Measurement of $\phi_s$ in $B_s \to J/\psi f_0(980)$

Measurement of mixing induced CP violation in $B_s$ decays is of prime importance in probing new physics. So far only the channel $B_s \to J/\psi\phi$ has been used. Here we report on a measurement from the 2010 and 2011 LHCb data, a total of 378 pb$^{-1}$, using the CP odd eigenstate $J/\psi f_0(980)$, where $f_0(980)\to \pi^+\pi^-$. There are 1428 $\pm$ 47 signal events within $\pm$ 20 MeV of the $B_s$ mass and $\pm$ 90 MeV of the $f_0(980)$ mass. Using a time dependent fit of the data with the $B_s$ lifetime and the difference in widths of the heavy and light eigenstates constrained to values measured in the $J/\psi\phi$ analysis yields a value of the CP violating phase of $-0.44 \pm 0.44 \pm 0.02~{\rm \,rad}$, consistent with the Standard Model expectation.