Archive for June, 2012

CMS puts new constraints on Dark Matter

CMS recently released a paper on the search for monojets (arXiv:1206.2664 25-June-2012). Normally one thinks of monojets (events with one energetic jet and large missing energy or “MET'') in the context of supersymmetry or maybe large extra dimensions. But this paper follows a treatment first published by CDF (arXiv:1106.4775 23-June-2011) in which the monojet search is reinterpreted as a limit on the production of dark matter particles in hadron colliders. Credit for this idea should probably go to Beltran, Hooper, Kolb, Krusberg, and Tait (arXiv:1002.4137 22-Feb-2010) among others.

Setting aside supersymmetry, one can expect dark matter particles to be produced in pairs at the LHC or the Tevatron if they have weak-scale couplings to standard model particles. Crudely put, producing DM particles would be a lot like producing a pair of heavy neutrinos (with minor modifications of the DM particles are not fermions; CMS assumes they are spin-1/2 fermions). It makes sense to calculate production cross sections in a generic framework – perhaps even taking just a contact interaction to couple a pair of DM particles to matter. In any case, the DM particle pair may sometimes recoil against an energetic jet, and since they themselves leave the apparatus undetected, a nice monojet signature arises.

The kicker is that the acceptance and efficiency for a DM signal hardly depends on the mass of the DM particle, after one has asked for an energetic jet and large MET. This leads to interesting bounds for very light DM particles at masses inaccessible to most direct-detection DM searches.

Events were selected in a straight-forward way: ask for a good-quality jet consistent with the primary vertex. The jet should have pT > 110 GeV and |η| < 2.4. The MET should be at least 250 GeV. Any second jet should not be back-to-back with the leading jet. Events with leptons or isolated, energetic tracks are vetoed.

The contamination is mainly Z→νν, as one would guess. It is monitored and estimated using Z→μμ events: one predicts 5106±271 events. There is also significant contamination from W+jet events; again this contamination is estimated by finding W→μν events: one predicts 2632±237 events. Much smaller contaminations come from top and Z→LL events. The total number of events expected from SM sources is 7842±367, and CMS observes 7584 events. No signs of DM particle production. Systematic uncertainties are at the 20% level.

The non-observation of monojets implies upper limits on the contact interactions (or whatever theoretical framework you want to use), that can be translated into upper limits for DM-nucleon scattering. Here is the result:

limits on dark matter – nucleus scattering inferred from the CMS monojet search

Similar results were obtained by CDF, as shown in the plot. Also, the CMS results on photon+MET give results not quite as stringent as those obtained from the monojet search.

I like this result because it nicely illustrates the synergy between collider and direct-detection searches for dark matter particles. What we would really love to see, of course, is a signal in both. Alas, that has not yet happened…

June 28, 2012 at 5:56 pm 1 comment

Independence-Day Higgs Seminar

The CERN management have decided to convene a special seminar on July 4th at 9am Geneva time (2am in Chicago) at which the results from CMS and ATLAS will be presented. According to the official announcement, the main auditorium will be reserved for CERN personnel, so reporters and visitors will have to view the proceedings via live feed into another large hall at CERN. Of course there will also be a live broadcast that you might be able to access from the announcement page.

The timing of the seminar has nothing to do with U.S. history, of course – it is dictated by the beginning of the ICHEP Meeting in Melbourne, Australia (4-11 July). Nonetheless, American particle physicists might, in the end, have a good extra reason to celebrate this holiday. We will see…

June 23, 2012 at 3:20 am Leave a comment

Do you like to spread rumors?

Everyone is excited about the coming ICHEP conference and what will be shown by CMS and ATLAS concerning the search for the standard model Higgs boson. It is hard to be patient, and the urge to indulge one’s curiosity and to speculate without bound is hard to control.

One must ask, though: is it really OK to spread rumors?

The main rumor-monger these days is Peter Woit at Not Even Wrong — a formerly excellent blog that in recent months sometimes seems more devoted to scandal and gossip than to education. You can read there that ATLAS and CMS are seeing “about 4 sigma” in the H→γγ channel. Peter’s statements were quickly echoed over on viXra log and even Tommaso Dorigo posted an elliptical entry on Quantum Diaries. I’m sure other blogs are posting rumors – I am too lazy to go look for them now.

As a member of the CMS Collaboration, I know precisely what we have. But my loyalty remains with my collaboration, especially the people who are working right now to carry out the analysis and verify the results, as well as to the people at the top who have to chart strategy and make difficult decisions. A little splash in a blog is not worth the bother it would cause all these people.

I don’t mean to sound sanctimonious. I am just stating who I am and what choice I have made. Other people obviously make other choices, for all sorts of reasons. The education of the interested public is essential and I applaud those who do it well. The desire to get off of high horses and to step out of ivory towers is good and I value that, too.

But where does one draw the line? At what point does a blogger make a transition from expressing his/her enthusiasm, and explaining something to the general reader, toward increasing ratings and readership of his/her blog? Maybe I am too straight-laced, hailing from New England, while others prefer to hang loose. I guess the only thing to say is, as one says in this part of the world : vive la différence!

June 17, 2012 at 2:47 pm 15 comments

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