Angular and CP-violation analyses of Bˉ→D∗+l−νˉl\bar{B}\to D^{*+} l^-\bar{\nu}_{l} decays at hadron collider experiments

The $\bar{B}\to D^{*+} l^-\bar{\nu}_{l}$ branching fraction ratio $R(D^{*})$ has shown intriguing discrepancies between the Standard Model prediction and measurements performed at BaBar, Belle and LHCb experiments, a possible sign of beyond the Standard Model physics. Theoretical studies prove how observables related to the $\bar{B}\to D^{*+} l^-\bar{\nu}_{l}$ differential decay distribution can be used to further constrain New Physics contributions, but their experimental measurements is lacking to date. This article presents the attainable precision on the measurement of $\bar{B}\to D^{*+} l^-\bar{\nu}_{l}$ angular and CP-violating observables by exploiting approximate reconstruction algorithms using information from detectable final-state particles only, a case of special interest for hadron collider experiments. The resolution on the phase space variables is studied using $\bar{B}\to D^{*+} l^-\bar{\nu}_{l}$ decays simulated in a forward detector geometry like LHCb. A novel method to correct the observable values for the reconstruction inaccuracies based on detector simulation is successfully tested on simulated data and the decrease in precision with respect to a perfect reconstruction is evaluated. The $D^{*+}$ longitudinal polarization fraction and one of the CP-violating observables can be measured losing a factor 2 and 5 in precision, respectively. The extraction of phase space distributions from the template fit selecting $\bar{B}\to D^{*+} l^-\bar{\nu}_{l}$ decays and associated systematic uncertainties are also discussed.Comment: 22 pages, 13 figure

Extracting maximum information from polarised baryon decays via amplitude analysis: the Λc+→pK−π+\Lambda^+_c \to pK^-\pi^+ case

We consider which is the maximum information measurable from the decay distributions of polarised baryon decays via amplitude analysis in the helicity formalism. We focus in particular on the analytical study of the $\Lambda^+_c \to pK^-\pi^+$ decay distributions, demonstrating that the full information on its decay amplitudes can be extracted from its distributions, allowing a simultaneous measurement of both helicity amplitudes and the polarisation vector. This opens the possibility to use the $\Lambda^+_c \to pK^-\pi^+$ decay for applications ranging from New Physics searches to low-energy QCD studies, in particular its use as absolute polarimeter for the $\Lambda^+_c$ baryon. This result is valid as well for baryon decays having the same spin structure and it is cross-checked numerically by means of a toy amplitude fit with Monte Carlo pseudo-data.Comment: 35 pages, 5 figure

Helicity amplitudes for generic multi-body particle decays featuring multiple decay chains

We present the general expression of helicity amplitudes for generic multi-body particle decays characterised by multiple decay chains. This is achieved by addressing for the first time the issue of the matching of final particle spin states among different decay chains in full generality for generic multi-body decays, proposing a method able to match the exact definition of spin states relative to the decaying particle ones. We stress the importance of our result by showing that one of the matching method used in the literature is incorrect, leading to amplitude models violating rotational invariance. The results presented are therefore relevant for performing numerous amplitude analysis, notably those searching for exotic structures like pentaquarks

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

Search for C ⁣PC\!P violation in Λb→pK−μ+μ−\Lambda_b\to pK^-\mu^+\mu^- decays

In this thesis the first search for $C\!P$-violation on rare heavy beauty baryon $\Lambda_b\to pK^-\mu^+\mu^-$ decays is described. This analysis is carried out on the whole dataset recorded by the LHCb experiment during 2011 and 2012. The $C\!P$ symmetry violation study is one of the most promising method for searching physics beyond the standard model, as the measured amount of $C\!P$-violation in high-energy physics experiments, even though compatible with standard model expectations, is not sufficient for explaining the observed matter-antimatter asymmetry of our universe. The $\Lambda_b\to pK^-\mu^+\mu^-$ decays occur via electroweak loop diagrams which allow possible new physics fields to give this process a sensible contribution. For the $\Lambda_b\to pK^-\mu^+\mu^-$ decay a limited quantity of $C\!P$-violation is expected in the standard model, making this transition suited to look for beyond standard model physics. In this thesis, $C\!P$-violation is searched exploiting direct $C\!P$ asymmetries and triple-products asymmetries, two complementary measurements sensitive to different new physics contributions. Experimentally, the study of $\Lambda_b\to pK^-\mu^+\mu^-$ transition is very clean thanks to the high performance of the muon detection and particle identification systems of LHCb experiment

Amplitude analysis and polarisation measurement of the Λc+\Lambda^+_c baryon in pK−π+pK^-\pi^+ final state for electromagnetic dipole moment experiment

The first part of the thesis presents an experiment proposal for short-lived heavy baryon electromagnetic dipole moments measurement, exploiting spin precession of particles channeled in bent crystals, produced from fixed-target collisions of multi-TeV energy protons extracted from the main beam of the Large Hadron Collider accelerator. The LHCb detector is considered to measure the baryon polarisation from its decay distribution. The same methodology is further extended to the measurement of $\tau$ lepton electromagnetic dipole moments, which is complicated by the presence of undetectable neutrinos. The second part of the thesis presents the amplitude analysis of $\Lambda^+_c\to pK^-\pi^+$ decays from $\Lambda^0_b \to \Lambda^+_c \mu X$ production recorded by the LHCb experiment, including the extraction of the $\Lambda^+_c$ polarisation vector. This analysis is an essential tool in light of the $\Lambda^+_c$ baryon dipole moments measurement, providing an unprecedented knowledge of the main $\Lambda^+_c$ decay mode and the $\Lambda^+_c$ polarisation. Order one million events are selected from LHCb data with negligible background contributions. The $\Lambda^+_c\to pK^-\pi^+$ decay model for generic polarisation is written in the helicity formalism and fitted to data. An amplitude model describing $\Lambda^+_c\to pK^-\pi^+$ decays has been obtained from two-dimensional Dalitz plot fits, giving valuable information on the main intermediate resonances, including the observation of new $\Lambda^*$ resonant contributions. The third part of the thesis deals with the $\Lambda^+_c$ polarisation measurement in proton-neon collisions at 68.6 GeV centre-of-mass energy recorded by LHCb, which provides information on the $\Lambda^+_c$ baryon structure. A significant $\Lambda^+_c$ polarisation is also crucial for the proposed dipole moments experiment. The feasibility of the measurement has been proved achieving two essential results: a few hundreds $\Lambda^+_c\to pK^-\pi^+$ decays are selected from proton-neon LHCb data, separated from background contributions; the $\Lambda^+_c$ polarisation measurement from the selected sample has been proved feasible on simulated pseudo-experiments, showing that the main associated systematic uncertainty is negligible compared to statistical fluctuations

Helicity Amplitudes for Generic Multibody Particle Decays Featuring Multiple Decay Chains

We present the general expression of helicity amplitudes for generic multibody particle decays characterised by multiple decay chains. This is achieved by addressing for the first time the issue of the matching of the final particle spin states among different decay chains in full generality for generic multibody decays, proposing a method able to match the exact definition of spin states relative to the decaying particle ones. We stress the importance of our result by showing that one of the matching methods used in the literature is incorrect, leading to amplitude models violating rotational invariance. The results presented are therefore relevant for performing numerous amplitude analyses, notably those searching for exotic structures like pentaquarks