Search

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

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

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 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

A search for CP violation in the decay Λ0b → pK − μ + μ − is presented. This decay is mediated by flavour-changing neutral-current transitions in the Standard Model and is potentially sensitive to new sources of CP violation. The study is based on a data sample of proton-proton collisions recorded with the LHCb experiment, corresponding to an integrated luminosity of 3 fb−1. The Λ0b → pK − μ + μ − decay is observed for the first time, and two observables that are sensitive to different manifestations of CP violation are measured, ΔACP≡ACP(Λ0b→pK−μ+μ−)−ACP(Λ0b→pK−J/ψ) and a aTˆ-oddCP, where the latter is based on asymmetries in the angle between the μ + μ − and pK − decay planes. SI

The production cross-section of J/ψ pairs is measured using a data sample of pp collisions collected by the LHCb experiment at a centre-of-mass energy of √s=13 TeV, corresponding to an integrated luminosity of 279 ±11 pb−1. The measurement is performed for J/ψ mesons with a transverse momentum of less than 10 GeV/c in the rapidity range 2.0 < y < 4.5. The production cross-section is measured to be 15.2 ± 1.0 ± 0.9 nb. The first uncertainty is statistical, and the second is systematic. The differential cross-sections as functions of several kinematic variables of the J/ψ pair are measured and compared to theoretical predictions. SI

The observation of the decay Ξ−b→J/ψΛK−is reported, using a data sample corresponding to an integrated luminosity of 3 fb−1, collected by the LHCb detector in ppcollisions at centre-of-mass energies of 7and 8 TeV. The production rate of Ξ−bbaryons detected in the decay Ξ−b→J/ψΛK−is measured relative to that of Λ0bbaryons using the decay Λ0b→J/ψΛ. Integrated over the b-baryon transverse momentum pT

A search for the non-resonant decays B 0s → μ + μ − μ + μ − and B 0 → μ + μ − μ + μ − is presented. The measurement is performed using the full Run 1 data set collected in proton-proton collisions by the LHCb experiment at the LHC. The data correspond to integrated luminosities of 1 and 2 fb−1 collected at centre-of-mass energies of 7 and 8 TeV, respectively. No signal is observed and upper limits on the branching fractions of the non-resonant decays at 95% confidence level are determined to be B(B0s→μ+μ−μ+μ−)

The charmless three-body decays B 0( s) → K0Sh+h′ − (where h(′) = π, K) are analysed using a sample of pp collision data recorded by the LHCb experiment, corresponding to an integrated luminosity of 3 fb−1. The branching fractions are measured relative to that of the B0 → K0Sπ+π− decay, and are determined to be: B(B0→K0SK±π∓)B(B0→K0SK+π−)=0.123±0.009(stat)±0.015(syst),B(B0→K0SK+K−)B(B0→K0Sπ+π−)=0.549±0.018(stat)±0.033(syst),B(B0s→K0Sπ+π−)B(B0→K0Sπ+π−)=0.191±0.027(stat)±0.031(syst)±0.011(fs/fd),B(B0s→K0SK±π∓)B(B0→K0Sπ+π−)=1.70±0.07(stat)±0.11(syst)±0.10(fs/fd),B(B0s→K0SK+K−)B(B0→K0Sπ+π−)∈[0.008−0.051]at 90% confidencelevel, where fs/fd represents the ratio of hadronisation fractions of the B0s and B0 mesons. SI

A test of lepton universality, performed by measuring the ratio of the branching fractions of the B 0 → K *0 μ + μ − and B 0 → K *0 e + e − decays, RK∗0, is presented. The K *0 meson is reconstructed in the final state K + π −, which is required to have an invariant mass within 100 MeV/c 2 of the known K *(892)0 mass. The analysis is performed using proton-proton collision data, corresponding to an integrated luminosity of about 3 fb−1, collected by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. The ratio is measured in two regions of the dilepton invariant mass squared, q 2, to be RK∗0={0.66+−0.110.07(stat)±0.03(syst)for0.045

