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XENON1T: our excess is due to tritium junk, axions, or magnetic neutrinos

This result has been known for a year or so. Grad students were tortured by the requirement of their silence. But the news got out today. XENON1T claims a 3.5-sigma excess in the search for neutrinos.

As of 2020, XENON1T is the leading detector of dark matter. Elena Aprile designed the xenon-based detectors years ago – and she is still the boss of the 163 folks. 1T indicates "one ton"; previous generations of the detector had lower masses of that inert gas. They really used 2 tons for detection and another 1.2 tons around it. They claim to have the world-class tiny background rate, 76 events per ton, year, and keV.

OK, they looked at the "electronic recoil" and at very low energies, below 7 keV (which they treated as a single bin), there is a 3.5-sigma excess (53 events above the 232 events that were expected). There are three basic explanations of the excess:

  • it's some tritium (very heavy hydrogen) dirt
  • the electrons were kicked an axion from the Sun (or another boson)
  • the electrons were kicked by neutrinos but those must have a high magnetic moment to interact with the electrons (through a virtual photon)
Of course, 3.5 sigma isn't too much so it may be a fluke, too: they should get to 5 sigma rather soon if it's not a fluke.

Here you have some basic links: I will avoid my guess which of the possibilities is most likely. As far as I can say, it's almost pure prejudice or the reflection of one's optimism or pessimism.

The tritium contamination is totally possible but the experimenters refuse to discuss this possibility – which should be impossible due to their xenon purification process – because by admiting that the xenon can be this dirty, they would basically admit to be about as dirty as your favorite homeless man (who also has some tritium on his body, aside from the leftovers of the feces). Aprile et al. are willing to admit some krypton-83 due to decaying rubidium-83 (half-life 86.2 days) in the xenon circulation system but that's just on par with admiting fleas.

The exciting two possibilities are generally disfavored or disbelieved by the argument that if such things (interacting neutrinos, new axion-like bosons or dark photons) were emitted by the stars, lots of the stars and other celestial bodies that we see would cool far more quickly and we couldn't see them (or they wouldn't look as hot and sexy as they look).

Note that the "magnetic neutrino" possibility would assume a collision of a neutrino and the electron through (prepare your stomach) dimension-7 operator of the type \(d/v \cdot \psi_e \cdot \nu_\mu \cdot F_{\mu\nu}\). If the neutrinos were this strong magnets, we would also know that they have to be Majorana fermions, not Dirac fermions.

The new particle could also be axions or dark photons. Axions are typically light scalar particles, quanta of the field \(a\), that deserve the name especially if they couple to the instanton number of some gauge field via \(a\cdot F\wedge F\). They may arise from many \(p\)-forms in compactification of string theory and they may even be used as a detergent. ;-)

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