Friday, 20 March 2015

LHCb: B-meson anomaly persists

Today LHCb released a new analysis of the angular distribution in  the B0 → K*0(892) (→K+π-) μ+ μ- decays. In this 4-body decay process, the angles between the direction of flight of all the different particles can be measured as a function of the invariant mass  q^2 of the di-muon pair. The results are summarized in terms of several form factors with imaginative names like P5', FL, etc. The interest in this particular decay comes from the fact that 2 years ago LHCb reported a large deviation from the standard model prediction in one q^2 region of 1 form factor called P5'. That measurement was based on 1 inverse femtobarn of data;  today it was updated to full 3 fb-1 of run-1 data. The news is that the anomaly persists in the q^2 region 4-8 GeV, see the plot.  The measurement  moved a bit toward the standard model, but the statistical errors have shrunk as well.  All in all, the significance of the anomaly is quoted as 3.7 sigma, the same as in the previous LHCb analysis. New physics that effectively induces new contributions to the 4-fermion operator (\bar b_L \gamma_\rho s_L) (\bar \mu \gamma_\rho \mu) can significantly improve agreement with the data, see the blue line in the plot. The preference for new physics remains remains high, at the 4 sigma level, when this measurement is combined with other B-meson observables.

So how excited should we be? One thing we learned today is that the anomaly is unlikely to be a statistical fluctuation. However, the observable is not of the clean kind, as the measured angular distributions are  susceptible to poorly known QCD effects. The significance depends a lot on what is assumed about these uncertainties, and experts wage ferocious battles about the numbers. See for example this paper where larger uncertainties are advocated, in which case the significance becomes negligible. Therefore, the deviation from the standard model is not yet convincing at this point. Other observables may tip the scale.  If a  consistent pattern of deviations in several B-physics observables emerges,  only then we can trumpet victory.

Plots borrowed from David Straub's talk in Moriond; see also the talk of Joaquim Matias with similar conclusions. David has a post with more details about the process and uncertainties. For a more popular write-up, see this article on Quanta Magazine. 

Saturday, 14 March 2015

Weekend Plot: Fermi and more dwarfs

This weekend's plot comes from the recent paper of the Fermi collaboration:

It shows the limits on the cross section of dark matter annihilation into tau lepton pairs. The limits are obtained from gamma-ray observations of 15 dwarf galaxies during 6 years. Dwarf galaxies are satellites of Milky Way made mostly of dark matter with few stars in it, which makes them a clean environment to search for dark matter signals. This study is particularly interesting because it is sensitive to dark matter models that could explain the gamma-ray excess detected from the center of the Milky Way.  Similar limits for the annihilation into b-quarks have already been shown before at conferences. In that case, the region favored by the Galactic center excess seems entirely excluded. Annihilation of 10 GeV dark matter into tau leptons could also explain the excess. As can be seen in the plot, in this case there is also  large tension with the dwarf limits, although astrophysical uncertainties help to keep hopes alive.  

Gamma-ray observations by Fermi will continue for another few years, and the limits will get stronger.   But a faster way to increase the statistics may be to find more observation targets. Numerical simulations with vanilla WIMP dark matter predict a few hundred dwarfs around the Milky Way. Interestingly, a discovery of several new dwarf candidates was reported last week. This is an important development, as the total number of known dwarf galaxies now exceeds the number of dwarf characters in Peter Jackson movies. One of the candidates, known provisionally as DES J0335.6-5403 or  Reticulum-2, has a large J-factor (the larger the better, much like the h-index).  In fact, some gamma-ray excess around 1-10 GeV is observed from this source, and one paper last week even quantified its significance as ~4 astrosigma (or ~3 astrosigma in an alternative more conservative analysis). However, in the Fermi analysis using  more recent reconstruction Pass-8 photon reconstruction,  the significance quoted is only 1.5 sigma. Moreover the dark matter annihilation cross section required to fit the excess is excluded by an order of magnitude by the combined dwarf limits. Therefore,  for the moment, the excess should not be taken seriously.