Mar 27, 2012
Much food for thought at Moriond
The first week of the 47th Rencontres de Moriond, devoted to weak interactions and unified theories, came to a close on 10 March, leaving participants not only impressed but also puzzled by the new results presented at the conference held in La Thuile. The focus this year was to look at the results on searches for the Higgs boson, exclusion limits, searches for dark matter, precision measurements, flavour and neutrino physics, and to assess their impact on theoretical models, in particular those based on supersymmetry (SUSY) and extra dimensions.
The first excitement came from new measurements of the branching ratio for the decay Bs→ μμ from the LHCb, CMS and ATLAS experiments at CERN's LHC. LHCb and CMS have a sensitivity within a factor of around two of the rate expected in the Standard Model for this extremely rare decay, where contributions from new physics could be detected. LHCb is setting the best limit to date, of less than 4.5 × 10–9, barely above the Standard Model prediction of around 3.5 × 10–9. This leaves little room for new physics. However, David Straub, a theorist affiliated with Scuola Normale Superiore and INFN in Pisa, showed that finding a branching ratio smaller than predicted by the Standard Model would also open the door to new physics, something that has previously received little attention but is now becoming possible with the increase in precision at the LHC.
The ATLAS and CMS collaborations showed updates to the results reported in December 2011 (CERN Courier January/February 2012 p6). These include further analyses of the full 2011 data sample. In the low-mass Higgs region, the ranges not excluded at 95% confidence level (CL) have shrunk a little more. For ATLAS, all possible Higgs masses below 122.5 GeV (except at 118 GeV) are now excluded, together with those from 129 GeV up to 539 GeV; for CMS all masses between 127.5 and 600 GeV are excluded. This leaves only a small range where the Higgs boson could still be found.
The small excesses reported in December are still there, coming mostly from H → γγ for both experiments and also from Higgs → llll for ATLAS. Having analysed the whole 2011 data set and included new decay channels, CMS observes a 2.8 σ deviation at 125 GeV, while ATLAS has a 2.5 σ excess at 126 GeV. When the "look-elsewhere effect" is taken into account in the 110–145 GeV range, the significance of this excess goes down to about 2.1 σ.
Fermilab's Tevatron experiments provided a surprise. Having analysed almost all of their data and greatly improved their analyses, the DØ collaboration sees a slight excess of events in the Higgs mass range of 115–145 GeV while CDF sees it for mH < 150 GeV, coming mostly from the H → b-b and H → WW channels. The combined effect corresponds to a 2.2 σ excess above the predicted background. In addition, CDF and DØ greatly improved the precision on the masses of the W boson and the top quark. Both play an important role in determining the consistency of the Standard Model. In particular, CDF has measured the W mass to be 80.387 ± 0.019 GeV, while DØ finds a mass of 80.375 ± 0.023 GeV. These recent measurements now confine the Higgs mass to mH = 94+29–24 GeV.
While all four collaborations – ATLAS, CMS, CDF and DØ – insisted that it was too early to jump to conclusions about the Higgs boson, theorists have already been checking the effects of the mass of the Higgs and find that the currently allowed range is already putting constraints on SUSY models.
Away from the colliders, the announcement during the conference of the measurement of the neutrino mixing angle θ13 caused excitement (Daya Bay experiment measures θ13). Another highlight concerned the 8 σ annual modulation observed by the DAMA/LIBRA dark-matter experiment, which the collaboration interprets as a signal of dark matter. It has been suggested that the effect could be caused by cosmic muons, but new calculations show that the data are inconsistent with the cosmic muon hypothesis at 99% CL.
Possible signs of a Higgs boson with production cross-sections and branching ratios compatible with the Standard Model coupled with no signs of new physics despite extremely precise tests, left all of the participants of this first week of "Moriond" rather puzzled. Perhaps it is time to go back to the drawing board.