Wednesday, 17 June 2020

Hail the XENON excess

Where were we...  It's been years since particle physics last made an exciting headline. The result announced today by the XENON collaboration is a welcome breath of fresh air. It's too early to say whether it heralds a real breakthrough, or whether it's another bubble to be burst. But it certainly gives food for thought for particle theorists, enough to keep hep-ph going for the next few months.

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.

Assuming it's new physics, what does this mean? XENON shows that there is a flux of light relativistic particles arriving into their detector.  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. Several theoretical models contain particles matching that description. Axions are the primary suspects, because today they are arguably the best motivated extension of the Standard Model. They are naturally light, because their mass is protected by built-in symmetries, and for the same reason their coupling to matter must be extremely suppressed.  For QCD axions the defining feature is their coupling to gluons, but in generic constructions one also finds the pseudoscalar-type interaction between the axion and electrons e:

To explain the excess, one needs the coupling g to be of order 10^-12, which is totally natural in this context. But axions are by no means the only possibility. A related option is the dark photon, which differs from the axion by certain technicalities, in particular it has spin-1 instead of spin-0. The palette of viable models is certainly much broader, with  the details to be found soon on arXiv.           

A distinct avenue to explain the XENON excess is neutrinos. Here, the advantage is that we already know that neutrinos exist, and that the Sun emits some 10^38 of them every second. In fact, the background model used by XENON includes 220 neutrino-induced events in the 1-210 keV range.
However, 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). For example, neutrinos could interact with electrons via a photon intermediary. While neutrinos do not have an electric charge, uncharged particles can still couple to photons via dipole or higher-multipole moments. It is possible that new physics (possibly the same that generates the neutrino masses) also pumps up the neutrino magnetic dipole moment. This can be described in a model-independent way by adding a non-renormalizable dimension-7 operator to the Standard Model, e.g.
   
To explain the XENON excess we need d of order 10^-6. That mean new physics responsible for the dipole moment must be just behind the corner, below 100 TeV or so.

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 g 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. On the experimental side, the near future is very optimistic, with the XENONnT, LUX-ZEPLIN, and PandaX-4T experiments all jostling for position to confirm the excess and earn eternal glory. On the theoretical side, the big question is whether the stellar cooling constraints can be avoided, without too many epicycles. It would be also good to know whether the particle responsible for the XENON excess could be related to dark matter and/or to other existing anomalies, in particular to the B-meson ones. For answers, tune in to arXiv, from tomorrow on. 

42 comments:

Anonymous said...

OMG, a post in Resonaances. Thanks.

Marc said...

Actually, the first study on using LXe detectors to search for axions is from Arisaka et al (https://arxiv.org/pdf/1209.3810.pdf) and the XENON100 collaboration published the result of a first search in 2014 (https://arxiv.org/pdf/1404.1455.pdf).

Jester said...

Sorry Marc, my bad. I changed the reference to earlier work in the text.

Anonymous said...

Is 750 eV a magic number, like the diphoton excess at 750 GeV? Why is it an orange flag for the new physics interpretation?

Jester said...

750 will be a traumatic number forever ;)

Anonymous said...

Hi, happy to see a new post at ResonAAnces! You say "it is not unheard of that they are relaxed by an order of magnitude (which is what is needed here)"; what examples (ideally "modern" ones) do you have in mind? Best,

Jester said...

For example the bounds on black hole dark matter. The CMB limits on solar mass black holes have been oscillating wildly, while those on 10^-10 solar mass black hole disappeared completely.

Ervin Goldfain said...

Hi Jester,

Great to see you back, it's been two years since your last post!

Here is an OT question: What is your view on the X17 boson claim by the Hungarian group?

Ervin

Jester said...

Much less exciting than the XENON excess, because of 1) smaller credibility of the collaboration, and 2) the lack of attractive models that could explain it.

Anonymous said...

Thanks for the post! Can someone elaborate on why having an excess close to a threshold should be a reason for concerns (the post mentions DAMA and CoGeNT)? Also, if that is a true signal, do you have an estimate of the number of events from sources other than solar we should be seeing here and there in the energy spectrum (perhaps it is negligible?)? Best,

Jester said...

