Wednesday 23 December 2009

Closed for Winter

Another year gone. In history books it will be marked as the year of the first LHC collisions, and in a few years noone will remember that 2.36$\ll$14. Meantime particle theorists have dwelled mostly on the dark side. The great expectations on the part of the Fermi experiment have not been fulfilled so far: their result may contain interesting physics but at the moment they are far from conclusive. The CDMS bubble inflated by certain irresponsible bloggers ;) has burst last week with a loud smack. In short, we remain in darkness.

This is the last post this year. While you gobble I vanish into jungle. Will be back next year, with more reckless rumors.

Thursday 17 December 2009


  • 16:41. Welcome to the live commentary from the CDMS seminar starting today at 2pm Pacific time at SLAC. It might be the event of the year, or the flop of the decade. Though most likely it will be a hint of the century.
  • 16:50 Our seminar room is starting to get packed. The webcast from SLAC will be projected on a big screen.
  • 17:00. The SLAC seminar has started! The speaker is Jodi Cooley.
  • 17:06. Dark matter history. Zwicky, Ruben, rotation curves, bullet clusters. No way around it, we have to suffer through that....
  • 17:09. There is also a webcast of the Fermilab seminar here. They are ahead of SLAC...
  • 17:12. Now talking at length about WIMPs. Does it mean they see a vanilla-flavor WIMP?
  • 17:14. A picture of a cow on one of the slides.
  • 17:15. Fermilab already got to the gamma rejection. We are watching the wrong webcast, booo.
  • 17:18. Jodi starts describing the CDMS experiment.
  • 17:20. Previous CDMS results at Fermilab. They're getting close.
  • 17:30. We switched to the Fermilab talk here in Rutgers. Now the speaker is Lauren Hsu. She seems faster.
  • 17:35. Very technical details about phonon timing and data quality monitoring.
  • 17:38. Expected backgrounds. Finally some important details.
  • 17:39. Estimated cosmogenic neutron background 0.04, and similarly for radiogenic ones.
  • 17:40. Surface event background estimated at 0.6.
  • 17:45. They are talking about expected limits. Scaring. They don't have a signal? If there were no signal they would obtain two times better bounds than the last time.
  • 17:47. Rumors reaching me, of 2 events at 11 and 15 keV.
  • 17:49. It's official: 2 events. One at 12 keV, the other at 15 kev.
  • 17:49. There are additional 2 events very close to the cut window, approximately at 12 keV.
  • 17:58. Now discussing the post-unblinding analysis and the statistical significance.
  • 18:00. Both events were registered on weekends. Grad students having parties?
  • 18:01. The significance of the signal is less than two sigma.
  • 18:04. One of the events has something suspicious with the charge pulse. A long discussion unfolds.
  • 18:12. After post-unblinding analysis the signal significance drops to 1.5sigma (23 percent probability of the background fluctuation).
  • 18:14. The new limits on dark matter $4x10^{-44} cm^2$ for a 70 GeV WIMP. Slightly better (factor 1.5) than the last ones.
  • 18:17. Inelastic dark matter interpretation of the DAMA signal is not excluded by the new CDMS data.
  • 18:18. Nearing the end. The speaker discusses super-CDMS, the possible future upgrade of the experiment.
  • 18:20. Summarizing, no discovery. Just a hint of a signal but with a very low statistical significance. Was fun anyway.
  • 18:20. So much for now. Good night and good luck. The first theory papers should appear on Monday.
A summary by CDMS here, and the paper here (on Arxiv password-protected untill Sunday evening; if you can't wait click here). See also a much better live commentary from eye-witnesses on Cosmic Variance.

