Search

The LHCb experiment foresees a new era of high luminosity, aiming to clarify persistent tensions between experiments and the SM predictions, such as those related to the flavour sector. This thesis is devoted to the measurement of the LFU ratios R(D0) and R(D*0), with the 3-prong hadronic tau decay modes, using LHCb Run 2 data. Even though the final result is blinded and the systematics uncertainties need to be computed, the signal branching fraction are obtained with a competitive precision. Moreover, the computation of the statistical and external uncertainties of the R(D(*)0) ratios hints that the analysis documented here will be a relevant contribution to the LFU sector. Furthermore, a contribution to the LHCb detector upgrade is documented, based on testing the new hybrid pixel VELO sensors' performance.

The first full angular analysis of the B0→D∗−D∗+s decay is performed using 6 fb−1 of pp collision data collected with the LHCb experiment at a centre-of-mass energy of 13 TeV. The D∗+s→D+sγ and D*− → D¯¯¯¯0π− vector meson decays are used with the subsequent D+s → K+K−π+ and D¯¯¯¯0 → K+π− decays. All helicity amplitudes and phases are measured, and the longitudinal polarisation fraction is determined to be fL = 0.578 ± 0.010 ± 0.011 with world-best precision, where the first uncertainty is statistical and the second is systematic. The pattern of helicity amplitude magnitudes is found to align with expectations from quark-helicity conservation in B decays. The ratio of branching fractions [ℬ(B0→D∗−D∗+s) × ℬ(D∗+s→D+sγ)]/ℬ(B0 → D*−D+s) is measured to be 2.045 ± 0.022 ± 0.071 with world-best precision. In addition, the first observation of the Cabibbo-suppressed Bs → D*−D+s decay is made with a significance of seven standard deviations. The branching fraction ratio ℬ(Bs → D*−D+s)/ℬ(B0 → D*−D+s) is measured to be 0.049 ± 0.006 ± 0.003 ± 0.002, where the third uncertainty is due to limited knowledge of the ratio of fragmentation fractions SI

The nomenclature and geometry of the various constituents of velo upgrade hybrid sensors are defined, as well as their position within the modules. The modules nomenclature and their position along the z-axis are defined.

A search for the decay B0S→K¯∗0μ+μ− is presented using data sets corresponding to 1.0, 2.0 and 1.6 fb−1 of integrated luminosity collected during pp collisions with the LHCb experiment at centre-of-mass energies of 7, 8 and 13 TeV, respectively. An excess is found over the background-only hypothesis with a significance of 3.4 standard deviations. The branching fraction of the B0S→K¯∗0μ+μ− decay is determined to be B(B0s→K¯∗0μ+μ−)=[2.9±1.0(stat)±0.2(syst)±0.3(norm)]×10−8, where the first and second uncertainties are statistical and systematic, respectively. The third uncertainty is due to limited knowledge of external parameters used to normalise the branching fraction measurement. SI

Amplitude models are constructed to describe the resonance structure of D0 → K− π + π + π − and D0 → K+ π − π − π + decays using pp collision data collected at centre-of-mass energies of 7 and 8 TeV with the LHCb experiment, corresponding to an integrated luminosity of 3.0 f b−1. The largest contributions to both decay amplitudes are found to come from axial resonances, with decay modes D0 → a1(1260) +K− and D0 → K1(1270/1400) + π − being prominent in D0 → K− π + π + π − and D0 → K+ π − π − π +, respectively. Precise measurements of the lineshape parameters and couplings of the a1(1260) +, K1(1270) − and K(1460) − resonances are made, and a quasi model-independent study of the K(1460) − resonance is performed. The coherence factor of the decays is calculated from the amplitude models to be RK3π = 0.459 ± 0.010 (stat) ± 0.012 (syst)±0.020 (model), which is consistent with direct measurements. These models will be useful in future measurements of the unitary-triangle angle γ and studies of charm mixing and CP violation SI

A Dalitz plot analysis of B0 → ηc(1S)K +π− decays is performed using data samples of pp collisions collected with the LHCb detector at centre-of-mass energies of √s = 7, 8 and 13 TeV, corresponding to a total integrated luminosity of 4.7 fb−1. A satisfactory description of the data is obtained when including a contribution representing an exotic ηc(1S)π− resonant state. The significance of this exotic resonance is more than three standard deviations, while its mass and width are 4096 ± 20 +18 −22 MeV and 152±58 +60 −35 MeV, respectively. The spin-parity assignments J P = 0+ and J P = 1− are both consistent with the data. In addition, the first measurement of the B0 → ηc(1S)K +π− branching fraction is performed and gives B(B0→ ηc(1S)K +π−) = (5.73 ± 0.24 ± 0.13 ± 0.66) × 10−4, where the first uncertainty is statistical, the second systematic, and the third is due to limited knowledge of external branching fractions SI

