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

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

Recherche de désintégration rares à quatre corps de mesons charmés avec électrons dans l'état final et reconstruction de longues traces pour le système de déclenchement de LHCb

The rare four-body rare charm decays D0->pipiee and D0->KKee receive contributions from flavour-changing neutral current c->all transitions. Measurements of such decays are unique probes of beyond-the-standard-model effects as new heavy particles can contribute in electroweak quantum loops. The search for rare charm D0->pipiee and D0->KKee decays is presented using a sample of proton-proton collisions data, corresponding to an integrated luminosity of 6 fb^-1 and recorded with a center-of-mass energy of 13 TeV at the LHCb experiment. The presented analysis is statistically limited and will greatly benefit from the upgrade of the LHCb detector which will record data with a five times greater instantaneous luminosity. The trigger system was completely redesigned by removing the first-level hardware trigger stage and selecting events with a fully software-based trigger directly at the 30 MHz collision rate of the LHC. This demanding task is performed by the first high-level trigger optimised for Graphic Processing Units (GPUs). In order to select interesting events, the trigger must reconstruct the long tracks traversing the whole LHCb detector and leaving hits in the three tracking detectors: the vertex locator, the upstream tracker and the scintillating fiber tracker. However, during the 2022 data taking, the upstream tracker was not installed, leading to an immediate necessity of modification of the long track reconstruction strategy. A new algorithm presented in this thesis allows the reconstruction of long tracks without using the upstream tracker information.Les désintégrations rares à quatre corps de mésons charmés D0->pipiee et D0->KKee reçoivent des contributions par les transitions à courant neutre avec changement de saveur de type c->ull. Les mesures de ces désintégrations sont fondamentales pour la recherche de physique au-delà du modèle standard parce que des particules plus massives peuvent contribuer dans les electroweak loops quantiques. La recherche de désintégrations charmées D0->pipiee et D0->KKee est présentée sur ce manuscrit en utilisant des données de collisions proton-proton, qui correspondent à une luminosité intégrée de 6 fb^-1 obtenues avec une énergie au centre de masse de 13 TeV avec l'expérience LHCb. L'analyse est toutefois limitée par la statistique et peut bénéficier de l'upgrade du détecteur LHCb qui va prendre des données avec une luminosité instantanée cinq fois plus grande. Le système de déclenchement a été complètement changé en enlevant le premier niveau du système qui était développé sur hardware. Le nouveau système de déclenchement doit donc sélectionner les événements avec un système fully-software, directement sur le taux de collisions de 30 MHz du LHC. Ceci a été développé et optimisé sur des Graphic Processing Units (GPUs). Pour sélectionner les événements intéressant et réduire le taux en entrée, le système de déclenchement doit reconstruire les traces dites longues, qui traversent tout le détecteur LHCb et laissant du signal sur les trois trajectographes de LHCb: le vertex locator, le upstream tracker et le scintillating fiber tracker. Cependant, pendant la prise de données de 2022, le upstream tracker n'a pas été installé, ce qui veut dire que la stratégie de reconstruction des traces longues devait être modifié. Un nouveau algorithme de reconstruction présenté sur ce manuscrit permet la reconstruction des traces longues sans utiliser l'information de l'upstream tracker

Tracking on GPU at LHCb’s fully software trigger

The LHCb experiment will use a fully software-based trigger to collect data from 2022 on, at an event rate of 30 MHz. During the first stage of the High-Level Trigger (HLT1), a partial track reconstruction, using efficient parallelisation techniques on GPU cards, is performed. This stage will reduce the event rate by around a factor 30. Reconstructing tracks at 30 MHz represents a challenge which needs to be faced with very performant tracking algorithms. These proceedings focus on reconstruction and performance aspects of HLT1, giving particular attention to the algorithms reconstructing particles traversing the whole LHCb detector

