Xenon Exhibition

What makes up the largest part of our universe? What influences the movement of whole galaxies without being visible? What is the Xenon Experiment, anyway, and why does it have to be done underground?

You may know xenon as the gas in modern car headlights, but what does it have to do with dark matter? Xenon plays a crucial role in the search for one of the great mysteries of the universe, and UZH is part of  international experiments trying to shed light on it.
In particular, UZH developed a detector filled with XENON gas. Meanwhile, further research is being carried out deep underground. Why? How? You can find out here above ground, and have a look at an earlier version of the detector.

1.4. km under the mountain range ...

Dark matter is present in the Universe, and its mass influences the trajectories of stars in galaxies and the movement of the galaxies themselves. Yet, no telescope or experiment has ever been able to directly observe this mysterious matter that makes up 85 percent of the Universe. That’s because it does not emit or reflect any light – hence why astrophysicists call it dark matter. For more than 20 years, astroparticle physicists have been developing detection instruments with ever increasing resolutions to figure out the nature of dark matter. Une of the most powerful detectors is located at the Gran Sasso underground laboratory in Italy, where scientists are hoping to detect dark matter directly from the cosmos.

The site: 1400 meters below ground level

The XENON experiments are being conducted at the Gran Sasso underground laboratory in Italy. The experiment is largely shielded from the interference of cosmic and radioactive radiation thanks to its location beneath a layer of rock which is 1,400-meters thick.

The Gran Sasso underground laboratory is in the Italian region of Abruzzo. It is located in the side branch of a highway tunnel that crosses the Gran Sasso massif and links the provinces of Teramo, Pescara, and L’Aquila.

 

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© XENON Collaboration | 2015

What we know. And what not yet.

In 1933, Swiss astrophysicist Fritz Zwicky observed the Coma galaxy cluster with a telescope at Mount Wilson Observatory in California. He measured its light and calculated its mass. He then determined the speeds of the individual galaxies and made a confusing discovery: the galaxies were moving faster than they should have based on their visible mass. They were moving so fast that they should have been escaping the gravity of the galaxy cluster. To explain the discrepancy between the observation and the calculation, Zwicky postulated that there was an additional mass acting on the galaxies that could not be detected with the telescope: dark matter. Zwicky’s observations laid forgotten until the 1970s, when astronomers, in particular Vera Rubin and her team, encountered the same problem as Zwicky: spiral galaxies such as our Milky Way spin so fast that they should actually be torn apart, were it not for an additional invisible mass – dark matter. Increasingly accurate measurements and computer simulations of galaxy formation and larger structures in the universe have been indirectly confirming the existence of dark matter ever since. However, the fundamental question regarding its composition remains unanswered.

The XENON experiments: light into the dark

The XENON experiments are based on the assumption that dark matter interacts with standard matter. Unlike in the CMS detector at CERN, the XENON experiments are on the direct hunt for dark matter and WIMPs (weakly interacting massive particles). Liquid xenon is used as the medium for the interaction. By employing larger and larger detectors and greater quantities of liquid xenon, the team is improving the chances of directly detecting dark matter.

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View inside the water tank with the suspended steel tank containing the actual detector (left side). The steel tank is filled with liquid and gaseous xenon, which is used as a detection medium for elementary particles. Water is used as a medium for shielding against the  radioactive and cosmic radiation. On the right of the image, you can see the technical area with the infrastructure for operating the detector and recording the data. © XENON Collaboration | 2015

The Xenon-Experiments in numbers

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The University of Zurich has been involved in the XENON experiments since 2007 through Prof. Laura Baudis’ research group.

Research means teamwork

Laura Baudis and her group built key components of the time projection chamber in the detector for the XENON1T experiment between 2014 and 2015. The team also had a leading role in the testing and construction of the 248 photosensors and in measuring the radioactivity of the detector materials. Prof. Baudis and her team were involved in the calibration, operation and data analysis.

Prof. Baudis’ group has also built components for the time projection chamber for the XENONnT experiment. Once again, they were involved in the testing and construction of the 494 photosensors and in measuring the radioactivity of the detector materials. The group is also responsible for the readout electronics of the photosensors, the cables, the light calibration system, and the new light signal amplifier.

Prof. Baudis’ group has also been working on the development of Xenoscope – the prototype for the planned DARWIN detector – since 2017. DARWIN will one day be built in the Gran Sasso underground laboratory. With its 50 metric tons of liquid xenon, the detector will be the most sensitive dark matter detector ever built.

Laura Baudis Portraitvideo

Michelle Galloway Portraitvideo

Working principle of a time projection chamber

The original time projection chamber (TPC), which was on display at the Science Pavilion UZH, was the heart of the XENON-1T experiment. It was built at the UZH and operated at Gran Sasso from 2013 to 2018. The XENON-1T experiment searched for elusive dark matter particles that have not yet been detected. In operation, the TPC was filled with 2000 kg of liquid xenon. Liquid xenon is particularly suitable for searching for Dark Matter because it has both high density and high atomic number.

The way the TPC works is illustrated in the figure. When a particle interacts with xenon, scintillation light and free electrons (ionisation) are produced. The light signal is registered by the light detectors (photomultipliers, PMTs) installed at the lower and upper ends of the detector. The copper rings surrounding the TPC serve to create a uniform electric field in the TPC. Therefore the free electrons drift with a constant velocity into the upper part of the TPC, where there is gaseous xenon. When the electrons enter the gaseous phase, they also generate light, which is detected. The white spacers and holders for the PMTs are made from teflon.

The position of the interaction point can be determined from the time difference between the first and the second light signal and the distribution in the upper light sensors. This and the ratio of the magnitude of the two light signals allows background signals to be filtered out.

First results and an assumption

The XENON1T detector was used to record the extremely rare radioactive decay of the xenon-124 isotope for the first time. This isotope has the incredibly long half-life of 1.8 x 1022 years. (To put this into perspective, the universe is 1.38 x 1010 years old.) The result shows how accurately the detector can detect very rare decay processes and filter out unwanted signals.

An unexpected signal was found when analyzing the XENON1T data; this may point to the existence of new particles or previously unknown properties of neutrinos.

Building the future

XENONnT will be fully commissioned in 2020. The first series of measurements are expected to start in 2021. Laura Baudis and her group are developing and building Xenoscope, the prototype for the planned 50 t DARWIN xenon detector, with  the help of an ERC Advanced Grant. The plans for the project include the use of new types of photosensors.