Thursday, June 18, 2020

Xenon1T Didn't Discover Axions Or An Extraordinary Neutrino Magnetic Moment

The pre-print of a new paper by the XEON1T dark matter detection experiment (press release available here) didn't discovery anything cool and basically admits that in its abstract, which should have read as follows (emphasis added):
We report results from searches for new physics with low-energy electronic recoil data recorded with the XENON1T detector. With an exposure of 0.65 tonne-years and an unprecedentedly low background rate of 76±2stat events/(tonne×year×keV) between 1-30 keV. . . .  
The excess can . . .  be explained by β decays of tritium, which was initially not considered, at 3.2σ significance with a corresponding tritium concentration in xenon of (6.2±2.0)×10^−25 mol/mol. Such a trace amount can be neither confirmed nor excluded with current knowledge of production and reduction mechanisms. . . . 
This analysis also sets the most restrictive direct constraints to date on pseudoscalar and vector bosonic dark matter for most masses between 1 and 210 keV/c2.
The bottom chart in Figure 10 of the paper, below, shows the constraints mention in the last paragraph above:
The necessary level of contamination need to produce this effect with a substance that wasn't tightly screened for because it wasn't pertinent to the primary mission of the experiment to detect WIMP dark matter (which it didn't), is truly tiny. It is roughly one tritium atom (each of which has a mass of 3.016 atomic mass units) per 1,000,000,000,000,000,000,000,000 xenon atoms (each of which has a mass of 131.293 atomic mass units). So, it would take roughly a mere 26 tiny tritium atoms interspersed into each ten kilograms of xenon, to produce the effect observed. This is an irreducible background effect that means that this experimental result is meaningless for telling us about new physics.

What other background effects were accounted for?

But, somebody threw the following language into the abstract (with similar breathless language in the press release), which practically rules out the possibility of new physics in same breath that it announces it, which shouldn't have been more than either a footnote in the body text, or a pre-print by the one or two members of the collaboration who came up with these hypotheses, never to be published, because they don't pass the smell test.
An excess over known backgrounds is observed below 7 keV, rising towards lower energies and prominent between 2-3 keV. The solar axion model has a 3.5σ significance, and a three-dimensional 90% confidence surface is reported for axion couplings to electrons, photons, and nucleons. This surface is inscribed in the cuboid defined by g(ae)<3.7×10^−12, g(ae)g(effan)<4.6×10−18, and g(ae)g(aγ)<7.6×10^−22 GeV^−1, and excludes either g(ae)=0 or g(ae)g(aγ)=g(ae)g(effan)=0. 
The neutrino magnetic moment signal is similarly favored over background at 3.2σ and a confidence interval of μν∈(1.4,2.9)×10^−11μB (90% C.L.) is reported. 
Both results are in tension with stellar constraints.  
. . . The significances of the solar axion and neutrino magnetic moment hypotheses are decreased to 2.1σ and 0.9σ, respectively, if an unconstrained tritium component is included in the fitting.  
Both results contradict the Standard Model. 

The paper also fails to adjust for the "look elsewhere effect", and is engaged in something akin to p hacking.

One of the authors of the paper in a statement made to the New York Times clarifies:
“We want to be very clear that all we are reporting is observation of an excess (a fairly significant one) and not a discovery of any kind,” said Evan Shockley of the University of Chicago in an email.
This very public statement undermining the cymbal crash that is getting attention for this paper, and the self-contradictory language in the abstract and press release, strongly suggests that the 163 scientists in the collaboration were sharply divided over whether such a highly speculative and ill supported claim of new physics should be included in the abstract and press release accompanying the paper. Shockley is doing his best not to have his professional reputation tarnished by this paper by making this statement.

Can we test the most plausible "no new physics" hypothesis of tritium contamination?

