Charmed matter–antimatter flips clocked by LHCb

The ability of certain neutral mesons to oscillate between their matter and antimatter states at distinctly unworldly rates is a spectacular feature of quantum mechanics. The phenomenon arises when the states are orthogonal combinations of narrowly split mass eigenstates that gain a relative phase as the wavefunction evolves, allowing quarks and antiquarks to be interchanged at a rate that depends on the mass difference. Forbidden at tree level, proceeding instead via loops, such fl avour-changing neutral-current processes offer a powerful test of the Standard Model and a sensitive probe of physics beyond it.

Only four known meson systems can oscillate

Predicted by Gell-Mann and Pais in the 1950s, only four known meson systems (those containing quarks from different generations) can oscillate. K⁰–K⁰ oscillations were observed in 1955, B⁰–B⁰ oscillations in 1986 at the ARGUS experiment at DESY, and B_s⁰–B_s⁰ oscillations in 2006 by the CDF experiment at Fermilab. Following the first evidence of charmed-meson oscillations (D⁰–D⁰) at Belle and BaBar in 2007, LHCb made the first single-experiment observation confirming the process in 2012. Being relatively slow (more than 100 times the average lifetime of a D⁰ meson), the full oscillation period cannot be observed. Instead, the collaboration looked for small changes in the flavour mixture of the D⁰ mesons as a function of the time at which they decay via the Kπ final state.

On 4 June, during the 10th International Workshop on CHARM Physics, the LHCb collaboration reported the first observation of the mass difference between the D⁰–D⁰states, precisely determining the frequency of the oscillations. The value represents one of the smallest ever mass differences between two particles: 6.4 × 10^–6 eV, corresponding to an oscillation rate of around 1.5 × 10⁹ per second. Until now, the measured value of the mass-difference between the underlying D⁰ and D⁰ eigenstates was marginally compatible with zero. By establishing a non-zero value with high significance, the LHCb team was able to show that the data are consistent with the Standard Model, while significantly improving limits on mixing-induced CP violation in the charm sector.

“In the future we hope to discover time-dependent CP violation in the charm system, and the precision and luminosity expected from LHCb upgrades I and II may make this possible,” explains Nathan Jurik, a CERN fellow who worked on the analysis.

The latest measurements of neutral charm–meson oscillations follow hot on the heels of an updated LHCb measurement of the B_s⁰–B_s⁰ oscillation frequency announced in April, based on the heavy and light strange-beauty-meson mass difference. The very high precision of the B_s⁰–B_s⁰ measurement provides one of the strongest constraints on physics beyond the Standard Model. Using a large sample of B_s⁰ → D_s^– π⁺ decays, the new measurement improves upon the previous precision of the oscillation frequency by a factor of two: Δm_s = 17.7683 ± 0.0051 (stat) ± 0.0032 (sys) ps^–1 which, when combined with previous LHCb measurements, gives a value of 17.7656 ± 0.0057 ps^–1. This corresponds to an oscillation rate of around 3 × 10¹² per second, the highest of all four meson systems.