It's just that bad thing happen near the threshold, as experimental control is diminished. Of course, I cannot point my finger at any concrete problem, so you are free to ignore this comment.

Various components of the background are shown in Table 1 of the XENON paper: https://www.science.purdue.edu/xenon1t/wp-content/uploads/2020/06/xenon1tlowersearches.pdf
Lead-214 is in fact the most important component in the region of interest.

Anonymous said...

Cool post! with possibly ~50 events, could we expect any mild temporal modulation (day/night; seasonal -- I think the speaker said the time window/statistics was not long/large enough though)? Any comments on the 18keV >3sigma downward modulation (too large to be related to the 1keV excess?)? Best,

Ervin Goldfain said...

"Much less exciting than the XENON excess"

Not surprisingly, others disagree, claiming that the dark photon is actually a promising candidate for explaining away the anomaly (https://arxiv.org/pdf/2006.01151.pdf)

Alex Small said...

Hasn't CERN had an axion telescope pointed at the see un for a while? Does their experiment have the sensitivity to detect axions with the energy and coupling strength proposed here?

Jester said...

The CAST experiment at CERN was using the axion-photon conversion in magnetic fields, so it was probing the axion coupling to photons. Their bounds are relevant and complementary to the XENON ones, which mostly constrain the axion-electron coupling (see Fig 8a of the XENON paper). Future CAST-like experiments could well see a signal, but it's a model-dependent question.

Anonymous said...

Probably naive, but...why can't they exclude tritium background? Is the signal below the level of quantitation of other methods that could independently measure the tritium concentration?

rfl said...

@Anonymous, yes, it's waaaaaay below that. 10^-25 (mol Tritium)/(mol Xenon). Or, just a handful of tritium atoms per kg of xenon. No idea how one could measure that directly.

Anonymous said...

Looking at the sigma`s shown at the bottom of the figure, the deficit around 17 keV looks as significant as the excess at small energies.

Anonymous said...

@Ervin Goldfain: the vector mediator models in 2006.01151 couple to non-conserved currents (i.e. not EM or B-L), and would almost certainly run into other experimental constraints if you wrote down a UV completion.

Ervin Goldfain said...

@ Anonymous at 21:30,

I agree that 2006.01151 has issues, and the dark photon proposal should be taken with a serious grain of salt.

My only point is that one should keep an open mind when it comes to liking or disliking claimed "breakthroughs", like the XENON excess or the X17 anomaly. Until the smoke clears, nobody knows for sure how our current models will need to change to accommodate these "would-be" discoveries.

Anonymous said...

The extraordinary evidence on backgrounds might include: publishing their 37Ar spectrum as a function of keV... makes a 2.8 keV peak... 1/2 life about 40 days... they injected 37Ar... they have the data but doesn't seem to be in their talks... also estimating/measuring the cosmogenic 37Ar from their surrounding rock which might have entered their system... then is there actual hard data on the dominant background (214Pb beta decay) and how it really looks at these super-low energies? Or just theory?

Anonymous said...

With the look-elsewhere effect the significance of the result would considerably reduce.... does the collaboration take LEE into account?

Thomas said...

The first theory paper explaining the excess was posted to the arxiv within 8 hours, successfully beating the arxiv submission deadline to appear on the same day. The only real question here is whether the impending deluge of Xenon1T excess papers will overtake the number of diphoton papers.

Jester said...

Anon, solar axions have a fixed spectrum (up to the freedom of choosing the relative couplings g_{ae} and g_{a\gamma \gamma}) so there is no LEE in this case. Idem for solar neutrinos. They take LEE into account for ALP and dark photon searches, see their Fig 10.

rfl said...

@Anonymous 18 June 2020 at 00:38:

XENON doesn't claim a discovery of BSM physics. Their 37Ar spectrum is shown in slide 67 of their presentation, available at xenon1t.org. XENON injected 37Ar only after that data taking. Cosmogenic 37Ar data from an air leak is incompatible with the purity of the xenon target. Fair point on 214Pb beta decay data, though the systematic uncertainty from the various models is too small by an order of magnitude at least.

rfl said...

