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Xenon100 approaching

March 12, 2011

NEUTEL is a series of conferences oriented on neutrino physics but for me, like for most theorists working on physics beyond the Standard Model,  neutrinos are boring. Of course, this sentiment cannot be supported by logical arguments. Neutrinos provides one of the few experimental hints that the Standard Model of particle physics is not a final theory.  Moreover neutrino physics  has a number of interesting questions left unanswered (absolute mass scale, 1-3 mixing angle, CP violation), it connects to exciting phenomena in cosmology and astrophysics, it enjoys a rich experimental program, and it holds a promise of new discoveries in near future. But what can I help… it’s boring. I am looking at NEUTEL with interest for another reason:  Wednesday at high noon the Xenon100 collaboration is expected to reveal new results of dark matter searches. Either dark matter will be discovered or new, much more stringent limits will be set.

The field of direct detection of dark matter has already a long history. The idea to look for scattering of dark matter particles on ordinary matter was put forward  back in 1986 in a theoretical paper by good man Witten (this Witten). First experimental limits on the cross section of dark matter on nucleons soon followed.  The remaining history is a painful process of improving the exclusion limits with a few entertainment breaks thanks to unconfirmed positive signals. In a sense,  being a decades-long series of disappointments, the field is similar to collider physics. The important  difference between the 2 fields is the much quicker rate of progress of the former. Here are 2 plots, one from 1997, one from 2010:

The left-hand side shows the experimental situation in the late 90s, with the solid red line denoting the limits from the DAMA experiment (the lower lying green lines refer to a projected sensitivity); the right-hand shows the current limits from CDMS and Xenon100.  Thus the gain in sensitivity was 2 orders of magnitude in a course of  the decade.  New Xenon100 limits are expected to move the limits down by another order of magnitude, if no signal is not found.  For comparison, in collider physics it took 8 years to vamp up the energy by a meager factor of 3.5, from 1.96 TeV in Tevatron’s run II to 7 TeV in the LHC.

When should the dark matter experiments expect to see the signal? Or when should they quit?  Unfortunately, the answer to these questions is not clear at present. The most appealing possibility – a weak scale dark matter particle interacting, like neutrinos, via  Z-boson exchange –  leads to the cross section of order 10^-37 cm^2 which  has been excluded in the 80s. There exists another natural possibility for WIMP dark matter:   a particle interacting via Higgs boson exchange. This would lead to the cross section in the ballpark of 10^-43cm^2 (however strongly depending on the Higgs mass, and also on the coupling of dark matter to the Higgs) which is currently being probed by experiment. If Xenon100 see nothing, it is a good moment to seriously start worrying whether the WIMP paradigm corresponds to reality, although models that predict even lower cross sections do exist. In the worst case dark matter may be very weakly interacting (axions, gravitinos) or very light (keV-MeV scale dark matter), in which case the direct detection searches are doomed from the start.

For the next decade or so the direct detection will go in the direction of monster detectors that will allow us to improve the sensitivity down to 10^-47 cm^2. Preparations for a 1 ton xenon detector are already well underway, while 10 ton xenon detectors are on the drawing board. Actually, it’s not the size of the experiment that is the limit here.  When the sensitivity reaches 10^-48cm^2, probably sometime in the next decade,  these searches will encounter the background of atmospheric neutrinos and diffuse supernova neutrinos. Thus, asymptotically, dark matter detection will merge into neutrino physics, which is probably why dark matter results are presented at NEUTEL 😉 . Well, that does not necessarily have to be the nightmare scenario. Recall that Kamiokande was actually meant to look for proton decay; instead, it got the Nobel for discovering neutrino oscillations.

(posted by Jester)

6 Comments leave one →
  1. March 12, 2011 6:57 pm

    The rumor circulating over here across the Atlantic is that the XENON100 box remains unopened and new results are still a month away. Maybe the rumor is wrong?

    • March 12, 2011 11:32 pm

      Hi MR,

      maybe the rumor is one month old ? Or maybe not, and we will not hear from Elena Aprile anything about the data just yet. Still, hearing about expected backgrounds in the box is almost as interesting.


  2. March 12, 2011 11:39 pm

    It might be true. Until recently the plan was to release it at NEUTEL. But I can see now that the title has been changed from “New Results of the XENON100 Experiment” to “Results from the XENON100 Experiment” 😦 It might be related to the problems with Kr-85 contamination that Nature wrote about.

    • March 14, 2011 12:58 am

      I don’t think that there are any “problems” with Kr-85, per se, except that it is a well-known background. It’s not as though the XENON collaboration just discovered Krypton. Indeed, it’s something they’ve known about for a long time. Their (predicted) limits take into account what they can expect from Krypton (which, with the preliminary run, they should have a pretty good idea of). That said, it’s not the expected backgrounds in this game, it’s the unexpected ones…

  3. Shantanu permalink
    March 13, 2011 6:26 am

    When the first evidence for neutrino mass was announced way back in 1998
    there were many quotes by famous physicsits. See
    One of them is by string theorist Pierre Ramond

    The Super-Kamiokande measurements strongly suggest neutrino oscillations with parameters (masses and mixing angle) that imply, through the seesaw mechanism, the existence of a scale commensurate with that at which the gauge couplings unify. This is in line with the string/grand-unified view of the world at short distances, and provides additional evidence for low energy supersymmetry.”

    Since 1998, we know a lot more about neutrinos. However I don’t think anyone nowadays talks about neutrinos pointing to evidence for low energy supersysmmetry. If I was at this meeting, I would have asked this question to some of the theorists. Anyhow maybe one of you can ask this question.

  4. April 15, 2011 4:36 pm

    Dark matter is like Count Dracula. It will take more than a few nails in the coffin to put this neo-aether idea to rest. Fortunately, there are now three studies linking the temperature of a test mass to its gravitational weight. These studies suggest the possibility that luminosity is the source of gravitational attraction. Astronomers all know that luminosity emanates from all gravitationally bound bodies in the universe such as binary stars, planetary systems, galaxies and clusters. My studies show a 1.9%, 8.9%, 9.6% and a 16% increase in the weight of the test mass when these test masses were placed between a 1000 W hot source and a cold source. Such unexpected anomalous results challenge the principle of equivalence upon which General Relativity depends and Einstein’s interpretation of E = mc^2. But go a ahead and ignore them. Maybe someday they will find that dark matter (and dark energy).

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