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## Search for violations of Lorentz invariance and $CPT$ symmetry in $B^0_{(s)}$ mixing
Violations of $\CPT$ symmetry and Lorentz invariance are searched for by studying interference effects in $\Bd$ mixing and in $\Bs$ mixing. Samples of $\Bd\to\jpsi\KS$ and $\Bs\to\jpsi\Kp\Km$ decays are recorded by the LHCb detector in proton--proton collisions at centre-of-mass energies of 7 and 8\tev, corresponding to an integrated luminosity of 3\invfb. No periodic variations of the particle-antiparticle mass differences are found, consistent with Lorentz invariance and $\CPT$ symmetry. Results are expressed in terms of the Standard Model Extension parameter $\Delta a_{\mu}$ with precisions of $\order(10^{-15})$ and $\order(10^{-14})$\gev for the $\Bd$ and $\Bs$ systems, respectively. With no assumption on Lorentz (non-)invariance, the $\CPT$-violating parameter $z$ in the $\Bs$ system is measured for the first time and found to be $\Rez = -0.022 \pm 0.033 \pm 0.005$ and $\Imz = 0.004 \pm 0.011\pm 0.002$, where the first uncertainties are statistical and the second systematic.