Monday, 15 November 2010

Wooden Stake for CoGeNT

There is a new interesting paper from CDMS that excludes an important region of the parameter space of dark matter models.

First, a short summary of previous episodes. Earlier this year CoGeNT made a claim of possible detection of dark matter. CoGeNT is a relatively small dark matter experiment using a germanium detector located in the Soudan mine. The spectrum of events they registered during the first months of operation is consistent with scattering of dark matter particles with the mass of order 10 GeV and the cross section on nucleons of order 10^-40cm2. Dark matter in this mass ballpark could also fit 1) the long-standing DAMA modulation signal, 2) the 2 events observed by CDMS last year, and 3) the oxygen band excess reported by the CRESST experiment. These developments came somewhat unexpected to most of us, as the dominant theoretical prejudice would place dark matter at a somewhat heavier scale, 100 GeV or so. Following this prejudice, the majority of dark matter experiments were optimizing their search strategies for the weak scale dark matter, neglecting the light mass region. The typical recoil resulting from a 10 GeV particle scattering in a detector would be too small to pass the threshold set by most experiments. The advantage of CoGeNT is a very low energy threshold, 0.4 keV in ionization energy translating to about 2 keV true recoil energy. This is the key reason why they could achieve a better sensitivity for light dark matter than the big guys in the detection business such as the CDMS and Xenon collaborations whose analysis threshold had been higher.

Nevertheless, the big guys didn't despair, but have worked toward improving sensitivity in the CoGeNT region. First came Xenon100. Using their early data they were able exclude the region of the parameter space consistent with the CoGeNT signal. The precise extent of their exclusion region is however controversial, because it strongly depends on poorly measured scintillation efficiency in xenon at low recoil energies, the so-called Leff parameter. Using more conservative assumptions about Leff, some of the CoGeNT parameter space remains allowed. Furthermore, the limits on light dark matter critically depend on certain unknown properties of dark matter, such as its velocity distribution in our galaxy; changing some assumptions could result in an enhanced event rate in a germaniun detector as compared to a xenon detector. For all these reasons, the Xenon100 exclusion was not considered conclusive.

Now the situation is clarified when the CDMS collaboration has recycled their old data so as to improve sensitivity for light dark matter. They lowered their recoil energy threshold down to 2 keV (as compared to 10 keV in their previous analysis). Lowering the threshold comes with the price, as at such low recoil CDMS cannot use the phonon timing cuts to better differentiate nuclear recoils (expected from dark matter scattering events) from electron recoils produced by all sorts of pesky backgrounds. The discriminating variable that remains available is the ionization yield (nuclear recoils typically produce small ionization, in a well-defined band) but that is not enough to get rid all of the events, see the plot above. Thus, whereas previous CDMS searches were expecting less than 1 background event, the new analysis has to deal with hundreds. Still, the dark matter cross section on nucleons that would be consistent with the CoGeNT signal would produce many more events than CDMS has observed. Assuming that all observed events come from dark matter (this is very conservative, as they are able to assign these events to known sources, such as surface events or noise) allows them to set pretty tight limits on the cross section in the low mass, see the solid black line in the upper plot. The CoGeNT region (shaded blue) is now comfortably excluded.

CDMS uses the the same germanium target as CoGeNT, so even theorists may find it hard to come with an explanation how dark matter could produce a signal in one and not in the other. Therefore it seems safe to pronounce the CoGeNT signal dead. Too bad. However the dark matter detection business regularly produces new entertainment; maybe the much-expected soon-to-come one-year Xenon100 results will provide us some?

14 comments:

Anonymous said...

Jester, you should really speak to someone in the DM field to get an appreciation of what "comfortably excluded" means for us...

Anonymous said...

Another negative result... Isn't that frustrating?

Chris Austin said...

Hi Jester,

Thanks very much for the report.