Wednesday 16 December 2009

A little update on CDMS

I guess I owe you a short summary to straighten out what I messed up. This Thursday CDMS is going to announce their new results on dark matter detection based on the 2008 and 2009 runs. The collaboration has scheduled two simultaneous talks, one in Fermilab and one in SLAC, for 5pm Eastern time (23:00 in Europe). The SLAC talk will be webcasted here. An ArXiv paper is also promised, and it will probably get posted Thursday evening. These facts are based on the news from the official CDMS page, so they may turn out to be facts after all, unlike the previous facts I called facts even though they were unfacts :-)

Having rendered unto Caesar, I can go on indulging in completely unfounded speculations. It is pretty clear that no discovery will be announced this week, in the formal scientific sense of the word "discovery". Earlier expectations of a discovery that I was reporting on were based on the rumors of a CDMS paper accepted in Nature, which turned out to be completely false.
Moreover, some CDMS members seem to play down the hopes. But the secrecy surrounding the announcement of the new results may suggest that CDMS has seen at least a hint of a signal:
not enough for a 3 sigma evidence, but enough to send us all into an excited state.

So I see two possible scenarios:
  • Scenario #1
    CDMS has detected 2-3 events with the expected background of order 0.5. All eyes will turn to XENON100 - a more sensitive direct detection experiment that is kicking off as we speak - who should provide the definitive answer by the next summer. In the meantime, theorists will produce a zillion of papers fitting their favorite recoil spectrum to the 3 events.
  • Scenario #2
    All this secrecy was just smoke and mirrors. CDMS has found 0 or 1 events, thus setting the best bounds so far on the dark matter-nucleon cross section. Given the expectations they raised in the physics community, the Thursday speakers will be torn to pieces by an angry mob, and their bones will be thrown to undergrads.
It's fifty-fifty: either they have seen dark matter or they have not ;-) We'll see in 2 days, stay tuned.

Wednesday 9 December 2009


0.9, 2.36, 7, 10, 14, 40....oh, I got carried away again :-)

Now that they got the highest energy particle collisions, the next step is the highest energy parton collisions. Recall that Tevatron has seen dijet events with the invariant mass as high as 1.2 TeV, so we haven't beaten them yet :-)

Monday 7 December 2009

What the hell is going on in CDMS???

The essence of blogging is of course spreading wild rumors. This one is definitely the wildest ever. The particle community is bustling with rumors of a possible discovery of dark matter in CDMS.

CDMS is an experiment located underground in the Soudan mine in Minnesota. It consists of two dozens of germanium and silicon ice-hockey pucks cooled down to 40 mK. When a particle hits the detector it produces both phonons and ionization, and certain tell-tale features of these two signals allow the experimenters to sort out electron events (expected to be produced by mundane background processes) from nuclear recoils (expected to be produced by scattering of dark matter particles, as the apparatus is well shielded from ordinary nucleons). The last analysis, published early 2008, was based on a data set collected in the years 2006--2007. After applying blind cuts they saw zero events that look like nuclear recoils, which allowed them to set the best limits so far on the scattering cross section of a garden variety WIMP (for WIMPs lighter than 60 GeV the bounds from another experiment called XENON10 are slightly better).

By now CDMS must have acquired four times more data. The new data set was supposed to be unblinded some time last autumn, and the new improved limits should have been published by now. They were not.

And then...
  • Fact #1: CDMS submitted a paper to Nature, and they were recently accepted. The paper is embargoed until December 18 (embargo is one of these relics of the last century that somehow persists until today, along with North Korea and Michael Jackson fans) - the collaboration is not allowed to speak publicly about its content. Consequently, CDMS has canceled all seminars scheduled before December 18.
  • Fact #2: A film crew that was supposed to make a reportage from unblinding the CDMS data was called off shortly before the scheduled date. They were told to come back in January, when the unblinding will be restaged.
OK, we're done with facts :-), the rest is just crazy speculations.

