The most interesting non-Higgs results coming from the LHC in 2011 was the observation CP violation in neutral D-meson decays delivered by the LHCb collaboration. Recall that LHCb studied the decays of the D and anti-D mesons into a pair of charged pions and a pair of charged kaons. For each D → π+π- and D → K+K- one can measure the decay rate, and subtract the analogous decay rate of anti-D (and normalize by the sum), thus obtaining the asymmetries A(π+π-) and A(K+K-). If either of the asymmetries is non-zero, that means the particular partial decay width is different for the particle and the anti-particle, which is a signal of CP violation. Since the total number of D and anti-D produced in LHC collisions is different and hard to estimate, LHCb quoted only the difference of the asymmetries ΔA = A(K+K-) - A(π+π-) where that uncertainty cancels out. They found ΔA = (-0.82 ± 0.24)%, significantly away from zero, which constituted the first evidence for CP violation in the charm sector at the level of 1%. What distinguishes this result from 1000 other flavor physics observables presented by the LHC, Tevatron, and B factories is that, until last year, theorists expected Standard Model processes to provide much smaller contributions to the CP asymmetry in D meson decays, estimates lying in the ballpark of 0.01-0.1%.

Exactly the same processes can also be studied at the Tevatron. CDF measured the same observable last year even before LHCb (they also gave separate CP asymmetries for the π+π- and K+K- final states), but at the time no statistically significant CP violation was observed. Now CDF has an update using the full Tevatron data set of almost 10 inverse femtobarn. They find ΔA = (-0.62 ± 0.23) % which is 2.7 sigma away from zero. This is an independent confirmation of the LHCb result. Combining all experimental measurements of CP violating observables in the D meson sector one arrives at the following graph

One can read off that the world average of the direct ΔA = (-0.67 ± 0.16)% is clearly non-zero, while there's currently no hint of the so-called indirect CP violation in D-Dbar meson mixing. Thus, we now know for sure CP is violated in decays of D mesons, an interesting fact in itself. However, we're still in the dark whether this result is a signal of new physics beyond the Standard Model, or just an unexpected enhancement of some difficult-to-calculate Standard Model contributions. I wish I could say that more data will clarify the situation, but even that is not clear at the moment...

See also the CDF talk in La Thuile.

## 4 comments:

You say "What distinguishes this result .. is that, until last year, theorists expected Standard Model processes to provide much smaller contributions to the CP asymmetry in D meson decays, estimates lying in the ballpark of 0.01-0.1%." That depends on *which* theorists. In 1987 Mitch Golden and I wrote a paper (Phys.Lett. B222 (1989) 501) in which we predicted the CP asymmetry that was now finally measured to be very large. We in fact predicted it to be larger than is observed, near 100% in the exact su(3)-flavor symmetry limit, but cautioned that we expected it to be diluted when su(3)-breaking corrections were included. When these are introduced one no longer has a prediction but can see if the resulting parametrization in reasonable. The analysis was carried out recently by Pirtskhalava and Uttayarat (arXiv:1112.5451) and shows the observation is easily accomodated in the SM.

Right, I should have said "most theorists". I don't think the authors of 1111.5000 agree with "easily accommodated". It seems to me the current consensus is that the effect may or may not be accommodated in the SM...

the low-energy QCD contributions to these amplitudes are simply much harder to compute than their relatives in K or B decays. it'll be some time until we have reasonably solid SM computations to elaborate on...

So, are the CP violations from all sources converging on a single value for the CP violating parameter (whose symbol I can't figure out how to put in a blog comment) in the CKM matrix? Or are they diverging?

It seems like a year or two ago, the big buzz pointing a new physics was that the CKM matrix was "broken" because there was no point that could fit all the confidence intervals for the parameters from the experimental results. Ergo, BSM physics like 4th generation models were looking attractive.

But, that buzz seems to have died down as LHC has produced a steady stream of more or less consistent and smaller confidence interval results.

I also concur with Benjamin Grinstein that the mere existence of CP violation in charm is such a pedestrian expected result that it rates a "meh" while the real issue is whether the magnitude of CP violation we see if the predicted result (which poses practical challenges both experimentally and in terms of doing the theoretical calculation), and of course, he got the right prediction and when.

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