On the topic of (much) lighter dark matter, what is the general opinion on the claims of de Vega, Sanchez, and colleagues, e.g. in arXiv:1009.3494, that wimps, i.e. dark matter heavier than 1 GeV, are very strongly disfavoured by astrophysical data on galactic scales, and that warm dark matter, specifically a right-handed sterile neutrino of mass a few keV, is the most interesting DM candidate? Do these experiments, CDMS, Xenon100, CoGeNT, DAMA, ... , have any chance at all of detecting such light dark matter? I have to admit that I find some of the arguments of de Vega et al quite impressive, for example compare the pictures on pages 42 to 44 of the above article.

Jester said...

@Anon1, hmmm, two leading experiments in the field using different techniques both claim that there's nothing there. One of them uses the same target and comparable threshold as the culprit (even sits in the same cavern). And I didn't mention here Peter's reanalysis of Xenon10 data who also found nothing. So what would "comfortably excluded" mean in that field? A direct message from heaven? ;-)

Jester said...

Chris, if dark matter is that light all direct detection experiments are of course screwed. Whether there is a hint for keV dark matter from astrophysics - I know little about it. But I know there is some conflicting evidence, for example the Lyman Alpha forest points to cold (therefore heavier than keV) dark matter. I'd rather be cautious about astrophysics and their dark matters, it's a dirty business ;-)

Luboš Motl said...

Sorry, Jester, I don't see from the picture how you came to the idea that it's "comfortably excluded". Do you mean that the region that is both blue and dashed belongs to the grey region?

It obviously doesn't.

What are the confidence levels of the regions? Even if they were 99% CL regions, I wouldn't say that it's comfortably excluded. If they are 68%, there's nothing to talk about. In that case, this is just an experiment with a few units of evidence in the negative direction.

But you surely don't consider such "touching" of the relevant region to be settling the issue, do you? With such standards, you could do climate science, but not real science.

Jester said...

The CDMS excluded region is above the solid line, not the grey shaded region (that one is the DAMA favored region). Both the CDMS exclusion and the CoGeNT region is at 90%cl.

vladimirkalitvianski said...

This particle theory blog is about bare or real particles?

Kea said...

Jester, you should clarify that what is being excluded is a WIMP-like localised classical recoil. This does not preclude a real (say) DAMA/LIBRA signal with another theoretical origin.

Luboš Motl said...

Dear Jester,
thanks for your clarification of the regions.

Still, the CDMS-excluded region is not covering the Cogent-favored region too safely - it's not really far from the 90% border.

90% is not even 2-sigma. You can't "comfortably exclude" hypothesis by 2-sigma evidence, especially if there are multiple sets of positive data that the hypothesis could actually be correct.

So this remains almost as unsettled as before.

Best wishes
Lubos

Anonymous said...

I think the CDMS result is quite inconclusive. If a modest systematic uncertainty in energy scale of around 10-20% were included, the CoGeNT region would no longer be excluded even at 90% C.L.

Anonymous said...

You physicists make me chuckle. Arguing over what comfort means. Seriously go back to doing physics and stop beating the guy up. If you aren't comfortable with what Jester is comfortable with too bad. Whats wrong with comfort at the 90% level? I can certainly think of 20 adjectives that would be stronger than comfortable that could take us .5% at a time closer to 99.999999999999999999 which would be virtual certainty.

Till Sawala said...

Cosmological constraints, based on Lyman Alpha (structure at intermediate redshifts) puts lower limits on sterile neutrinos of the order of a few keV (but see e.g. : http://arXiv:1008.0992, which suggests that the limits may not be as tight). Simulations of galaxy formation, when combined with pure dark matter simulations in order to compare the distribution of galaxies in an appreciable volume of the universe, tend to favour warm dark matter with masses around 0.5-2 keV. This corresponds to free-streaming masses (the scale below which structure deviates significantly from cold dark matter) on the order of 10^10 solar masses. In my opinion, there is currently some evidence that the abundance of galaxies favours WDM, but the astrophysical assumptions that go into the galaxy formation models still have a lot of uncertainties that need to be resolved, before we can claim that CDM has a real problem.

In any case, for cosmologists working on galaxies, 10-20 keV is basically "cold". From our point of view, the upper limits would have to go down a couple of orders of magnitude to get exciting. Unless of course, you find something up in the GeV range.

Cakeproof said...

I wish Xenon100 results were already published... why is it taking so long?