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Dark matter under Black Hills

Thanks to new arrival Björn Penning, the University of Zurich is now a member of another epic dark matter experiment.

Deep beneath the Black Hills in the USA, scientists are using a highly sensitive detector to search for dark matter. Since the start of the year, the University of Zurich joined the programme through its new professor Björn Penning. This means that Zurich is now involved in both leading dark matter experiments, and there are already plans for the future...


Deep down below these hills, several particle physics experiments are underway to uncover the secrets of dark matter and neutrinos.

Deep down below these hills, several particle physics experiments are underway to uncover the secrets of dark matter and neutrinos.

Somewhere in the middle of the United States, a remote mountain range called the "Black Hills" rises out of the vast plains of the Midwest. Somewhere in the middle of these Black Hills - so called because they appear black from a distance due to the evergreen fir trees - at 1600 metres above sea level lies the town of Lead, South Dakota. "Miles beyond ordinary" is the town's slogan, and by European standards there really is not much that is ordinary up here. There are log cabins made of thick tree trunks, few buildings are taller than two storeys, the shopping street features wooden works of art carved with chainsaws and the Moonshine Gulch Saloon looks like something out of a wild west picture book. It feels a bit like being in a theme park. In Lead and its neighbouring town of Deadwood, however, this is completely normal - almost like stepping back in time to the "Wild West".


South Dakota
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The area around Lead, South Dakota, has retained much of its old Wild West charm. For example neighbouring Spearfish Canyon was one of the filming locations of Dances with Wolves.


Bar in South Dakota
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This is not a staged event for tourists, just a regular bar in the neighbourhood.

At the time of the gold rush, almost 10,000 people lived in Lead and sought their fortune in the gold mines that cut deep into the rock of the Black Hills. Today, Lead has just under 3,000 inhabitants, most of whom are white, and time seems to have come to a standstill. The Black Hills are actually sacred ground for the Native Americans and it was guaranteed by treaty with the Native Americans in 1868 that no white settlers would be allowed to settle there. However, laws get rewritten when gold comes into play. After gold was discovered during an expedition into the Black Hills in 1874, the US government quickly took possession of the area, relocated the local Lakota tribe to reservations and authorised the establishment of the Homestake Mine. It is considered the largest and deepest gold mine in North America; its tunnels reach down to a depth of around 2,400 metres. Until 2001, it was used for gold mining.

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Sunrise over the headframes of the Homestake Mine, where the LZ experiment is searching for dark matter. Image: Sanford Underground Research Facility / © South Dakota Science and Technology Authority

Access to the mine

The access shaft to the "cage", the lift that takes people two kilometres underground in twelve minutes.

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A full cage

Lorry System
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Drives like these connect the two main shafts using the old lorry system

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The Sanford Underground Research Facility may still look like a gold mine in parts, but it is now a research laboratory

The gold rush may be long over, but the search is still on in the mines - not for gold, however, but for rare particles. UZH newcomer Björn Penning explains why people want to conduct particle physics experiments in such a remote and inaccessible place. The particle physicist took up his professorship at the University of Zurich at the beginning of the year and not only added his experiment to the university's portfolio, but also brought a whole team with him from Ann Arbor in Michigan. Together with colleagues from all over the world, the team is searching for evidence of dark matter in the deep drives and shafts of the Homestake mine. The many tonnes of rock around it create a unique setting: excellent shielding against the constant rain of cosmic particles that can otherwise interfere with measurements of rare physical events. "Our detector constitutes one of the least radioactive places in the world," says Penning. "Everything about it is ultra-clean." And that doesn't just mean the tonnes of rock that keep the cosmic particles out, but also possible contamination from used materials, dust and noise.

Bjoern Penning
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Björn Penning, a new addition to the UZH team, brought everything with him: his experiment, his team and his family

Access Control
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Every person in the shafts and tunnels must be accounted for…the tags for the scientists

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The DUNE neutrino experiment will also be installed in the Sanford Underground Research Facility. The picture shows excavations for that.


All this makes the Lux-Zeplin or LZ detector the most sensitive detector in the world for dark matter particles called WIMPs. WIMP stands for "weakly interacting massive particle", and researchers suspect that the universe could be full of WIMPs. However, since they hardly interact, we cannot see or study them using conventional methods. Their detection would be quite a sensation because it would prove the existence of dark matter - a substance that is thought to be more than five times more abundant than conventional matter, but has yet to be found. With its WIMP-sensitive system, LZ specialises in one of the possible candidates for dark matter.

