Pursuing forbidden beauty:Search for the lepton-flavour violating decays B0 → e± μ∓ and Bs0 → e± μ∓ and study of electron-reconstruction performance at LHCb

Physicists have developed a successful theory, called the Standard Model, describing all known fundamental particles and interactions, except gravity, up to high energies and small length scales. Despite its success, it is when extrapolating to astronomical length and time scales that the theory fails. To alleviate these issues, physicists search for more fundamental laws of physics by exploring higher energies. One such approach is the study of heavy and therefore energetic particles called beauty mesons. Since interactions of higher energies mediating decays of particles are relatively suppressed, decays of particles that are rare or forbidden according to the Standard Model are of particular interest, as such interactions can have a relatively large effect. At the particle accelerator LHC at CERN, beauty mesons are produced by the trillions a year. Hence, rare and forbidden beauty decays can be searched for and studied to great precision.This dissertation covers the search for the in the Standard Model forbidden decays of neutral beauty mesons to an electron and muon (a heavier sibling of the electron) with the data collected in 2011 and 2012 by the LHCb detector operating at the LHC. No such decays were found, but its increased precision of the upper limit of its probability is used to constrain new models. In addition, a study of the performance of the reconstruction of electrons at the LHCb experiment is presented, which will allow to determine and reduce systematic uncertainties in future analyses of beauty decays with electrons in the finalstate

Measurement of Ξcc++{{\varXi_{cc}^{++}}} production in pp collisions at s=13{\sqrt{ s}=13} TeV

The production of Ξcc ++ baryons in proton-proton collisions at a centre-of-mass energy of √s = 13TeV is measured in the transverse-momentum range 4 < PT < 15 GeV/c and the rapidity range 2.0 < y < 4.5. The data used in this measurement correspond to an integrated luminosity of 1.7 fb-1, recorded by the LHCb experiment during 2016. The ratio of the Ξcc ++ production cross-section times the branching fraction of the Ξcc ++ → Λc + K-π+ π+ decay relative to the prompt Λc + production cross-section is found to be (2.22 ± 0.27 ± 0.29) × 10-4, assuming the central value of the measured Ξcc ++ lifetime, where the first uncertainty is statistical and the second systematic

First observation of the decay Λ 0 b → η c ( 1 S ) p K −

The decay Λ0b→ηc(1S)pK− is observed for the first time using a data sample of proton-proton collisions, corresponding to an integrated luminosity of 5.5  fb−1, collected with the LHCb experiment at a center-of-mass energy of 13 TeV. The branching fraction of the decay is measured, using the Λ0b→J/ψpK− decay as a normalization mode, to be B(Λ0b→ηc(1S)pK−)=(1.06±0.16±0.06+0.22−0.19)×10−4, where the quoted uncertainties are statistical, systematic and due to external inputs, respectively. A study of the ηc(1S)p mass spectrum is performed to search for the Pc(4312)+ pentaquark state. No evidence is observed and an upper limit of B(Λ0b→Pc(4312)+K−)×B(Pc(4312)+→ηc(1S)p)B(Λ0b→ηc(1S)pK−)<0.24 is obtained at the 95% confidence level

Observation of New Resonances in the Λ 0 b π + π − System

We report the observation of a new structure in the Λ0bπ+π− spectrum using the full LHCb data set of pp collisions, corresponding to an integrated luminosity of 9  fb−1, collected at √s=7, 8, and 13 TeV. A study of the structure suggests its interpretation as a superposition of two almost degenerate narrow states. The masses and widths of these states are measured to bemΛb(6146)0=6146.17±0.33±0.22±0.16  MeV,mΛb(6152)0=6152.51±0.26±0.22±0.16  MeV,ΓΛb(6146)0=2.9±1.3±0.3  MeV,ΓΛb(6152)0=2.1±0.8±0.3  MeV,with a mass splitting of Δm=6.34±0.32±0.02  MeV, where the first uncertainty is statistical, the second systematic. The third uncertainty for the mass measurements derives from the knowledge of the mass of the Λ0b baryon. The measured masses and widths of these new excited states suggest their possible interpretation as a doublet of Λb(1D)0 states

Updated measurement of decay-time-dependent CP asymmetries in D0 → K+K− and D0 → π + π − decays

A search for decay-time-dependent charge-parity (CP) asymmetry in D0→K+K− and D0→π+π− decays is performed at the LHCb experiment using proton-proton collision data recorded at a center-of-mass energy of 13 TeV, and corresponding to an integrated luminosity of 5.4  fb−1. The D0 mesons are required to originate from semileptonic decays of b hadrons, such that the charge of the muon identifies the flavor of the neutral D meson at production. The asymmetries in the effective decay widths of D0 and ¯D0 mesons are determined to be AΓ(K+K−)=(−4.3±3.6±0.5)×10−4 and AΓ(π+π−)=(2.2±7.0±0.8)×10−4, where the uncertainties are statistical and systematic, respectively. The results are consistent with CP symmetry and, when combined with previous LHCb results, yield AΓ(K+K−)=(−4.4±2.3±0.6)×10−4 and AΓ(π+π−)=(2.5±4.3±0.7)×10−4

Measurement of the B − c meson production fraction and asymmetry in 7 and 13 TeV p p collisions

The production fraction of the B−c meson with respect to the sum of B− and ¯B0 mesons is measured in both 7 and 13 TeV center-of-mass (c.m.) energy pp collisions produced by the Large Hadron Collider (LHC), using the LHCb detector. The rate, approximately 3.7 per mille, does not change with energy, but shows a transverse momentum dependence. The B−c−B+c production asymmetry is also measured and is consistent with zero within the determined statistical and systematic uncertainties of a few percent

First observation of the decay B 0 → D 0 ¯ D 0 K + π −

The first observation of the decay B0→D0D¯0K+π- is reported using proton-proton collision data corresponding to an integrated luminosity of 4.7 fb-1 collected by the LHCb experiment in 2011, 2012 and 2016. The measurement is performed in the full kinematically allowed range of the decay outside of the D∗- region. The ratio of the branching fraction relative to that of the control channel B0→D∗-D0K+ is measured to be R=(14.2±1.1±1.0)%, where the first uncertainty is statistical and the second is systematic. The absolute branching fraction of B0→D0D¯0K+π- decays is thus determined to be B(B0→D0D¯0K+π-)=(3.50±0.27±0.26±0.30)×10-4, where the third uncertainty is due to the branching fraction of the control channel. This decay mode is expected to provide insights to spectroscopy and the charm-loop contributions in rare semileptonic decays