The New XENON 100 Results On Dark Matter
    By Tommaso Dorigo | April 14th 2011 08:47 AM | 34 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 Xenon 100 collaboration has finally released the results of their data analysis, and the results are saying that there is no Dark Matter in sight so far. Since we live in an age where time is precious, I think many of you are only interested in the bottomline. I can give it to you straight away, in the form of the plot which summarizes the results.

    Xenon 100 finds three events compatible with a dark matter signal, with a background expected from more mundane sources amounting to 1.8+-0.6 events. The limit they extract on the cross section versus mass of the hypothetical particle are shown below by a thick blue curve, which cuts into the flesh of the preferred parameter space of constrained minimal supersymmetric theories (in grey), pushing them farther away.

    Ok, now the busiest of you can leave, and let's discuss some details. XENON 100 is a liquid xenon detector located in the underground laboratories of Gran Sasso, in central Italy. Thanks to the massive rock above the lab, which shields it from cosmic ray muons by a factor of a million, and thanks to the ultra-pure active material, plus the excellent position reconstruction of the charged and light signals obtained in the time projection chamber, XENON is an almost background-free probe of the faint signal that dark matter particles could deposit.

    The data analyzed was recorded during the first six months of last year. The current configuration of XENON had 99 kilograms of liquid as a veto for radioactivity and cosmic rays, and 62 kilograms as active target. Photomultiplier tubes read the prompt signal of a interaction in the liquid (S1) as well as the delayed one due to ionization drift (S2), and by a combination of pattern analysis and timing information determine the 3-Dimensiona position of the primary interaction in the volume, allowing for a very good discrimination of backgrounds.

    Before the analysis of real data, a pre-defined signal region had been selected in the plane constructed with the signal energy and logarithm of the ratio of delayed versus prompt signals: this discriminates well the potential dark matter signal (low S2/S1 ratio) from backgrounds due to radioactivity (gamma rays or beta rays). You can see the signal region as the one delimited by the dashed lines in the figure below. The three red points are the signal candidates.

    I have to say I am not an expert of these detection techniques for dark matter signals, so my opinion on the trustworthiness of the results is not particularly useful; still, here it is. I think the analysis is quite careful, and I believe the results are correct. Further, one cannot fail to observe a posteriori that two of the three events lie very close to the boundary of the signal box....

    INLINE UPDATE: After posting this article, I realized I had not provided a link to the preprint, which is here. Also, a collaborator of Xenon sent me a kind private email to point out that it does not matter for the limit where the three events lay, since they extract the limit using the probability density functions of backgrounds. That's quite okay, and I am happy to hear they took the distributions into account. However, that rings a bell.

    In the limit plot above, there is a blue band which shows 1- and 2-sigma expected limits together with the actual limit observed. You should know that we live in a very uncertain time with respect to the methods experiments use to set upper limits, and even more the methods they use to compute 1- and 2-sigma bands. The matter is indeed a complicated one, and I do not want to discuss it here. However, I cannot fail to notice that the band appears to be quite narrow in the high-neutralino mass region, if compared with the fact that the observed data (3 events, two of which are close to the boundary and thus very background-like) are not very far from the 1.8+-0.6 expected from background sources (the observed limit is higher than the expectation for high masses because that's where the energetic events weigh in the most).

    What I mean to say is that the observed limit lays at the upper 2-sigma boundary for masses above 100 GeV. Their paper, on the other hand, says clearly that there is a 28% Poisson probability of 1.8 events fluctuating to three or more. So the bands must have been computed without accounting for the possibility of Poisson fluctuations, by conditioning in some way. The paper is not enough detailed to figure this out, but I think the bands are deceiving.

    [Further inline update: with a clearer mind this morning I am re-evaluating the band. The fact they use the energy of their candidates in the limit extraction made me have second thoughts about this. Perhaps it is a little too presumptuous (I mean, even too much for a presumptuous person like me) to imagine that we can tell by eye whether the band is correctly computed or not. My default answer to such a question has recently became "no", but that has nothing to do with Xenon100...

    Elaborating this a bit: if the limit is at the 2-sigma limit at high mass, that could indeed be taken to mean that Xenon has a first faint signal of dark matter particles in their data! I have seen people getting excited for two-sigma deviations in the past, so if you are a SUSY enthusiast you might well take the stand that the result is actually encouraging, contrarily to what I conclude at the bottom of this post...]

    Another concern is the fact that Xenon uses the Profile Likelihood method to set upper limits, but it is known that the limits one obtains that way are not precise in the case of small event counts. But again, these are details... In the end, whether they excluded a bit more or a bit less with their data is only important now - in a few years, these regions of the parameter space will be excluded by other searches anyway.

