Study of the B−→Λc+Λˉc−K−B^{-} \to \Lambda_{c}^{+} \bar{\Lambda}_{c}^{-} K^{-} decay

The decay $B^{-} \to \Lambda_{c}^{+} \bar{\Lambda}_{c}^{-} K^{-}$ is studied in proton-proton collisions at a center-of-mass energy of $\sqrt{s}=13$ TeV using data corresponding to an integrated luminosity of 5 $\mathrm{fb}^{-1}$ collected by the LHCb experiment. In the $\Lambda_{c}^+ K^{-}$ system, the $\Xi_{c}(2930)^{0}$ state observed at the BaBar and Belle experiments is resolved into two narrower states, $\Xi_{c}(2923)^{0}$ and $\Xi_{c}(2939)^{0}$, whose masses and widths are measured to be $$m(\Xi_{c}(2923)^{0}) = 2924.5 \pm 0.4 \pm 1.1 \,\mathrm{MeV}, \\ m(\Xi_{c}(2939)^{0}) = 2938.5 \pm 0.9 \pm 2.3 \,\mathrm{MeV}, \\ \Gamma(\Xi_{c}(2923)^{0}) = \phantom{000}4.8 \pm 0.9 \pm 1.5 \,\mathrm{MeV},\\ \Gamma(\Xi_{c}(2939)^{0}) = \phantom{00}11.0 \pm 1.9 \pm 7.5 \,\mathrm{MeV},$$ where the first uncertainties are statistical and the second systematic. The results are consistent with a previous LHCb measurement using a prompt $\Lambda_{c}^{+} K^{-}$ sample. Evidence of a new $\Xi_{c}(2880)^{0}$ state is found with a local significance of $3.8\,\sigma$, whose mass and width are measured to be $2881.8 \pm 3.1 \pm 8.5\,\mathrm{MeV}$ and $12.4 \pm 5.3 \pm 5.8 \,\mathrm{MeV}$, respectively. In addition, evidence of a new decay mode $\Xi_{c}(2790)^{0} \to \Lambda_{c}^{+} K^{-}$ is found with a significance of $3.7\,\sigma$. The relative branching fraction of $B^{-} \to \Lambda_{c}^{+} \bar{\Lambda}_{c}^{-} K^{-}$ with respect to the $B^{-} \to D^{+} D^{-} K^{-}$ decay is measured to be $2.36 \pm 0.11 \pm 0.22 \pm 0.25$, where the first uncertainty is statistical, the second systematic and the third originates from the branching fractions of charm hadron decays.Comment: All figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2022-028.html (LHCb public pages

Measurement of the ratios of branching fractions R(D∗)\mathcal{R}(D^{*}) and R(D0)\mathcal{R}(D^{0})

The ratios of branching fractions $\mathcal{R}(D^{*})\equiv\mathcal{B}(\bar{B}\to D^{*}\tau^{-}\bar{\nu}_{\tau})/\mathcal{B}(\bar{B}\to D^{*}\mu^{-}\bar{\nu}_{\mu})$ and $\mathcal{R}(D^{0})\equiv\mathcal{B}(B^{-}\to D^{0}\tau^{-}\bar{\nu}_{\tau})/\mathcal{B}(B^{-}\to D^{0}\mu^{-}\bar{\nu}_{\mu})$ are measured, assuming isospin symmetry, using a sample of proton-proton collision data corresponding to 3.0 fb${ }^{-1}$ of integrated luminosity recorded by the LHCb experiment during 2011 and 2012. The tau lepton is identified in the decay mode $\tau^{-}\to\mu^{-}\nu_{\tau}\bar{\nu}_{\mu}$. The measured values are $\mathcal{R}(D^{*})=0.281\pm0.018\pm0.024$ and $\mathcal{R}(D^{0})=0.441\pm0.060\pm0.066$, where the first uncertainty is statistical and the second is systematic. The correlation between these measurements is $\rho=-0.43$. Results are consistent with the current average of these quantities and are at a combined 1.9 standard deviations from the predictions based on lepton flavor universality in the Standard Model.Comment: All figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2022-039.html (LHCb public pages

Multidifferential study of identified charged hadron distributions in ZZ-tagged jets in proton-proton collisions at s=\sqrt{s}=13 TeV

Jet fragmentation functions are measured for the first time in proton-proton collisions for charged pions, kaons, and protons within jets recoiling against a $Z$ boson. The charged-hadron distributions are studied longitudinally and transversely to the jet direction for jets with transverse momentum 20 $< p_{\textrm{T}} < 100$ GeV and in the pseudorapidity range $2.5 < \eta < 4$. The data sample was collected with the LHCb experiment at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 1.64 fb$^{-1}$. Triple differential distributions as a function of the hadron longitudinal momentum fraction, hadron transverse momentum, and jet transverse momentum are also measured for the first time. This helps constrain transverse-momentum-dependent fragmentation functions. Differences in the shapes and magnitudes of the measured distributions for the different hadron species provide insights into the hadronization process for jets predominantly initiated by light quarks.Comment: All figures and tables, along with machine-readable versions and any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2022-013.html (LHCb public pages

