Breaking News: Two Great New Measurements

Two new ground-breaking measurements reported results in the last 24 hours!  Here are very quick summaries.

A group of atomic physicists, called the ACME collaboration, has performed the best search so far for the electric dipole moment (EDM) of the electron.  Unfortunately they didn’t find the EDM, but the limit

  • |de| < 8.7  10-29 e cm

is 12 times stronger than the previous one.  While this is still a billion times larger than what is expected in the Standard Model of particle physics (the equations used for the known elementary particles and forces), there are various types of as-yet unknown particles and forces that could easily produce a much larger electron EDM, through new violations of T symmetry (or, almost equivalently, CP symmetry).  These effects could have been large enough to have been discovered by this experiment, so those types of possible phenomena are now more constrained than before.  Fortunately, there’s more to look forward to; the method these folks are using can eventually be improved by another factor of 10 or so, meaning that a discovery using this technique is still possible.

This morning the LUX dark matter experiment reported new results, and knocked everyone’s socks off.  They have understood their backgrounds from radioactivity much better and more quickly than most of us expected, using new calibration methods and a much better characterization of their backgrounds than has previously been possible.  Although they have a detector only a bit larger than XENON100 and have only run the detector underground for three months, compared to the year or so that XENON100 ran previously, their limits on the rate for a dark matter particle to hit a Xenon nucleus beats XENON100’s results by a factor of 2 for a dark matter particle of mass 1000 GeV/c², increasing to about a factor of 3 for a dark matter particle in the 100 GeV/c² mass range, and soaring to a factor of 20 for a dark matter particle in the 10 GeV/c² mass range.  Consequently, LUX pretty definitively rules out the possibility, hinted at by several dark matter experiments (as discussed in the second half of the article I wrote about this in April), of a dark matter particle in the 5 – 20 GeV/c² mass range.  (See the figure below.) While XENON100 seemed to contradict this possibility already, it didn’t do so by a huge factor, so there were questions raised as to whether their result was convincing. But the sort of ~10 GeV/c² dark matter that people were talking about is ruled out by LUX by such a large factor that finding ways around their result seems nigh impossible.   And again, there’s more to look forward to; by 2015 their results should improve by another factor of 5 or so… so they get another shot at a discovery, as will XENON1T, the successor to XENON100.

Congratulations to both groups for their spectacular achievements!

Results from the LUX paper, with labels added (hopefully correctly) by me; the shaded blue area is the range that LUX expected to reach, and the blue line their actual result, which exceeds the XENON100 result (red line).  The left plot shows the range 10 - 1000 GeV/c^2; the right plot is an inset showing details of the low-mass region, along with the hints of signals from DAMA/LIBRA, COGENT, CMDS and CRESST, all of which now appear entirely implausible.

Results from the LUX paper, showing excluded regions as a function of dark-matter particle mass (horizontal axis) and dark-matter/nucleus collision rate (vertical axis), with labels added (hopefully correctly) by me.  The shaded blue area is the range that LUX expected to reach, and the blue line their actual limit, which considerably exceeds the XENON100 result (red line) at all masses. The left plot shows the mass range 3 – 4000 GeV/c^2; the right plot is an inset showing details of the 5 – 12 GeV/c^2 mass region, along with the hints of signals from DAMA/LIBRA, COGENT, CMDS and CRESST, all of which now appear entirely implausible.

17 responses to “Breaking News: Two Great New Measurements

  1. “But the sort of ~10 GeV/c² dark matter that people were talking about …”

    Matt, regarding what “people were talking about”, are we speaking here of sheer speculation on their part? Or mathematical/statistical predictions based on something more concrete (though still having a speculative foundation).

  2. Does the EDM result confirms the existence of unknown particles and forces or not yet ?
    What is the impact of excluding 5-20GeV WIMPS on Supersymmetry ?

    • 1) Since the EDM is not observed, but only bounded from above, the result simply excludes some types of unknown particles and forces; it confirms nothing.

      2) Not much.

  3. kashyap vasavada

    Matt,interesting report. What is the range of believable theoretical models prediction of WIMP-nucleon cross section? Or one can get any answer by fudging couplings?!! This will be a serious problem for dark matter theories. If I understand MOND has been ruled out by terrestrial experiments.

    • There are plenty of dark-matter theories that can evade the XENON100 and LUX results, which are putting increasing pressure specifically on WIMP-type models, where the particle in question interacts with matter via the Higgs or via the Z particle. If dark matter is of a very different sort, these measurements may not be relevant.

    • I have seen figures of 10^-39 cited for a neutrino cross-section (which should be similar to any Z mediated interactions with dark matter) and 10^-42 to 10^-46 for a Higgs mediated cross-section in an old post by Jester that I have lost the link to since I accidentally linked to the entire blog rather than the pertinent post.

        • “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 10^-42-10^-46 cm2 ballpark (depending on the Higgs mass and on the coupling of dark matter to the Higgs). This generic possibility is now getting disfavored thanks to Xenon100’s efforts, unless the Higgs is heavier than we expect. Therefore, even though models predicting the cross section below 10^-44 cm2 certainly do exist, it may be a good moment to start thinking more seriously about alternatives to WIMP.”

          So this is pre-Higgs discovery and could be made far more specific now?

  4. to seek supersymmetry might occur strongest violation of operator T or CP to levels greater than given by standard model.i believe will appear chanes in the spacetime structures to 4-dimension manifolds as observed by s. donaldson-that implies differents smooth topological 4-dimension manifolds with spin tensor-with torsion

  5. Is there not a case that the EDM is in fact what we now believe is the Higgs Particle. Is there any chance of ambiguities here?

  6. It was suggested at the time of the Higgs measurement that it is only the same thing ECM? Please indicate

  7. But heavy WIMPS does not solve the hierarchy problem, in addition we are still waiting for your explanation of naturalness problem .

  8. Anthony Barker

    What limits does the LUX result impose on LSP production at LHC energies?

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