No. Why not?
There might have been undetected traces of radioactive tritium (a version of hydrogen with two neutrons) in XENON1T, causing the surrounding liquid to sparkle. The XENON team worked hard to avoid this sort of noise from the beginning, Martens said. Still, he said, the tiny levels of tritium in question here would be impossible to perfectly screen out. And with XENON1T now taken apart to build a bigger future experiment, it’s impossible to go back and check. 
The tritium hypothesis fits the data to a confidence level of 3.2 sigma. Joey Neilsen, a physicist at Villanova University in Pennsylvania, who is not involved in XENON, said that corresponds to about a 1 in 700 chance that random fluctuations would have produced the signal.
The half-life of tritium is 12.3 years. The experiment ran for roughly two years.

Are there other problems with the solar axion hypothesis?

One telltale clue would suggest whether the solar axions hypothesis should be taken seriously: seasonal changes in the data, Yu said.  
“If the signal were indeed from solar axions, one would expect a modulation in the signal due to the relative position of the sun to the Earth,” she said.  
As our planet gets a bit more distant from the star it orbits, the solar axion stream should weaken. As Earth gets closer to the sun, Yu said, the signal should get stronger.  
Martens said that no seasonal variation is visible in the XENON1T signal. The signal is too faint, and the experiment ran too briefly at just two years, for XENON1T to have picked it up.
The argument that axions are "well motivated" is also overstated.

Axions are a hypothesis proposed to explain why the strong force doesn't have CP violation (i.e., to oversimplify, the strong force of the Standard Model, like the electromagnetic force, but unlike the weak force, has no arrow of time and treats particles and antiparticles with the same electric charge in the same way). 

But, the "strong CP problem" isn't a problem in any meaningful sense of the word, with anything but a preconceived notion about how Nature should have chosen its physical constants that it is not obliged to honor. 

Also, the lack of CP violation in the strong force already has a natural and reasonable explanation, which is that gluons which transmit the strong force are massless in the Standard Model. And, massless particles (the photon, the gluon and the hypothetical graviton) because they travel at the speed of light, don't experience time. But, since CP violation is equivalent to time direction violation (if CPT is perfectly conserved as there is every reason to believe it is), you wouldn't expect a particle that doesn't experience time to work differently going forward in time and going backward in time. 

Moreover, the axions needed to explain this experimental result does not have the properties of the axion needed to explain the "strong CP problem" if it was a problem. It would instead be a theoretical variation on the original axion concept with no real physical or mathematical motivation.

Jester (a.k.a. Adam Falkowski) at his Resonances blog is likewise unconvinced (emphasis added):
The XENON collaboration was operating a 1-ton xenon detector in an underground lab in Italy. Originally, this line of experiments was devised to search for hypothetical heavy particles constituting dark matter, so called WIMPs. For that they offer a basically background-free environment, where a signal of dark matter colliding with xenon nuclei would stand out like a lighthouse. However all WIMP searches so far have returned zero, null, and nada. 
Partly out of boredom and despair, the xenon-based collaborations began thinking out-of-the-box to find out what else their shiny instruments could be good for. One idea was to search for axions. These are hypothetical superlight and superweakly interacting particles, originally devised to plug a certain theoretical hole in the Standard Model of particle physics. If they exist, they should be copiously produced in the core of the Sun with energies of order a keV. This is too little to perceptibly knock an atomic nucleus, as xenon weighs over a hundred GeV. However, many variants of the axion scenario, in particular the popular DFSZ model, predicts axions interacting with electrons. Then a keV axion may occasionally hit the cloud of electrons orbiting xenon atoms, sending one to an excited level or ionizing the atom. These electron-recoil events can be identified principally by the ratio of ionization and scintillation signals, which is totally different than for WIMP-like nuclear recoils. This is no longer a background-free search, as radioactive isotopes present inside the detector may lead to the same signal. Therefore collaboration have to search for a peak of electron-recoil events at keV energies.
This is what they saw in the XENON1t data

Energy spectrum of electron-recoil events measured by the XENON1T experiment. 

