Precise measurement of the charged B meson mass at the LHCb experiment

In the SM framework hadrons are colourless particles composed of quarks and gluons that interact by means the strong interactions described by Quantum chromodynamics (QCD). In this framework the theoretical predictions have to be compared with experimental data in order to verify if QCD is the correct theory of strong interactions. $b$ physics is considered a stringent Standard Model (SM) test and recent developments in $b$ hadron physics gave significant improvement in experimental determination of SM parameters. $b$ mass is a fundamental parameter of SM but, since quarks are confined inside hadrons and are not observed as physical particles, it has to be extrapolated by theoretical calculations: a non-perturbative tool formulated on a discrete Euclidean space time grid for calculating the hadronic spectrum and the matrix elements of any operator called lattice QCD (LQCD) has been developed. One of the purposes of LQCD is to provide predictions on $B$ meson mass starting from $b$ quark value: a fundamental request is that LQCD predictions have to match with experimental data. In this light accurate experimental measurements are essential and thanks to the luminosity reached at LHC it is now possible to improve the measurements recorded in literature in particular for $B$ mesons since LHCb provides a unique $b$ physics laboratory. The LHCb experiment is a forward spectrometer designed to take advantage of the characteristic $b \bar b$ pair angular distribution. Between $2010$ and $2013$, when Run I took place, the LHCb experiment recorded data collisions both at $\sqrt{s} = 7$ and 8 TeV and collected about $10^{12}$ heavy flavor decays. In this study the $B^{\pm}$ mass measurement has been performed in the $B^{+} \rightarrow J/\psi \phi K^{+}$ decay channel (charge-conjugate state is implied) using the total $3$ fb$^{-1}$ data collected by LHCb during 2011 at a center-of-mass energy $\sqrt{s} = 7$ TeV and 2012 at a center-of-mass energy $\sqrt{s} = 8$ TeV obtaining a signal yield of $3285 \pm 72$. From the invariant mass distribution fit and from systematic uncertainties analysis the mass of $B^{\pm}$ meson is measured to be: \begin{equation*} m(B^{\pm}) = 5279.14 \pm 0.11_{stat} \pm 0.25_{sys}\hbox{ MeV}/c^{2} \hbox{.} \end{equation*} This value is in good agreement with the world average and is more precise than the best previous measurement.