Saturday, 24 July 2010

D0 says: neither dead nor alive

This year CP violation in the Bs meson system has made the news, including BBC News and American Gardener. The D0 measurement of the same-sign dimuon asymmetry in B decays got by far the largest publicity. Recall that Tevatron's D0 reported 1 percent asymmetry at the 3.1 sigma confidence level, whereas the standard model predicts a much smaller value. The results suggests a new source of CP violation, perhaps new heavy particles that we could later discover at the LHC.

The dimuon asymmetry is not the only observable sensitive to CP violation in the Bs system. Another accessible observable is the CP violating phase in time-dependent Bs decays into the J/ψ φ final state. In principle, the dimuons and J/ψ φ are 2 different measurements that do not have to be correlated. But there are theoretical arguments (though not completely bullet-proof) that a large deviation from the standard model in one should imply a large deviation in the other. This is the case, in particular, if new physics enters via a phase in the dispersive part of the Bs-Bsbar mixing amplitude ($M_{12}$, as opposed to the absorptive part $\Gamma_{12}$), which is theoretically expected if the new particles contributing to that amplitude are heavy. The previous, 2-years old combination of the CDF and D0 measurements displayed an intriguing 2.1 sigma discrepancy with the standard model. CDF updated their result 2 months ago and, disappointingly, the new results is perfectly consistent with the standard model. D0 revealed their update today in an overcrowded room at ICHEP. Here is their new fit to the CP violating phase vs. the width difference of the 2 Bs mass eigenstates
Basically, D0 sees the same 1.5 sigmish discrepancy with the standard model as before. Despite 2 times larger statistics, the discrepancy is neither going away nor decreasing, leaving us children in the dark. Time will tell whether D0 found hints of new sources of CP violation in nature,
or merely hints of complicated systematical effects in their detector.


Kea said...

Wow, what a conference! Thanks for the fantastic reporting. Don't forget that phrase in the two D0 arxiv papers ... "assuming CPT invariance".

Kea said...

oops, sorry ... I was thinking of the OTHER recent D0 result on Bs mesons.

Anonymous said...


1.5 sigma is not a hint of anything. This is dead.