Measurement of Bs meson lifetime in the Bs->JPsiPhi decay channel at CMS experiment

The physics of B-meson offers a wide field, at the TeV energy scale, of opportunities in the search for New Physics and in the Standard Model tests.The CP symmetry violation in the Bs decay offers important tests of the Standard Model through the measurement of quantities like the weak phase and the light and heavy Bs mass eigenstates decay width difference. During the measurement a high precision is required in the analysis process due to the small difference between light and heavy mass eigenstates; particular attention is required in the reconstruction of the Bs creation and decay point.This work presents an estimation of the systematic effects depending on the measurement of the Bs creation point through the study of its mean lifetime in the Bs->JPsiPhi decay channel which is highly affected by those effects.Using data collected during 2018, period "D", by the CMS experiment, the Bs decays are reconstructed from the tracks of the decay products and filtered applying a set of selection cuts. The creation point is then estimated through several procedures, the possible choices have been compared with a data sample of events obtained from a Monte Carlo simulation in order to choose the best candidate. At last the mean lifetime of the Bs is estimated fitting the flight distance distribution through a multi-step fit procedure with an unbinned maximum likelihood method.The analysis has shown that the highest precision is obtained from the creation point selected with the pointing angle minimization method, in a data sample with a "Displacement" request; the systematic error associated with the primary vertex choice is estimated to be an one percent order of magnitude.