Searching for CP violation in the $B_s^0\to \phi \phi$ decay at LHCb

The study of flavour physics allows for the Standard Model (SM) to be tested to higher energies than can be accessed through direct searches. The SM is known not to provide enough of a difference between matter and anti-matter, termed CP violation, to explain the dominance of matter in our universe. One of the main purposes of the LHCb experiment is to search for new sources of $CP$ violation in the decays of B mesons. Flavour changing neutral current (FCNC) interactions are forbidden at tree level in the SM, and can therefore only be accessed through quantum loops. In New Physics scenarios such as Supersymmetry, new particles could appear in those loops introducing new sources of $CP$ violation. The $B_s^0 \to \phi\phi$ decay proceeds via the $b\to s\bar{s}s$ FCNC transition. Triple products provide a method of exploiting the angular distributions of $P\to VV$ decays to create $T$-odd observables. Asymmetries of these $T$-odd observables, averaged over the initial flavour of the $B_s^0$ meson provide a measure of $T$ violation. Assuming $CPT$ conservation, violation of time reversal infers $CP$ violation. The $CP$-violating weak phase in the interference between $B_s^0$ mixing and the decay to two $\phi$ mesons is predicted to be close to zero in the SM. The measurements of the triple product asymmetries and the $CP$-violating weak phase have been performed using 1.0 fb$^{−1}$ of LHCb data. Events where kaon pairs originate from a spin-0 or non-resonant state are accounted for with the associated angular distributions. Triple product asymmetries are measured to be $A_U = −0.055\pm 0.036({\rm stat}) \pm 0.018({\rm syst})$ and $A_V =0.010\pm 0.036({\rm stat})\pm 0.018({\rm syst})$. The $CP$-violating phase is found to be in the interval $[ −2.46, −0.76]$ rad at 68% confidence level. The p-value for the hypothesis of zero radians is found to be 16%. These results represent the most accurate measurements of the triple product asymmetries and the first measurement of the $CP$-violating weak phase.