@Anonymous 18 June 2020 at 03:34

This is a blind analysis so there is no look-elsewhere effect for the axion or neutrino magnetic moment hypothesis. The look-elsewhere effect for the bosonic dark matter hypotheses is taken into account as the peak position is unconstrained, as you can see in Fig 10 of the paper. And I don't think much of the residuals between 150-175keV, that's just oscillations up and down from the spectral peak shape not perfectly matching, quite common in any spectral fit.

rfl said...

...and obviously I should read Jester's replies before posting my own *g* ;)

Anonymous said...

@rfl 00:38... thanks... slide 67 of their talk was only labeled in raw signal units, not keV. As well as the raw to keV conversion, not clear how thresholding due to full selection criteria modifies, particularly when projected onto the keV axis. As for 214Pb... the low energy betas from heavy nuclei/large Z atoms are a very weird corner of parameter space, where it is fair to question the accuracy of calculations... actual measurements that test calculations would be extremely welcome... systematic errors determined entirely by theory without testing/comparison with actual experiment aren't the most convincing. The one comparable heavy that the calcs rest on is 241Pu... isn't that a non-unique beta decay, while 214Pb is a unique one?

Anonymous said...

I wonder what else one can learn from it if this is the world's most sensitive tritium detector.

Jester said...

Katrin may feel offended

Anonymous said...

You say the axion coupling to electrons is “totally natural” in this context. But that is wrong. The term you wrote breaks the discrete shift symmetry of the axion phi — phi +2 pi f. It also breaks the continuous shift symmetry of the axion, which can at most be broken by extremely small effects; much smaller than this amount. So unfortunately the axion explanation does not look good....it is wishful thinking.

Jester said...

The interaction I wrote is, by equations of motion, equivalent to (\bar e \gamma^\mu \gamma^5 e) d_\mu a/f with g = 2 m_e/f and f the axion decay constant. In this form the shift symmetry is manifest. The smallness of g is just a manifestation of f >> m_e.

Anonymous said...

for the QCD axion, in order to get g=10^{-12}, and using the relationship between m and f, one needs much higher values for axion mass than they quote in conclusions. values that are essentially ruled out by a range of other experiments. the paper seems somewhat dishonest in how it is written.

curious george said...

What do error bounds on the number of events mean?

Jester said...

It's the Poisson error. Basically for N events in each bin the error bars show +/-Sqrt[N] to guide the eye.

Chapette said...

Jester, we need more of your posts, for at least two different reasons.

David Straub said...

3.5 sigma and the "possible explanations in order of likelihood" are all systematic in nature? OK, maybe this one is too obvious, but ...

bertie said...

I have visited your site, feeling increasingly forlorn, for 2 years and finally am rewarded!
Great post!

Jester said...

Agree David, there should be a statistical fluke after the ghost. But 3.5 sigma here is a bit different than 3.5 sigma at the LHC, where you *expect* a 5/10000 fluke to occur once in a while in some distribution. The XENON collaboration have made O(10) new physics searches in their whole history, so, imo, systematics is more of a worry than statistics in this case.

Lentotaidoton said...

ATLAS measures light scattering on light and constrains axion-like particles

https://atlas.cern/updates/physics-briefing/light-scattering-light-constrains-axion-particles

LHC does not study possible axions only by CAST. Here the latest news concerning light-by-light scattering news by ATLAS.

Anonymous said...

Dear Jester,

in you article in Le Monde, you write that the absence of large quantities of antimatter in the universe is a phenomenon not described by the standard model.

Could you point to a review about this topic? Or, more humorously: this is repeated so often without citing a reference that it is time to change the situation...

Thank you
Gianni

Jester said...

Well, it's hard to give a reference, because everybody knows it so nobody bothers to write it up :) More seriously, this is a non-trivial question, because the Standard Model in principle does have all the necessary ingredients for baryogenesis. So one cannot give a qualitative answer. It's rather a quantitive issue: if you calculate the baryon asymmetry predicted by the Standard Model, you find it way smaller than what is observed. You can look e.g. at this review for details: https://arxiv.org/pdf/hep-ph/0609145.pdf But, as I said, the issue is rather technical, so it's not an easy read.