Search for new physics with rare charm decays at LHCb

International audienceThe LHCb experiment is playing a crucial role in the study of rare and forbidden decays of charm hadrons, which are unique probes for hints of physics beyond the Standard Model. These proceedings present results of the search for semileptonic three-body decays of the form $D_{(s)}^+$ \rightarrow&nbsp;$h^\pm$ \ellp $\ell^{(')\mp}$, where $h^\pm$ is a charged pion or kaon and $\ell^{(')\mp}$ is an electron or a muon. Furthermore, studies of the four-body neutral meson charm decays D^0 \rightarrow \pi^+ \pi^- \mu^+ \mu^- and D^0 \rightarrow K^+ K^- \mu^+ \mu^- are discussed. Prospects for leptonic and semileptonic rare charm decays at LHCb in the future are presented in the end

Test of Lepton Flavour Universality using Bs semileptonic decays

The Large Hadron Collider beauty experiment (LHCb) focuses its studies on measurements of the decay properties of heavy-flavour hadrons, i.e. mesons or baryons containing charm ($c$) or beauty ($b$) quarks. LHCb is a single-arm spectrometer situated on the LHC ring with a forward angular acceptance, built this way to detect the majority of the $b\bar{b}$ quarks pair produced in the $pp$ collisions. One interesting topic LHCb has been studying is related to Lepton Flavour Universality of the Standard Model (LFU) testing. In case this symmetry is violated experimentally, this would be a clear sign of New Physics with contributions that favor couplings with one lepton family rather than the others. LFU tests can be performed in $b$-hadron decays by comparing branching fractions to final states with different lepton species and also by checking the angular distributions of the decays of interest. Experimental results on LFU tests performed by Belle, BaBar and LHCb show some tensions with respect to the SM. In particular, measurements of branching fraction ratios of decays involving $b \to c \ell^- \bar{\nu}_\ell$ transitions, as $R(D)$ and $R(D^*)$ show a limited compatibility with the SM predictions of about $3.08 \sigma$. Similarly, measurements of branching fraction ratios of decays involving $b \to s \ell^- \ell^+$ rare decays, show a compatibility with the SM predictions of about $2.5 \sigma$. To contribute in clarifying this interesting picture, LHCb is planning and currently performing measurements to test LFU using different $b$-hadrons decays produced at LHC, including different decay modes. This thesis focuses on the $R(D_s)$ measurement, defined as: $$R(D_s) = \frac{{\cal B}(B_s^0 \rightarrow D_s^- \tau^+ \nu_{\tau})}{{\cal B}(B_s^0 \rightarrow D_s^- \mu^+ \nu_{\mu})}\, .$$ The $B_s^0 \rightarrow D_s^- \tau^+ \nu_{\tau}$ decay is reconstructed through the $\tau$ three-prong hadronic decay, $\tau^+ \rightarrow \pi^+ \pi^- \pi^+ (\pi^0) \nu_{\tau}$, while the $D_s^-$ meson is reconstructed through the $D_s^- \rightarrow K^+ K^- \pi^-$ decay chain, so the visible final state consists of six charged tracks. In order to achieve the most precise experimental result, the measurement is performed by introducing a normalization channel similar to the signal and redefining the ratio as: $$R(D_s) = \frac{{\cal B}(B_s^0 \rightarrow D_s^- \tau^+ \nu_{\tau})}{{\cal B}({\rm norm})} \times \frac{{\cal B}({\rm norm})}{{\cal B}(B_s^0 \rightarrow D_s^- \mu^+ \nu_{\mu})} = {\cal K} \times \alpha \, .$$ Here the ratio $\alpha$ is completely determined by external inputs, i.e. measurements of branching fractions by independent analyses, while the ratio $\cal K$ is to be determined from the analysis of the signal and normalization channels as: $${\cal K} = \frac{N_{\rm sig}}{\epsilon_{\rm sig}} \frac{\epsilon_{\rm norm}}{N_{\rm norm}}\frac{1}{{\cal B}(\tau^+ \rightarrow \pi^+ \pi^- \pi^+ (\pi^0) \nu_{\tau}) \times {\cal B}(D_s^- \rightarrow K^+ K^- \pi^-)} \, .$$ Here, $N_{\rm sig}$ ($N_{\rm norm}$) and $\epsilon_{\rm sig}$ ($\epsilon_{\rm norm}$) represent, respectively, the signal (normalization) yield and selection efficiency. The main purpose of this thesis is to choose among possible normalization channels that are suitable for the $R(D_s)$ measurement. As a general requirement, the normalization and the signal channels should be reconstructed and selected in a similar way so that, when measuring the ratio ${\cal K}$, any systematic uncertainties related to the reconstruction and selection efficiency will be strongly reduced. The normalization channels studied in this thesis are $B_s^0 \rightarrow D_s^- \pi^+\pi^+\pi^-$ with $D_s^- \rightarrow K^+ K^- \pi^-$ and $B^0 \rightarrow D^- \pi^+\pi^+\pi^-$ with $D^- \rightarrow K^+ \pi^- \pi^-$. Chapter 1 is an introduction to the LHCb experiment and its apparatus. In Chapter 2, an overview of the LFU and of the experimental tests performed so far are presented, leading to the motivations for the $R(D_s)$ measurement, that is discussed in Chapter 3. Chapter 4 details several aspects of the analysis concerning the signal and normalization channels selection, the efficiency calculation and a quantitative evaluation of the main systematic uncertainties contributing to the $R(D_s)$ measurement. Final conclusions are drawn at the end

