Observation of CP Violation in Charm Decays

A search for charge-parity (CP) violation in D-0 -> K-K+ and D-0 -> pi(-)pi(+) decays is reported, using pp collision data corresponding to an integrated luminosity of 5.9 fb(-1) collected at a center-of-mass energy of 13 TeV with the LHCb detector. The flavor of the charm meson is inferred from the charge of the pion in D* (2010)(+) -> D-0 pi(+) decays or from the charge of the muon in (B) over bar -> D-0 mu(-)(nu) over bar X-mu decays. The difference between the CP asymmetries in D-0 -> K-K+ and D-0 -> pi(-)pi(+) decays is measured to be Delta A(CP) = [-18.2 +/- 3.2(stat) +/- 0.9(syst)] x 10(-4) for pi-tagged and Delta A(CP) = [-9 +/- 8(stat) +/- 5(syst)] x 10(-4) for mu-tagged D-0 mesons. Combining these with previous LHCb results leads to Delta A(CP) = (-15.4 +/- 2.9) x 10(-4), where the uncertainty includes both statistical and systematic contributions. The measured value differs from zero by more than 5 standard deviations. This is the first observation of CP violation in the decay of charm hadrons

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

Tetra- and pentaquark spectroscopy

Recent results on exotic spectroscopy from the LHCb experiment are introduced in these proceedings. They contain the evidence for a new pentaquark candidate decaying to $J/\psi p$ and $J/\psi \bar{p}$ in the $B^0_s\to J/\psi p \bar{p}$ decays and the observation of four new resonances decaying to $J/\psi K^+$ and $J/\psi\phi$ in the $B^+ \to J/\psi \phi K^+$ decays, two of which are tetraquarks with strangeness

Search for pentaquark candidates in B(s)0→J/ψppˉB^0_{(s)}\to J/\psi p \bar{p} decays at LHCb

This thesis presents the analysis of the $B^0_{(s)}\to J/\psi p \bar{p}$ decays, which have been performed on the data collected by the LHCb experiment at CERN, during Run~1 and Run~2. These decay modes are particularly attractive for the searches of exotic states, such as pentaquarks and glueballs, which could occur in the final states $J/\psi p$ ($J/\psi \bar{p}$) and $p \bar{p}$, respectively. Pentaquarks are states composed of five quarks, observed for the first time by LHCb in the decay of $\Lambda_b \to J/\psi p K^-$ in 2015. Glueballs are states made up only of gluons, which are the mediators of the strong force. Although they are predicted by the Standard Model of particle physics, they have not been observed yet. The field of exotic spectroscopy has gained increasing interest over the last few years, notably after the first observation of pentaquark states made by LHCb. Since Quantum Chromodynamics (QCD) does not work in the perturbative regime at distance scales that are characteristic of observable hadrons, a number of non-perturbative QCD-inspired models have been developed to describe the substructures of these exotic states. Depending on the model, multiquark states could either be tight bound states or molecules made up of hadrons. Therefore, it is crucial to find new experimental evidence that could help discriminate among different theoretical models. In this thesis, the observation of $B^0_{(s)}\to J/\psi p \bar{p}$ decays [Phys. Rev. Lett. 122 (2019) 191804] is presented using data collected by LHCb between 2011 and 2016 and corresponding to a luminosity of 5.2 $fb^{-1}$. The branching ratio of these modes is measured to be: \begin{align*} \mathcal{B}(B^0\to J/\psi p \bar{p})&= (4.51\pm 0.40\; \text{(stat)} \pm 0.44\; \text{(syst)}) \times 10^{-7},\\ \mathcal{B}(B^0_{s}\to J/\psi p \bar{p})&= (3.58\pm 0.19\; \text{(stat)} \pm 0.33\; \text{(syst)}) \times 10^{-6}, \end{align*} where the first uncertainty is statistical and the second one is systematic. A statistical significance greater than 10$\sigma$ is obtained for these modes. The limited phase space, which characterizes this decay, allowed us to perform the most precise $B^0_{(s)}$ mass measurements at the publication date. The $B^0_{(s)}$ masses are measured to be $5366.85 \pm 0.19\; ({\rm stat})\pm 0.13\; ({\rm sys}) ~{\rm MeV}$ and $5279.74 \pm 0.30\; ({\rm stat})\pm 0.10\; ({\rm sys}) ~{\rm MeV}$ for the $B^0_{s}$ and $B^0$, respectively. To investigate the presence of exotic resonances, we perform a full amplitude analysis of the $B^0_{s}\to J/\psi p \bar{p}$ decay using the helicity formalism and study the properties of intermediate resonances, such as masses, widths and quantum numbers, from the description of the angular distribution of the decays. The study is based on about 900 signal candidates, selected from the data collected by LHCb in the period of time between 2011 to 2018 and corresponding to 9 $fb^{-1}$ of luminosity. We find evidence of a new state decaying to $J/\psi p$ and $J/\psi \bar{p}$ with a statistical significance of $3.1 \sigma$, including systematic uncertainties. The excess can be interpreted as a pentaquark state candidate with quark content $c\bar{c}u u d$. Its mass, width and fit fraction are measured to be: \begin{align*} \text{M}_{P_c} &= 4337 ^{+7}_{-4} (\text{stat})^{+3}_{-3} (\text{syst}) MeV, \nonumber\\ \text{$\Gamma$}_{P_c} &= 29 ^{+26}_{-12} (\text{stat}) ^{+14}_{-13} (\text{syst}) MeV, \nonumber\\ f(P_c) &= 14.4^{+8.0}_{-4.0}(\text{stat}) \pm 8.2 (\text{syst})\%, \end{align*} where the first uncertainty is statistical and the second one is systematic. The preferred spin hypothesis for $J/\psi p$ corresponds to quantum numbers equal to $J^P = 1/2^+$, even though other $J^P$ assignments cannot be excluded. Concerning the theoretical interpretation, the $P_c$ state occurs around 10 MeV below the threshold of an hidden-charm molecule composed by $\chi_{c0}(1P)p$, which has preferred spin-parity value equal to $1/2^+$. This fact could favour the molecular interpretation for this state. However, given the limited statistics up to date, it is not possible to rule out other hypotheses. In order to confirm the nature of such excess we need to wait for more data to be collected by the experiment. Finally, we find no evidence for glueball states decaying to $p \bar{p}$, which were predicted by Ref. [Eur. Phys. J. C75 (2015) 101] to have mass around 2.23 GeV. During the period of the PhD, I have also been involved in hardware activities related to the upgrade of the LHCb detector to be installed in 2021. My main contribution was to the test of the front-end electronics of the Upstream Tracker (UT) silicon strip detector, in particular to the SALT ASIC chip designed to operate at the $pp$ collision frequency of 40 MHz. We have found a suboptimal performance in terms of signal-to-noise ratio, which led to redesigning the chip to meet the requirements

Spectroscopy and hadronic structure measurements

The study of properties of conventional and exotic states is crucial for investigating the internal structure of hadrons. Recent results on conventional and exotic spectroscopy obtained from the LHC experiments will be presented in this work. As for the conventional spectroscopy, it concerns the observation of excited $\Xi_b$ states and the search for baryons in the $bc$ sector. Regarding exotic states, several analyses of the exotic state, $X(3872)$, will be discussed, combining the results of the LHCb, ATLAS and CMS collaborations. Finally, recent results on tetraquarks and pentaquarks obtained by the LHCb collaboration will be presented, with a focus on the doubly charm tetraquark, $T_{cc}^{+}$, and the pentaquark $P_{\psi}^N(4337)$