Mesures précises des propriétés des baryons charmés avec le détecteur LHCb au LHC

Charmed baryon polarization is not predicted by theory and it is a necessary input for the measurement of the charmed baryons magnetic dipole moment (MDM) which is foreseen at the LHC. Baryon’s polarization has been measured for strange (Lambda) and beauty (Lambda_b) baryons in different colliding systems, however, no measurement exists for charmed baryons as of today. In this thesis, the Lambda_c polarization is measured by mean of a five-dimensional amplitude analysis of the three-body weak decay Lc->pKpi of Lc produced in pp collisions at a center of mass energy of 13 TeV.The Lc->pKpi decay passes through intermediate resonant states which interfere with each other, and which need to be included in the amplitude. First, the equations describing the amplitude of this three-body decay have been derived within the helicity formalism. The polarization is accounted for by mean of the spin density matrix and the intermediate resonant states are described using the isobar model factorization. This work allowed to better understand the helicity amplitudes and can be easily extended to other three-body baryonic decays featuring particles with spin in the final state.Then, the helicity amplitudes obtained are used to describe the data collected by the LHCb detector at CERN during the 2016 data taking period (Run 2), corresponding to an integrated luminosity of 1.7 fb^-1. Since the polarization depends on the production mechanism involved, it is important to separate the Lc produced directly after the pp collisions via strong interactions (prompt production), from the Lc produced via a weak decay of other baryons (secondary production). In this analysis, the promptly produced Lc are studied.The Lc->pKpi decay, with a branching ratio of 6.28 +- 0.32 %, is the most abundant Lc decay mode and the final data sample, after the optimization of the selection chain, contains around ~500 000 signal events, it has a signal purity of ~97% and the contamination due to secondary Lc is less than 2 %. The asymmetry parameters of the intermediate decays, which are combinations of the helicity couplings contained in the amplitude, are also measured along with the fit fractions, which describe the contribution of each resonance to the total amplitude. The results of the Lc->pKpi amplitude analysis will be used to measure the polarization of Lc baryons produced in proton gas (pNe) collisions, using a data sample collected by the LHCb detector during 2017 at a center of mass energy of 68 GeV.The next data acquisition phase, foreseen in 2022, will see an increase of the collision rate by a factor of 5 at LHCb. A new detector, called PLUME, has been designed to perform a luminosity measurement in the new running conditions. In this thesis, the front-end electronics of the LHCb calorimeter has been tested to prove that it is adapted for the PLUME detector needs and it is now the baseline electronics for PLUME. Finally, a measurement of the LHCb clock shift using the PLUME detector is proposed. The LHCb clock can be desynchronized from the LHC main clock; a shift up to 1 ns has been measured during Run 1 and Run 2, using the Outer Tracker (OT), with a time resolution of 0.5 ns. During Run 3 the OT will be removed and LHCb will collect data at 40 MHz with a new triggering scheme based on an entirely software trigger. Stable running conditions are essential for such a scheme to work, and the clock shift could have a large impact on the LHCb detector performances. PLUME could be used to monitor the LHCb clock shift and in this thesis a preliminary timing measurement is performed probing the feasibility of such a measurement and opening the route to further studies.La polarisation des baryons charmés n'est pas prédite par la théorie et constitue une donnée nécessaire pour la mesure du moment dipolaire magnétique (MDM) des baryons charmés qui est prévue au LHC. Elle a été mesurée pour les baryons étranges (Lambda) et beaux (Lambda_b) dans différents systèmes de collision, mais aucune mesure n'existe à ce jour pour les baryons charmés. Dans cette thèse, la polarisation du baryon Lambda_c est mesurée au moyen d'une analyse d'amplitude à cinq dimensions de la désintégration faible à trois corps Lc->pKpi pour des Lc produits dans des collisions pp à une énergie de 13 TeV dans le centre de masse.La désintégration Lc->pKpi passe par des états résonants intermédiaires qui interfèrent entre eux et qui doivent être inclus dans l'amplitude. Tout d'abord, les équations décrivant l'amplitude de cette désintégration à trois corps ont été dérivées dans le cadre du formalisme d’hélicité. La polarisation est prise en compte au moyen de la matrice de densité de spin et les états résonants intermédiaires sont décrits à l'aide du modèle isobare. Ce travail a permis de mieux comprendre les amplitudes d'hélicité et peut être facilement étendu à d'autres désintégrations baryoniques à trois corps comportant des particules avec spin dans l'état final.Ensuite, les amplitudes d'hélicité obtenues sont utilisées pour décrire les données collectées par le détecteur LHCb au CERN en 2016 (Run 2), correspondant à une luminosité intégrée de 1,7 fb^-1. Comme la polarisation dépend du mécanisme de production impliqué, il est important de séparer les Lc produits directement après les collisions pp via des interactions fortes (production dite « prompt »), des Lc produits via une désintégration faible d'autres baryons (production secondaire) ; dans cette analyse, les Lc « prompt » sont étudiés. La désintégration Lc->pKpi, avec un rapport d'embranchement de 6,28 +- 0,32 %, est le mode de désintégration de Lc le plus abondant et l'échantillon de données final, après l'optimisation de la chaîne de sélection, contient environ ~500 000 événements de signal ; la pureté du signal est de ~97 % et la contamination due aux Lc venant de B est inférieure à 2 %. Les paramètres d'asymétrie, qui sont des combinaisons des couplages d'hélicité contenus dans l'amplitude, sont également mesurés ainsi que les contributions individuelles des résonances à l'amplitude totale. Les résultats de l'analyse en amplitude de la désintégration Lc->pKpi seront utilisés pour mesurer la polarisation des Lc produits dans des collisions proton-gaz (pNe), en utilisant un échantillon de données collectées par le détecteur LHCb en 2017, à une énergie de 68 GeV dans le centre de masse.La prochaine phase d'acquisition de données, prévue en 2022, verra une augmentation du taux de collision d'un facteur 5 à LHCb. Un nouveau détecteur, appelé PLUME, a été conçu pour effectuer une mesure de luminosité dans les nouvelles conditions de fonctionnement. Dans cette thèse, l'électronique frontale du calorimètre de LHCb a été testée pour prouver qu'elle est adaptée aux besoins du détecteur PLUME et elle est maintenant l'électronique de base pour PLUME. Enfin, une mesure du décalage de l'horloge de LHCb à l'aide du détecteur PLUME est proposée. L'horloge de LHCb peut être désynchronisée de l'horloge principale du LHC ; un décalage allant jusqu'à 1 ns a été mesuré pendant les Run 1 et 2, en utilisant le Outer Tracker (OT), avec une résolution temporelle de 0,5 ns. Au cours du Run 3, l'OT sera supprimé et LHCb collectera des données à 40 MHz avec un nouveau schéma de déclenchement entièrement « software ». Des conditions de fonctionnement stables sont essentielles pour qu'un tel schéma fonctionne, et le décalage de l'horloge pourrait avoir un impact important sur les performances du détecteur LHCb. PLUME pourrait être utilisé pour surveiller le décalage de l'horloge du LHCb. Dans cette thèse, la faisabilité de la mesure du décalage en temps est étudiée

