Observation of $CP$ violation in $B^0\text{\hspace{0.17em}⁣}\to J\text{\hspace{0.17em}⁣}\mathrm{/}\psi \rho (770{)}^0$ decays

The time-dependent $CP$ asymmetry in $B^0\text{\hspace{0.17em}⁣}\to J\text{\hspace{0.17em}⁣}\mathrm{/}\psi \rho (770{)}^0$ decays is measured using proton-proton collision data corresponding to an integrated luminosity of $6{\text{fb}}^{-1}$, collected with the LHCb detector at a center-of-mass energy of $13\text{TeV}$ during the years 2015-2018. The $CP$-violation parameters for this process are determined to be $2{\beta }_{c\bar{c}d}^{\mathrm{e}\mathrm{f}\mathrm{f}}=0.710\pm 0.084\pm 0.028\text{rad}$ and $\mathrm{\mid }\lambda \mathrm{\mid }=1.019\pm 0.034\pm 0.009$, where the first uncertainty is statistical and the second systematic. This constitutes the first observation of time-dependent $CP$ violation in $B$ meson decays to charmonium final states mediated by a $b\text{\hspace{0.17em}⁣}\to c\bar{c}d$ transition. These results are consistent with, and two times more precise than, the previous LHCb measurement based on a data sample collected at 7 and $8\text{TeV}$ corresponding to an integrated luminosity of $3{\text{fb}}^{-1}$. Assuming approximate SU(3) flavor symmetry, these two measurements are combined to set the most stringent constraint on the enguin contribution, $\mathrm{\Delta }{\phi }_s$, to the $CP$-violating phase ${\phi }_s$ in $B_s^0\text{\hspace{0.17em}⁣}\to J\text{\hspace{0.17em}⁣}\mathrm{/}\psi \phi (1020)$ decays, yielding $\mathrm{\Delta }{\phi }_s=5.0\pm 4.2\text{mrad}$.