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The Sciences

Dark Matter Detected, or Not? Live Blogging the Seminar

Cosmic VarianceBy JoAnne HewettDecember 18, 2009 12:22 AM

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1:10 PM: As previously reported by Sean, the physics community has been all atwitter recently with rumors that the CDMS collaboration has observed dark matter events. Personally, I have heard rumors that they have either 0, 1, 3, or 4 signal events above the 0.5 events expected from background, with the number being dependent on who is propagating the rumor. The journal Nature has denied that a paper will be published, but everyone seems to agree that a CDMS paper will appear on the arXiv tonight. And now, finally, the day of reckoning has arrived. Members of the CDMS collaboration are giving at least 2 simultaneous seminars this afternoon (SLAC and Fermilab). 2:00 PM PST. The excitement in the air is palpable. Not much work is being done – everyone is pretty much talking in the hallways, trying to pass the time until 2:00. David E. Kaplan is here from Johns Hopkins University to film the event for a documentary he is making. Rumors are spreading that at least 3 groups of theorists have papers ready to submit to the ArXiv after the seminars are concluded. The era of data has begun! While we wait, let’s recall how CDMS detects dark matter. The Cryogenic Dark Matter Search consists of Ge detectors, situated deep underground in the Soudan mine in northern Minnesota. The depth is required to reduce background from cosmic rays. As the earth travels through the sky, it presumably sweeps through clumps of dark matter which subsequently scatters with the particles in the Ge. The scattering is expected to be elastic, meaning the initial and final scattering particles are the same. The detector measures the ionization and phonons from these interactions. Background events mainly create electron recoils, and so a nuclear recoil gives a good DM signal. The energy of this recoil gives information about the mass of the DM particle and the number of events compared to the total amount of data taken gives the interaction rate. 1:30: So what does a theorist do while waiting for the seminar? A quick calculation, of course! What else? The energy of the highest observed recoil event, with an assumed DM velocity distribution, gives a bound on the mass of the DM particle. Using some DM velocity assumptions from 0912.2358, one could do a quick and dirty calculation! My student Randy did so, and here's the plot (vertical axis is WIMP mass in units of GeV and horizontal axis is the recoil energy (in GeV):

cdms1.jpg

Taking this to the talk, and then can immediately read off the WIMP mass bound! Wanna bet how many theory papers appearing tonight have this plot? 1:45: In my seat - front and center. The auditorium is packed - SRO. Folks from UC Santa Cruz and Berkeley are here. The speaker is Jodi Cooley from Sounthern Medthodist University and is wired and ready to go. 1:54: JoAnne beat me to it, but I’ll join in on the live blogging with the view from the back of the Auditorium! –Risa 1:55: Learned that there's a group of theorists at the KITP in Santa Barbara watching this event over skype. 1:58: You can watch it yourself -- Live Streaming Video starting at 2 PM: http://www-group.slac.stanford.edu/kipac/cdms_live.html -- Risa 2:00: A hush just came over the room...JoA 2:01: Jodi is showing her outline - starts with an intro, then describes the detector, then gives the results, and then discusses future plans for the collaboration. My colleague sitting next to me just leaned over and whispered "It looks like a signal talk." - JoA 2;05: liveblogging appears to be deleting some of our comments, so you gotta pay close attention! --Risa 2:12: Quick comment on JoAnne's nice plot while we are still in the intro. It's important to remember that the actual value of the WIMP mass depends, as JoAnne says, on "some DM velocity assumptions". The paper she cites is eminently reasonable, but the values are still wildly uncertain (see e.g. arXiv:0906.5361), so if we do actually have a dark matter detection, understanding the expected density and velocity distribution of dark matter in our own galaxy becomes much more important. --Risa 2:15: slide 14 discusses the direct detection event rate. Expected signal event rate is < 0.01 events/kg-day. kg-day is the unit used by the experiments to measure their sensitivity - kg denotes the size of the detector, and day is the number of days the detector was taking data. -JoA 2:20: Each detector consists of 230 g Ge and 100 g Si. There are 30 detectors stacked into 5 towers. -JoA 2:23 Experiment is designed to have ~0.5 background events. -- Risa 2:24 1/2 mile Underground in the Soudan mine. Note that this is in Minnesota, not the Sudan. --Risa 2:23: Slide 24: they can select or tune the background level as desired by their cuts. They choose 0.5 background events in the signal box as a target. The depth is at 6060 mwe (meters water equivalent) - JoA 2:27: About to open the signal box... - JoA 2:30: November 5, 2009 opened the box in a big conference call. --Risa 2:31: Measured surface event background in 0.6 plus/minus 0.1 (stat). They use 3 techniques to determine this. Slide 30. Neutron bckgrnd is 0.04 plus 0.04 minus 0.03. Radiogenic bckgrnd in 0.03-0.06 events. - JoA 2:32: Experiment has 194.1 kg-days WIMP equivalent @ 60 GeV/c^2 --Risa 2:33: THE NUMBER IS TWO!!!! - JoA 2:34: I consistently heard 3 events, so interesting comment on the rumor mill -- Risa 2:35: The two events happened at different times, different months, different detectors. -- Risa 2:38: Were in inner region of the detector where there is better background rejection. -- Risa 2:40 Probability of observing these two events is 23%. Clearly not the solid detection we were all hoping for! --Risa 2:41: Refined calculation of the surfact background including timing, post unblinding leakage estimate is now 0.8 plus/minus 0.1 (stat) plus/minus 0.2 (sys) -JoA 2;42: The 23% probability of observing the 2 events includes the post-unblinding leakage. -JoA 2:45 Summarizing now: cross section limit is < 3.8 x 10^{-44} for WIMP mass of 70 GeV at 90% CL. -JoA 2:47: More towers of detectors have recently been installed, with more to be installed in 2010. -JoA 2:48: arXiv number is 0912.3320 -JoA 2:50: Bottom line: "The results cannot be interpreted as significant evidence for WIMP interactions, but we cannot reject the possibility that either event is signal." --Risa 2:51: Question about other unanalyzed data. Answer: next step is really SuperCDMS. -- Risa 2:52: The events took place on 8/5/07 and 10/27/07. --Risa 2:55: Very solid, careful work, a new upper limit, and a new thing to add to "hints of dark matter". But could easily be a statistical fluctuation. Going to be a bit longer until something solid. -- Risa First event was detected on 10/27/07 with recoil energy of roughly 12 keV, and the second was on 08/05/07 with roughly 15 keV. A 3^rd event lies just outside their box with recoil of 12 keV. -JoA 2:59: The crowd is filing out amid much discussion, while Dimopoulos and crowd are being interviewed. There will be much discussion all afternoon! - JoA 3:01: Given the above figure, if these events are interpreted as signal, the lower bound on the WIMP mass for these recoil energies is roughly 2 GeV. Not that constraining! -JoA 3:18: Summary in CDMS's own words here (pdf). -Sean 4:00: Corrected figure now posted. :) -JoA 6:40: Can't find the paper on the arXiv and 0912.3320 is a nice paper (on DM) written by my good friend and collaborator Hooman Davoudiasl, but has nothing to do with CDMS... Jodi told me it was submitted at 1, just before the talk, which means it won't be out until the next round (Sunday night, I think). --Risa

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