Measurement of the direct $CP$ asymmetry in $B^0 \to K^{*0} \gamma$ decays with the LHCb detector

The radiative decay $B^0 \to K^{*0} \gamma$ proceeds through a FCNC, being sensitive to physics beyond the Standard Model that can modify the transition. A particularly interesting observable is the direct $CP$ asymmetry of this decay. Prior to the measurement of physical quantities, the detector needs to be calibrated to guarantee the maximum quality of the data. The calibration and alignment of the SPD sub-detector has been studied using cosmic rays and the first LHC collisions. The stability of the SPD efficiency due to the ageing of the detector has been monitored with 2011 data. The full 2011 data sample corresponding to 1 fb$^{-1}$ at a centre-of-mass energy of $7 \mathrm{TeV}$ has been used to study the direct $CP$ asymmetry in $B^0 \to K^{*0} \gamma$ decays, and it is measured to be $0.8 \pm 1.7(\mathrm{stat.})\pm 0.9(\mathrm{syst.})$. This result is compatible with the Standard Model prediction and improves by a 20 % the previous best measurement.