Guy Wilkinson

Abstract:

Decays of B0 and Bs0 mesons to the charmless baryonic final states pp-h+h′-, where h and h′ each denote a kaon or a pion, are searched for using the LHCb detector. The analysis is based on a sample of proton-proton collision data collected at center-of-mass energies of 7 and 8 TeV, corresponding to an integrated luminosity of 3 fb-1. Four-body charmless baryonic Bs0 decays are observed for the first time. The decays Bs0→pp-K+K-, Bs0→pp-K±πâ", B0→pp-K±πâ" and B0→pp-π+π-A re observed with a significance greater than 5 standard deviations; evidence at 4.1 standard deviations is found for the B0→pp-K+K-decay and an upper limit is set on the branching fraction for Bs0→pp-π+π-. Branching fractions in the kinematic region m(pp-) < 2850 MeV/c2 are measured relative to the B0→J/ψ(→pp-)K∗(892)0 channel.

Abstract:

A highly significant structure is observed in the Λ+cK−π+π+ mass spectrum, where the Λ+c baryon is reconstructed in the decay mode pK−π+. The structure is consistent with originating from a weakly decaying particle, identified as the doubly charmed baryon Ξ++cc. The difference between the masses of the Ξ++cc and Λ+c states is measured to be 1334.94±0.72(stat.)±0.27(syst.)  MeV/c^2, and the Ξ++cc mass is then determined to be 3621.40±0.72(stat.)±0.27(syst.)±0.14(Λ+c)  MeV/c^2, where the last uncertainty is due to the limited knowledge of the Λ+c mass. The state is observed in a sample of proton-proton collision data collected by the LHCb experiment at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 1.7  fb^−1, and confirmed in an additional sample of data collected at 8 TeV.

Abstract:

he production of J/ψ mesons is studied in proton-lead collisions at the centre-of-mass energy per nucleon pair √sNN =8.16 TeV with the LHCb detector at the LHC. The double differential cross-sections of prompt and nonprompt J/ψ production are measured as a function of the J/ψ transverse momentum and rapidity in the nucleon–nucleon centre-of-mass frame. Forward-to-backward ratios and nuclear modification factors are determined. The results are compared with theoretical calculations based on collinear factorisation using nuclear parton distribution functions, on the colour glass condensate or on coherent energy loss models.

Abstract:

Using decays to ϕ-meson pairs, the inclusive production of charmonium states in b-hadron decays is studied with pp collision data corresponding to an integrated luminosity of 3.0fb-1, collected by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. Denoting by BC≡B(b→CX)×B(C) the inclusive branching fraction of a b hadron to a charmonium state C that decays into a pair of ϕ mesons, ratios RC2C1≡BC1/BC2 are determined as Rηc(1S)χc0=0.147±0.023±0.011, Rηc(1S)χc1=0.073±0.016±0.006, Rηc(1S)χc2=0.081±0.013±0.005, Rχc0χc1=0.50±0.11±0.01, Rχc0χc2=0.56±0.10±0.01 and Rηc(1S)ηc(2S)=0.040±0.011±0.004. Here and below the first uncertainties are statistical and the second systematic. Upper limits at 90% confidence level for the inclusive production of X(3872), X(3915) and χ c 2 (2 P) states are obtained as Rχc1X(3872) < 0.34, Rχc0X(3915) < 0.12 and Rχc2χc2(2P) < 0.16. Differential cross-sections as a function of transverse momentum are measured for the η c (1 S) and χ c states. The branching fraction of the decay Bs0 is measured for the first time, B(Bs0)=(2.15±0.54±0.28±0.21B)×10-6. Here the third uncertainty is due to the branching fraction of the decay Bs0, which is used for normalization. No evidence for intermediate resonances is seen. A preferentially transverse ϕ polarization is observed. The measurements allow the determination of the ratio of the branching fractions for the η c (1 S) decays to ϕϕ and pp¯ as B(ηc(1S))/B(ηc(1S)→pp¯)=1.79±0.14±0.32.