LHCb squeezes D-meson mixing

The weak force, unlike other fundamental forces, has a distinctive feature: its interactions slightly differ when involving quarks or antiquarks. This phenomenon, known as CP violation, allows for an asymmetry in the likelihood of a process occurring with matter compared to its antimatter counterpart, which is an essential requirement to explain the large dominance of matter in the universe. However, the size of CP violation predicted by the Standard Model (SM), and in accordance with experimental measurements so far, is not large enough to explain this cosmological imbalance. This is why physicists are actively searching for new sources of CP violation and striving to improve our understanding of the known ones. The phenomenology offered by the quantum-mechanical oscillations of neutral mesons into their antimatter counterparts, the antimesons, provides a particularly rich experimental ground for such studies.

The LHCb collaboration recently measured a set of parameters that determine the matter–antimatter oscillation of the neutral D⁰ meson into the D⁰ anti­meson with unprecedented precision. This enables the search for the predicted hitherto unobserved CP violation in this oscillation.

D⁰ mesons are composed of a charm quark and an up antiquark. Their oscillations are extremely slow, with an oscillation period over a thousand times longer than their lifetimes. As a result, only a very few D⁰ mesons transform before they decay. Oscillations are therefore identified as extremely small changes in the flavour mixture – matter or antimatter – as a function of the time at which the D⁰ or the D⁰ decays.

In LHCb’s analysis, the initial matter–antimatter flavour of the neutral meson is experimentally inferred from the charge of the accompanying pion in the CP-conserving decay chains D^*(2010)⁺ → D⁰π⁺ and D^*(2010)^– → D⁰π^–. The mixing effect (or oscillation) then appears as a decay-time dependence of the ratio, R, of the number of “suppressed” and “favoured” decay processes of the neutral meson. The suppressed decays can occur with or without a net oscillation of the D⁰ meson, while the favoured decays are largely dominated by the direct process. In the absence of mixing, this ratio is predicted to be constant as a function of the D⁰ decay time while, in the case of mixing, it approximately follows a parabolic behaviour, increasing with time. Figure 1 shows the ratio R, including data for both matter (R⁺ for D⁰ → K⁺π^–) and antimatter (R^– for D⁰ → K^–π⁺) processes, and corresponding model predictions. The variation depends not only on the oscillation parameters but also on the various observables of CP violation, which differentiate between matter and antimatter.

This analysis is the most precise measurement of these parameters to date, improving the uncertainty on both mixing and CP-violating observables by a factor of 1.6 compared to the previous best result, also by LHCb. This improvement is largely due to an unpre­cedentedly large sample of about 1.6 million suppressed decays and 421 million favoured decays collected during Run 2, making LHCb unique in probing up-type quark transitions. The results confirm the matter–antimatter oscillation of the D⁰ meson and show no evidence of CP violation in the oscillation.

These findings call for future analyses of this and other decays of the D⁰ meson using data from the third and fourth run of the LHC, exploiting the potential of the currently operating detector upgrade (Upgrade I). The detector upgrade proposed for the fifth and sixth runs of the LHC (Upgrade II) would provide a six-times-bigger sample, yielding the precision needed to definitively test the predictions of the SM.