Blogging ICHEP 2010

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

Saturday, July 24, 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. That would suggest 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 (not completely bullet-proof) theoretical arguments that a large deviation from the standard model in one should results in an observable deviation in the other. This is the case, in particular, if new physics enters via a phase in $M_{12}$ (the so-called dispersive part of the mixing amplitude, as opposed to the absorptive part $\Gamma_{12}$), which is 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, their results seems 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 difference of the widths 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 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.


  1. This is the most frustrating thing: it doesn't get bigger and it doesn't go away. :( -Gordon.

  2. When they combine results from same-sign dimuon asymmetry and from J/psi Phi decay, they have already made an assumption with a non-zero contribution from new physics to CP violation in the Bd system. New physics can even predict the phase for the same-sign dimuon asymmetry different from the phase for the J/psi Phi decay. Modifying M12 is not the only option from new physics. -Yang

  3. Oh, that's cool to know. So it looks like we have (i) the SM value for beta_s (= 0.019) and (ii) some real new physics in the oscillations ... say from CPT violation?

  4. CPT violation is an interesting option ... though perhaps bizarre. Or is it after the recent MINOS and MiniBooNE results? There are models like R-parity violating SUSY (and what can't you do with it?) that gives an absorptive part Gamma_12, so it changes both beta_s and dimuon asymmetry.


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