New high-precision constraints on charm CP violation

There are four neutral mesons that allow particle–antiparticle transitions – mixing – and so make ideal laboratories for studies of matter–antimatter asymmetries (CP violation). Indeed, such an asymmetry has already been observed for three of these mesons: K⁰, B⁰ and B⁰_s. So far, searches for CP violation in the fourth neutral meson – the charm meson D⁰ – have not revealed a positive result. However, being the only one of the four systems to contain up quarks, the D⁰ mesons provide unique access to effects from physics beyond the Standard Model.

The LHCb collaboration presented recently two new sets of measurements at the CHARM 2013 conference, held in Manchester on 31 August–4 September. Both measurements use several million decays of D⁰ mesons into two charged mesons. The first is based on D⁰ → K⁺π^– decays and their charge conjugate, from data recorded in 2011 and 2012. Owing to the Cabibbo-Kobayashi-Maskawa mechanism, the direct decay is suppressed relative to its Cabibbo-favoured counterpart. However, the final state can also be reached through mixing of the D⁰ meson into its antimeson, followed by the favoured decay D⁰ → K⁺π^–.

These two components and their interference are distinguished through analysis of the decay-time structure of the decay – comparison of the structure for D⁰ and D⁰ decays measures CP violation. The results give the best measurements to date of the mixing parameters in this system and are consistent with no CP violation at an unprecedented level of sensitivity (LHCb 2013a).

The second measurement is based on decays into a pair of kaons or a pair of pions and uses data that were recorded in 2011. The asymmetry between the mean lifetimes measured in D⁰ and D⁰ decays is related to a parameter, A_Γ, which is the asymmetry between the inverse effective lifetimes of decays to the specific final state. It is a measurement of so-called indirect CP violation. The results for the two final states are A_Γ(KK) = (–0.35±0.62±0.12) × 10^–3 and A_Γ(ππ) = (0.33±1.06±0.14) × 10^–3 (LHCb 2013b). This is the first time that a search for indirect CP violation in charm mesons has reached a sensitivity of better than 10^–3.

The combination of previous measurements performed by the Heavy Flavor Averaging Group hinted at potentially nonzero values for the parameters of CP-violation in D⁰ mixing, |q/p| and φ. As the figure shows, the new results from LHCb do not support this indication. However, they provide extremely stringent limits on the underlying parameters of charm mixing, therefore constraining the room for physics beyond the Standard Model.