The essence of rare beauty:Studying B0(s) → μ+μ− decays with the LHCb experiment

The model developed by particle physicists which describes our Universe up to very high energies and to the smallest scales, called the Standard Model, has performed excellently to explain the results of measurements up to now. The Standard Model is built on symmetries, matter and interactions; a curious property of the matter in the Standard Model is that it appears in three generations. Particles from different generations are almost identical, but differ greatly in their couplings to the Higgs particle, and the structure of the couplings is unexplained; this is called the flavour puzzle. For example, their masses differ greatly: third-generation particles are at least a thousand times heavier than those from the first generation. The third-generation particles called beauty quarks are especially sensitive to possible differences between the generations. Extra sensitive to such effects are decays of beauty quarks that occur seldomly according to the Standard Model, for which even small effects from particles beyond the Standard Model can be significant. A special class of these decays are B decays to two muons (siblings of the electron), which occur at the level of one time every billion B decays; they are extra sensitive to new Higgs particles or dark matter particles. At the Large Hadron Collider, trillions of beauty quarks per year are produced and even such very rare decays can be tested. This dissertation describes the study of B decays to two muons with data collected by the LHCb experiment from 2011 to 2016, resulting in the discovery of the decay of a strange beauty particle or "Bs meson" to two muons and the first measurement of the lifetime of this decay, the strongest limit on the decay of the "Bd meson" to two muons. By itself, the measurement is consistent with the Standard Model. Combined with similar measurements, hints are found of deviations from the Standard Model. Future measurements are needed to shed light on this possibility to solve the flavour puzzle

Observation of the B + → Jψη′K + decay

The B + → Jψη′K + decay is observed for the first time using proton-proton collision data collected by the LHCb experiment at centre-of-mass energies of 7, 8, and 13 TeV, corresponding to a total integrated luminosity of 9 fb −1. The branching fraction of this decay is measured relative to the known branching fraction of the B + → ψ(2S)K + decay and found to be B(B+→Jψη′K+)B(B+→ψ(2S)K+)=(4.91±0.47±0.29±0.07)×10−2, where the first uncertainty is statistical, the second is systematic and the third is related to external branching fractions. A first look at the J/ψη′ mass distribution is performed and no signal of intermediate resonances is observed

Search for the doubly charmed baryon Ω + cc

A search for the doubly charmed baryon Ω+cc with the decay mode Ω+cc → Ξ+cK−π+ is performed using proton-proton collision data at a centre-of-mass energy of 13 TeV collected by the LHCb experiment from 2016 to 2018, corresponding to an integrated luminosity of 5.4 fb−1. No significant signal is observed within the invariant mass range of 3.6 to 4.0GeV/c2. Upper limits are set on the ratio R of the production cross-section times the total branching fraction of the Ω+cc → Ξ+cK−π+ decay with respect to the Ξ++cc→Λ+cK−π+π+Ξcc++→Λc+K−π+π+ decay. Upper limits at 95% credibility level for R in the range 0.005 to 0.11 are obtained for different hypotheses on the Ω+cc mass and lifetime in the rapidity range from 2.0 to 4.5 and transverse momentum range from 4 to 15 GeV/c

Branching Fraction Measurements of the Rare B 0 s → ϕ μ + μ − and B 0 s → f ′ 2 ( 1525 ) μ + μ − Decays

The branching fraction of the rare B0s→ϕμ+μ− decay is measured using data collected by the LHCb experiment at center-of-mass energies of 7, 8, and 13 TeV, corresponding to integrated luminosities of 1, 2, and 6  fb−1, respectively. The branching fraction is reported in intervals of q2, the square of the dimuon invariant mass. In the q2 region between 1.1 and 6.0  GeV2/c4, the measurement is found to lie 3.6 standard deviations below a standard model prediction based on a combination of light cone sum rule and lattice QCD calculations. In addition, the first observation of the rare B0s→f′2(1525)μ+μ− decay is reported with a statistical significance of 9 standard deviations and its branching fraction is determined

Observation of the B0 → D ¯ ∗0K+π- and Bs0 → D ¯ ∗0K-π+ decays

The first observations of B0→D¯∗(2007)0K+π- and Bs0→D¯∗(2007)0K-π+ decays are presented, and their branching fractions relative to that of the B0→D¯∗(2007)0π+π- decay are reported. These modes can potentially be used to investigate the spectroscopy of charm and charm-strange resonances and to determine the angle γ of the Cabibbo-Kobayashi-Maskawa unitarity triangle. It is also important to understand them as a source of potential background in determinations of γ from B+→DK+ and B0→DK+π- decays. The analysis is based on a sample corresponding to an integrated luminosity of 5.4 fb-1 of proton-proton collision data at 13 TeV center-of-mass energy recorded with the LHCb detector. The D¯∗(2007)0 mesons are fully reconstructed in the D¯0π0 and D¯0γ channels with the D¯0→K+π- decay. A novel weighting method is used to subtract background while simultaneously applying an event-by-event efficiency correction to account for resonant structures in the decays

Observation of the Mass Difference between Neutral Charm-Meson Eigenstates

A measurement of mixing and CP violation in neutral charm mesons is performed using data reconstructed in proton-proton collisions collected by the LHCb experiment from 2016 to 2018, corresponding to an integrated luminosity of 5.4  fb−1. A total of 30.6 million D0→K0Sπ+π− decays are analyzed using a method optimized for the measurement of the mass difference between neutral charm-meson eigenstates. Allowing for CP violation in mixing and in the interference between mixing and decay, the mass and decay-width differences are measured to be xCP=[3.97±0.46(stat)±0.29(syst)]×10−3 and yCP=[4.59±1.20(stat)±0.85(syst)]×10−3, respectively. The CP-violating parameters are measured as Δx=[−0.27±0.18(stat)±0.01(syst)]×10−3 and Δy=[0.20±0.36(stat)±0.13(syst)]×10−3. This is the first observation of a nonzero mass difference in the D0 meson system, with a significance exceeding seven standard deviations. The data are consistent with CP symmetry and improve existing constraints on the associated parameters.</p

Search for the decay B0 → ϕμ+μ−

A search for the decay B0 → ϕμ+μ− is performed using proton-proton collisions at centre-of-mass energies of 7, 8, and 13 TeV collected by the LHCb experiment and corresponding to an integrated luminosity of 9 fb−1. No evidence for the B0 → ϕμ+μ− decay is found and an upper limit on the branching fraction, excluding the ϕ and charmonium regions in the dimuon spectrum, of 4.4 × 10−3 at a 90% credibility level, relative to that of the B0s → ϕμ+μ− decay, is established. Using the measured B0s → ϕμ+μ− branching fraction and assuming a phase-space model, the absolute branching fraction of the decay B0 → ϕμ+μ− in the full q2 range is determined to be less than 3.2 × 10−9 at a 90% credibility level