A search for the decay K0S→μ+μ− is performed, based on a data sample of proton-proton collisions corresponding to an integrated luminosity of 3 fb −1 , collected by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV . The observed yield is consistent with the background-only hypothesis, yielding a limit on the branching fraction of BF(K0S→μ+μ−)

The decays of B 0s and B¯0s mesons into the J/ψK + K − final state are studied in the K + K − mass region above the ϕ(1020) meson in order to determine the resonant substructure and measure the CP-violating phase, ϕ s , the decay width, Γ s , and the width difference between light and heavy mass eigenstates, ΔΓ s . A decay-time dependent amplitude analysis is employed. The data sample corresponds to an integrated luminosity of 3 fb−1 produced in 7 and 8 TeV pp collisions at the LHC, collected by the LHCb experiment. The measurement determines ϕ s = 119 ± 107 ± 34 mrad. A combination with previous LHCb measurements using similar decays into the J/ψπ + π − and J/ψϕ(1020) final states gives ϕ s = 1 ± 37 mrad, consistent with the Standard Model prediction. SI

The first observation of the decay ηc(2S) →pp¯ pis reported using proton–proton collision data corre-sponding to an integrated luminosity of 3.0fb−1recorded by the LHCb experiment at centre-of-mass energies of7 and 8TeV. The ηc(2S)resonance is produced in the decay B+→[c¯ c]K+. The product of branching fractions normalised to that for the J/ψintermediate state, Rηc(2S), is measured to be Rηc (2S) ≡ B(B+→ηc(2S)K+ )×B(ηc(2S)→ p¯p ) B(B+→ J/ψ K+)×B( J/ψ → p¯p ) = (1.58±0.33±0.09)×10−2, where the first uncertainty is statistical and the second systematic. No signals for the decays B+→X(3872)(→p¯ p)K+and B+→ψ(3770)(→p¯ p)K+are seen, and the 95% confidence level upper limits on their relative branching ratios are found to be RX(3872)

An amplitude analysis of the decay Λ0b → D0 pπ − is performed in the part of the phase space containing resonances in the D0 p channel. The study is based on a data sample corresponding to an integrated luminosity of 3.0 fb−1 of pp collisions recorded by the LHCb experiment. The spectrum of excited Λ +c states that decay into D0 p is studied. The masses, widths and quantum numbers of the Λc(2880)+ and Λc(2940)+ resonances are measured. The constraints on the spin and parity for the Λc(2940)+ state are obtained for the first time. A near-threshold enhancement in the D0 p amplitude is investigated and found to be consistent with a new resonance, denoted the Λc(2860)+, of spin 3/2 and positive parity. SI

Using decays to φ-meson pairs, the inclusive production of charmonium states in b-hadron decays is studied with pp collision data corresponding to an integrated luminosity of 3.0 fb−1, collected by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. Denoting by BC ≡ B(b → CX) × B(C → φφ) the inclusive branching fraction of a b hadron to a charmonium state C that decays into a pair of φ mesons, ratios RC1 C2 ≡ BC1/BC2 are determined as Rχc0 ηc(1S) = 0.147 ± 0.023 ± 0.011, Rχc1 ηc(1S) = 0.073 ± 0.016 ± 0.006, Rχc2 ηc(1S) = 0.081 ± 0.013 ± 0.005, Rχc1 χc0 = 0.50 ± 0.11 ± 0.01, Rχc2 χc0 = 0.56 ± 0.10 ± 0.01 and Rηc(2S) ηc(1S) = 0.040 ± 0.011 ± 0.004. Here and below the first uncertainties are statistical and the second systematic. Upper limits at 90% confidence level for the inclusive production of X(3872), X(3915) and χc2(2P) states are obtained as RX(3872) χc1 < 0.34, RX(3915) χc0 < 0.12 and Rχc2(2P) χc2 < 0.16. Differential cross-sections as a function of transverse momentum are measured for the ηc(1S) and χc states. The branching fraction of the decay B0 s → φφφ is measured for the first time, B(B0 s → φφφ) = (2.15±0.54± 0.28±0.21B)×10−6. Here the third uncertainty is due to the branching fraction of the decay B0 s → φφ, which is used for normalization. No evidence for intermediate resonances is seen. A preferentially transverse φ polarization is observed. Themeasurements allow the determination of the ratio of the branching fractions for the ηc(1S) decays to φφ and p p as B(ηc(1S)→ φφ)/B(ηc(1S)→ p p) = 1.79 ± 0.14 ± 0.32. SI

