Thursday 14 April 2011

Xenon100: Nothing

The most expected experimental result of Spring 2011 is out now. XENON100 just released the results of the dark matter search based on 100 days of data-taking with xenon target in 2010. Here is what they see:
The plot shows all events that pass the quality cuts. The x-axis corresponds to the measured recoil energy determined by counting the number of scintillation photons in the event, the so-called S1. (There is an important companion paper fixing the relation between recoil and S1 at low energies where previous experimental results have been somewhat confusing). Most of the events in the plot are due to photons scattering on electrons from the xenon atoms. The way to distinguish those from the more interesting nuclear recoils (expected when a dark matter particle scatters) is by simultaneously measuring the number of ionization electrons, the so-called S2. Nuclear recoils typically lead to a smaller ratio of S2/S1 (the grey area in the plot). Therefore one makes a cut on S2/S1 (the dashed horizontal line) defining the signal region such that most electron recoils are rejected while the bulk of nuclear recoils is retained. At the end of the day one finds 3 events in the signal window (red points) while the expected background, mostly from spillover of electron recoils, is estimated to be 1.8 ± 0.6. Once again, no signal :-( Instead, we have new limits on the dark matter - nucleon cross section
For a 100 GeV dark matter particle the limit is around 10^-44 cm2, 3 times better than the previous limits from CDMS and Edelweiss. For light dark matter the improvement seems to be even better, more than an order of magnitude, which further disfavors dark matter interpretations of the CoGeNT and DAMA signals.

Actually, the paper mentions in passing that the analysis leading to these limits was not completely blind. After opening the box, there were many events at small recoil energy of which 3 fell into the signal region, which would make 6 signal events in total. However after investigating these 3 additional events the collaboration decided they were static from the electric can opener ;-), and devised additional cuts to get rid of them.

So what do these results tell us about the WIMP dark matter? At which point should we start to worry that we're on the wrong track? Unfortunately, there is no sharp prediction for the dark matter cross section. The most appealing possibility – a weak scale dark matter particle interacting with matter via Z-boson exchange - leads to the cross section of order 10^-39 cm2 which was excluded back in the 80s by the first round of dark matter experiments. There exists another natural possibility for WIMP dark matter: a particle interacting via Higgs boson exchange. This would lead to the cross section in the 10^-42-10^-46 cm2 ballpark (depending on the Higgs mass and on the coupling of dark matter to the Higgs). This generic possibility is now getting disfavored thanks to Xenon100's efforts, unless the Higgs is heavier than we expect. Therefore, even though models predicting the cross section below 10^-44 cm2 certainly do exist, it may be a good moment to start thinking more seriously about alternatives to WIMP. In the worst case dark matter may be very weakly interacting (axions, gravitinos) or very light (keV-MeV scale dark matter), in which case the current approach to direct detection is doomed from the start.

See also Peter's and Tommaso's blogs.

12 comments:

NW said...

Jester - are you talking about cross section, or cross section per nucleon? The cross section per nucleon for a Z-boson is I think around 10^-39 (see Essig 0710.1668 eq 1) and for a Higgs boson I think it's more like 10^-45 (ish) see e.g., Davoudiasl et al 0405097 fig 2. So I think the region we're about to get into is the Higgs mediated region.

Jester said...

I meant per nucleon...indeed I screwed with Z. I'll doublecheck the Higgs...

Anonymous said...

Of course there's always the possibility that we are living in a region of low dark matter density inside the halo.
This is always a possibility to revive your favorite weak scale dark matter model.

Kea said...

The poor zombie stringy susy just keeps on going regardless, heh? What we need now is a susy reduction combination plot for multiple experiments: LHC, Xenon, etc.

Anonymous said...

It is statistically improbable for us to be in a region of abnormally low dark matter density. It seems cowardly to invoke particles with arbitrarily low cross section. The whole thing starts to feel like aether - something that absolutely ought to be there, but aint. And it can get even worse - http://physics.aps.org/articles/v4/23

chris said...

"It is statistically improbable for us to be in a region of abnormally"

anthropic principle to the rescue ^_^

Anonymous said...

Maybe the dark halo is very different to the standard one. See http://arxiv.org/abs/1103.6091: a WIMP with mass in the TeV range and a rotating dark disk could be a viable solution for DAMA and the recoils measured by the other experiments.

Jester said...

@NW: Concerning the Higgs exchange, for example hep-ph/0011335 quotes σ ~(DM-Higgs coupling)^2*(120 GeV/mh)^4*(100 GeV/mDM)^2*10^-42cm2 . Davoudiasl et al must be using a smaller coupling, but I can't figure out what exactly they put in.

Anonymous said...

There is no large under/overdensity in the DM halo, according to formation with the CDM paradigm with newtonian gravity: the halo turns out to be quite smooth (there are also experimental constraints from stellar kinematics.) The local density can then be estimated as 0.4+-0.2 GeV/cm3 (at 1 sigma!). Ref is http://inspirebeta.net/record/849062

Alejandro Rivero said...

Kea, do you include my version of susy between the zoombie stringy ones?

Kea said...

Alejandro, only if you predict zombie superpartners.

NW said...

Hi Jester - I think Davoudiasl is consistent with 0011335. Note that if you look at the plots, the characteristic cross section when normalizing relic abundance is typically ~10^-45 or a bit lower for heavier WIMPs and MH=O(120). But, I agree, it's a funny way of writing the cross section because the coupling is characteristically smaller than O(1) to DM.

The way I think about it (which is certainly a feeble approximation) is that Z's have O(1) couplings to matter, while the effective Higgs Yukawa is ~1/3 x mp/v ~ 10^-3 (where the 1/3 is uncertain and comes from people who know how to calculate such things). So if Z's give ~10^-39, I expect Higgses to give (10^-3)^2 x 10^-39 ~ 10^-45. That's obviously super hand-wavy but from my perspective we're just now getting into what I think of as the rich part of the "Higgs-mediated" region for thermal WIMPs.