Probing New Physics via the $B^0_s\to \mu^+\mu^-$ Effective Lifetime
  We have recently seen new upper bounds for $B^0_s\to \mu^+\mu^-$, a key decay to search for physics beyond the Standard Model. Furthermore a non-vanishing decay width difference $\Delta\Gamma_s$ of the $B_s$ system has been measured. We show that $\Delta\Gamma_s$ affects the extraction of the $B^0_s\to \mu^+\mu^-$ branching ratio and the resulting constraints on the New Physics parameter space, and give formulae for including this effect. Moreover, we point out that $\Delta\Gamma_s$ provides a new observable, the effective $B^0_s\to \mu^+\mu^-$ lifetime $\tau_{\mu^+\mu^-}$, which offers a theoretically clean probe for New Physics searches that is complementary to the branching ratio. Should the $B^0_s\to \mu^+\mu^-$ branching ratio agree with the Standard Model, the measurement of $\tau_{\mu^+\mu^-}$, which appears feasible at upgrades of the LHC experiments, may still reveal large New Physics effects. 
Kristof De Bruyn, Robert Fleischer, Robert Knegjens, Patrick   Koppenburg, Marcel Merk, Antonio Pellegrino, Niels Tuning
hep-ph hep-ex