Measurement of the branching fraction ratio R K at large dilepton invariant mass

A test of lepton universality between muons and electrons is performed using B+ → K+ℓ+ℓ− decays (where ℓ = e, μ), in the dilepton invariant-mass-squared region above 14.3 GeV2/c4. The data used for the measurement consists of beauty meson decays produced in proton-proton collisions, corresponding to an integrated luminosity of 9 fb−1, collected by the LHCb experiment between 2011 and 2018. The ratio of branching fractions for B+→ K+μ+μ− and B+→ K+e+e− decays is measured to be RK=1.08−0.09+0.11stat−0.04+0.04syst, which is consistent with the Standard Model prediction of unity. This constitutes the most precise test of lepton flavour universality using B+ → K+ℓ+ℓ− decays with dilepton invariant-mass-squared above the ψ(2S) mass, whilst being the first of its kind at a hadron collider

Search for the lepton-flavour-violating decays B 0 → K *0 τ ± e ∓

A first search at LHCb for the lepton-flavour-violating decays B0 → K*0τ±e∓ is presented. The analysis is performed using a sample of proton-proton collision data, collected with the LHCb detector at a centre-of-mass energy of 13 TeV between 2016 and 2018, corresponding to an integrated luminosity of 5.4 fb−1. No significant signal is observed, and upper limits on the branching fractions are determined to be BB0→K∗0τ−e+<5.97.1×10−6 and BB0→K∗0τ+e−<4.95.9×10−6 at the 90% (95%) confidence level. These results correspond to the current most stringent upper limits for b → sτl transitions

Measurement of ψ (2 S ) to J/ψ cross-section ratio as function of multiplicity in p Pb collisions at s NN = 8.16 TeV

The production ratio of ψ(2S) to J/ψ charmonium states is presented as a function of multiplicity in proton-lead collisions at a centre-of-mass energy of sNN=8.16 TeV, for both prompt and nonprompt sources. The total luminosity recorded by the LHCb experiment corresponds to 13.6 nb−1 for pPb collisions and 20.8 nb−1 for Pbp collisions, where the first particle corresponds to the particle traveling towards the detector. Measurements are performed in the dimuon final state at forward (backward) centre-of-mass rapidity, with respect to the proton direction, 1.5 < y*< 4.0 (−5.0 < y*< −2.5) for pPb (Pbp) collisions. A multiplicity dependence of the prompt production ratio is observed in pPb collisions, whereas no dependence is found in nonprompt production, nor in either prompt or nonprompt production in Pbp collisions. These results suggest that in the Pb-going direction additional suppression mechanisms beyond comover effects may be present, possibly related to the formation of quark-gluon plasma. This highlights a transition from small to large collision systems and provides important insight into the suppression of charmonia in proton-nucleus collisions

Rapidity and multiplicity dependence of charged-particle flow in p Pb collisions at s NN = 8.16 TeV

The elliptic and triangular flow of charged particles are measured using two-particle angular correlations in pPb collisions in the pseudorapidity range 2.0 < |η| < 4.8. The data sample was collected by the LHCb experiment in 2016 at a centre-of-mass energy per nucleon pair of sNN=8.16 TeV, containing in total approximately 1.5 billion collision events. Non-flow contributions are obtained in low-multiplicity collisions and subtracted to extract the flow harmonics. The results are presented as a function of event multiplicity and hadron transverse momentum. Comparisons with a full (3+1)D dynamic model indicate that it overestimates the measured elliptic flow. A comparison between the forward and backward regions reveals no significant differences in flow parameters, suggesting that final-state effects may dominate over initial-state effects in the origin of flow in small systems

