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 Bs0–Bs0 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 Bs0–Bs0 oscillation frequency announced in April, based on the heavy and light strange-beauty-meson mass difference. The very high precision of the Bs0–Bs0 measurement provides one of the strongest constraints on physics beyond the Standard Model. Using a large sample of Bs0 → Ds– π+ decays, the new measurement improves upon the previous precision of the oscillation frequency by a factor of two: Δms = 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.