2$\beta_s$ measurement using B$_s^0$ $\rightarrow$ J$\psi\phi$ at LHCb

A measurement of the phase of the $\rm B_s$-$\bar{\rm B}_s$ oscillation amplitude with respect to that of the ${\rm b} \rightarrow {\rm c^{+}}{\rm W^{-}}$ tree decay amplitude is one of the key goals of the LHCb experiment with first data. In the Standard Model (SM), this phase equals $-2\beta_s$, which is predicted to be small, i.e. $\sim$-0.04 rad. Although the statistical uncertainties are large, the current results hint at a possible contribution of New Physics in the $\rm B_s$-$\bar{\rm B}_s$ box diagram. Already with 2 fb$^{-1}$ of data, corresponding to one nominal year of data taking, the LHCb experiment is expected to achieve a statistical uncertainty of $\sigma(2\beta_s)\simeq 0.03$ rad, similar to the $2\beta_s$ value predicted by the SM