Nicoḷ Magini
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Analysis of the angular distributions of the final products of the 
B_s -> J/psi phi -> l+l- K+K- decay chain allows the determination of B_s mixing parameters,
in particular the width difference Delta_Gamma_s which is at present not precisely measured.
It also allows to measure the CP-violating phase phi_s of the B_s system, which is expected
to be very small (~0.03) in the Standard Model but might show large non-SM enhancements. The
 high production rate of B_s mesons at the LHC will permit precise measurements of these 
quantities; the performances of the three LHC experiments with a B physics programme (ATLAS,
CMS and LHCb) in the reconstruction and analysis of this decay will be presented.