Measurement of $R$($D$(∗)) in semileptonic $B$ decays and upgrade of the LHCb Upstream Tracker

The LHCb experiment at the Large Hadron Collider provides a unique opportunity to study flavor physics with high luminosity. One topic in flavor physics is lepton flavor universality (LFU), a property of the standard model (SM) which requires the three generations of leptons (e, μ, τ ) couple to gauge bosons of the eletroweak interactions with the same strength. It is an important probe for testing the validity of the SM and possibly providing hints to new physics beyond the SM. This thesis presents a preliminary framework for the measurement of $R$($D$∗)), a proxy to test LFU, defined as the ratio of branching fractions $B$($B$ → $D$(∗)τ −ντ )/$B$($B$ → $D$(∗)μ−νμ), with LHCb 2016 data. Another topic of the thesis is the upgrade of the LHCb Upstream Tracker (UT) which greatly increases the readout rate of the detector and removes limitations due to hardware trigger, paving the way for future precision measurements with even higher luminosity.