• 08.07.2020
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Zurich particle physicists make headlines with XENON collaboration

Do they really exist, the lightweights?

XENON
Image: XENON collaboration
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XENON
XENON (Image: XENON collaboration)

Axions are elementary particles with extremely low mass. So far they exist only in the brains of theoretical physicists, nobody has observed them. Really? Measurement results at detector XENON1T in Italy make one sit up and take notice.

The current results of the XENON1T detector, which were published a few days ago, have the potential to cause a sensation. In the best case they could put our understanding of matter on a new basis and change our view of the world. But the physicists who were involved in the latest measurements do not want to put words like "sensation" into their mouths. They know: Perhaps the measurement data containing the alleged sensation can be explained in a completely different, quite banal way: As a statistical fluctuation that soon disappears into thin air. Or as a disturbing influence that distorts the measurement.

Surplus of events

The XENON1T detector was in operation from 2016 to the end of 2018 in the Italian underground laboratory Gran Sasso. It is used to search for previously unknown elementary particles. The detector basically consists of a tank filled with liquid xenon. If a particle enters the tank from the outside, it can collide with a xenon atom, triggering weak light signals and knocking electrons out of the xenon atom that is hit. The walls of the tank are lined with highly reflective synthetic material PTFE, while at the top and bottom 248 highly sensitive photosensors that can absorb the light flashes are placed.

The scientists involved in the XENON1T experiment have a precise expectation as to how many light flashes can be expected in a given time from the particles known so far. When the researchers analysed the data recorded between 2016 and 2018, they were astonished: they had expected 232 flashes of lightning for the period in question. In fact, they found 285, 53 more than expected.

Three possible explanations

This deviation does not seem to be spectacular for the outsider. It is quite different for the scientists involved. "The latest results of the XENON1T detector are very exciting," says Laura Baudis, physics professor at the University of Zurich, who is a leading participant in the experiment. "The observed excess signal allows various explanations, which could be very spectacular or relatively banal."

The explanation would be banal if the excess signal was due to the fact that extremely small amounts of the radioactive hydrogen isotope tritium are present in the liquid xenon. In this case, the excess signal could be explained by the radioactive decay of tritium and would hardly bring about any new physical findings.

The second possible explanation would be much more exciting: In this case, the excess light flashes would come from neutrinos, those practically massless elementary particles that penetrate matter, practically without making themselves noticeable. If the excess signal really does come from neutrinos – as particle physicists deduce from their observations – this would mean that the magnetic moment of the neutrinos is greater than previously assumed.

Reference to dark matter

The third explanatory hypothesis is the proof of the axion - and that would be the scientific sensation mentioned at the beginning. The axion is at least 500,000 times lighter than an electron. and about the same weight as a neutrino. The energy spectrum measured in the XENON1T experiment actually looks similar to that predicted for axions that are generated in the Sun and travel from there to Earth. But how did particle physicists even come up with the idea that there could be axions? "The particles were postulated by theoretical physicists because their existence could explain why time reversal symmetry is violated in the weak interaction but not in the strong interaction (nuclear force). So the existence of axions would solve an existing problem of the Standard Model of particle physics," says Baudis.

This is not the only reason why the discovery of this elementary particle would be spectacular. Axions could also be a component of dark matter, i.e. the form of matter that is likely to fill a large part of our universe, which has not yet been detected. But here too, Laura Baudis warns against jumping to hasty conclusions: "If we could detect axions, it would be a breakthrough in our understanding of matter. But this would not yet prove that these particles are dark matter. This would have to be proven independently with new experiments."

University of Zurich at the forefront

University of Zurich played a leading role in the XENON1T experiment. Its scientists built the so-called time projection chamber inside the detector and tested its mechanical stability in a large nitrogen cryostat in the Physics Department. Together with the Japanese company Hamamatsu Photonics, the researchers developed the 248 photodetectors with extremely low radioactivity levels, and built the associated readout electronics and light calibration system. They also used a specially built radioactivity measuring device to show which materials were suitable for building the XENON1T detector. Finally, together with research groups from Chicago and San Diego, they carried out the data analyses that led to the latest results.

It goes without saying that the scientists of the XENON experiment want to clarify as quickly as possible which of the three interpretations actually applies. This should be possible with the XENONnT detector, which will start operation in Gran Sasso at the end of 2020. "In the new detector, we will detect particles with four tons of liquid xenon instead of one, which will allow us to record more events and achieve more meaningful statistics," says Laura Baudis. "In addition, since we can reduce the background effects by a factor of 6 and more, we will be able to see events more clearly. We assume that we will have certainty within a year's time as to which of the three possible explanations applies – and whether we can actually observe axions in our detector".

Author: Benedikt Vogel

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