LHCb

Abstract:

The Standard Model of particle physics—the theory of particles and interactions at the smallest scale—predicts that matter and antimatter interact differently due to violation of the combined symmetry of charge conjugation (C) and parity (P). Charge conjugation transforms particles into their antimatter particles, whereas the parity transformation inverts spatial coordinates. This prediction applies to both mesons, which consist of a quark and an antiquark, and baryons, which are composed of three quarks. However, despite having been discovered in various meson decays, CP violation has yet to be observed in baryons, the type of matter that makes up the observable Universe. Here we report a study of the decay of the beauty baryon Λ0b$${\varLambda }_{0}^{b}$$ to the pK−π+π− final state, which proceeds through b → u or b → s quark-level transitions, and its CP-conjugated process, using data collected by the Large Hadron Collider beauty experiment1 at the European Organization for Nuclear Research (CERN). The results reveal significant asymmetries between the decay rates of the Λ0b$${\varLambda }_{0}^{b}$$ baryon and its CP-conjugated antibaryon, providing, to our knowledge, the first observation of CP violation in baryon decays and demonstrating the different behaviours of baryons and antibaryons. In the Standard Model, CP violation arises from the Cabibbo–Kobayashi–Maskawa mechanism2, and new forces or particles beyond the Standard Model could provide further contributions. This discovery opens a new path in the search for physics beyond the Standard Model.

Abstract:

The first evidence for the decay B-→D**0τ-ν¯τ is obtained using proton-proton collision data collected by the LHCb experiment, corresponding to an integrated luminosity of 9 fb-1, at centre-of-mass energies of 7, 8, and 13 TeV. Here, the D**0 meson represents any of the three excited charm mesons D1(2420)0, D2*(2460)0, and D1′(2400)0. The B-→D**0τ-ν¯τ signal is measured with a significance of 3.5σ, including systematic uncertainties. The combined branching fraction B(B-→D1,2**0τ-ν¯τ)×B(D1,2**0→D*+π-), where D1,2**0 denotes both D1(2420)0 and D2*(2460)0 contributions, is measured to be [0.051±0.013(stat)±0.006(syst)±0.009(ext)]%, where the last uncertainty reflects that of the branching fraction of the normalization channel B-→D1,2**0Ds-(*). The ratio between the tauonic and muonic semileptonic B decays, with the latter taken from world average values, is also determined and found to be R(D1,2**0)=0.13±0.03(stat)±0.01(syst)±0.02(ext).

Abstract:

A study of the charmless baryonic decays Bs0→K0pp¯ is presented, where Bs0 denotes either a B0 or a Bs0 meson. The analysis is based on proton-proton collision data collected by the LHCb experiment at centre-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of 9 fb−1. The decay Bs0→K0pp¯ is observed for the first time, with a measured branching fraction of (9.14 ± 1.69 ± 0.90 ± 0.33 ± 0.20) × 10−7 and a significance of 5.6σ. The uncertainties respectively account for statistical and systematic contributions, the precision of the branching fraction of the normalisation channel B0→ K0π+π− and the fragmentation fraction ratio fs/fd. The branching fraction determined for B0→K0pp¯ is (2.82 ± 0.08 ± 0.12 ± 0.10) × 10−6, which is the most precise measurement to date.

Abstract:

An angular analysis of the decay Bs0→ ϕe+e− is presented, using proton-proton collision data collected with the LHCb detector between 2011 and 2018 at centre-of-mass energies of 7, 8 and 13 TeV. The combined dataset corresponds to an integrated luminosity of 9 fb−1. Observables are determined by fitting time-integrated projections of the angular distribution in three bins of dielectron mass squared, q2, corresponding to [0.1, 1.1], [1.1, 6.0] and [15.0, 19.0] GeV2/c4. The results are compatible with predictions based on the Standard Model of particle physics.