Thursday, May 02, 2013

Two dark matter papers

When it comes to the existence of dark matter, I have two news – good and bad. Let me begin with the bad news.

We often discuss provoking statements by some of the dark matter direct search experiments that claim that they see evidence of a dark matter particle under the ground. But there's still one staunch experiment in the "Dark Matter Is Not Seen" axis of the dark matter wars that refuses all these claims, XENON100 (soon to be upgraded to XENON1T: yes, the numbers stand for 100 kilograms and 1 ton of liquid xenon).

The latest exclusion plot described in this paper (click) is rather cheeky.

Well, it looks like this:

Click to magnify the chart.

You see various potato ellipses in the chart that summarize the positive statements – there seems to be a dark matter (WIMP) particle with the mass given on the \(x\)-axis and the cross section with the nucleons encoded by the \(y\)-axis. You see various claims of this kind – not really compatible with each other – made by DAMA/I, DAMA/Na, CoGeNT, CRESST-II. An extra shape could perhaps be added to reflect the data from PAMELA, Fermi, and the newest three events from CDMS II (which wouldn't be too far from the CoGeNT potato).

Tony Zee is releasing his new, playful and cool 900-page book on general relativity in a nutshell!

On the other hand, the long lines depict the statements by the "negative experiments" that claim that all the points above their curve are excluded: SIMPLE, COUPP, ZEPLIN-III, EDELWEISS, CDMS (2010/2011: later betrayed the axis), and – most famously – XENON100. I say "most famously" because XENON100 is by far the most powerful experiment of this kind, at least among the negative ones.

The newest exclusion curve makes this priority even more obvious. Note the blue line inside the green-and-yellow (Brazil) band at the bottom (the exclusion is about a sigma stronger than expected). It is safely below all the "positive" potato ellipses and it is also well below the other exclusion curves. The contradiction between the latest XENON100 results and the "positive" experiments couldn't be stronger. Well, it could but it's already strong enough! ;-) One may say that pretty much all the preferred regions are disfavored by XENON100 at 5 sigma or more.

Note that the liquid xenon is a relatively diverse mixture of many isotopes (or do they filter which ones they use?). So the absence of signals is probably not due to some special properties of a xenon nucleus. On the other hand, the absence could be explained if the signals ultimately involved the interaction of a particle with the electrons – because xenon (unlike germanium, silicon, and all the other elements used in the experiments) is an inert gas with full electron shells and \(L=S=J=0\), when it comes to atomic physics. The events don't look like interactions with the electrons but there could be some subtleties. The tension between XENON100 and others seems so strong that the inert character of xenon seems "almost necessary" for me to understand the apparent xenophobia of the dark matter particle – but the de Broglie wavelength of the new particle would have to be of atomic size or longer for the vanishing atomic angular momentum to matter at all (which seems like an insanely low momentum, too). Also note that the collisions with the electrons are supposed to be "background" and distinguished from the dark-matter-like collisions with the nuclei but there could be a reason why some particle's collisions with the electrons look nucleus-like.

Now, the good news.
AMS-02 Positrons from Decaying Wino in the Pure Gravity Mediation Mode
A Japanese group is discussing sort-of-positive recent claims by AMS-02 that could be even more positive if we were told everything that Sam Ting knows. The authors clearly believe that in wino veritas.

Note that a wino is a superpartner of the three W-bosons associated with the electroweak \(SU(2)\) gauge symmetry. I say three – we're only used to the two charged W-bosons (plus and minus) but there's also the third, \(z\)-component that gets combined to the photon and the Z-boson. And it's the superpartner of this, neutral W-boson (a superposition of the zino and the photino), that is perhaps the most popular dark matter candidate. This particle is a neutralino but a specific one because a general neutralino may be composed of other mixtures than the neutral winos – it may have different ratios of the zino and photino i.e. it may have an admixture of the bino, the superpartner of the \(U(1)_Y\) hypercharge gauge boson; and it may also contain a flavor (or lots of) the neutral higgsinos.

At any rate, the Japanese authors claim that a neutral wino of the mass\[

m_{\tilde W}\sim 1\TeV

\] is great to explain the positron excess seen by AMS-02 (and, less accurately, by previous observations by PAMELA and Fermi-LAT). They go beyond the identification of the wino as their pet dark matter particle. In fact, they extend the quote to
In vino veritas, in aqua sanitas
which is approximately a Latin translation of "in the wino there is the truth and the gravity is its sane and healthy cause".

There are different mechanisms of supersymmetry breaking – or how its cause looks from the Standard Model viewpoint (mediation). Gauge mediation is arguably the most popular one but I would say that this philosophy has weakened due to the negative results from the LHC on any new physics. I have always preferred more "universalistic" causes of supersymmetry breaking and the so-called gravity mediation belongs among the mainstream representatives of this idea.

The authors discuss their PGM (pure gravity mediation) model. The wino dark matter discussed above is unstable due to the R-parity-violating decay through \(LLE^c\) interactions (the two first factors refer to the lepton doublet and the last one is the charged lepton singlet).

We hear that there is a natural way to reproduce all the \(\gamma\)-ray and positron data from the new \(1\TeV\) wino whose lifetime is something like \(10^{27}\) seconds. That's over \(10^{19}\) years, billions times the current age of the Universe, but enough to produce a detectable trace of the positrons for our thirsty telescopes (they're thirsty because they also believe that in vino veritas, in aqua sanitas). Note that while this lifetime seems very long, it is \(10^{15}\) times shorter than the current lower bound on the lifetime of the proton.

