Wednesday, 30 October 2013

Fiat LUX

A new episode of the dark matter detection saga was just broadcast live from Sanford Lab.  LUX is a new direct detection experiment located in a South Dakota mine, not far from Mt Rushmore. Today they presented their results based on the first 3 months of data taking.

LUX is very similar to its direct competitor Xenon100. They both use a dual-phase xenon target, and  a combination of scintillation and ionization photons to detect collisions of dark matter particles with nuclei, estimate their recoil energies, and separate dark-matter-like events from gamma- and beta-ray backgrounds.  The active target region of LUX  is about 4 times larger than that of Xenon100, but the shorter time of data taking means the amount of data collected so far is similar.  However, one advantage of LUX is better collection of photons appearing inside the  detector, which allows them to lower the energy threshold for detection down to 3 keV nuclear recoils (compared to the 6.6 keVnr threshold in the latest Xenon100 analysis). Thanks to this feature, they already managed to beat the Xenon100 sensitivity significantly, especially for low mass (below 10 GeV) dark matter which produces few photons when it scatters on nuclei. This last aspect was what made today's announcement so interesting. Recently there has been several (partly contradictory) reports of possible detection of low mass dark matter, the most serious of which was the 3 events seen by the CDMS silicon detector. These signals were in tension with the  Xenon100 results, but the low mass region is experimentally difficult and an independent confirmation was more than welcome. If the CDMS signal was really dark matter LUX should be literally swamped with dark matter events. Instead, what they see is this:

There's 160 events surviving the experimental  cuts. They are plotted according to how many scintillation (S1) and ionization (S2) photons they produce. The background is expected to  show up in the blue band, and the dark matter signal (characterized by a lower ratio of ionization to scintillation) in the red band. What can be found in the red band is perfectly consistent with leakage from the background region (note in particular that there is no events below the center of the band), with  the p-value for the  background only hypothesis at the fairly large value of 35%. Translating that into limits on the scattering cross section of vanilla-type dark matter on nucleons results in this plot: 

The red line is the previous best limit from Xenon100. The blue line is the current 90% CL limit from LUX,  which puts them at the pole position in the entire mass range above GeV. They are the first to break the 10^-45 cm^2 cross section barrier: the limit goes down to 7.6*10^-46 cm^2  for dark matter mass of 33 GeV. To put it into perspective, the LHC can currently study processes with a cross section down to 10^-39 cm^2 (1 femtobarn).  The inlay shows the low mass region where positive signals were claimed by CDMS-Si (green), CoGeNT (orange), CRESST (yellow) and DAMA (grey). All of these regions are now comfortably excluded, at least in the context of simple models of dark matter.   

So,  the light dark matter signal that has been hanging around for several years is basically dead now.  Of course, theorists will try to reconcile the existing positive and negative results, just because it's their job.  For example, by playing with the relative couplings of dark matter to protons and neutrons one can cook up xenophobic models where dark matter couples much more strongly to silicon and germanium than to xenon.  But seriously, there's now little reason to believe that we are on the verge of a discovery. Next time, maybe.  

The LUX paper is here.


Anonymous said...

There is probably no reason to go "xenophobic". The semi-empirical models of LXe response that LUX has used are neither conservative, nor based on what could be called "quality data", from the way they have been shifting over the years. Their low-energy limits are exquisitely sensitive to all of this.

Anonymous said...

Juan Collar, you're not fooling anyone with your "Anonymous" identity.

Anonymous said...

Why in this day and age should we take a non-blinded analysis from a dark matter experiment seriously?

Robert L. Oldershaw said...

There is not a shred of empirical evidence that suggests that the dark matter is in the form of any mythical WIMP, axion, or other hypothetical particle.

A far better candidate for the dark matter is low-mass black holes with masses in the 0.1 to 0.6 solar mass range. The MACHO microlensing experiment apparently detected billions of objects in this mass range that were inconsistent with conventional stars. Primordial black holes were considered the best interpretation of the empirical results.

