LHCb recently reported an anomaly in the angular distribution of B0 → K*0 (→K+π-) μ+ μ- decays. The discreet charm of flavor physics is that even trying to understand which process is being studied may give you a serious migraine. So let's first translate to English. B0 is a pseudoscalar meson made of an anti-b- and a d-quark that is easily found in the junk produced by LHC collisions. K*0, actually K*0(892) because they come in variety of masses, is a vector meson made of an anti-s and a d-quark which promptly decays to a usual charged kaon and a pion. B0 → K*0(→K+π-) μ+ μ-, in the following simply referred to as B → K*μμ, is a rare decay occurring with the branching fraction of order 10^-7.
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Basically, LHCb measured all these Sn and FL coefficients as a function of q^2. The largest anomaly is observed at low q^2 in the parameter S5 (also presented as P5' which is S5 rescaled a function of FL). LHCb quantifies it is a 3.7 sigma deviation from the Standard Model in the region 4.3≤q^2≤8.68 GeV^2; this is downgraded to 2.5 sigma if the look-elsewhere effect is taken into account. Theorists fitting the data quote the deviation between 1 and 4.5 sigma, depending on theoretical assumptions and how the data are sliced and cooked.
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The interesting question is whether new physics could be responsible for the anomaly. To go beyond a yes/no answer one has to, unfortunately, go through a bit of technicalities. At the parton level, the relevant process is the b→sμ+μ- decay. Theorists computing the B → K*μμ decay thus start from an effective interaction Lagrangian with 4-fermion and dipole operators involving the b- and s-quarks. The operators relevant for this process are
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where Λref≈35 TeV. This set of operators allows one to describe the B → K*μμ decays in a completely model independent way, whether within the Standard Model or in some new physics scenario. In the Standard Model a subset of these operators is generated (see the diagrams) with the coefficients C7, C9 and C10 of order 1 (the suppression scale of the effective operators is tens of TeV due to the loop suppression, and also due to the CKM suppression via the small Vbs matrix element; this is why B → K*μμ is so sensitive to new physics).
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So, the verdict is... well, at this point the anomaly is not utterly solid yet. One warning flag is that it shows up in a complicated angular analysis rather than in a clean and simple observable, which gives more opportunities for theories and experimenters alike to commit a subtle error in the analysis. Moreover, in order to explain the anomaly, new physics contributions to the B → K*μμ amplitude need to be of the same order of magnitude as the Standard Models ones, which requires a certain degree of conspiracy. Most likely, the experimental data and the Standard Model predictions will approach each other when more data is analyzed, as it has happened countless times in the past. Nevertheless, we're looking forward to the future updates on B → K*μμ with a little more anticipation than usual. Note that the current LHCb analysis includes only the 7 TeV run data; the twice as large 8 TeV sample is still waiting to see the light...
[Most pictures stolen from Nicola Serra's talk at EPS]