    [End of inline update]

    In any case, it is quite exciting to see supersymmetric theories being gradually pushed off the board. In the paper, the XENON collaboration does provide a comment on this:

    This result excludes a large fraction of previously unexplored WIMP parameter space, and cuts into the region where supersymmetric WIMP dark matter is accessible by the LHC. Moreover, the new result challenges the interpretation of the DAMA and CoGeNT results as being due to light mass WIMPs.
    With the LHC experiments actively excluding large swaths of SUSY parameter space, and the coming of age of XENON, it seems we have entered a very sorrowful era for SUSY aficionados...


    "it seems we have entered a very sorrowful era for SUSY aficionados..."
    For once you got it right: SUSY aficionados are very worried, while SUSY experts are eagerly waiting for discovery.

    Yawn. Oh, did anyone still believe in the WIMP scenario? Ah yes, I see all those zombies terrorising the good citizens.

    Kea, do you have something constructive to say? If not, wy don't you shut up? (Please)

    Though Zombie WIMPS causing mass hysteria was a pretty good visual.
    Are you a zombie?

    "it seems we have entered a very sorrowful era for SUSY aficionados..."
    Well, the SM is still fighting hard to be the one theory that describes everything we have in the field of particle physics.
    I believe these results strengthen hopes that the Higgs Boson may really exist!

    That's the strangest interpretation I've heard yet. Improved exclusion limits on WIMPs as evidence of the Higgs...

    They see a excess compatible with 'expected' (Buchmueller et al.) susy and the main susy motivation is the higgs mass :-)
    First susy LHC exclusion is also improving the agreement between 'expected' susy and the LEP higgs bound :-)
    Don't underestimate the bad faith of susy aficionados, even if their arguments are quite correct. :-)

    Maybe it's time for people to think about Alejandro Rivero's extension of "hadronic supersymmetry" to the leptons, which would make the leptons superpartners of certain mesons. It might be achievable in a preon theory.

    Anonymous commenters asking other people on a blog to Please shut up? You gotta be kidding me. Lol.

    I have to agree with Kea (I would even if she were not a friend). Anonymous commenter, please find a nickname and qualify yourself, or behave (especially avoid telling identifiable individuals what they should or should not do). You should realize that your failure to do so puts you in a situation where you have fewer rights here.

    Hi all!
    I'm not the anon commenter. just someone who follows this blog and others.
    And i have to say that, to someone who doesn't know well kea (as the majority of whom comes across these blogs), she sounds like a "troll".
    Yes a "troll". And, keeping the internet slang, very often her posts are just flames (i.e. "zombie susy", "stupid susy followers" and so on).
    It's not difficult to understand that it may sound quite annoying (and offensive?) to certain kind of people. Moreover she is not helping out offering some argument or other "compelling" views...

    Hi Gaf,

    I admit Kea is not renowned for her good e-manners. She's blunt, so to speak. But I never censor people for being blunt, especially if their comments have content.

    You are right about the censorship. But then you cannot expect everyone to be fine with kea's manners.
    By the way, where is content in this comment:
    "Yawn. Oh, did anyone still believe in the WIMP scenario? Ah yes, I see all those zombies terrorising the good citizens."

    I see only good ol' trolling!

    Gaf, I don't expect that everybody reacts the same way to sarcastic comments. However, sarcastic comments do have content -namely, sarcasm. It is different from trolling because
    1) it comes from a well-known individual (with a name and surname, and with a PhD)
    2) it does not target other readers in particular.

    The 2011 results seems to have constrained the parameter space quite a bit as compared to 2010 results. Do you have any understanding of how much the 2012, or including the second half of 2010, will have on the graph? Is there a significant diminishing return?

    Xenon100 and SUSY...
    Figure 5 of their paper (which is shown here above) includes the prediction for the cross section and the neutralino mass in the most simple model (the CMSSM), taken from
    The prediction is based on a fit to all other available existing data.

    Yes, some of the parameter space favored in this model at the 95% CL is now excluded, whereas the 68% CL
    area is not touched yet, especially because of the slightly higher number of observed than expected events.
    Things would look a bit worse for the CMSSM if they had excluded what they had expected.
    And one has to keep in mind that we are talking about the most restrictive realization of the MSSM.

    Therefore, any claims like "SUSY is dead now/again/finally/..." can only be based on personal prejudice (I admit, I also have some... ;-) but not on experimental physics results.

    Cheers, Sven

    Thanks Sven. It's worrying that such common sense statements as yours are so rare in some blogs.

    Well, with a slightly stronger limit things would only look worse for the Buchmueller et al vision of the CMSSM. They don't believe in the more fine-tuned parts of the CMSSM parameter space, which from a frequentist point of view fit perfectly well, even better, than the low m0 m12 region they like to focus on. See for instance, figure 7. The part of the parameter space Buchmueller et al enjoy is just a little island in one corner, not even containing the maximum likelihood point (probably; it is difficult to say for sure with these horribly complex parameter spaces). Of course Buchmueller et al think they legitimately excluded these regions already (, figure 1) so who knows, but I think I lean towards believing the more recent Multinest results.
    And of course as you say there is a lot more to the full MSSM anyway.

