Measurement of the CP violating phase $\phi_s$ in $\overline{B}^0_s \to J/\psi f_0(980)$

Measurement of mixing-induced $CP$ violation in $\overline{B}^0_s$ decays is of prime importance in probing new physics. So far only the channel $\overline{B}^0_s\to J/\psi\phi$ has been used. Here we report on a measurement using an LHCb data sample of 0.41\,fb$^{-1}$, in the $CP$ odd eigenstate $J/\psi f_0(980)$, where $f_0(980)\to\pi^+\pi^-$. A time dependent fit of the data with the $\overline{B}^0_s$ lifetime and the difference in widths of the heavy and light eigenstates constrained to the values obtained from $\overline{B}^0_s \to J/\psi\phi$ yields a value of the $CP$ violating phase of $-0.44\pm 0.44\pm0.02~{\rm\,rad}$, consistent with the Standard Model expectation.