The Effect of Penguins in the B_d to J/psi K⁰ CP asymmetry

Performing a fit to the available experimental data, we quantify the effect of long-distance contributions from penguin contractions in B^0 to J/psi K^0 decays. We estimate the deviation of the measured S_CP term of the time-dependent CP asymmetry from sin(2 beta) induced by these contributions and by the penguin operators. We find Delta S = 0.000 +- 0.012 ([-0.025,0.024] @ 95% probability).