The First measurements of J/𝜓 production in proton-proton collisions at √s=13 TeV and study of the B→𝜇𝜇𝜋𝜋 decays at LHCb: two tests of QCD predictions

We present a search for the $B^0_s\to\pi^+\pi^-\mu^+\mu^-$ and $B^0\to\pi^+\pi^-\mu^+\mu^-$ decays using data collected by the LHCb detector in proton-proton collisions at centre-of-mass energies of 7 and 8TeV and corresponding to an integrated luminosity of $3\ \text{fb}^{-1}$. We obtain the first observation of the $B^0_s\to\pi^+\pi^-\mu^+\mu^-$ decay with a statistical significance of $7.2\sigma$ and the first evidence of the $B^0\to\pi^+\pi^-\mu^+\mu^-$ decay with a statistical significance of $4.8\sigma$. The branching ratios of these decays are measured to be $\mathcal{B}(B^0_s\to\pi^+\pi^-\mu^+\mu^-)=(8.6\pm 1.5\,({\rm stat}) \pm 0.7\,({\rm syst})\pm 0.7\,({\rm norm}))\times 10^{-8}$ and $\mathcal{B}(B^0_s\to\pi^+\pi^-\mu^+\mu^-)=(2.11\pm 0.51\,({\rm stat} \pm 0.15\,({\rm cyst})\pm 0.16\,({\rm norm}) )\times 10^{-8}$, where the third uncertainty is due to the branching fraction of the decay $B^0\to J/\psi(\to\mu\mu)K^*(892)^0(\to K \pi)$, used as a normalisation.\\ We present also the first measurement of the production cross-section of $J/\psi$ mesons in proton-proton collisions at centre-of-mass energy of 13TeV using the data collected by LHCb in 2015 and corresponding to an integrated luminosity of $3.02\pm0.12\ \text{pb}^{-1}$. The production cross-section is measured as a function of the transverse momentum \pt and the rapidity $y$ of the $J/\psi$ meson in the region $p_T<14$GeV/$c^2$ and $2.0<y<4.5$, for both prompt $J/\psi$ mesons and $J/\psi$ mesons from $b$-hadron decays. The production cross-sections integrated over the kinematic coverage are found to be $15.30\pm 0.03\text{(stat)}\pm 0.86\text{(syst)}\mu b$ for prompt $J/\psi$ and $2.34\pm 0.01\text{(stat)}\pm 0.13\text{(syst)}\mu b$ for $J/\psi$ from $b$-hadron decays, assuming zero polarisation of the $J/\psi$ meson. The ratios between these cross sections and the ones measured at the centre-of-mass-energy of 8\tev are also determined and compared with theoretical expectations. This measurement also validated a novel approach for online collection of ready-to-use data, which was implemented for the first time.\\ The study of the performances of the Silicon Tracker of the LHCb experiment during the 2012 and 2015 years of operation is also reported, and my contribution to a novel monitoring system of this sub-detector is presented.