    Dark matter rationalizes galactic rotation vs. radius. To criticize is to volunteer. Let us end dark matter in a 90 day bench top experiment, empirically providing a testable alternative that contradicts no prior observation and is compatible with classical theory.

    Mirror symmetric postulates create Newton (Green's function), general relativity (Equivalence Principle; formally fails for fermions and spin-orbit coupling), string theory (BRST invariance), Calabi-Yau manifolds, Standard Model, supersymmetry. The massed universe is different: fermions (matter) have anti-symmetric wavefunctions; chiral electroweak interaction. The universe is observed to be chiral at all scales. Massless boson photons see mirror symmetry, arxiv:0912.5057, 0905.1929, 0706.2031. Excluding the former for seeing the latter is invalid reasoning.

    1) Noether's theorems couple vacuum isotropy to conservation of angular momentum through continuous symmetries. Discrete symmetry geometric parity is outside Noether. Fermion angular momentum has an intrinsic minimum non-zero value at large scales and small angular accelerations. Galactic rotation vs. radius requires no particulate agent, only demonstration of non-conservation of angular momentum for odd-parity atomic mass distributions.

    2) Local opposite shoes do not vacuum free fall identically. One lags the other in parallel minimum action trajectories. Photon opposite circular polarizations traversing a homogeneous chiral electronic medium have different refractive indices. A massed sector chiral vacuum background must be probed - empirically validated or falsified.

    An atom is the smallest configurable fermion set. Of 230 3-space crystallographic space groups, 11 enantiomorphic pairs are opposite shoes. Discard single space groups containing racemic or opposite sense screw axes. Grow untwinned single crystals of P3(1)21 | P3(2)21 right- and left-handed alpha-quartz or P3(1) | P(3)2 gamma-glycine. Chemically and macroscopically identical single crystals in enantiomorphic space groups violate the Equivalence Principle,
    two geometric parity Eotvos experiments

    A geometric parity Eotvos experiment recalls Yang and Lee observing beta-decay mirror asymmetry. It was impossible and ridiculous (Christmas 1956) until it was true (New Year's Day 1957). The greatest obstacle to understanding reality is not ignorance but the illusion of knowledge (elegant theory rigorously derived from convenient but defective postulates). Somebody should look. The worst it can do is succeed.

    Sounds pretty adventurous but not too expensive to test. Why not?

    1) Atom configuration is chemistry not physics. Since chemistry can be derived from physics, there is no reason to look. Ask a physicist for an aspirin.

    2) Theory says it is irrelevant. PAM Dirac told Otto Stern (of th Stern-Gerlach experiment) that the proton's magnetic moment could be exquisitely calculated but only crudely measured. Stern measured it, good to only 30%, and received a Nobel Prize/Physics because PAM Dirac was terrifically wrong. Yang and Lee were crackpots, then Nobel Laureates. Theory by itself is not good enough.

    3) Physicists cannot look into stereograms, know little about crystallography, and most of all do not tolerate risk . An experimentalist is remembered for his worst work, e.g., Ephraim Fischbach.



    I loaded quartz in that balanced Eötvös rotor
    And by some chance found a signal in its chart,
    And there it hung, rotor torquing out the scandal,
    But the physicists all were loathe to take part...
    For professors all said, "Beware!
    You're on a gravity trip."
    The profs all said, "Beware!
    Keep your hands off scholarship."
    And a chemist will drag you under
    By the fancy shoes on his wicked feet.
    Sit down, sit down, sit down, sit down,
    Sit down you're rockin' the boat.

    Uncle Al thanks Tommaso Dorigo for his patence and tolerance with dissenting ideas. One can only seize a blue rose by grasping where it is not. Streetlight Fallacy. Etc.

    Maybe the dark halo is very different to the standard one.
    See a WIMP with mass in the TeV range and a rotating dark disk could be a viable solution for DAMA and the recoils measured by the other experiments.

    Interesting. I'll check it.
    Everyone says that "WIMPS" are the "leading dark matter candidate".

    This is a bit bizarre. Over the last 35 years particle physicists,
    astrophysicists and cosmologists have searched deep space,
    the Moon, the bottoms of mines , Antarctica, accelerator debris,
    etc. for "WIMPS".

    Result: Not a single "WIMP" or axion, or "sparticle", or magnetic
    monopole, or boojum has ever been detected. Nor has any "extra
    dimension" beyond the 4 we know of.

    Perhaps nature is repeatedly giving us the less-than-subtle hint
    that the dark matter is NOT in the form of unobservable
    imaginary particles.