The heart of the detector is a chamber filled with seven tonnes of liquid xenon. When a WIMP hits a xenon atom and interacts with it, the particle collisions in the xenon produce flash of light, which the researchers can use to determine what kind of particle has interacted in the xenon. Surrounding this chamber there are various shields, two cryostats to keep the xenon liquid and prevent heat transfer, and an external detector. The entire detector, which is around 1.50 metres high and almost as wide, is located in an enormous water tank - like an oversized, highly sensitive Matryoshka doll.  (Video)

LZ Experiment
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The LZ outer detector and central cryostat

Penning himself is responsible for the outer liquid scintillator detector, also known as the "veto" detector, because that is precisely its task: to exclude certain events – to veto them. "We tell the inner detector which events it doesn't even need to look at," explains Penning. The detector chamber, which is surrounded by light sensors, is filled with a scintillator liquid that interacts very strongly. The walls are white so that the particles of light, photons, are reflected until they hit a sensor. "The problem is that sometimes a neutron interacts in the inner detector, which looks quite similar to a WIMP in its light signal. If our detector has recognised it as a neutron, it vetoes it." This way, nobody gets unnecessarily excited because they think they have discovered dark matter...

At work
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Penning working on the detector. Everything has to be kept very clean sto not introduce radioactive contaminations that would interfere with the signals

Veto Team
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Björn Penning's veto team in their detector

The veto detector is so sensitive that it can detect individual photons. It has been in operation since 2020 and is constantly reading out data. "We are dealing with an insanely large amount of data," says Penning. "We see billions of events per year and have to filter out the ones that could be interesting. That's one or two events per year."

The LZ experiment was constructed at the Sanford Underground Research Facility SURF in 2019. As if that wasn't adventure enough - imagine a twelve-minute journey in complete darkness in the 100-year-old cage and the careful transport of high-tech equipment by gold mine carts into the underground caverns hijacked by experimental physics - the pandemic hit shortly afterwards, making commissioning quite difficult. Nevertheless, the researchers at LZ were able to publish their first results in July 2022 and are presently the world's most sensitive dark matter experiment.


Event Display
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An event display from LZ. The inner detector photosensors are displayed in yellow and the outer detector ones in blue. One can see the cryostat and outer detector vessels in light green. The bottom shows light pulses measured in the different subdetectors.

Penning is actually a Higgs specialist with research experience at the Tevatron in the USA and the Large Hadron Collider in Geneva. He did his first dark matter studies on colliders there when he became a research fellow at Imperial College in London. A permanent position at the University of Bristol brought him into contact with direct detection experiments for dark matter for the first time. With a move to Brandeis University in the United States came a complete switch to the LZ experiment; large detector components were built at Brandeis by Penning and a team of students and postdocs. This was followed by a call to the University of Michigan - a dual appointment for him and his wife, an astrophysicist - where the whole team joined. A dual career appointment also brought him to Zurich, again with the whole team plus their son, something that also makes the German grandparents very happy. When you grow up in the Black Forest, which is also named after fir trees that look black from afar, it is almost a matter of predestination to work on an experiment in the Black Hills that searches for dark matter...

With its involvement in LZ and the XENONnT experiment based at the Gran Sasso laboratory in Italy, the University of Zurich is the only one in the world to be involved in the two leading direct detection experiments. So it will definitely be involved if one of them detects dark matter! "We need each other to confirm the results. The dark matter community is cooperative," says Penning. The various collaborations around the world are working on a concept for a future joint project, the "XENON DARWIN LUX-ZEPLIN" experiment - which, with a target mass of many tens of tonnes, would not only be even more sensitive to WIMPs, but could also work on other areas of research such as other dark matter candidates, neutrinos, supernovae and more. Like the small town of Lead in South Dakota, this experiment promises to be anything but ordinary...


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Rendering of the planned XLZD experiment, a future experiment planned by Laura Baudis and Björn Penning with international collaborators to extend the search for dark matter.

Author: Barbara Warmbein
© images: Björn Penning, Adam Gomez / Sanford Underground Research Facility, Matthew Kapust / Sanford Underground Research Facility