The expected background is approximately flat from 30 keV down to the detection threshold at 1 keV, below which it falls off abruptly. On the other hand, the data seem to show a signal component growing towards low energies, and possibly peaking at 1-2 keV. Concentrating on the 1-7 keV range (so with a bit of cherry-picking), 285 events is observed in the data compared to an expected 232 events from the background-only fit. In purely statistical terms, this is a 3.5 sigma excess. . . . 
The peak of the excess corresponds to the temperature in the core of the Sun (15 million kelvin = 1.3 keV), so our star is a natural source of these particles (but at this point XENON cannot prove they arrive from the Sun). Furthermore, the particles must couple to electrons, because they can knock xenon's electrons off their orbits 
. . . For QCD axions the defining feature is their coupling to gluons, but in generic constructions one also finds the interaction between the axion and electrons. . . .
in the standard picture, the interactions of neutrinos with electrons are too weak to explain the excess. To that end one has to either increase their flux (so fiddle with the solar model), or to increase their interaction strength with matter (so go beyond the Standard Model). . . . 
How confident should we be that it's new physics? 
Experience has shown again and again that anomalies in new physics searches have, with a very large confidence, a mundane origin that does not involve exotic particles or interactions. In this case, possible explanations are, in order of likelihood, 1) small contamination of the detector, 2) some other instrumental effect that the collaboration hasn't thought of, 3) the ghost of Roberto Peccei, 4) a genuine signal of new physics. 
In fact, the collaboration itself is hedging for the first option, as they cannot exclude the presence of a small amount of tritium in the detector, which would produce a signal similar to the observed excess. Moreover, there are a few orange flags for the new physics interpretation:
1. Simplest models explaining the excess are excluded by astrophysical observations. If axions can be produced in the Sun at the rate suggested by the XENON result, they can be produced at even larger rates in hotter stars, e.g. in red giants or white dwarfs. This would lead to excessive cooling of these stars, in conflict with observations. The upper limit on the axion-electron coupling from red giants is 3*10^-13, which is an order of magnitude less than what is needed for the XENON excess. The neutrino magnetic moment explanations faces a similar difficulty. Of course, astrophysical limits reside in a different epistemological reality; it is not unheard of that they are relaxed by an order of magnitude or disappear completely. But certainly this is something to worry about.  
2. At a more psychological level, a small excess over a large background near a detection threshold.... sounds familiar. We've seen that before in the case of the DAMA and CoGeNT dark matter experiments, at it didn't turn out well.  
3. The bump is at 1.5 keV, which is *twice* 750 eV.  
So, as usual, more data, time, and patience is needed to verify the new physics hypothesis.
Roberto Peccei, whose ghost is mentioned, was the physicist at UCLA who first proposed the axion as a hypothetic particle to explain the lack of CP violation in the strong force back in 1977. He died on June 1, 2020 at age 78 of non-COVID related causes. It isn't implausible that the collaboration allowed the axion claim to be highlighted despite only marginal support for it in this experiment, to honor and call attention to his recent passing.

The third point about 750 eV is also a bit of an inside joke, referencing a 750 GeV diphoton excess of events noted in late 2015 and early 2016 that turned out to be nothing more than a statistical fluke after much undeserved hype.

Lubos Motl is more neutral in his assessment and provides additional links in a blog post on the topic at The Reference Frame, also provides a humanizing lede to the story:
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.


Ryan said...

Shouldn't tritium decay away pretty quickly though?

andrew said...

The experiment was only run for a couple of years before they tore it down, and tritium has a half life of 12.3 years.

Also, even if there were say, seepage in a tiny fissure in the rock immediately adjacent to the xenon tank, that could probably do it and provide a replenished supply. 2600 tritium atoms dispersed in a ton of material is something you'd need a scanning electron microscope to see and could search for days for and still miss.

At a minimum they'd need to have some a priori protocol to establish a baseline for tritium radiation as part of their background from trial and error.

Ryan said...

Gotcha. Hopefully the next generation experiment runs longer / can otherwise rule this in or out.