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 ModeA 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 sanitaswhich 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 ResultIn 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.
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