Precise measurements of CP-violating and mixing parameters of B0s and B0d systems with the LHCb detector.

In this dissertation, the analysis strategies and the preliminary results on the ongoing studies of CP-violating and mixing parameters of the B0s and B0d systems are presented. The analysed dataset corresponds to the proton-proton interactions collected by the LHCb experiment from 2015 to 2018 at a centre-of-mass energy of √s = 13 TeV/c^2, during the so-called Run 2, corresponding to an integrated luminosity of 5.9 fb−1. In particular, the determination of the CP-violating phase φs, of the decay-width difference ∆Γs and of the decay widths Γs and ΓH in the decay channels B0s → J/ψφ and B0s → J/ψπ+π− are discussed, together with the measurement of the mixing parameter ∆Γd, exploiting the decay channels B0d → J/ψK∗(892)0 and B0d → J/ψK0s. These measurements represent an important test of the Standard Model (SM), the current most complete theoretical description of elementary particles and their interactions, and a sensitive probe of physics phenomena beyond the SM. The analysis presented in this thesis, using the B0s → J/ψφ decay, allows to obtain a statistical precision on the φs measurement of 0.022 rad and thus it will be the world’s most precise single measurement of this parameter, with the possibility to observe CP violation in the interference between a decay with and without mixing. The preliminary measurement of ∆Γd using data collected in 2016 shows a statistical precision of 0.008 ps−1 and an achievable precision, with the full Run 2 dataset, of 0.004 ps−1. This final measurement will supersede the precision reached in the previous LHCb analysis and of the current world average value, ∆Γd = 0.0007 ± 0.0066 ps−1, representing the major contribution to the future world average combination

Measurement of the Λb0→J/ψΛ\Lambda^0_b\rightarrow J/\psi\Lambda angular distribution and the Λb0\Lambda^0_b polarisation in pppp collisions

International audienceThis paper presents an analysis of the ${\Lambda}_b^0$→ J/ψΛ angular distribution and the transverse production polarisation of ${\Lambda}_b^0$ baryons in proton-proton collisions at centre-of-mass energies of 7, 8 and 13 TeV. The measurements are performed using data corresponding to an integrated luminosity of 4.9 fb$^{−1}$, collected with the LHCb experiment. The polarisation is determined in a fiducial region of ${\Lambda}_b^0$ transverse momentum and pseudorapidity of 1 < p$_{T}$< 20 GeV/c and 2 < η < 5, respectively. The data are consistent with ${\Lambda}_b^0$ baryons being produced unpolarised in this region. The parity-violating asymmetry parameter of the Λ → pπ$^{−}$ decay is also determined from the data and its value is found to be consistent with a recent measurement by the BES III collaboration.[graphic not available: see fulltext

Measurement of the electron reconstruction efficiency at LHCb

International audienceThe single electron track-reconstruction efficiency is determined using a sample corresponding to $1.3~\mathrm{fb}^{-1}$of $pp$ collision data recorded with the LHCb detector in 2017. This measurement exploits $B^+\to J/\psi (e^+e^-)K^+$ decays, where one of the electrons is fully reconstructed and paired with the kaon, while the other electron is reconstructed using only the information of the vertex detector. Despite this partial reconstruction, kinematic and geometric constraints allow the $B^+$-meson mass to be reconstructed and the signal to be well separated from backgrounds. This in turn allows the electron reconstruction efficiency to be measured by matching the partial track segment found in the vertex detector to tracks found by LHCb's regular reconstruction algorithms. The agreement between data and simulation is evaluated, and corrections are derived for simulated electrons in bins of kinematics. The presented method allows LHCb to measure branching fractions involving single electrons with a an electron reconstruction systematic uncertainty below $1\%$

Measurement of fs/fuf_s / f_u Variation with Proton-Proton Collision Energy and BB-Meson Kinematics

