Sunday, 17 June 2007

Xenon10 taking the lead

It is a bit belated news but i still find it worthy to point out. Two weeks ago the XENON collaboration posted a paper with new limits on dark matter from direct detection. The paper follows earlier announcements and a note in Nature.

There must be a flux of dark matter passing constantly through the Earth. If the dark matter particle belongs to the WIMP category, it is supposed to have weak strength interactions with the Standard Model matter. In such a case, once in a while it should collide with an atom and we may observe a recoil energy transferred to the nucleus. The XENON10 expreriment uses xenon as a target. They apply some clever techniques to reduce the background from other particles (e.g. neutrons or photons) penetrating the tank. Interested engineers and hobbyists may find the details in the paper. For all the rest, the important stuff is summarized in this plot:
It shows the limit on the spin-independent scattering cross section of dark matter on nucleons as a function of the dark matter particle mass. The new limits are better by a factor of six than the previous ones from the CDMS-II experiment. I guess that at this point the controversial DAMA detection signal can be forever buried in oblivion. For reference, the plot shows expected cross sections in the constrained MSSM scenario.

Of course, the importance of the XENON10 results goes far beyond constraining the parameter space of some obsolete models. The exciting point is that we are really probing the cross sections expected for WIMP particles. In the coming years we either confirm the WIMP hypothesis or make it implausible. No need to tell that an eventual positive signal would have a huge impact on the LHC program and science fiction literature.

Thursday, 14 June 2007

Nima's Horizons

Nima Arkani-Hamed is yet another soul that has fallen to the spell of the landscape. Landscape is a perfect framework for predicting things we already know. Nima is a bit more clever than that. He is concerned with a bigger picture. I mean, with conceptual questions associated with the existence of multiple vacua in a gravity theory.

Nima shared some remarks on the subject yesterday in a theory seminar. He drew an analogy between the present situation in particle physics and the early days of quantum mechanics. In the latter case, quantum effects turned out to imply loss of predictivity concerning the results of individual measurements. Now, he believes, we should again accept certain loss of predictivity due to the landscape and the anthropic selection.

That was an introduction. The bulk of the talk was about the Standard Model landscape. It turns out that the landscape pops up in the minimal Standard Model coupled to Einstein gravity with a small cosmological constant. Such a system has, of course, a unique 4D de Sitter vacuum, but there exist many more vacua with compactified spatial dimensions. One example is the class of vacua with the AdS3xS1 geometry. The radius of the circle is stabilized by the interplay of the small cosmological constant and the Casimir energy induced by the photon, the graviton and the light neutrinos. The funny accident is that these vacua would not exist if the cosmological constant were a factor of 10 larger or if the solar neutrino mass squared difference were a factor of 10 smaller.

Nima went on discussing some more technical details of this setup:
  • A near-moduli space of the photon Wilson line wrapping the circle.
  • Black string solutions interpolating between 4D and 3D vacua.
  • The two-dimensional CFT dual to the Standard Model.
All that you can find in his recent paper.

Up to this point, i've been almost fair. Now it's time for a few snide remarks. Listening to this talk was like visiting a flea market. It was colourful and entertaining, but most of the things on display seemed utterly useless. Nima forgot to say what can the Standard Model landscape teach us about the big questions he had addressed at the beginning. Well, certainly the landscape may be present in far simpler setups than string theory. But in this very example it seems totally irrelevant, both from the theoretical and the experimental points of view. I had this guilty feeling that i wouldn't even bother to listen if the name of the speaker were different. Judging from the looks on the others' faces, i wasn't all alone...

Transparencies not available, as usual, though this time it isn't so much of a waste.