Alas, primordial black holes are not fashionable today, while fairytale WIMPs are. So it goes in the pseudo-science era.

Robert L. Oldershaw
Discrete Scale Relativity/Fractal Cosmology

andrew said...

The results are really not surprising given that astronomy data seem to already favor warm dark matter of the keV scale, rather than cold dark matter of the GeV scale (both of which are consistent with "cold" dark matter as it is defined in the lamdaCDM model).

Also, given the absence of any indication of such particles in W and Z boson decays, it seems likely that any new dark matter candidate, if it exists, cannot participate in weak force interactions (i.e. it must be "sterile" and interact only via gravity and possibly also via some as yet undiscovered force peculiar to the dark sector). Such particles would be expected to have very low cross-sections of interaction with ordinary matter, far below the 10^-39 or so of the weakly interacting neutrino.

It is not at all obvious how you design a direct detection experiment that would detect what is functionally, if not taxonomically, a keV sterile neutrino of the kind favored by WDM models.

Dan D. said...

Disappointing, to be sure. I guess time to watch IceCube and AMS-02 like a hawk while waiting for the next run of LUX, XENON1T, and the LHC 2015 startup? Or, maybe axion searches will turn up something soon? #graspingatstraws

Anonymous said...

It's getting tough to reconcile reality with most current DM models.

Usually the hallmark for a good scientific theory is its ability to make accurate, testable predictions. There is one theory that accurately predicts galactic rotation curves and especially galactic satellite features, an area where DM theories can only be charitably described as "poor":

Yeah, MOND and its derivative theories are evil/not true/an affront to "science" (unlike Dark Matter, String Theory and SUSY) but it's a tool that can predict real, observable features in galaxies.

Maybe DM is just neutrinos. Or maybe it's a combination of boring "normal" particles and exotic physics like MOND (which has issues in the galaxy cluster and larger scale.)

Ru said...

When you talk about cold or warm dark matter, I understand that it's a measure of velocity, but is it possible to put a figure on it in Kelvin? or would that not make sense..

Jester said...

As you say, WDM is defined by the velocity distribution or the power spectrum (an intermediate case between cold and hot dark matter). You don't expect DM to be in thermal equilibrium now so one cannot assign temperature to it.

Hal Swyers said...

It seems like the particle desert is getting larger all the time. At some level this is both disappointing and exciting, it could also be evidence of R-parity violation at very high energy scales. Unfortunately we aren't likely to know in my lifetime...bummer.

Mitchell said...

Hal Swyers, why does R-parity violation help?

Anonymous said...

two things
1) XENON100 has 34 kg of FV and 225 days while LUx has 118kf FV and 85 days, so they have ~1.3 more exposure not like written above.
2) I agree we that it is starting to get frustrating for WIMP search, however when talking about MOND or its GR extension TeVes one should take into account that it can't explain weak lensing nor cluster of galaxies phenomenas

Jester said...

True. I meant the total size (370 vs 161kg) but it's more appropriate to compare the active target region (250 vs 62kg) or the fiducial volume in the most recent analyses as you do. I'll correct the text.

Anonymous said...

Anonymous said:
" I agree we that it is starting to get frustrating for WIMP search, however when talking about MOND or its GR extension TeVes one should take into account that it can't explain weak lensing nor cluster of galaxies phenomenas "

True, but it is remarkably good at predicting the rotational curves of galaxies and their satellites, whereas DM fails at making predictions that match observations (retroactive predictions by manipulating where and how the DM is clustered and how it does/doesn't decay or interact with normal matter isn't really science).

The problems with MOND/TeVes at the larger scales seem relatively minor in comparison. Warm (or hot) neutrinos could have been an answer (at least in large clusters) but the probable non-existence of sterile neutrinos makes WDM and HDM problematic.

The Secret Experimentalist said...

