LHCb makes first observations of interesting B⁰_s decays

Using data collected in proton–proton collisions at the LHC at a centre-of-mass energy of 7 TeV, the LHCb experiment has observed new rare decay modes of B⁰_s mesons for the first time. The decay B⁰_s → J/ψ f₀(980) will be important for studying CP violation in the B⁰_s system, while the semileptonic decay B⁰_s → D^*–_s2Xμ⁺ν will be valuable for testing QCD-based theoretical predictions.

The first new decay mode observed is of the hadronic decay B⁰_s → J/ψ f₀(980). This is particularly interesting because it is to a CP eigenstate, which means that it can be used in measuring mixing-induced CP violation. The B⁰_s consists of a b antiquark (b) bound with an s quark, and can decay to a J/ψ (cc) together with an ss state, which can be a φ or, more rarely, an f₀. While the φ decays to K⁺K^–, the f₀ decays to π⁺π^–. The collaboration analysed J/ψK⁺K^– and J/ψπ⁺π^– events to search for the relevant decay candidates. Finally, using a fit to the π⁺π^– mass spectrum with two interfering f₀ resonances (f₀(980) and f₀(1370)), they measured a ratio of the B⁰_s decays to J/ψ f₀(980) and J/ψ φ of 0.252+0.046-0.032(stat.)+0.027-0.033 (syst.) (LHCb collaboration 2011a). The events close to the f₀(980) could be used to measure the CP-violating phase for B⁰_s decays, which is some 20 times smaller than in B⁰ mixing and hence much more sensitive to physics beyond the Standard Model.

The LHCb collaboration has also made the first observation of another decay, B⁰_s → D^*–_s2Xμ⁺ν. The most frequent decays of the B⁰_s involve the b quark changing into a c quark, resulting in a cs charm hadron, such as a D^–_s or D^*–_s , or other excited states. The relative proportion of such final states provides valuable information for testing theoretical models based on QCD. To investigate decays of this kind, the collaboration looked for final states in which the decay D⁰ → K⁺π^– formed a vertex with a K^– and a μ⁺. The analysis revealed two structures in the D⁰ K^– mass spectrum at masses consistent with the D_s1(2536)^– and D^*_s2(2573)^– mesons (LHCb collaboration 2011b). While the D_s1(2536)^– has been observed previously in B⁰_s decays by the DØ collaboration at Fermilab’s Tevatron, LHCb’s result marks the first observation of the D^*_s2(2573)^– state in B⁰_s decays. The measured branching fraction relative to the total B⁰_s semileptonic rate for the D^*_s2(2573)^– comes out at 3.3±1.0(stat.)±0.4(syst.)%, while the value for the D_s1(2536)^– is measured to be 5.4±1.2 ±0.5 %. These values agree well the prediction of the updated Isgur-Scora-Grinstein-Wise quark model, ISGW2.

The observation of these two new decay modes demonstrates that the LHCb experiment is already competitive in the field of heavy flavour physics. Great progress is expected with the larger data sample due from the coming run, with the potential to constrain, or even observe, new physics.