A study of B0s → η c ϕ and B0s → η c π + π − decays is performed using pp collision data corresponding to an integrated luminosity of 3.0 fb−1, collected with the LHCb detector in Run 1 of the LHC. The observation of the decay B 0s → η c ϕ is reported, where the η c meson is reconstructed in the pp¯, K + K − π + π −, π + π − π + π − and K + K − K + K − decay modes and the ϕ(1020) in the K + K − decay mode. The decay B 0s → J/ψϕ is used as a normalisation channel. Evidence is also reported for the decay B 0s → η c π + π −, where the η c meson is reconstructed in the pp¯decay mode, using the decay B 0s → J/ψπ + π − as a normalisation channel. The measured branching fractions are B(B0s→ηcϕ)=(5.01±0.53±0.27±0.63)×10−4,B(B0s→ηcπ+π−)=(1.76±0.59±0.12±0.29)×10−4, where in each case the first uncertainty is statistical, the second systematic and the third uncertainty is due to the limited knowledge of the external branching fractions. SI

The polarization of the Y(1S), Y(2S) and Y(3S) mesons, produced in pp collisions at centre-of-mass energies √s=7 and 8 TeV, is measured using data samples collected by the LHCb experiment, corresponding to integrated luminosities of 1 and 2 fb−1, respectively. The measurements are performed in three polarization frames, using Y → μμ decays in the kinematic region of the transverse momentum p ΥT

The B0, B0s, B+ and Λ0b hadron production asymmetries are measured using a data sample corresponding to an integrated luminosity of 3.0fb−1, collected by the LHCb experiment in proton–proton collisions at centre-of-mass energies of 7 and 8TeV. The measurements are performed as a function of transverse momentum and rapidity of the bhadrons within the LHCb detector acceptance. The overall production asymmetries, integrated over transverse momentum and rapidity, are also determined. SI

A search for CPviolation in D±→η π±and D±s→η π±decays is performed using proton–proton collision data, corresponding to an integrated luminosity of 3fb−1, recorded by the LHCb experiment at centre-of-mass energies of 7and8TeV. The measured CP-violating charge asymmetries are ACP(D±→η π±) =(−0.61 ±0.72 ±0.53 ±0.12)%and ACP(D±s→η π±) =(−0.82 ±0.36 ±0.22 ±0.27)%, where the first uncertainties are statistical, the second systematic, and the third are the uncertainties on the ACP(D±→K0Sπ±)and ACP(D±s→φπ±)measurements used for calibration. The results represent the most precise measurements of these asymmetries to date. SI

A measurement of the production cross-section of Z bosons in pp collisions at √s=13 TeV is presented using dimuon and dielectron final states in LHCb data. The cross-section is measured for leptons with pseudorapidities in the range 2.0 < η < 4.5, transverse momenta p T > 20 GeV and dilepton invariant mass in the range 60 < m(ℓℓ) < 120 GeV. The integrated cross-section from averaging the two final states is σ ℓℓZ = 194.3 ± 0.9 ± 3.3 ± 7.6 pb, where the first uncertainty is statistical, the second is due to systematic effects, and the third is due to the luminosity determination. In addition, differential cross-sections are measured as functions of the Z boson rapidity, transverse momentum and the angular variable ϕ *η. SI

