Lux: No Dark Matter In Sensitive Direct Search
    By Tommaso Dorigo | October 30th 2013 12:44 PM | 14 comments | Print | E-mail | Track Comments
    About Tommaso

    I am an experimental particle physicist working with the CMS experiment at CERN. In my spare time I play chess, abuse the piano, and aim my dobson...

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    The results of the LUX experiment are out - and they are negative: no dark matter signal has been spotted by the extra sensitive detector. This is a normal day for you and me, but a gloomy day for those that counted on the neutralino to be the first supersymmetric particle to show up and redeem decades of claims.

    Flippant comments aside, the experiment is a technological marvel. Located one mile underground at Sanford Laboratory, it is a water-shielded tank containing 250 kg of liquid xenon as active material. The deep underground location, combined with the exceptional purity of the xenon and the high efficiency of electron recoil detection, guarantees a high sensivity to the weak signal of neutralinos, or whatever other neutral, weakly-interacting particle dark matter is made of.

    The search was described, along with a number of technical details, nice pictures, and complicated graphs, in a talk given today by the spokespersons of the experiment, Rick Gaitskell and  Dan McKinsey. You can find the slides at this link.

    They also produced a draft of a Physical Review Letters article, but the article was taken off the web very quickly after it appeared. The abstract however reads as follows:

    The Large Underground Xenon (LUX) experiment, a dual-phase xenon time-projection chamber operating at the Sanford Underground Research Facility (Lead, South Dakota), was cooled and filled in February 2013. We report results of the first WIMP search dataset, taken during the period April to August 2013, presenting the analysis of 85.3 live-days of data with a fiducial volume of 118 kg. A profile-likelihood analysis technique shows our data to be consistent with the background-only hypothesis, allowing 90% confidence limits to be set on spin-independent WIMP-nucleon elastic scattering with a minimum upper limit on the cross section of 7.6 × 10−46 cm2 at a WIMP mass of 33 GeV/c2. We find that the LUX data are in strong disagreement with low-mass WIMP signal interpretations of the results from several recent direct detection experiments."
    So, LUX reachs down to extremely small cross sections, and still there is no hint of neutralinos permeating the universe and hitting the xenon atoms, not even for ready money. I do not rejoice - I would be as happy as you both if we found Supersymmetry in WIMP-matter interactions or in LHC collision data, so the result is not welcome. However, it confirms my beliefs that Supersymmetry is not the explanation of the composition of matter in the universe any more than it is a solution to the hierarchy problem.


     The detector was fully commissioned in April this year, and the results reported are based on three months of data taking; a year-long run will follow in 2014.


    Hi Tommaso,
    thanks for the link to the slides. Please have a closer look at page 6, which gives a nice overview of the present status. It also shows very schematically where possible DM candidates could be expected. In particular, while the MSSM dots are just examples, notice that their cross sections are still below the present exclusion limits.
    I musst add that I respect other people's beliefs ;).
    More seriously, I think that the important messange here isn't excluding SUSY or any of the other models en vogue but confirming XENON vs Ge-Si-... in the low energy range, unless something exotic pops out. Dilemas are the beauty of science!

    Actually, LUX only confirms XENON up to a certain point, since it doesn't have a better nuclear recoil calibration at low energies.

    By the way, their article is now public:

    Thanks Federico for your note and the link. Yes, it is true that Susy survives. It can't be killed anyway ;-)
    I predicted this result, on the grounds that whenever Lubos Motl gets excited about the imminent discovery of supersymmetric particles, the result is always null. He has made such predictions at least a dozen times that I remember, and of course he was wrong every single time. Why should this time be different?

    Dark Matter and Dark Energy are Mirage

    Here is one which pleases me more: :

    "The significance of past-pointing four-vectors and negative energies in general relativity is discussed. The sign of the energy is not absolute, but relative to the four-velocity of the observer, and every particle/observer always measures its own mass as positive. It is shown that the description of the interaction of past-pointing and future-pointing matter in general relativity requires two metric tensors for self-consistency. This aspect of general relativity might account for the observations that led to the proposal of "dark energy" and non-baryonic "dark matter"."

    Who knows. More null experiments are clearly needed.

    Atoms do emit photons, but in effect it are the oscillating electrons that do this. They do this only when they suddenly decrease their energy because they switch to a lower energy oscillation mode.