Recherche de désintégration rares à quatre corps de mesons charmés avec électrons dans l'état final et reconstruction de longues traces pour le système de déclenchement de LHCb

Les désintégrations rares à quatre corps de mésons charmés D0->pipiee et D0->KKee reçoivent des contributions par les transitions à courant neutre avec changement de saveur de type c->ull. Les mesures de ces désintégrations sont fondamentales pour la recherche de physique au-delà du modèle standard parce que des particules plus massives peuvent contribuer dans les electroweak loops quantiques. La recherche de désintégrations charmées D0->pipiee et D0->KKee est présentée sur ce manuscrit en utilisant des données de collisions proton-proton, qui correspondent à une luminosité intégrée de 6 fb^-1 obtenues avec une énergie au centre de masse de 13 TeV avec l'expérience LHCb. L'analyse est toutefois limitée par la statistique et peut bénéficier de l'upgrade du détecteur LHCb qui va prendre des données avec une luminosité instantanée cinq fois plus grande. Le système de déclenchement a été complètement changé en enlevant le premier niveau du système qui était développé sur hardware. Le nouveau système de déclenchement doit donc sélectionner les événements avec un système fully-software, directement sur le taux de collisions de 30 MHz du LHC. Ceci a été développé et optimisé sur des Graphic Processing Units (GPUs). Pour sélectionner les événements intéressant et réduire le taux en entrée, le système de déclenchement doit reconstruire les traces dites longues, qui traversent tout le détecteur LHCb et laissant du signal sur les trois trajectographes de LHCb: le vertex locator, le upstream tracker et le scintillating fiber tracker. Cependant, pendant la prise de données de 2022, le upstream tracker n'a pas été installé, ce qui veut dire que la stratégie de reconstruction des traces longues devait être modifié. Un nouveau algorithme de reconstruction présenté sur ce manuscrit permet la reconstruction des traces longues sans utiliser l'information de l'upstream tracker.The rare four-body rare charm decays D0->pipiee and D0->KKee receive contributions from flavour-changing neutral current c->all transitions. Measurements of such decays are unique probes of beyond-the-standard-model effects as new heavy particles can contribute in electroweak quantum loops. The search for rare charm D0->pipiee and D0->KKee decays is presented using a sample of proton-proton collisions data, corresponding to an integrated luminosity of 6 fb^-1 and recorded with a center-of-mass energy of 13 TeV at the LHCb experiment. The presented analysis is statistically limited and will greatly benefit from the upgrade of the LHCb detector which will record data with a five times greater instantaneous luminosity. The trigger system was completely redesigned by removing the first-level hardware trigger stage and selecting events with a fully software-based trigger directly at the 30 MHz collision rate of the LHC. This demanding task is performed by the first high-level trigger optimised for Graphic Processing Units (GPUs). In order to select interesting events, the trigger must reconstruct the long tracks traversing the whole LHCb detector and leaving hits in the three tracking detectors: the vertex locator, the upstream tracker and the scintillating fiber tracker. However, during the 2022 data taking, the upstream tracker was not installed, leading to an immediate necessity of modification of the long track reconstruction strategy. A new algorithm presented in this thesis allows the reconstruction of long tracks without using the upstream tracker information