International audienceThe ratio of the Bs0 and B+ fragmentation fractions fs and fu is studied with Bs0→J/ψϕ and B+→J/ψK+ decays using data collected by the LHCb experiment in proton-proton collisions at 7, 8, and 13 TeV center-of-mass energies. The analysis is performed in bins of B-meson momentum, longitudinal momentum, transverse momentum, pseudorapidity, and rapidity. The fragmentation-fraction ratio fs/fu is observed to depend on the B-meson transverse momentum with a significance of 6.0σ. This dependency is driven by the 13 TeV sample (8.7σ), while the results for the other collision energies are not significant when considered separately. Furthermore, the results show a 4.8σ evidence for an increase of fs/fu as a function of collision energy

Measurement of the branching fraction of the decay Bs0→KS0KS0B_s^0\to K_S^0 K_S^0

International audienceA measurement of the branching fraction of the decay Bs0→KS0KS0 is performed using proton–proton collision data corresponding to an integrated luminosity of 5  fb-1 collected by the LHCb experiment between 2011 and 2016. The branching fraction is determined to be B(Bs0→KS0KS0)=[8.3±1.6(stat)±0.9(syst)±0.8(norm)±0.3(fs/fd)]×10-6, where the first uncertainty is statistical, the second is systematic, and the third and fourth are due to uncertainties on the branching fraction of the normalization mode B0→ϕKS0 and the ratio of hadronization fractions fs/fd. This is the most precise measurement of this branching fraction to date. Furthermore, a measurement of the branching fraction of the decay B0→KS0KS0 is performed relative to that of the Bs0→KS0KS0 channel, and is found to be B(B0→KS0KS0)B(Bs0→KS0KS0)=[7.5±3.1(stat)±0.5(syst)±0.3(fs/fd)]×10-2

Test of lepton universality with Λb0→pK−ℓ+ℓ− {\Lambda}_b^0\to {pK}^{-}{\mathrm{\ell}}^{+}{\mathrm{\ell}}^{-} decays

International audienceThe ratio of branching fractions of the decays ${\Lambda}_b^0$ → pK$^{−}$e$^{+}$e$^{−}$ and ${\Lambda}_b^0$ → pK$^{−}$μ$^{+}$μ$^{−}$,${R}_{pK}^{-1}$, is measured for the first time using proton-proton collision data corresponding to an integrated luminosity of 4.7 fb$^{−1}$ recorded with the LHCb experiment at center-of-mass energies of 7, 8 and 13 TeV. In the dilepton mass-squared range 0.1 < q$^{2}$< 6.0 GeV$^{2}$/c$^{4}$ and the pK$^{−}$ mass range m(pK$^{−}$) < 2600 MeV/c$^{2}$, the ratio of branching fractions is measured to be ${R}_{pK}^{-1}={1.17}_{-0.16}^{+0.18}\pm 0.07$, where the first uncertainty is statistical and the second systematic. This is the first test of lepton universality with b baryons and the first observation of the decay ${\Lambda}_b^0$ → pK$^{−}$e$^{+}$e$^{−}$.[graphic not available: see fulltext

Precision measurement of the Ξcc++\Xi_{cc}^{++} mass

International audienceA measurement of the ${\varXi}_{cc}^{++}$ mass is performed using data collected by the LHCb experiment between 2016 and 2018 in pp collisions at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.6 fb$^{−1}$. The ${\varXi}_{cc}^{++}$ candidates are reconstructed via the decay modes ${\varXi}_{cc}^{++}\to {\varLambda}_c^{+}{K}^{-}{\pi}^{+}{\pi}^{+}$ and ${\varXi}_{cc}^{++}\to {\varXi}_c^{+}{\pi}^{+}$. The result, 3621.55 ± 0.23 (stat) ± 0.30 (syst) MeV/c$^{2}$, is the most precise measurement of the ${\varXi}_{cc}^{++}$ mass to date.[graphic not available: see fulltext

Observation of new resonances in the Λb0π+π−\Lambda_{b}^0\pi^+\pi^- system

International audienceWe report the observation of a new structure in the Λb0π+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 be mΛb(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 Λb0 baryon. The measured masses and widths of these new excited states suggest their possible interpretation as a doublet of Λb(1D)0 states