Thursday, 5 May 2011

CoGeNT observes annual modulation!

What a year... Previously I had to think hard to make up a blogging subject that would not be too boring. But these days there's hardly a week without a new discovery, a new rumor of a discovery, or a refutal of the previous week's rumor. This year the particle community was already electrified by the CDF forward-backward asymmetry, the CDF W+2j bump, the would-be Higgs decaying to photons in ATLAS, and now there is CoGeNT... The rumor that CoGeNT observes the annual modulation of the signal has been circulating for a while, but only recently it was officially announced, first at the APS April Meeting in Disneyland last Monday, and today at the symposium in STSI Baltimore.

CoGeNT is a dark matter experiment located in the Soudan mine in Minesotta. In spite of a relatively small size and limited background rejection its germanium detector has certain advantages, e.g. a low threshold (0.4 keVee, corresponding to true recoil energy of about 2 keV) and a very good energy resolution. This makes it particularly sensitive to light GeV-scale dark matter whose scattering cannot produce nuclear recoils far above keV. CoGeNT was running continuously since December 2009 until March 2011 when the power was cut off due to a fire in the Soudan mine. The new results based on 145 kg.day of data continue to show an excess of events at low recoil energies which can be interpreted as the scattering of light dark matter particles in the detector. The preferred parameter region has shrunk and now points to 7-8 GeV particle with the cross section on nucleons around 10-40 cm2. More importantly they were able to measure the annual modulation of the signal. Because the velocity of the Earth with respect to the dark matter sea changes anually due to the orbital motion around the Sun, the event rate of dark matter scattering is expected to oscillate with a peak in June and a minimum in December. And here is what CoGeNT observes.
The solid line is the expectation from dark matter, and the dashed line is the best modulation fit to the signal. The phases of the two are within 1 sigma. CoGeNT estimates that the modulation hypothesis is preferred at 2.8 sigma. The modulation is most pronounced in the 0.5-2 keV region while it is absent for surface events.

Well, I don't know what to think about it. The parameter region consistent with the CoGeNT signal is naively excluded by CDMS, Xenon10 and Xenon100. One would have to assume that these 3 experiments are terribly wrong about their energy scale in order to reconcile their limits with the CoGENT signal. Maybe CoGeNT is just wrong. On the other hand, the observed modulation is very intriguing, especially in combination with the long-standing DAMA modulation signal and the oxygen band excess in CRESST. On the third hand, maybe nobody is wrong, but dark matter is simply different than what we've expected it to be. Prepare for a new wave of dark papers on arXiv.

The video of CoGeNT's presentation is here. See also this post on Cosmic Variance.

17 comments:

AS said...

both CoGent and CDMS use germanium and they are in the same place, so there seems to be a direct contradiction between their results

Jester said...

but they have different background rejection, for example CoGeNT does not cut on the ionization fraction. What if dark matter mimics electron recoils... unlikely, but not completely excluded.

Chris said...

Thanks for the update. What is your opinion on the claim on pages 7 to 16 of this talk by Rouven Essig that a new neutral "dark force" gauge boson A' with mass \sim MeV to GeV and a kinetic mixing with the photon with coefficient 10^{-2} to 10^{-5} might explain both CoGeNT and DAMA, as well as PAMELA and Fermi cosmic ray excesses and the muon (g - 2) anomaly? The second half of the talk indicates that a direct search for the A' produced via bremsstrahlung by an electron beam on a tungsten target has been provisionally approved for a 30 day run at the Jefferson Lab possibly some time this year. Is there any way that something like this, with the unusual coupling via the kinetic mixing, could make CoGeNT and DAMA consistent with CDMS, Xenon10, and Xenon100?

Jester said...

That explanation is subject to the CDMS and Xenon constraints, i think... you need something much more crazy.

Anonymous said...

Jester, has it ever ocurred to you that some experimental groups may be greatly exaggerating their sensitivity?

Jester said...

I think it's unlikely that all three are so badly wrong.

Anonymous said...

Lorentz would be exhilarated to see this. Lumineferous aether at last?

Anonymous said...

Sorry, some naive question, but where does it pick? does it agree with Dama or not?

Matti Pitkanen said...

For a connection between the more than two year old
CDF anomaly and the recent dark matter discrepancy see
this.

Anonymous said...

Actually, I think you'll find that the A' hypothesis is not ruled out by CDMS and Xenon.

Jester said...

Anon-3, it's hard to say. DAMA and CoGeNT can be explained using similar dark matter masses, but the cross sections don't match very well. One needs to assume the sodium quenching factor is much larger than what is measured to get a better agreement.
Anon-1, as long as it's the usual spin-independent elastic scattering the CDMS and Xenon limits apply, whether the force carrier is A' or something else.

Anonymous said...

CDMS and CoGeNT have very similar spectra.

Anonymous said...

Jester, large enough quenching factors for sodium recoils have been measured to bring DAMA and CoGeNT in near agreement, you should look at the large spread in those measurements. You are thinking about the value obtained by DAMA themselves, which is somewhere in the middle of the measured range. This uncertainty is however not enough to make CoGeNT and DAMA overlap. Something else is missing there.

Jester said...

I was referring to 1007.1005 where they get an overlap of CoGeNT and DAMA, but for Q_Na \sim 0.5, whereas DAMA quotes Q_Na = 0.3 \pm 0.01. Even assuming that the true error is a few times bigger than that, I agree that the quenching factor cannot be the whole story.

Chris said...

Anon at 07:19, could you elaborate a little or suggest a ref?

Anonymous said...

"The modulation is most pronounced in the 0.5-2 keV region while it is absent for surface events."

that pretty much says it all...

Anonymous said...

Curve of 7 GeV WIMP prediction in 0.5-3 keV bin has amplitude too high by factor 2. Then fitted amplitude of 16 % can't be accounted by a 7 GeV WIMP expected to give modulation of 6 %. Check Schwetz at http://indico.cern.ch/conferenceTimeTable.py?confId=116616#all.detailed.