The decays Λ0b → ψ(2S)pK− and Λ0b → J/ψπ + π −pK− are observed in a data sample corresponding to an integrated luminosity of 3 fb−1, collected in proton-proton collisions at 7 and 8 TeV centre-of-mass energies by the LHCb detector. The ψ(2S) mesons are reconstructed through the decay modes ψ(2S) → μ+μ− and ψ(2S) → J/ψπ + π −. The branching fractions relative to that of Λ0b → J/ψpK− are measured to be B(Λ0b→ψ(2S)pK−)B(Λ0b→J/ψpK−)=(20.70±0.76±0.46±0.37)×10−2, where the first uncertainties are statistical, the second are systematic and the third is related to the knowledge of J/ψ and ψ(2S) branching fractions. The mass of the Λ 0b baryon is measured to be M(Λ0b)=5619.65±0.17±0.17 MeV/c2, where the uncertainties are statistical and systematic. SI

The Λ0b→Λφdecay is observed using data corresponding to an integrated luminosity of 3.0fb−1recorded by the LHCbexperiment. The decay proceeds at leading order via a b →sssloop transition and is therefore sensitive to the possible presence of particles beyond the Standard Model. A first observation is reported with a significance of 5.9standard deviations. The value of the branching fraction is measured to be (5.18 ±1.04 ±0.35+0.67−0.62) ×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. SI

Measurements of the differential branching fraction and angular moments of the decay B 0 → K +π− μ + μ − in the K +π− invariant mass range 1330 < m(K +π−) < 1530 MeV/c 2 are presented. Proton-proton collision data are used, corresponding to an integrated luminosity of 3 fb−1 collected by the LHCb experiment. Differential branching fraction measurements are reported in five bins of the invariant mass squared of the dimuon system, q 2, between 0.1 and 8.0 GeV2 /c 4. For the first time, an angular analysis sensitive to the S-, P- and D-wave contributions of this rare decay is performed. The set of 40 normalised angular moments describing the decay is presented for the q 2 range 1.1-6.0 GeV2 /c 4. SI

A measurement of the cross-section for W → eν production in pp collisions is presented using data corresponding to an integrated luminosity of 2 fb−1 collected by the LHCb experiment at a centre-of-mass energy of √s=8 TeV. The electrons are required to have more than 20 GeV of transverse momentum and to lie between 2.00 and 4.25 in pseudorapidity. The inclusive W production cross-sections, where the W decays to eν, are measured to be σW+→e+νe=1124.4±2.1±21.5±11.2±13.0pb, σW−→e−ν¯e=809.0±1.9±18.1±7.0±9.4pb, where the first uncertainties are statistical, the second are systematic, the third are due to the knowledge of the LHC beam energy and the fourth are due to the luminosity determination. Differential cross-sections as a function of the electron pseudorapidity are measured. The W + /W − cross-section ratio and production charge asymmetry are also reported. Results are compared with theoretical predictions at next-to-next-to-leading order in perturbative quantum chromodynamics. Finally, in a precise test of lepton universality, the ratio of W boson branching fractions is determined to be B(W→eν)/B(W→μν)=1.020±0.002±0.019, where the first uncertainty is statistical and the second is systematic. SI

A measurement of the differential branching fraction of the decay B 0 → K ∗(892)0 μ + μ − is presented together with a determination of the S-wave fraction of the K + π − system in the decay B 0 → K +π− μ + μ −. The analysis is based on pp-collision data corresponding to an integrated luminosity of 3 fb−1 collected with the LHCb experiment. The measurements are made in bins of the invariant mass squared of the dimuon system, q 2. Precise theoretical predictions for the differential branching fraction of B 0 → K ∗(892)0 μ + μ − decays are available for the q 2 region 1.1 < q 2 < 6.0 GeV2 /c 4. In this q 2 region, for the K +π− invariant mass range 796 < m Kπ < 996 MeV/c 2, the S-wave fraction of the K +π− system in B 0 → K +π− μ + μ − decays is found to be FS=0.101±0.017(stat)±0.009(syst), and the differential branching fraction of B 0 → K ∗(892)0 μ + μ − decays is determined to be dB/dq2=(0.392+0.020−0.019(stat)±0.010(syst)±0.027(norm))×10−7c4/GeV2. The differential branching fraction measurements presented are the most precise to date and are found to be in agreement with Standard Model predictions. SI