SMOG at LHCb: experimental results

We report the latest results obtained by the LHCb experiment in its fixed-target configuration using the System for Measuring Overlap with Gas (SMOG).These results use the $p\text{He}$(2016), $p\text{Ne}$(2017) and $\text{PbNe}$ (2018) samples collected at a center of mass energy of $\sqrt{s_\text{NN}}= 110$ GeV and $\sqrt{s_\text{NN}}= 68.5$ GeV (for $p\text{Ne}$ and $\text{PbNe}$) respectively. The implications of these experimental results on theory are discussed, with a focus on the impact on quantum chromodynamics (QCD) and on theoretical predictions on the antiprotons flux originating from cosmic rays spallation on the interstellar medium

Precise measurements of charmed baryon properties with the LHCb detector at the LHC

Charmed baryon polarization is not predicted by theory and it is a necessary input for the measurement of the charmed baryons magnetic dipole moment (MDM) which is foreseen at the LHC. Baryon's polarization has been measured for strange ($\Lambda$) and beauty ($\Lambda^0_b$) baryons in different colliding systems, however, no measurement exists for charmed baryons as of today. In this thesis, the $\Lambda_c^+$ polarization is measured by means of a five-dimensional amplitude analysis of the three-body weak decay $\Lambda_c^+\rightarrow pK^-\pi^+$ of $\Lambda_c^+$ produced in $pp$ collisions at a center of mass energy of 13 TeV. The $\Lambda_c^+\rightarrow pK^-\pi^+$ decay passes through intermediate resonant states which interfere with each other, and which need to be included in the amplitude. First, the equations describing the amplitude of this three-body decay have been derived within the helicity formalism. The polarization is accounted for by means of the spin density matrix and the intermediate resonant states are described using the isobar model factorization. This work allowed to better understand the helicity amplitudes and can be easily extended to other three-body baryonic decays featuring particles with spin in the final state. The helicity amplitudes obtained are used to describe the data collected by the LHCb detector at CERN during the 2016 data taking period (Run 2), corresponding to an integrated luminosity of $1.7fb^{-1}$. The analysis is performed on $\Lambda_c^+$ produced directly after the $pp$ collisions via strong interactions (prompt production). The next data acquisition phase, foreseen in 2022, will see an increase of the collision rate by a factor of 5 at LHCb. A new detector, PLUME, has been designed to perform a luminosity measurement in the new running conditions. In this thesis, the front-end electronics of the LHCb calorimeter has been tested to prove that it is adapted for the PLUME detector needs and it is now the baseline electronics for PLUME. Finally, a preliminary measurement of the LHCb clock shift using the PLUME detector is proposed