First observation of B0 → D*(2007)0 K+π- and Bs0 → D*(2007)0 K-π+ decays in LHCb

Measurements of the γ angle of the unitarity triangle formed from elements of the CKM matrix are achieved through the study of B hadron decays featuring interference between b → cus and b → ucs processes. Such measurements are affected by a systematic uncertainty due to the limited knowledge of yet to be observed partially reconstructed decays featuring soft neutral final state particles. Unless knowledge of these decays is improved, the precision of future determination of γ may become limited by this source of uncertainty. In this thesis such decays are studied, resulting in the first observation of B0 → D∗ (2007)0K+π − and B0 s → D∗ (2007)0K−π + decays, and measurements of their branching fractions with respect of that of B0 → D∗ (2007)0π +π − decays. In addition, results on the phase space distribution of such decays are given, which directly benefit the understanding of these decays as partially reconstructed backgrounds in measurements of γ in B+ → DK+ and B0 → DK+π − decay studies. The B0 → D∗ (2007)0K+π − and B0 s → D∗ (2007)0K−π + decay channels also provide interesting insight for the study of resonances in the charm sector and could themselves be used in the future for determinations of γ. The study presented in this thesis is based on the data recorded by the LHCb experiment during the years from 2016 to 2018, corresponding to a sample of 5.4 fb−1 . It constitutes the first analysis within the LHCb collaboration featuring fully reconstructed D∗ (2007) → D0γ and D∗ (2007) → D0π 0 decays, and pioneers the usage of a novel weighting method while implementing an event-by-event efficiency correction to account for resonant structures in the decays under study

Measurement of the Λ0b→ J/ψΛ angular distribution and the Λ0b polarisation in pp collisions

This paper presents an analysis of the Λ0b→ J/ψΛ angular distribution and the transverse production polarisation of Λ0b baryons in proton-proton collisions at centre-of-mass energies of 7, 8 and 13 TeV. The measurements are performed using data corresponding to an integrated luminosity of 4.9 fb−1, collected with the LHCb experiment. The polarisation is determined in a fiducial region of Λ0b transverse momentum and pseudorapidity of 1 < pT< 20 GeV/c and 2 < η < 5, respectively. The data are consistent with Λ0b baryons being produced unpolarised in this region. The parity-violating asymmetry parameter of the Λ → pπ− decay is also determined from the data and its value is found to be consistent with a recent measurement by the BES III collaboration

Search for lepton-flavour-violating decays of Higgs-like bosons.

A search is presented for a Higgs-like boson with mass in the range 45 to 195 GeV/c2 decaying into a muon and a tau lepton. The dataset consists of proton-proton interactions at a centre-of-mass energy of 8 TeV , collected by the LHCb experiment, corresponding to an integrated luminosity of 2 fb-1 . The tau leptons are reconstructed in both leptonic and hadronic decay channels. An upper limit on the production cross-section multiplied by the branching fraction at 95% confidence level is set and ranges from 22 pb for a boson mass of 45 GeV/c2 to 4 pb for a mass of 195 GeV/c2

Observation of an Excited Bc+ State

Using pp collision data corresponding to an integrated luminosity of 8.5 fb-1 recorded by the LHCb experiment at center-of-mass energies of s=7, 8, and 13 TeV, the observation of an excited Bc+ state in the Bc+π+π- invariant-mass spectrum is reported. The observed peak has a mass of 6841.2±0.6(stat)±0.1(syst)±0.8(Bc+) MeV/c2, where the last uncertainty is due to the limited knowledge of the Bc+ mass. It is consistent with expectations of the Bc∗(2S31)+ state reconstructed without the low-energy photon from the Bc∗(1S31)+→Bc+γ decay following Bc∗(2S31)+→Bc∗(1S31)+π+π-. A second state is seen with a global (local) statistical significance of 2.2σ (3.2σ) and a mass of 6872.1±1.3(stat)±0.1(syst)±0.8(Bc+) MeV/c2, and is consistent with the Bc(2S10)+ state. These mass measurements are the most precise to date

Measurement of the nuclear modification factor and prompt charged particle production in p-Pb and pp Collisions at sqrt[s_{NN}]=5 TeV

The production of prompt charged particles in proton-lead collisions and in proton-proton collisions at the nucleon-nucleon center-of-mass energy √sNN=5  TeV is studied at LHCb as a function of pseudorapidity (η) and transverse momentum (pT) with respect to the proton beam direction. The nuclear modification factor for charged particles is determined as a function of η between −4.8<η<−2.5 (backward region) and 2.0<η<4.8 (forward region), and pT between 0.2<pT<8.0  GeV/c. The results show a suppression of charged particle production in proton-lead collisions relative to proton-proton collisions in the forward region and an enhancement in the backward region for pT larger than 1.5  GeV/c. This measurement constrains nuclear PDFs and saturation models at previously unexplored values of the parton momentum fraction down to 10−6

Angular analysis of the rare decay B s 0 → ϕμ + μ −

An angular analysis of the rare decay Bs0→ ϕμ+μ− is presented, using 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 8.4 fb−1. The observables describing the angular distributions of the decay Bs0→ ϕμ+μ− are determined in regions of q2, the square of the dimuon invariant mass. The results are consistent with Standard Model predictions

Measurement of the Bs0 →μ+μ- decay properties and search for the B0 →μ+μ- and Bs0 →μ+μ-γ decays

An improved measurement of the decay B0s→μ+μ− and searches for the decays B0→μ+μ− and B0s→μ+μ−γ are performed at the LHCb experiment using data collected in proton-proton collisions at √s=7, 8 and 13 TeV, corresponding to integrated luminosities of 1, 2 and 6  fb−1, respectively. The B0s→μ+μ− branching fraction and effective lifetime are measured to be B(B0s→μ+μ−)=(3.09+0.46+0.15−0.43−0.11)×10−9 and τ(B0s→μ+μ−)=(2.07±0.29±0.03)  ps, respectively, where the uncertainties include both statistical and systematic contributions. No significant signal for B0→μ+μ− and B0s→μ+μ−γ decays is found and the upper limits B(B0→μ+μ−)4.9  GeV/c2. Additionally, the ratio between the B0→μ+μ− and B0s→μ+μ− branching fractions is measured to be Rμ+μ−<0.095 at 95% confidence level. The results are in agreement with the Standard Model predictions