Recherche de désintégration rares à quatre corps de mesons charmés avec électrons dans l'état final et reconstruction de longues traces pour le système de déclenchement de LHCb

The rare four-body rare charm decays D0->pipiee and D0->KKee receive contributions from flavour-changing neutral current c->all transitions. Measurements of such decays are unique probes of beyond-the-standard-model effects as new heavy particles can contribute in electroweak quantum loops. The search for rare charm D0->pipiee and D0->KKee decays is presented using a sample of proton-proton collisions data, corresponding to an integrated luminosity of 6 fb^-1 and recorded with a center-of-mass energy of 13 TeV at the LHCb experiment. The presented analysis is statistically limited and will greatly benefit from the upgrade of the LHCb detector which will record data with a five times greater instantaneous luminosity. The trigger system was completely redesigned by removing the first-level hardware trigger stage and selecting events with a fully software-based trigger directly at the 30 MHz collision rate of the LHC. This demanding task is performed by the first high-level trigger optimised for Graphic Processing Units (GPUs). In order to select interesting events, the trigger must reconstruct the long tracks traversing the whole LHCb detector and leaving hits in the three tracking detectors: the vertex locator, the upstream tracker and the scintillating fiber tracker. However, during the 2022 data taking, the upstream tracker was not installed, leading to an immediate necessity of modification of the long track reconstruction strategy. A new algorithm presented in this thesis allows the reconstruction of long tracks without using the upstream tracker information.Les désintégrations rares à quatre corps de mésons charmés D0->pipiee et D0->KKee reçoivent des contributions par les transitions à courant neutre avec changement de saveur de type c->ull. Les mesures de ces désintégrations sont fondamentales pour la recherche de physique au-delà du modèle standard parce que des particules plus massives peuvent contribuer dans les electroweak loops quantiques. La recherche de désintégrations charmées D0->pipiee et D0->KKee est présentée sur ce manuscrit en utilisant des données de collisions proton-proton, qui correspondent à une luminosité intégrée de 6 fb^-1 obtenues avec une énergie au centre de masse de 13 TeV avec l'expérience LHCb. L'analyse est toutefois limitée par la statistique et peut bénéficier de l'upgrade du détecteur LHCb qui va prendre des données avec une luminosité instantanée cinq fois plus grande. Le système de déclenchement a été complètement changé en enlevant le premier niveau du système qui était développé sur hardware. Le nouveau système de déclenchement doit donc sélectionner les événements avec un système fully-software, directement sur le taux de collisions de 30 MHz du LHC. Ceci a été développé et optimisé sur des Graphic Processing Units (GPUs). Pour sélectionner les événements intéressant et réduire le taux en entrée, le système de déclenchement doit reconstruire les traces dites longues, qui traversent tout le détecteur LHCb et laissant du signal sur les trois trajectographes de LHCb: le vertex locator, le upstream tracker et le scintillating fiber tracker. Cependant, pendant la prise de données de 2022, le upstream tracker n'a pas été installé, ce qui veut dire que la stratégie de reconstruction des traces longues devait être modifié. Un nouveau algorithme de reconstruction présenté sur ce manuscrit permet la reconstruction des traces longues sans utiliser l'information de l'upstream tracker