In the appendix, they discuss a grand unified explanation for the tiny R-parity-violating term. The overall slogan for the paper gets enhanced to
In vino veritas, in aqua sanitas, in zythum unitas.
One may want to spend an hour by thinking how the positive dark-matter signals in the direct search experiments could actually be due to some \(1\TeV\) particle's collisions with the electrons rather than \(8.6\GeV\) particle's collisions with the nuclei...

A bonus...

Tomorrow, another paper on a similar topic (a model for the CDMS II events) will be released by four Indians (from Texas, Mexico, and Maryland):
A Supersymmetric Model for Dark Matter and Baryogenesis Motivated by the Recent CDMS Result
In their supersymmetric model (MSSM plus a color triplet, isosinglet chiral superfield \(X\) with \(Q_{\rm el}=\pm 2/3\), the opposite sign than the up-quark singlet, and a neutral superfield \(N\): \(\tilde N_1\), a boson, is the R-parity-odd LSP), the dark matter particle isn't really a WIMP but a non-thermally produced scalar with some extra modifications to baryogenesis etc.


  1. do you think that the experiments in CERN will see signs of supersymmetry or it is something difficult to predict?

    finding the Higgs mechanism should make no difference in making supersymmetry more likely to be found, right?

  2. Dear George,

    in recent 8 years or so, I have consistently rated the LHC's ability to find SUSY to be around 50 percent, perhaps a bit more, and I am still around that point.

    On one hand, the odds decreased as the previously plausible "early SUSY at the LHC" scenarios have been ruled out. On the other hand, the Higgs discovery made the accessible SUSY somewhat more likely and the dark matter hints are having a similar effects.

    So I still don't think it's obvious I will lose my SUSY bet (only $100 loss if I lose) that is to be decided after papers with at least 30/fb of data - luckily enough, that will only be in 2015. ;-)

  3. The bad news make me wanting to fetch the Gravitiy nutshell at once to cheer me up :-/ (why is the landscape on the cover picture melting or beading ...?) and the wino as a dark matter particle I like :-)

  4. Stephen Paul KingMay 3, 2013, 5:26:00 AM

    HA HA HA HA HA! It might as well be a sea of primordial black holes with vanishing scattering cross section...

  5. Primordial black holes are severely constrained by observations. I am not sure uf there US any parameter space left given the lask of gravitational lensing.

  6. Well, I am happy with the Xenon100 results. When will our scientist realize that the dark matter idea is just like the aether idea.
    Kepler thought it was the sun's light that was responsible for the orbital motion of the planets. Light from the sun like Newton's mass-based gravitational force varies inversely as the square of the distance from the sun. If light is gravitationally attractive, there ought to some experiments demonstrating this. Well there are. I have performed lots of them. But so far they are ignored. Why is this? It could be because I am an amateur and have no physics credentials or it could be my experimental results are so unwelcome it is easy is to assume I am just a crackpot or a careless experimenter See

  7. You say that you are an AMATEUR, but you nevertheless have the blatant audacity to tell perfectly capaple PROFESSIONAL PHYSICISTS who know their business perfectly well, that dark matter is just another aether and they should stop looking for and thinking about it ... ?!

    Wow, what a pompous fool you obviously are ...!

    Today, there are just too many pompous fools of this kind and other agressive dimwits, who have no clue what they are talking about, but nevertheless think they have the right to patronize much smarter people by telling them what they are allowed to consider interesting, investigate experimentally and theoretically, think about etc ...

  8. I know that I now have some points according to a well known crackpot index too, because of my three capitalized words ... :-D

  9. I am proud to be called an amateur Oh to be up there among the ranks of James Watt, Faraday, Cavendish, Darwin, Copernicus. And I would be ashamed to be called a Scholastic which to me our today's scientists are becoming more and more like with there with their blind faith in the unphysical notion that some yet-to-be described inherent property of mass gives it the capability to either warp space or attract other mass. I mean what yet-to-be described, inherent ability did the earth have to make all the objects in the sky revolve around it in a 24 hour period? You tell me oh certified one.

  10. Let me tell you what you should consider: the above picture of one of my experiments where the test mass loses 99% of its weight. The dark matter lovers after 20 years of trying and a billion dollars cannot experimentally the validity of their aether-like notion. I, the amateur, on the other hand can demonstrate with my various experiments that light is gravitationally attractive. Experiments are supposed to trump theory. Scientists are not supposed to act like the Scholastics who would not look through Galileo's telescope to see the phases of Venus which could not be explained by their 1500 year old belief that the universe revolved around the earth every 24 hours.

  11. Dear Lubos Motl,

    Apologies for yet another slightly off topic and late question, but I would love to hear (in brief of course :-) your perspective on M-Theory - and whether or not it still remains as a plausible theory, in the light of the LHC results, thus far.
    Kindest Regards,

  12. Heads up on the latest Fermi 130 Gev Gamma-ray line, dark matter paper: arXiv:1305.5597
    Search for Gamma-ray Spectral Lines with the Fermi Large Area Telescope and Dark Matter Implications,
    Fermi-LAT Collaboration.