Blogging ICHEP 2010


A collective forum about the 35th edition of
the International Conference on High Energy Physics (Paris, July 2010)

Saturday, May 29, 2010

Important update from CDF

Last week the D0 experiment at the Tevatron presented the new measurement of the same-sign dimuon charge asymmetry in B-meson decays. This asymmetry probes CP violation in B-mesons, including the $B_s$ mesons that have been less precisely studied than their $B_d$ friends and may still hold surprises in store. D0 claimed that their measurement is inconsistent with the standard model at the 3.2 sigma level and hints to a new physics contribution to the $B_s \bar B_s$ mixing. 3 sigma anomalies in flavor physics are not unheard of, but in this case there were reasons to get excited. One was that the $B_s$ system is a natural place for new physics to show up, because the standard model contribution to the CP-violating mixing phase is tiny, and theoretical predictions are fairly clean. The other reason was that the D0 anomaly seemed to go along well with earlier measurements of CP violation in the $B_s$ system. Namely, the measurement of CP violation in the time-dependent tagged $B_s \to J/\psi \phi$ decays displayed a 2.1 sigma discrepancy with the standard model, and some claimed the discrepancy is even higher when combined with all other flavor data. In other words, all measurements (except for $B_s \to D_s \mu X$ that however has a larger error) of the phase in the $B_s \bar B_s$ mixing consistently pointed toward new physics.

Disappointingly for most theorists, another Tevatron's experiment CDF recently presented an update that reverses that trend. CDF repeated the measurement of $B_s \to J/\psi \phi$ on a larger data sample of 5.2 inverse femtobarn, that is with 2 times larger statistics than in the previous measurement. The new result is consistent with the standard model at the 0.8 sigma level:
So at this moment only one experiment claims to see an anomaly in the $B_s$ system, while another measurement of the $B_s \bar B_s$ mixing phase is perfectly consistent with the standard model. Of course, further measurements of the mixing phase may bring another twist to the story...hopefully, new illuminating experimental results will be presented at ICHEP 2010.

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