  • Theory #1: The common lore is that particle physics papers appearing in Nature (the magazine, not the bitch) are those claiming a discovery. It is not at all impossible that the new data set contains enough events for an evidence or even a discovery. If the zero events in the previous CDMS paper was a downward fluke, several WIMP events could readily occur in the new data. Furthermore, in some fancy theories like inelastic dark matter, a large number of WIMP scattering events is conceivable because the new data were collected in summer when the wind is favorable.
  • Theory #2: Data-starved particle theorists once again are freaking out for no reason. There is no discovery; CDMS will just publish their new, improved bounds on the scattering cross section of dark matter. CDMS is acting strangely because they want to draw attention: the experimental community is turning toward noble liquid technologies and funding of solid-state detectors like the one in CDMS is endangered.
We'll see in 10 days. Disillusioned skeptics like myself bet on #2, but almost everybody else strongly believes in #1. If they are right, the Christmas holiday will be ruined.

Important update: I just received this in an email:
I was alerted to your blog of yesterday (you certainly don't make contacting you easy). Your "fact" #1, that Nature is about to publish a
CDMS paper on dark matter, is completely false. This would be instantly
obvious to the most casual observer because the purported date of publication is a Friday, and Nature is published on Thursdays. Your "fact" therefore contains as much truth as the average Fox News story, and I would be grateful if you would correct it immediately.

Your comments about the embargo are therefore, within this context, ridiculous. Peer review is a process, the culmination of which is publication. We regard confidentiality of results during the process as a matter of professional ethics, though of course authors are free to post to arxiv at any point during the process (we will not interfere with professional communication of results to peers).

Dr Leslie Sage
Senior editor, physical sciences
It is still true that the new CDMS data are scheduled to be released on December 18th, and there will be presentations in a number of labs around the world. But if there's no Nature paper then theory #2 becomes far more likely.

Sunday 6 December 2009

Back on track... what's next?

Unless you just came back from a trip to Jupiter moons you know that the LHC is up and running again. This time, each commissioning step can be followed in real time on blogs, facebook, or twitter, which demonstrates that particle physics has made a huge leap since the last year, at least in technological awareness. After the traumatic events of the last Fall (LHC meltdown, moving to New Jersey) I'm still a little cautious to wag my tail. But the LHC is back on track, no doubt about it, and one small step at a time we will reach high energy collisions early next year.

So what does this mean, in practice?
In the long run - everything.
In the short run - nothing.

The long run stands for 3-4 years. By that time, the LHC should acquire enough data for us to understand the mechanism of electroweak symmetry breaking. Most likely, Higgs will be chased down and its mass and some of its properties will be measured. If there is new physics at the TeV scale we will have some general idea of its shape, even if many details will still be unclear. The majority (all?) of currently fashionable theories will be flushed down the toilet, and the world will never be the same again.

In the short run, though, nothing much is bound to happen. In 2010 the LHC will run at 7 TeV center of mass energy (birds singing that there won't be an energy upgrade next year), which makes its reach rather limited. Furthermore, the 100 inverse picobarns that the LHC is planning produce next year is just one hundredth of what Tevatron will have acquired by the end of the end of 2010. These two facts will make it very hard to beat the existing constraints from Tevatron, unless in some special, very lucky circumstances. So the entire 2010 is going to be an engineering run; things will start to rock only in 2012, after the shutdown in 2011 brings the energy up to 10 TeV and increases the luminosity.


Unless, by the end of 2010 Tevatron has a 2-3 sigma indication of a light Higgs boson. In that case, I imagine, CERN might consider the nuclear option. The energy is not that crucial for chasing a light Higgs - the production cross section at 7 TeV is only a factor of two smaller than at 10 TeV, and moreover the background at 7 TeV is also smaller. So CERN might decide to postpone the long shutdown and continue running at 7 TeV for as long as it takes to outrace Tevatron and claim the Higgs discovery. That scenario is not impossible in my opinion, and it would be very attractive for bloggers because it promises blood. But even that, in any case, is more than one year away.

In the meantime, we have to look elsewhere for excitement. Maybe the old creaking Tevatron will draw the lucky number? Or maybe astrophysicists will find a smoking gun in the sky? Or, as I was dreaming early this year, the dark matter particle will be detected. Or maybe it already was? But that wild rumor deserves a separate post ;-)