Amplitude analysis of the radiative decay B s 0 → K + K − γ

A search for radiative decay of Bs0 mesons to orbitally excited K+K− states is performed using proton proton collisions recorded by the LHCb experiment, corresponding to an integrated luminosity of 9 fb−1. The dikaon spectrum in the mass range mKK < 2400 MeV/c2 is dominated by the ϕ(1020) resonance that accounts for almost 70% of the decay rate. Considering the possible contributions of f2(1270), f2′(1525) and f2(2010) meson states, the overall tensor contribution to the amplitude is measured to beFf2=16.8±0.5stat.±0.7syst.%, mostly dominated by the f2′(1525) state. Several statistically equivalent solutions are obtained for the detailed resonant structure depending on whether the smaller amplitudes interfere destructively or constructively with the dominant amplitude. The preferred solution that corresponds to the lowest values of the fit fractions along with constructive interference leads to the relative branching ratio measurementBBs0→f2′γBBs0→ϕγ=19.4−0.8+0.9stat.−0.5+1.4syst.±0.5B%, where the last uncertainty is due to the ratio of measured branching fractions to the K+K− final state. This result represents the first observation of the radiative Bs0→f2′1525γ decay, which is the second radiative transition observed in the Bs0 sector

Tracking of charged particles with nanosecond lifetimes at LHCb

A method is presented to reconstruct charged particles with lifetimes between 10ps and 10ns, which considers a combination of their decay products and the partial tracks created by the initial charged particle. Using the Ξ- baryon as a benchmark, the method is demonstrated with simulated events and proton-proton collision data at s=13TeV, corresponding to an integrated luminosity of 2.0fb-1 collected with the LHCb detector in 2018. Significant improvements in the angular resolution and the signal purity are obtained. The method is implemented as part of the LHCb Run 3 event trigger in a set of requirements to select detached hyperons. This is the first demonstration of the applicability of this approach at the LHC, and the first to show its scaling with instantaneous luminosity

Observation of the decay B s 0 → K 0 p p ¯ and measurement of the B s 0 → K 0 p p ¯ branching fractions

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

Measurement of the B 0 → ρ (770) 0 γ branching fraction

The ratio between the branching fractions of the B0 → ρ(770)0γ and B0 → K*(892)0γ decays is measured with 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 measured value isBB0→ρ7700γBB0→K∗8920γ=0.0189±0.0007±0.0005, where the first uncertainty is statistical and the second systematic. The branching fraction for B0→ ρ(770)0γ decays is hence obtained asBB0→ρ7700γ=7.9±0.3±0.2±0.2×10−7, where the last uncertainty is due to the branching fraction of the normalisation mode. This result assumes that both the ρ(770)0 and K*(892)0 decays saturate the dihadron mass spectra considered in the analysis. It is consistent with the current world-average value and by far the most precise measurement to date

Amplitude analysis and branching fraction measurement of B + → D ∗ − D s + π + decays

The decays of the B+ meson to the final state D∗−Ds+π+ are studied in proton-proton collision data collected with the LHCb detector at centre-of-mass energies of 7, 8, and 13 TeV, corresponding to a total integrated luminosity of 9 fb−1. The ratio of branching fractions of the B+→D∗−Ds+π+ and B0→D∗−Ds+ decays is measured to be 0.173 ± 0.006 ± 0.010, where the first uncertainty is statistical and the second is systematic. Using partially reconstructed Ds∗+→Ds+γ and Ds+π0 decays, the ratio of branching fractions between the B+→D∗−Ds∗+π+ and B+→D∗−Ds+π+ decays is determined as 1.31 ± 0.07 ± 0.14. An amplitude analysis of the B+→D∗−Ds+π+ decay is performed for the first time, revealing dominant contributions from known excited charm resonances decaying to the D*−π+ final state. No significant evidence of exotic contributions in the Ds+π+ or D∗−Ds+ channels is found. The fit fraction of the scalar state Tcs¯0∗2900++ observed in the B+→D−Ds+π+ decay is determined to be less than 2.3% at a 90% confidence level

Three-pion Bose-Einstein correlations measured in proton-proton collisions

A study on the Bose-Einstein correlations for triplets of same-sign pions is presented. The analysis is performed using proton-proton collisions at a centre-of-mass energy of s = 7 TeV, recorded by the LHCb experiment, corresponding to an integrated luminosity of 1.0 fb−1. For the first time, the results are interpreted in the core-halo model. The parameters of the model are determined in regions of charged-particle multiplicity. This measurement provides insight into the nature of hadronisation in terms of coherence, being consistent with the presence of coherent emission of pions