    Dark matter does not emit photons (or gluons, but it curves space. So any massive (free non-oscillating elementary) particle that is at low enough energy can act as dark matter.
    For example, why is the Higgs particle not considered as a dark matter particle?

    I am puzzled why these experiments only looked at neutralinos.
    If you think, think twice
    > I am puzzled why these experiments only looked at neutralinos.

    They didn't. One can find any particle that interacts via the weak force with the atoms in the detector mass. These may or may not be neutralinos, sterile neutrinos, whatever...

    If dark matter consists of particles that only interact via gravitation (not at all impossible, I can't remember what simulations of cosmological evolution say about that) well, tough luck, no-one is ever gonna detect them. If there are no particles but one needs some mathematical tuning of the mysterious sort, one can also wait a long time.

    Higgs is not "dark matter" because it is not stable. You need particles that have staying power of the order of the universe's lifetime and that can wander into your detector.

    > Atoms do emit photons, but in effect it are the oscillating electrons that do this.

    Properly, the energy in the electron configuration is dumped out as it changes (increasing entropy in the process), yes. But note that if you rearrange the nucleus you get a nice gamma quantum out of the "corequake".

    > So any massive particle that is at low enough energy can act as dark matter

    Well, I do think massive GUT things like magnetic monopoles could work too. But they don't seem to be very plentiful. I remember that someone even thought that mini black holes might be cloaked as electrons, incapable of radiating their mass as Hawking energy because that would imply erasure of some quantum numbers (can't remember the mechanism). Tests for seriously massive electrons or protons in bulk material didn't yield anything either.

    John Duffield
    Good stuff Tommaso. I went through the slides, it looks like good science, thanks for putting this up. But maybe your title should read No Dark Matter particles rather than No Dark Matter. Something as simple as a gravitational field is an example of dark matter in the wider sense because the field energy has a mass equivalence. See The Foundation of the General Theory of Relativity page 185 where Einstein said "the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy". Maybe LUX is a timely reminder that dark matter isn't necessarily composed of particles.
    The LUX physicists make no reference to the work of Milgrom. However, over the past 30 years, Milgrom has accumulated overwhelming empirical evidence in his favor.
    Kroupa, P.; Pawlowski, M.; Milgrom, M.; The failures of the standard model of cosmology require a new paradigm; International Journal of Physics D, vol.21, 31 Dec. 2012
    Can someone point out where Kroupa, Pawlowski, & Milgrom are mistaken?

    John Duffield
    I don't think they're totally mistaken, in that gravity ends up getting modified. But IMHO the mistake they make is to criticise the SMoC without going back to Einstein's original GR. For example postulate1 ignores the fact that Einstein described a gravitational field as inhomogeneous space, or that gravitational field energy doesn't appear in the EFEs. Einstein said "the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy" but postulate2 ignores the fact that any concentration of energy causes gravity, not just matter. So if the early universe had a high but uniform energy-density there's no gravity and no inhomogeneity and no curvature. But it would have had something like gravitational time dilation which depends on potential (=energy density) so even slow initial expansion would look very fast for any gedanken observers present. That's effectively inflation, which they pooh-pooh as postulate3. Their reference to postulate4 assumes that matter rather than any inhomogeneous energy density assists clumping, and they're forgetting about the pressure diagonal in the stress-energy tensor when they complain about postulate5. All in all IMHO they're being critical of modern misinterpretation then pulling a rabbit out of the hat called a0. That's another "constant", a patch, when what they ought to be looking at is a proper correction in the guise of a non-constant cosmological constant. That's "the energy-density of empty space". If that energy-density is not uniform, it has a mass-equivalence and a gravitational effect. And space, of course, is dark.
    Dear Tommaso

    May I ask something?
    In order to detect WIMPS one takes advantage of the expected diurnal (and annual) variation of their flux.
    If we had two SIMILAR detectors located in two sites with as same backgrounds as possible - antidiametrically in the earth - 
    then when one would measure diurnal excess, the other would measure deficit.
    Thus by measuring this anticorrelation , wouldn't we be able to conclude easier about whether there are WIMPs or not?

    Hi Emmanuel,

    sure, two detectors are better than one; but since one knows what kind of annual
    variation and diurnal variation to expect, given that the motion of the Earth in the galaxy is known, there is no need to have two detectors; besides, one can just fit the event counts as DAMA
    has been doing in the course of the last 10 years or so, using a sinusoidal of period 1y.

    In any case not all these experiments do this. Lux, or other small-background experiments,
    do not look at time variations, but at rare events...


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