Precision leap for B_s⁰ fragmentation and decay

How likely is it for a b quark to partner itself with an s quark rather than a light d or u quark? This question is key for understanding the physics of fragmentation and decay following the production of a b quark in proton–proton collisions. In addition, the number of B_s⁰ mesons to be produced, formed by a pair of b and s quarks, is required for measuring its decay probabilities, most notably to final states that are sensitive to physics beyond the Standard Model, such as the B_s⁰ → μ⁺μ^– decay.

The knowledge of f_s/f_d – the ratio of the fragmentation fraction of a b quark to a B_s⁰ or a B⁰ meson – is thus a key parameter at the LHC. So far it has been measured with limited precision and has been the dominant systematic uncertainty for most B⁰_s branching fractions. Now, however, the LHCb collaboration has, in a recent publication, combined the efforts of five different analyses with information on this parameter. The f_s/f_d ratio was measured in previous publications through semi­leptonic decays, hadronic decays with D mesons and hadronic decays with J/ψ mesons in the final state. Some of these measurements are only sensitive to the product of the fragmentation fraction and the branching fractions. This new work analyses these results simultaneously, obtaining a precise measurement of f_s/f_d as well as branching fraction measurements of two important decays, B⁰_s → D^–_s π⁺ and B⁰_s → J/ψ φ. These are golden channels for mixing and CP violation measurements in the B⁰_s sector.

Precision leap

The results reduce the uncertainty on f_s/f_d by roughly a factor of two for collisions at 7 TeV, and a factor of 1.5 for collisions at 13 TeV, yielding a precision of about 3%. They also confirm the dependence of f_s/f_d on the transverse momentum of the B⁰_s meson, and indicate a slight dependence on the centre-of-mass energy of proton–proton collisions (figure 1). The results are used in this work to update the previous branching-fraction measurements of about 50 different B⁰_s decay channels, significantly improving their precision, and boosting several searches for new physics.