Search for New Physics with an angular analysis of B0→D∗μνB^0 \to D^\ast \mu \nu decays and alignment of the LHCb Vertex Locator

A search for New Physics was performed with an angular analysis of $B^0 \rightarrow D^* \mu \nu$ decays in $pp$ collision data collected by the LHCb experiment during 2011 and 2012, corresponding to an integrated luminostiy of 3 $\rm fb^{-1}$. The signal is extracted using a multidimensional fit to data using templated distributions derived from simulation and from control samples in collision data. New Physics contributions are measured via their corresponding Wilson coefficients (real and imaginary part) and in several fit configurations that allow for different New Physics operators (vector, scalar and tensor). Form factor parameters using three parametrizations (BLPR, CLN and BGL) are measured in a Standard Model scenario. The the New Physics operators Im($V_{\rm qRlL}$), Re($V_{\rm qRlL}$), Im($V_{\rm qLlL}$), Im($S_{\rm qRlL}$), Re($S_{\rm qRlL}$), Im($T_{\rm qLlL}$), Re($T_{\rm qLlL}$) are measured with precision of 1.14e-02, 1.66e-02, 1.18e+00, 3.49e-01, 4.77e-01, 8.70e-03, 1.59e-02, respectively. This is the first full angular analysis of semileptonic $B$ hadron decays at LHCb and is the first measurement of New Physics search using LHCb data in semileptonic decays.The thesis also discusses a suite of alignment studies for the LHCb Vertex Locator (VELO). Thermal properties of the LHCb Vertex Locator (VELO) are studied using the real-time detector alignment procedure where the variation of the position and orientation of the detector elements as a function of the operating temperature of the VELO is presented. This study uses a dataset collected by the LHCb experiment during a VELO temperature scan performed at the end of LHC Run 2 (October 2018). Significant shrinkage of the VELO modules is observed at the operating temperature of $-30^\circ$C compared to the laboratory measurements on a single module taken at a range of temperatures from $+45^\circ$C to $-25^\circ$C. The thermal shrinkage expected from the extrapolation of laboratory measurements to lower temperatures and the results of this alignment study are in good agreement. A study of the VELO alignment for Run 3 is made in order to investigate the alignment performance with the Run 3 VELO detector and to establish the best alignment strategy to ensure the desired physics performance. The VELO detector is aligned using different configurations and degrees of freedom. The results of Run 3 alignment accuracy along with a set of monitoring plots that show sensitivity under different misalignments are presented

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

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

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

Adding timing to the VELO

The LHCb experiment is designed to perform high precision measurements of matter-antimatter asymmetries and searches for rare and forbidden decays, with the aim of discovering new and unexpected particles and forces. In 2030 the LHC beam intensity will increase by a factor of 50 compared to current operations. This means increased samples of the particles we need to study, but it also presents experimental challenges. In particular, with current technology it becomes impossible to differentiate the many (>50) separate proton-proton collisions which occur for each bunch crossing.\\ In this project a Monte Carlo simulation was developed to model the operation of a silicon pixel vertex detector surrounding the collision region at LHCb, under the conditions expected after 2030, after the second upgrade of the Vertex Locator(VELO).The main goal was studying the effect of adding '4D' detectors which save high-precision timing information, in addition to the usual three spatial coordinates, as charged particles pass through them. With the additional information on the particle timing, it is possible to separately reconstruct the individual 50+ collisions, allowing the next generation of high-precision measurements to be made at the LHCb

A Future Vertex Locator with Precise Timing for the LHCb Experiment

The LHCb experiment is designed to perform high precision measurements of matter-antimatter asymmetries and searches for rare and forbidden decays, with the aim of discovering new and unexpected particles and forces. In 2030 the LHC beam intensity will increase by a factor of 50 compared to current operations. This means increased samples of the particles we need to study, but it also presents experimental challenges. In particular, with current technology it becomes impossible to differentiate the many (>50) separate proton-proton collisions which occur for each bunch crossing. A Monte Carlo simulation was developed to model the operation of a silicon pixel vertex detector surrounding the collision region at LHCb, under the conditions expected after 2030, after the second upgrade of the Vertex Locator (VELO). The main goal was studying the effect of adding '4D' detectors which save high-precision timing information, in addition to the usual three spatial coordinates, as charged particles pass through them. With the additional information on the particle timing, it is possible to separately reconstruct the individual 50+ collisions, allowing the next generation of high-precision measurements to be made at the LHCb

The Alignment of the LHCb Vertex Detector: Performance in Run 2 and studies for the Upgrade

The LHCb detector at the LHC is a general purpose detector in the forward region designed to reconstruct decays of b and c hadrons. The Vertex Locator (VELO), consisting of a series of silicon strip sensors placed along the beam direction, allows the reconstruction of primary and secondary vertices and precise lifetime measurements. In Run 2 (2015-2018), a realtime alignment and calibration procedure was developed. Data collected at the start of each LHC fill are processed to evaluate the alignment using a Kalman filter method. An overview of the alignment algorithm and the real-time procedure is presented emphasizing the performance and its stability during the Run 2 data taking period. The VELO operates in a secondary vacuum with a bi-phase CO$_2$ running through cooling blocks attached to the module base, with heat conducted through a TPG core. The cooling temperature is -30 degrees and the sensors operate at -8 degrees. Tests involving a change of the operational temperature to higher values up to -20 degrees were made to take dedicated samples at each temperature. These samples were analysed to evaluate the dependency of the module position as a function of the VELO temperature and were compared with measurement on a single module. For Run 3 (2021-2023), a new tracking system is being built, including a new Vertex Locator based on silicon pixels (VP) and an improved alignment procedure is under development, building upon the success of the Run 2 strategy. From recent tests, misalignments can arise from potential distortions of the VELO upgrade modules when cooled to their operating temperature. A study on simulated data has been performed to evaluate the effect of the distortions on the physics performance. A broad variety of detector movements are simulated and the residual distortions are determined by the alignment procedure. Results of these studies and the on-going optimisation of the upgrade VELO alignment procedure are presente