    A far better candidate for the galactic dark matter is "primordial"
    planetary-mass and stellar-mass Kerr-Newman ultracompact
    objects, i.e., black holes and singularities. Gravitational microlensing
    experiments have turned up evidence for such a population of objects.
    Stellar-mass black holes are known to exist in well-observed
    binary systems.

    We also know that there are billions of pulsars, isolated neutron
    stars, RRATs, magnetars, and gamma-ray burst sources out there
    in the Milky Way Galaxy and they might be viewed as excited states
    of the ground state black holes.

    To those who say that stellar-mass black holes could only make up
    about 20% of the dark matter (current microlensing inference), I would
    reply that billions of detected objects are of more scientific interest
    than 100s of No-Shows for "WIMPs" and other hypothetical particles.

    When they "detect" one "bump", or find a bold new "limit",
    they hold press conferences.

    Why do they show no interest in billions of observed ultracompacts?

    Robert L. Oldershaw
    Discrete Scale Relativity; Fractal Cosmology

    Dear Robert,

    I understand that you do not understand the importance of particle dark matter, but this means very little, unfortunately.

    Science works by making inferences from the data, and trying to verify them. One very important inference cosmologists have made is that if one hypothesized the existence of a 100 GeV neutral, weakly-interacting particle, then the post-big-bang abundance would be exactly right to explain away the amount of dark matter we observe in the universe, single-handedly. The cross section for annihilation and production of such a particle would be exactly what it takes.

    This is a very important hint, and we must pursue it.
    But what if your assumptions are wrong? Does the dark matter have to be in the form of particles? I think not.

    Many have convinced themselves that "WIMPs" [or axions, or sparticles, ...] are the only possibility.

    I beg to differ, and since we are scientists, we consider all possibilities that are consistent with observations.

    Robert L. Oldershaw
    Discrete Scale Relativity; Fractal Cosmology

    No, it is not necessary that dark matter be made o subatomic particles in order to account for the observed features of the universe. But it is a very viable explanation, and as such it requires to be investigated. Or are you here to tell me that one should not bother ? You are entitled to investigate any other explanation you find viable, but criticizing the one most theorists believe to be the most likely answer is just not leading anywhere.

    I think you miss my main point, so I will repeat it.

    1. MACHOs are routinely ignored by the theoretical physics community.

    2. "WIMPs" are enthusiastically embraced by the theoretical physics community.

    Given the well-known observation and non-observation results, I think that 1 and 2 should be reversed.

    I am not saying that we should stop looking for any form of dark matter. Is that clear?

    I am saying that we should not be biased in our assumptions of what the dark matter "must be".

    Robert L. Oldershaw
    Discrete Scale Relativity; Fractal Cosmology

    It's worth remembering, I think, that in the 1990s (not so long ago) WIMPs and MACHOs were considered as roughly equally plausible dark matter candidates. But then microlensing experiments (+other, mostly older and more indirect astrophysical arguments, and the rise of the standard cosmological model) excluded MACHOs - across a broad range of masses - as the main contributor to the Galactic dark halo mass. The experimental interest in particle dark matter may never find anything, but it's not an unreasonable direction to pursue.

    That said, we will be in a somewhat uncomfortable situation if the experiments manage to exclude a large fraction of the plausible particle candidates. It would be good to root at least one of the dark matter / dark energy in an experimentally accessible corner of physics!

    Sorry to be such a pain, but:

    One solar mass primordial black holes are still very much in the running.
    And planetary-mass ultracompacts have not been fully tested, except to say that the DM could not ALL be in the form of planetary-mass ultracompacts.

    If the microlensing experiments indicate that only 20% of the dark matter objects could be stellar-mass black holes, that is still a huge number - about the same order of magnitude as the number of luminous stars in the Galaxy!

    So to date, I'd say the score is MACHOs: billions; "WIMPs", "axions", "sparticles", etc.: zero

    Dear Tommaso,

    the fact that You allow such destructive and negative comments by Kea and others which do nothing but poisoning the atmosphere of Your otherwise very interesting blog is a reason to avoid this site in the future. Why can`t people just respect each other or the work of other people even if they dont agree with it or them? In particular if somewhat "speculative" topics which are not immediately provable or disprovable are discussed, all bets are open and there`absolutely no reason to sling mud or to mock people who persuit other research directions. A bit more tolerance from these negative commenters would be desirabel :-( .

    This is a free place. People may come dressed as they like, and if they misbehave... too bad, I do not want to micromanage.

    I think that dark matter and limited galaxy size will be explained as energy ignored by Newton and Einstein.
    The problem of stars' speed in this site will be solved, too.
    From dE/dt=kE, ignored energy is
    "ignored" E(t)=E(0){1-e^(kt)}e^(kt)

    My two manuscripts is in below.
    "Energy ignored by Einstein: Dark Matter is not Required"
    "Limited Galaxy Sizes Shown by New Gravitation Equattion"