The decay Λ0b→Λ+cppπ−is observed using ppcollision data collected with the LHCb detector at centre-of-mass energies of √s=7and 8Tev, corresponding to an integrated luminosity of 3fb−1. The ratio of branching fractions between Λ0b→Λ+cppπ−and Λ0b→Λ+cπ−decays is measured to be B(Λ0 b →Λ + c ppπ− ) B(Λ0 b →Λ + c π−) = 0.0540±0.0023±0.0032. Two resonant structures are observed in the Λ+cπ−mass spectrum of the Λ0b→Λ+cppπ−decays, corresponding to the Σc(2455)0and Σ∗c(2520)0states. The ratios of branching fractions with respect to the decay Λ0b→Λ+cppπ−are B(Λ0 b →Σ0 c pp)×B(Σ0 c →Λ + c π− ) B(Λ0 b →Λ + c ppπ−) = 0.089±0.015±0.006, B(Λ0 b →Σ ∗0 c pp)×B(Σ ∗0 c →Λ + c π− ) B(Λ0 b →Λ + c ppπ−) = 0.119±0.020±0.014. In all of the above results, the first uncertainty is statistical and the second is systematic. The phase space is also examined for the presence of dibaryon resonances. No evidence for such resonances is found. SI

A search for CPviolation in Λ0b→pK−and Λ0b→pπ−decays is presented using a sample of ppcollisions collected with the LHCb detector and corresponding to an integrated luminosity of 3.0fb−1. The CP-violating asymmetries are measured to be ApK−CP=−0.020 ±0.013 ±0.019and Apπ−CP=−0.035 ±0.017 ±0.020, and their difference ApK−CP−Apπ−CP=0.014 ±0.022 ±0.010, where the first uncertainties are statistical and the second systematic. These are the most precise measurements of such asymmetries to date. SI

The production of ϒ(nS) mesons (n = 1, 2, 3) in pPb and Pbp collisions at a centre-of-mass energy per nucleon pair \( \sqrt{s_{\mathrm{NN}}}=8.16 \) TeV is measured by the LHCb experiment, using a data sample corresponding to an integrated luminosity of 31.8 nb−1. The ϒ(nS) mesons are reconstructed through their decays into two opposite-sign muons. The measurements comprise the differential production cross-sections of the ϒ(1S) and ϒ(2S) states, their forward-to-backward ratios and nuclear modification factors. The measurements are performed as a function of the transverse momentum pT and rapidity in the nucleon-nucleon centre-of-mass frame y* of the ϒ(nS) states, in the kinematic range pT < 25 GeV/c and 1.5 < y* < 4.0 (−5.0 < y* < −2.5) for pPb (Pbp) collisions. In addition, production cross-sections for ϒ(3S) are measured integrated over phase space and the production ratios between all three ϒ(nS) states are determined. Suppression for bottomonium in proton-lead collisions is observed, which is particularly visible in the ratios. The results are compared to theoretical models. SI

The B¯0s→χc2K+K− decay mode is observed and its branching fraction relative to the corresponding χc1 decay mode, in a ±15 MeV/c2 window around the ϕ mass, is found to be B(B¯0s→χc2K+K−)B(B¯0s→χc1K+K−)=(17.1±3.1±0.4±0.9)%, where the first uncertainty is statistical, the second systematic and the third due to the knowledge of the branching fractions of radiative χc decays. The decay mode B¯0s→χc1K+K− allows the B0s mass to be measured as m(B0s)=5366.83±0.25±0.27 MeV/c2, where the first uncertainty is statistical and the second systematic. A combination of this result with other LHCb determinations of the B 0s mass is made. SI

A measurement of Z → τ +τ − production cross-section is presented using data, corresponding to an integrated luminosity of 2 fb−1, from pp collisions at √s=8 TeV collected by the LHCb experiment. The τ +τ − candidates are reconstructed in final states with the first tau lepton decaying leptonically, and the second decaying either leptonically or to one or three charged hadrons. The production cross-section is measured for Z bosons with invariant mass between 60 and 120 GeV/c2, which decay to tau leptons with transverse momenta greater than 20 GeV/c and pseudorapidities between 2.0 and 4.5. The cross-section is determined to be σpp→Z→τ+τ−=95.8±2.1±4.6±0.2±1.1 pb, where the first uncertainty is statistical, the second is systematic, the third is due to the LHC beam energy uncertainty, and the fourth to the integrated luminosity uncertainty. This result is compatible with NNLO Standard model predictions. The ratio of the cross-sections for Z → τ+τ− to Z → μ+μ− (Z → e+e−), determined to be 1.01 ± 0.05 (1.02 ± 0.06), is consistent with the lepton-universality hypothesis in Z decays. SI