Analysis of charged particle production in proton-nucleus and proton-proton collisions at the LHCb experiment

This thesis presents the measurement of prompt charged particle production in proton-lead and proton-proton collisions at the nucleon-nucleon centre-of-mass energy of 5.02TeV in the LHCb experiment. Double-differential cross-sections of prompt charged particles are determined in the two systems as a function of pseudorapidity (eta) and transverse momentum (pT) with respect to the proton beam

Central exclusive production of J/ψ and ψ(2S) mesons in pp collisions at √s=13 TeV

Measurements are reported of the central exclusive production of J/ψ and ψ(2S) mesons in pp collisions at a centre-of-mass energy of 13 TeV. Backgrounds are significantly reduced compared to previous measurements made at lower energies through the use of new forward shower counters. The products of the cross-sections and the branching fractions for the decays to dimuons, where both muons are within the pseudorapidity range 2.0 < η < 4.5, are measured to be σJ/ψ→μ+μ−=435±18±11±17 pbσψ(2S)→μ+μ−=11.1±1.1±0.3±0.4 pb. The first uncertainties are statistical, the second are systematic, and the third are due to the luminosity determination. The cross-sections are also measured differentially for meson rapidities between 2.0 and 4.5. Good agreement is observed with theoretical predictions. Photoproduction cross-sections are derived and compared to previous experiments, and a deviation from a pure power-law extrapolation of lower energy data is observed.S

Evidence for the decay B0S→K¯∗0μ+μ−

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.S

Measurement of the CKM angle γ using B± → DK± with D → K0Sπ+π−, K0SK+K− decays

A binned Dalitz plot analysis of B± → DK± decays, with D → K 0Sπ+π− and D → K 0SK+K−, is used to perform a measurement of the CP-violating observables x± and y±, which are sensitive to the Cabibbo-Kobayashi-Maskawa angle γ. The analysis is performed without assuming any D decay model, through the use of information on the strong-phase variation over the Dalitz plot from the CLEO collaboration. Using a sample of proton-proton collision data collected with the LHCb experiment in 2015 and 2016, and corresponding to an integrated luminosity of 2.0 fb−1, the values of the CP violation parameters are found to be x− = (9.0 ± 1.7 ± 0.7 ± 0.4) × 10−2, y− = (2.1 ± 2.2 ± 0.5 ± 1.1) × 10−2, x+ = (−7.7 ± 1.9 ± 0.7 ± 0.4) × 10−2, and y+ = (−1.0 ± 1.9 ± 0.4 ± 0.9) × 10−2. The first uncertainty is statistical, the second is systematic, and the third is due to the uncertainty (on the strong-phase measurements. These values are used to obtain γ = (87 + 11− 12)∘, rB = 0.086 + 0.013− 0.014, and δB = (101±11)°, where rB is the ratio between the suppressed and favoured B-decay amplitudes and δB is the corresponding strong-interaction phase difference. This measurement is combined with the result obtained using 2011 and 2012 data collected with the LHCb experiment, to give γ = (80 + 10− 9)∘, rB = 0.080 ± 0.011, and δB = (110 ± 10)°.S

Measurement of D ±s production asymmetry in pp collisions at √s=7 and 8 TeV

The inclusive D ±s production asymmetry is measured in pp collisions collected by the LHCb experiment at centre-of-mass energies of √s=7 and 8 TeV. Promptly produced D ±s mesons are used, which decay as D ±s → ϕπ±, with ϕ → K+K−. The measurement is performed in bins of transverse momentum, pT, and rapidity, y, covering the range 2.5 < pT < 25.0 GeV/c and 2.0 < y < 4.5. No kinematic dependence is observed. Evidence of nonzero D ±s production asymmetry is found with a significance of 3.3 standard deviations.S

Observation of the decay B¯0s→χc2K+K− in the ϕ mass region

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.S

Evidence for an ηc(1S)π− resonance in B0 → ηc(1S)K+π− decays

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 fractionsS

Studies of the resonance structure in D0→K∓π±π±π∓ decays

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 violationS

Angular analysis of B0→D∗−D∗+s with D∗+s→D+sγ decays

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 fractionsS

Searches for 25 rare and forbidden decays of D+ and D+s mesons

A search is performed for rare and forbidden charm decays of the form D+(s)→h±ℓ+ℓ(′)∓, where h± is a pion or kaon and ℓ(′)± is an electron or muon. The measurements are performed using proton-proton collision data, corresponding to an integrated luminosity of 1.6 fb−1, collected by the LHCb experiment in 2016. No evidence is observed for the 25 decay modes that are investigated and 90 % confidence level limits on the branching fractions are set between 1.4 × 10−8 and 6.4 × 10−6. In most cases, these results represent an improvement on existing limits by one to two orders of magnitudeWe acknowledge support from CERN and from the national agencies: CAPES, CNPq, FAPERJ and FINEP (Brazil); MOST and NSFC (China); CNRS/IN2P3 (France); BMBF, DFG and MPG (Germany); INFN (Italy); NWO (Netherlands); MNiSW and NCN (Poland); MEN/IFA (Romania); MSHE (Russia); MICINN (Spain); SNSF and SER (Switzerland); NASU (Ukraine); STFC (United Kingdom); DOE NP and NSF (U.S.A.). We acknowledge the computing resources that are provided by CERN, IN2P3 (France), KIT and DESY (Germany), INFN (Italy), SURF (Netherlands), PIC (Spain), GridPP (United Kingdom), RRCKI and Yandex LLC (Russia), CSCS (Switzerland), IFINHH (Romania), CBPF (Brazil), PL-GRID (Poland) and OSC (U.S.A.). Individual groups or members have received support from AvH Foundation (Germany); EPLANET, Marie Skłodowska-Curie Actions and ERC (European Union); A*MIDEX, ANR, Labex P2IO and OCEVU, and Région Auvergne-Rhône-Alpes (France); Key Research Program of Frontier Sciences of CAS, CAS PIFI, Thousand Talents Program, and Sci. & Tech. Program of Guangzhou (China); RFBR, RSF and Yandex LLC (Russia); GVA, XuntaGal and GENCAT (Spain); the Royal Society and the Leverhulme Trust (United Kingdom)S