There’s a new white paper/review that just appeared on the arXiv about the possibility of keV-scale sterile neutrino dark matter. I haven’t read through it, but this is an interesting non-WIMP possibility for dark matter. Sterile neutrinos don’t interact at all with the Standard Model except maybe through neutrino oscillations, but if they have mass, they can still be a dark matter candidate if there’s a good production mechanism. Current dark matter experiments would generally be insensitive to this kind of dark matter, since the standard signal of low energy nuclear recoils would probably be disallowed, or at least heavily suppressed, but there are apparently still some paths toward finding sterile neutrino dark matter beyond just discovering keV-scale sterile neutrinos.
IceCube and Antares have looked for evidence of neutrino emission coincident with the gravitational wave signal seen by LIGO. They see no evidence of neutrinos being emitted by the gravitational wave source. That doesn’t mean there are no neutrinos, just that even if there are, not enough reached us to be able to see them. But, as the article points out, this means that gravitational wave and neutrino observatories can now work together to try to study rare astrophysical events.
While yesterday’s big news was clearly the gravitational wave result, LUX also put its first spin-dependent WIMP interaction limits on the arXiv. In direct detection experiments, the spin-independent limit is typically stronger because the amplitudes add together in coherent nuclear scattering, leading to a dependence that scales like a polynomial factor of the atomic number. Spin-dependent interactions, typically from axial vector interactions, give rise to terms related to the individual nucleons’ spins. Nucleons tend to arrange themselves so that the spins mostly cancel, so the spin-dependent terms tend to be smaller than the spin-independent terms by a factor of approximately A2 if you assume that the fundamental couplings are the same. Because the cross section ends up with some angular momentum factors in terms, you need to know the isotopic abundances very well to get a reliable spin-dependent measurement. In this result, LUX gets the best result of any direct detection experiment for WIMP-neutron spin-dependent scattering and is about an order of magnitude behind in the WIMP-proton channel (xenon is not the best nucleus to use for spin-dependent proton interactions).
Speaking of underground physics, Gizmodo has an article today on Snolab, with pictures of some of the facilities and a few detectors. Snolab is the deep underground lab in Sudbury, Ontario where a number of particle physics experiments are operating or are planned, such as SNO (and SNO+) and quite a few dark matter searches.
Today’s XKCD basically describes a sizable fraction of my career working underground in salt mines.
xkcd posted a particle physics related comic for Christmas this year.
In the latest LHC story I just wrote about earlier, the Nature news post I linked actually has a pretty embarrassing error. This is particularly bad since Nature is one of the premier journals in most fields. For reference, you can find the ATLAS and CMS slides here.
In case they edit the text, the Nature post says that each of the photons has an energy of 750 GeV, giving a total energy of 1.5 TeV. There is one problem here that I noticed (well, if I ignore the article saying that the photons have the same mass – technically true but not really what I think the author meant to say): The photons shouldn’t both have the same energy. While the two colliding protons have approximately the same energy, the momentum transferred is high enough that the collision isn’t really proton on proton but rather parton on parton. The center of mass frame relevant to the event is not in general going to be the lab frame, so everything needs to be boosted to get to the lab frame.
This isn’t really a problem since there’s an obvious explanation: The author is talking about the center of mass frame but didn’t explicitly say so. Unfortunately, this is not true in this case. If you look at both the ATLAS and CMS slides (look for the diphoton resonance/exotics search parts), it is very clear that the center of mass energy of the two photons is 750 GeV, not 1500 GeV.