A surprising asymmetry and more excited states

The flavour-changing neutral-current decays B → K(K^*)μ⁺μ^– provide important channels in searching for new physics, as they are highly suppressed in the Standard Model. The predictions in these channels suffer from relatively large theoretical uncertainties but these can be overcome by measuring asymmetries in which the uncertainties cancel. One example is the isospin asymmetry A_I, which compares the decays: B⁰ → K⁰(K^*0)μ⁺μ^– and B⁺ → K⁺(K^*+)μ⁺μ^–. In the Standard Model, A_I is predicted to be small, around –1%, for the decays to the excited K^*, and while there is no precise prediction for the decays to the K, a similar value is expected.

LHCb has measured A_I for these decays as a function of the dimuon mass (q²), using data corresponding to an integrated luminosity of 1.0 fb^–1, with a surprising result. While the measurements for B → K^*μ⁺μ^– are consistent with the prediction of negligible isospin asymmetry, the value for B → Kμ⁺μ^– is non zero. In particular, in the two q² bins below 4.3 GeV/c² and in the highest bin above 16 GeV/c² the isospin asymmetry is negative in the B → Kμ⁺μ^– channel. These q² regions are furthest from the charmonium regions and cleanly predicted theoretically. The measured asymmetry is dominated by the deficit observed in B⁰ → K⁰μ⁺μ^–. Integrated over the dimuon mass range, the result for A_I deviates from zero by more than 4σ.

These results were obtained with the full data sample for 2011, which should more than double by the end of 2012. In the meantime, theorists will analyse this puzzling result to establish whether this effect can be accommodated in the framework of the Standard Model – or whether its explanation requires new physics.

In a different study, LHCb observed two Λ_b excited states for the first time, as predicted within the context of the quark model. The excited states (see figure) were reconstructed in three steps. First, Λ_c⁺ particles were reconstructed through their decay Λ_c⁺ → pK^–π⁺; then the Λ_c particles were combined with π^– to look for Λ_b particles; finally the Λ_b particles were combined with π⁺π^– pairs. In this way the team found about 16 Λ_b(5912)→Λ_bπ⁺π^– decays (4.9σ significance) and about 50 Λ_b(5920)→Λ_bπ⁺π^– decays (10.1σ) among some 6 × 10¹³ proton–proton collisions detected during 2011.