Malcolm John

Abstract:

The production of W and Z bosons in association with jets is studied in the forward region of proton-proton collisions collected at a centre-of-mass energy of 8 TeV by the LHCb experiment, corresponding to an integrated luminosity of 1.98 ± 0.02 fb−1. The W boson is identified using its decay to a muon and a neutrino, while the Z boson is identified through its decay to a muon pair. Total cross-sections are measured and combined into charge ratios, asymmetries, and ratios of W +jet and Z+jet production cross-sections. Differential measurements are also performed as a function of both boson and jet kinematic variables. All results are in agreement with Standard Model predictions.

Abstract:

The branching fraction of the decay Bs0 → Ds(∗)+ Ds(∗)- is measured using pp collision data corresponding to an integrated luminosity of 1.0 fb^-1, collected using the LHCb detector at a center-of-mass energy of 7 TeV. It is found to be B(Bs0 → Ds(∗)+ Ds(∗)-) = (3.05 ± 0.10 ± 0.20 ± 0.34)%, where the uncertainties are statistical, systematic, and due to the normalization channel, respectively. The branching fractions of the individual decays corresponding to the presence of one or two Ds∗± are also measured. The individual branching fractions are found to be B(Bs0 → Ds∗± Ds∓) = (1.35 ± 0.06 ± 0.09 ± 0.15)%, B(Bs0 → Ds∗+ Ds∗-) = (1.27 ± 0.08 ± 0.10 ± 0.14)%. All three results are the most precise determinations to date.

Abstract:

We perform a search for near-threshold Ξb0 resonances decaying to Ξb−π+ in a sample of proton-proton collision data corresponding to an integrated luminosity of 3 fb−1 collected by the LHCb experiment. We observe one resonant state, with the following properties: m(Ξ∗0b) − m(Ξ−b) − m(π+) = 15.727 ± 0.068(stat) ± 0.023(syst) MeV/c2 Γ(Ξ∗0b) = 0.90 ± 0.16(stat) ± 0.08(syst) MeV. This confirms the previous observation by the CMS collaboration. The state is consistent with the JP = 3/2+  Ξb∗ 0 resonance expected in the quark model. This is the most precise determination of the mass and the first measurement of the natural width of this state. We have also measured the ratio σ(pp→Ξ∗0bX)B(Ξ∗0b→Ξ−bπ+)/σ(pp → Ξ−bX) =0.28 ± 0.03(stat.) ± 0.01(syst.).

Abstract:

The Λ0b → Λφ decay is observed using data corresponding to an integrated luminosity of 3.0 fb^-1 recorded by the LHCb experiment. The decay proceeds at leading order via a b → sss loop transition and is therefore sensitive to the possible presence of particles beyond the Standard Model. A first observation is reported with a significance of 5.9 standard deviations. The value of the branching fraction is measured to be (5.18 ± 1.04 ± 0.35^+0.67-0.62) X 10^-6, where the first uncertainty is statistical, the second is systematic, and the third is related to external inputs. Triple-product asymmetries are measured to be consistent with zero.

Abstract:

Violations of CPT symmetry and Lorentz invariance are searched for by studying interference effects in B^0 mixing and in Bs^0 mixing. Samples of B^0 → J/ψKS^0 and Bs^0 → J/ψK^+K^- decays are recorded by the LHCb detector in proton-proton collisions at center-of-mass energies of 7 and 8 TeV, corresponding to an integrated luminosity of 3 fb^-1. No periodic variations of the particle-antiparticle mass differences are found, consistent with Lorentz invariance and CPT symmetry. Results are expressed in terms of the standard model extension parameter Δaμ with precisions of O(10^-15) and O(10^-14)  GeV for the B^0 and Bs^0 systems, respectively. With no assumption on Lorentz (non)invariance, the CPT-violating parameter z in the Bs^0 system is measured for the first time and found to be Re(z) = -0.022 ± 0.033 ± 0.005 and Im(z) = 0.004 ± 0.011 ± 0.002, where the first uncertainties are statistical and the second systematic.