This is a nice result.  However, whether you believe in "light" dark matter or not, it doesn't change anything in a fundamental way: there was already tension between XENON and the various WIMP claims.  Though LUX has a lower threshold, it hasn't shown a better recoil calibration at low energies.

Robert L. Oldershaw said...

So would you say the WIMP hypothesis is unfalsifiable, given that the "parameter space" appears to be infinitely and arbitrarily adjustable?

Anonymous said...

Great article.
Could almost understand all of it.

What's a CL Limit ?

Have you tried Heligoland yet?

Michel Beekveld

Hal Swyers said...


I went ahead a wrote a response. Not sure if what you are looking for but does cover some of my current thoughts

vmarko said...

Can anyone recommend me some nice review paper about the state of the art of various theoretical DM explanations, and how much is each of them inline with (maybe not most recent) experiments and theoretical background?

I mean, I'd like to see a paper which discusses all (even dumb) proposals in turn, and answers questions like "why not neutrinos?", "why not black holes?", "why not WIMPs?" "why not axions?", "why not SUSY?", "why not TeVeS?", etc. Or gives pros and cons for each, experimental status, and so on.

Can anyone point me to something like that?

Thanks, :-)

Anonymous said...

Marko -- This presentation may be a good starting point for dark matter candidates --

Jester -- Another recent exciting experimental result you may want to discuss in the blog is the factor of 12 improvement in bound on electron EDM ( ). The past EDM experiments had already implied that the EW scale was fine-tuned by 1% or so -- now the finetuning implied by the latest results is a factor of 12 worse than before (0.1% fine-tuning) -- making it even more unlikely to discover anything at LHC14.

vmarko said...


Thanks! That presentation is very good, I went through all 67 slides in detail. The only issue is that (like with all presentations) I need to guess what the speaker wanted to say about the material written in a given slide. But nevertheless it's a nice summary. :-)

Best, :-)

Tienzen said...

@ vmarko --- "why not neutrinos?", "why not black holes?", "why not WIMPs?" "why not axions?", "why not SUSY?", "why not TeVeS?", etc.

The article “Dark Matter Candidates (by Marc Schumann , Spring 2012, ) does provide a detailed description about the pros and cons for different DM candidates. However, there are some more new data available now.

a. For SUSY (with s-particles, such as Neutralino) --- no SUSY below 1 Tev was discovered at LHC, and it received a deadly blow by the LHCb data.
b. For the other type WIMPs (LKP or sterile neutrinos, including Neutralino), the weak hints of DAMA/LIBRA, CRESST, CoGeNT and CDMS were in contradiction to the XENON100 and are now firmly ruled out by the LUX data.
c. The direct search of DM at ATLAS has produced limits stronger than the XENON100. See ( , By Tommaso Dorigo, September 23rd 2013).
d. For black holes --- see the article “The Mystery of Dark Matter Clarified—a Little, ... it's probably not: black holes. ( )”

Most importantly, the Planck data (dark energy = 69.2; dark matter = 25.8; visible matter = 4.82) can be fully accounted for by the Iceberg Model. See , that is, the non-SM-particle DM is not needed theoretically.

Anonymous said...

Get tired to stress that WIMP dark matter cannot exist, since it runs in too many problems at the galactic scale. Why do particle physicist never admit this problem?

andrew said...

A "90% CL limit" is a confidence level limit and is the range of values over which there is a 90% probability that the true value resides within given considerations of statistical and systemic error. It is a bit less than +/- 1 standard deviation from the measured value.

Anonymous said...

What do people think of Mukhanov's latest proposal
for dark matter?

Anonymous said...

Jester - Are you going to write about the electric dipole moment null experimental results? It's hard not to see a parallel with the LUX results. SUSY has received another blow and with that maybe a blow to exotic dark matter particles.

Robert L. Oldershaw said...

So how is it going with the exciting new results expected from the AMS-02 experiment?