Of Particular Significance

A Bit More on CRESST

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 09/07/2011

Two full non-stop days of work at Canada’s Perimeter Institute, filled with  intensive study of the papers, notes and conference talks from the ATLAS and CMS experiments at the Large Hadron Collider, have left me slightly exhausted but pretty well up-to-date.   This higher level of knowledge will start to percolate into these pages almost immediately.

Meanwhile, a bit more insight into the CRESST dark matter situation (see yesterday’s post). There  have already been silly press articles.  The BBC article says

  • Researchers at the Cresst experiment in Italy say they have spotted 67 events in their detectors that may be caused by dark matter particles called Wimps.

No, that’s not what they said.  (Not surprisingly, there are other serious scientific errors in the BBC article.)  The CRESST paper said they see 67 events that look like a dark matter particle hitting an atomic nucleus in the detector, but some number of them — probably most of them — are caused by backgrounds.  That is, they know there are other sources, perfectly conventional, for many of these events. 

They list four types of known backgrounds, from various forms of radioactivity, all of which can create effects in the detectors that are sometimes not distinguishable from a particle of dark matter striking an atomic nucleus in the detector.   The difficult task of the experimenters is to determine whether the known backgrounds could have given them 67 events.  They claim the known backgrounds could not have produced so many fake events, and that instead between 1/3 and just over 1/2 the events are extras, which might therefore be from dark matter collisions.

Maybe.  The question is whether the excess events that they see are from dark matter, or whether they are from a fifth type of background that they haven’t understood yet.  Both are plausible at this point.  Quoting from the paper:

We have estimated these four backgrounds and have found using a likelihood ratio test that, at a significance larger than 4 standard deviations, these backgrounds are not sufficient to explain all the observed events. Scatterings of WIMPs may be the origin of this e ffect and, under this assumption, we have derived the corresponding WIMP parameters.

I should add that there is something I currently find disconcerting about the way the analysis was done.  In the data analysis they assume a standard WIMP (weakly-interacting massive particle, where weakly-interacting means interacting via the weak nuclear interaction) but a standard WIMP of this class is ruled out by two independent experiments, CDMS and XENON100, which are quite different from each other and from CRESST.  To avoid CDMS and XENON100 constraints, presumably the dark matter particle should not be a standard WIMP.  However in that case wouldn’t the method that CRESST used to determine the  mass and interaction strength of the dark matter particle also need to be changed?  possibly altering the statistical significance and the number of events?  Will try to get insight from experts here.
Unfortunately CRESST did not plot all of their data in their paper.  They have eight  detectors, but they only showed the full data from two of them.  This makes it difficult to do more detailed analysis; one just has to take what they say at face value.  Perhaps they can be convinced to show the data from the remaining detectors.

I’m grateful to Neal Weiner of NYU, and Philip Schuster and Natalia Toro of the Perimeter Institute, who are more expert than I, for some insights and discussions.

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5 Responses

  1. Thank you for the informative updates on LHC
    I have a few fundamentally silly question on this subject which are causing me some logical problems.
    How does dark matter choose which stars it affects and which it does not affect gravitationally? In other words, how can it affect the outer stars of a galaxy but not the inner stars or the solar system?
    How can dark matter within hadrons (when they are created by supersymmetry superparticles and higgs) be located in a place other than where quarks (that is matter) exist? Would dark matter not have the same location and be proportional to mass of ordinary quark matter if that were so?
    If it were in the same location and proportional to ordinary mass would that not make its existance irrelevant with respect to the anomalous speed of outer stars.
    Regards Alf

    1. Dark matter affects all stars; the reason we detect its effects by looking at stars far from the center of a galaxy is that the amount of dark matter falls off more slowly with distance from the galactic core than does the number of stars. In the middle of a galaxy, the stars influence each other as much as the dark matter influences them, so we can’t detect the effects of dark matter. But in the outer regions, the dark matter has a larger effect on the stars than the stars have on each other. So it’s in the outer regions that dark matter has, *relatively* speaking, a large impact, and therefore that is where we can detect its presence.

      When (or rather, if, since we don’t yet know if this is possible) dark matter particles are created in a collision of two protons, they are free to go anywhere they like. They will not be affected almost at all by the presence of the surrounding proton. That is because their interactions with ordinary matter are very weak. You could ask the same question of neutrinos (which certainly can be produced in proton-proton collisions) and the answer is just about the same — neutrinos barely interact with the stuff inside the proton, and behave almost the same way inside a proton as they do in empty space.

      Your third question, I’m afraid, doesn’t make sense… it must be based on some misconceptions that are somewhat revealed by the second part of your second question. I do not know why you think that dark matter’s mass would be proportional to the mass of ordinary quark matter. The mass of ordinary quark matter comes from the strong nuclear force between the quarks, antiquarks and gluons in the proton. (If this is a confusing statement, read my article on What Is A Proton, http://profmattstrassler.com/articles-and-posts/largehadroncolliderfaq/whats-a-proton-anyway/.) Since dark matter particles do not feel that force, their masses are essentially unaffected by being inside a proton.

  2. Thanks for this update 🙂

    Such clarifying comments showing that things are not that simple should always be printed directly below the corresponding article written by the journalist …

    I`m curious about what is assumed that this standard WIMP used by CRESST exactly is (a LSP or something else ?) and what other classes of DM particles should be considered in the analysis instead if the standard WIMP is excluded already.

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