 m × 5 m × 4 m support structure of the demonstrator in the assembly hall of the Physik Institute of the University of Zürich. This first milestone  enables us to start the assembly of the various systems of the detector. 

Watch a time-lapse video of the assembly by clicking here!

Our DARWIN PhD student Yanina Biondi was selected to participate in the 57th International Erice School of Subnuclear Physics "In search for the unexpected". She presented a poster about "Xenoscope: Towards DARWIN – the Ultimate Dark Matter Detector" and was awarded the diploma for the best poster. She was a very active participant at the school and was also awarded the Bjorn Wiik diploma for new talents in physics. Congratulations Yanina!

From June 24 to June 26 we hosted the GERDA collaboration meeting in the Main Building of the University of Zurich. The first two days were spent to discuss the status of the experiment and analysis work (summary of data taking, the calibration and energy scale, the background model and the pulse shape analysis) as well as exotic searches (e.g., searches for SuperWIMPs, or for a bosonic component of neutrinos). We also had a guest talk from Patricia Sanchez on neutrinoless double beta decay searches with DARWIN. The last day was dedicated to an in-depth analysis discussion and plans for the unblinding of the last GERDA data set in spring 2020. GERDA will end its last run in December 2019, when the works for LEGEND-200 at LNGS will start.  All the participants enjoyed a lovely dinner and boat cruise on Lake Zurich with MS Bachtel in the evening of June 24!

Two-neutrino double electron capture (2νECEC) is a second-order weak-interaction process with a predicted half-life that surpasses the age of the Universe by many orders of magnitude. Until now, indications of 2νECEC decays have only been seen for two isotopes, ⁷⁸Kr and ¹³⁰Ba, and instruments with very low background levels are needed to detect them directly with high statistical significance. The 2νECEC half-life is an important observable for nuclear structure models and its measurement represents a meaningful step in the search for neutrinoless double electron capture—the detection of which would establish the Majorana nature of the neutrino and would give access to the absolute neutrino mass.

The XENON collaboration has detected the first direct observation of 2νECEC in ¹²⁴Xe with the XENON1T dark-matter detector. The significance of the signal is 4.4 standard deviations and the corresponding half-life of 1.8 × 10²² years (statistical uncertainty, 0.5 × 10²² years; systematic uncertainty, 0.1 × 10²² years) is the longest measured directly so far. This study demonstrates that the low background and large target mass of xenon-based dark-matter detectors make them well suited for measuring rare processes and highlights the broad physics reach of larger next-generation experiments.

The results are now published as a Nature Letter and can be found here: Nature Volume 568 Issue 7753, 25 April 2019 (arxiv link coming soon).

The XENON1T team has recently disassembled the Time Projection Chamber (TPC) at the Laboratori Nazionali del Gran Sasso to prepare for the installation of the new XENONnT detector. Shown at the right is a picture taken during the photomultiplier tube (PMT) array disassembly. After removal of the bottom array of PMTs, one can see the electrodes, reflectors, and top array of PMTs inside the detector. The PMTs will be safely stored until installation in XENONnT later this year.

The 2019 Science Info Day at the University of Zurich took place on 9 March at Irchel Campus. The Science Info Day is the Faculty of Science's orientation event for prospective students. During this day, high school students, their parents, and teachers could learn about the degree programs offered by the Faculty of Science. The program included guided tours, short presentations and experiment demonstrations. This year, two members of our group, Yanina Biondi and Patricia Sanchez, showed our labs and explained the R&D activities which we are working on.

Congratulations to our former master student Chris Marentini, who was honored with the Dectris-Award for the best Master Thesis in experimental physics. His Master Thesis was conducted within our group on "Characterization of Novel VUV-Silicon Photomultipliers and their Application in Xenon-Based Dual-Phase TPCs" (PDF, 10 MB), which was supervised by Dr. Julien Wulf. We wish him all the best in his future endeavors.

We are pleased to announce that the meeting of the DARWIN Collaboration will take place in Zürich from Monday, 17th December 2018, to Tuesday, 18th December 2018, and it is organised by the University of Zurich. The meeting programme consists of  plenary talks and discussions about R&D towards DARWIN, design considerations, MC simulations, sensitivity studies and science channels. All information related to the meeting and the final programme can be found in the indico website.