A time-dependent angular analysis of B0s→ψ(2S)φdecays is performed using data recorded by the LHCb experiment. The data set corresponds to an integrated luminosity of 3.0 fb−1collected during Run1 of the LHC. The CP-violating phase and decay-width difference of the B0ssystem are measured to be φs=0.23+0.29−0.28±0.02 radand s=0.066+0.041−0.044±0.007 ps−1, respectively, where the first uncertainty is statistical and the second systematic. This is the first time that φsand shave been measured in a decay containing the ψ(2S)resonance. SI

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. SI

The first measurement of CP asymmetries in the decay B0s→J/ψK∗(892)0 and an updated measurement of its branching fraction and polarisation fractions are presented. The results are obtained using data corresponding to an integrated luminosity of 3.0 fb−1 of proton-proton collisions recorded with the LHCb detector at centre-of-mass energies of 7 and 8 TeV. Together with constraints from B 0 → J/ψ ρ 0, the results are used to constrain additional contributions due to penguin diagrams in the CP -violating phase ϕ s , measured through B 0s decays to charmonium. SI

The production of Υ mesons in pp collisions at √s=7 and 8 TeV is studied with the LHCb detector using data samples corresponding to an integrated luminosity of 1 fb−1 and 2 fb−1 respectively. The production cross-sections and ratios of cross-sections are measured as functions of the meson transverse momentum p and rapidity y, for p < 30 GeV/c and 2.0 < y < 4.5 SI

The Standard Model of Particle Physics (henceforth, SM) is a very successful theory. Nevertheless, it fails to explain some important questions present in nature. Therefore, a new model beyond the SM is needed. This is often known as New Physics, NP. Some of these alternatives are Supersymmetry (SUSY) and models with extra dimensions. This thesis represents a comprehensive study of NP searches and its implications, performed in a threefold manner. Firstly, a study of NP implications in kaon physics is presented, through the $K_S^0 \rightarrow \pi^0 \mu^+ \mu^-$ sensitivity study at LHCb and phenomenological studies on $K_S^0 \rightarrow \mu^+ \mu^-$. The former serves as important input for the SM prediction of the $K_L^0 \rightarrow \pi^0 \mu^+ \mu^-$ branching fraction, that is a relevant channel in models with extra dimensions. Concerning the latter, it plays a key role in scenarios where flavour violation does not originate from the CKM matrix, \textit{non-minimal Flavour Violating} (non-MFV), where it carries information complementary to $B_{s(d)}^0 \rightarrow \mu^+ \mu^-$. A phenomenological study is shown under the simplest supersymmetrical extension of the SM, the \textit{Minimal Supersymmetric Standard Model} (MSSM). Secondly, a review of other $\Delta F = 2$ transitions is carried out. This is done via the measurement of the weak mixing phase $\phi_s$ in $B_s^0 \rightarrow J/\psi \phi$ at LHCb, accompanied by a phenomenological study of this and more physics observables. The weak mixing phase $\phi_s$ measures the $CP$ violation that happens in the interference between mixing and decay. The SM prediction for the $B_s^0 \rightarrow J/\psi \phi$ channel is very precise, making it a golden mode where to measure this observable. As for the phenomenological study, several physics observables, such as $\phi_s$ and the Electric Dipole Moments (EDMs), are computed under the MSSM paradigm. To finalize, a more general study is done within the \textit{minimal Anomaly Mediated Supersymmetry Breaking}, mAMSB, and the \textit{Constrained Minimal Supersymmetric Standard Model}, CMSSM. In these scenarios, the prospects for the detection of dark matter, as well as for the prediction of the $(g-2)_{\mu}$ anomaly and flavour observables are presented, together with the available parameter space in light of the present experimental constraints.