Axions, neutrinos, or noise? The XENON1T surplus
This month (June 2020), it was announced that the XENON1T experiment has found a surplus of counts during its 2016-2018 operations. Originally intended to search for WIMP-scale dark matter candidates, its modus operandi has been modified to search for axion-mass candidates.
The WIMP-mass search looked for nuclear recoils in a 3,200 kg tank of pure xenon. The concept was that a high-mass DM candidate particle might interact with a xenon nucleus, resulting in the production of photons, observed as a flash of light that would be detected by light-sensitive photomultiplier tubes facing into the tank of liquid xenon. However, as with several other similar experiments, no such signals were seen.
It was decided to change tack and look for axion-mass candidates instead, using electron recoils, where a particle interacts with an electron in a xenon atom, rather than with its nucleus.
Operating deep underground in Italy’s Laboratori Nazionali del Gran Sasso (LNGS), an excess of counts above the expected background level has been recorded. No practical particle-detection experiment is free from all sources of background signal; however careful we are to reduce the background noise as much as is possible, there will be some background events. What we’re looking for is a signal that is above the expected amount of events that we can fairly attribute to background sources.
Fortunately, this expected background can be modelled- sophisticated software simulations can be used to predict how many events we would expect from background sources such as natural radioactivity in the experiment’s environment, in the material from which the equipment is made, etc.
In this case, over the time that XENON1T was operating, around 232 counts were expected to be observed due to background sources; however, 285 counts were recorded, which is a large enough excess to be considered significant and well worth investigating.
Currently, there are three main possibilities for this excess:
it’s due to tritium atoms in the tank- the excess is consistent with a very small, but unmeasurable, contamination of tritium;
it’s due to neutrinos- but this would require the neutrino to have a magnetic moment, a property currently not required of the particle, and one which would indicate some physics beyond the Standard Model (and thus very interesting in its own right);
it’s due to axions produced in the sun- which would be the first definitive detection of axions as a class of particle, even though it wouldn’t be the primordial DM axions which must have been around since the start of the universe. (This option also requires revisions of our understanding of stellar astrophysics, which also would be a significant outcome.)
So although none of these indicates the detection of a particle which could be claimed as a DM candidate, the second and third options listed above do lead to new and interesting physics. And, possibly, the first detection of an axion of any sort.
The XENON experiment is to be upgraded, from XENON1T, to XENONnT, which will have 8,300kg of liquid xenon. This upgrade also will allow the investigation of the first option listed above- if the excess signal is due to tritium impurities, it should be seen in the XENONnT data as well.
Whatever the outcome, the results from XENONnT will be much anticipated.
A paper describing the XENON1T result can be found here.