The II South American Dark Matter Workshop was held in São Paulo last week, organised by the South American Institute for Fundamental Research (ICTP-SAIFR). Scientists from South America and all over the world met to discuss the latest and most important results in all Dark Matter search branches: theoretical, collider, direct and indirect, and astrophysical. Our group gave two talks: "DARWIN: the ultimate dark matter detector" (Yanina Biondi) and "Status and Results from the XENON1T Dark Matter Experiment" (Adam Brown).

Silicon Photomultipliers (SiPMs) are a promising solid-state alternative to the widely used Photomultiplier Tubes (PMTs) in rare-event searches with liquid xenon as target material. We have characterised a vacuum ultraviolet (VUV) sensitive SiPM down to temperatures of 110K and in liquid xenon at 185K. Furthermore, we studied the radioactivity of the raw SiPM materials. The detailed description of our cryogenic setup and the conducted SiPM measurements are published in JINST 13 (2018) no.10, P10022.

Congratulations to Dr. Julien Wulf, who very successfully defended his Ph.D. thesis on September 20th. Julien will stay with our group for another six months (starting October 1, 2018) as a postdoc on XENON/DARWIN.

During the last week of August, the annual meeting of the Swiss Physical Society (SPS 2018) and the TeV Particle Astrophysics conference (TeVPA 2018) took place in Lausanne and Berlin respectively. Our group was represented at both events. The current status of the projects XENON1T, GERDA and DARWIN was presented in 5 talks at the SPS meeting. Likewise, two contributions to TeVPA 2018 showed the latest results from GERDA and XENON1T.

• Contributions to the Nuclear, Particle and Astrophysics session at SPS meeting.
• Contributions to TeVPA conference (XENON1T and GERDA).

Dr. Roman Hiller, currently searching for the neutrinoless double beta decay with the GERDA experiment in our group, was awarded a Forschungskredit fellowship in 2018. The Forschungskredit award supports young scientists up to postdoctoral level to carry out a research project at the University of Zurich. Roman Hiller received the grant to continue his neutrino physics research in our group for one year starting in August 2018.

The XXVIII International Conference on Neutrino Physics and Astrophysics, Neutrino 2018, was held in Heidelberg last week, organised by MPIK and KIT. Scientists from all over the world met to discuss the last and most important results in the field. Our group showed three posters: Neutrino physics with DARWIN (Patricia Sanchez), Double beta decay with XENON1T (Chiara Capelli) and Calibration with GERDA (Rizalina Mingazheva).

• DARWIN: an excellent environment to probe neutrino physics
• Search for the neutrinoless double beta decay of 136-Xe with the XENON-1T experiment
• Energy calibration of the GERDA experiment

One of the highlights of this year's Neutrino conference were new results from the GERDA experiment: with a large data release, GERDA has doubled its exposure, for a total of 82.4 kg years. Improvements in background suppression techniques have brought down the background expectation to 0.1 events per exposure in the energy region of interest of the decay. GERDA is now the most sensitive experiment in the field, with a  half-life sensitivity of 10²⁶ yr for the neutrinoless double beta decay of ⁷⁶Ge.

No signal was detected yet, and an upper limit on the half-life could be set: a ⁷⁶Ge atom takes on average longer than ten quadrillion times the age of the universe to decay via this rare process which is not allowed in the Standard Model of particle physics and would violate lepton number conservation by ΔL = 2.

Since December 2015, the GERDA experiment restarted its search for lepton number violation via the neutrinoless double beta decay of germanium-76. The first 6 months of data were released in June 2016. With the extended detector mass of 36 kg and an unprecedented low background of 0.001 cts/keV kg yr, the highest sensitivity ever reached with germanium detectors was already equaled (from Phase I of the experiment, 2011-2013). This was achieved with a new background suppression technique, by using the liquid argon cryostat of the detector as a scintillator to veto background events. The new limit on the life time of the decay, as well as the success of the new technique and the record background were reported in the April issue of Nature 544, 47–52 (2017).