SEARCH FOR PENTAQUARK CANDIDATES IN B^0_{(S)} -> J/PSI P ANTI-P DECAYS AT LHCB

This thesis presents the analysis of the B^0_{(s)} to J/psi p anti-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 anti-p) and p anti-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 anti-p decays [1] 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: B^0 to J/psi p anti-p = (4.51 +/- 0.40 (stat) +/- 0.44 (syst)) x 10^{-7}, B^0_{s} to J/psi p anti-p = (3.58 +/- 0.19 (stat) +/- 0.33 (syst)) x 10^{-6}, 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 +/- 0.19 (stat)+/- 0.13 (sys) MeV and 5279.74 +/- 0.30 (stat) +/- 0.10 (sys) 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 BsJpp 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 anti-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 anti-c u u d. Its mass, width and fit fraction are measured to be: M_{P_c} = 4337 ^{+7}_{-4} (stat)^{+3}_{-3} (syst) Mev, Gamma_{P_c} = 29 ^{+26}_{-12} (stat) ^{+14}_{-13} (syst) Mev, f(P_c) = 14.4^{+8.0}_{-4.0}(stat) +/- 8.2 (syst)%, 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 anti-p, which were predicted by [2] to have mass around 2.23Gev. 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. References: [1] LHCb collaboration, R. Aaij et al., Observation of B0(s)->J/psi p anti-p decays and precision measurements of the B0(s) masses, Phys. Rev. Lett. 122(2019) 191804, arXiv:1902.05588. [2] Y. K. Hsiao and C. Q. Geng, fJ(2220) and hadronic B0s decays, Eur. Phys. J.C75 (2015), no. 3 101, arXiv:1412.4900.