The LHCb collaboration at CERN in Geneva has observed and measured a crucial oscillation in the measured mass of neutral charm mesons arising from the particles changing into their antiparticles and back again. Although the detection is fully consistent with the Standard Model of particle physics, it could provide a “very clean” window into physics beyond the Standard Model in future experiments, the researchers say.
Mesons are particles comprising bound states of a quark and an antiquark. They are vital to nuclear binding, as they mediate the strong interaction between protons and neutrons, but in nature, free mesons exist only as the short-lived decay products of cosmic-ray interactions. However, they can be readily produced in particle accelerators and have been studied intensively for decades.
Just as protons and neutrons have heavy counterparts such as sigma baryons, in which one or more of the up or down quarks is replaced by a heavier cousin, so do mesons. The Standard Model predicts that some of these heavy mesons exist as superpositions of both particle and antiparticle, with a quantum wavefunction that evolves as the particles propagate. As a result, the probability of detecting either the meson or the anti-meson should also evolve too.
Mass difference
For complex reasons governed by quantum mechanics and the weak interaction, there is a difference in the lifetime – or width – and the mass between the two allowed superpositions. Both these differences can affect the proportion of mesons and anti-mesons detected.
“The width difference just allows for a slow time evolution from the meson to the anti-meson,” explains LHCb spokesperson Guy Wilkinson of the University of Oxford. “Only the mass difference allows the particle to turn into another particle and back again.” Evolution due to the difference in width was first confirmed in 2007 in data from the Belle collaboration in Japan and the BaBar collaboration in California. The sinusoidal oscillation due to mass difference had been seen in other particles such as strange-beauty mesons, but never before in charm mesons.
Both the width and mass differences could be crucial for probing violations of charge-parity (CP) symmetry. This is the hypothesis that, if both charge and parity are interchanged simultaneously, the laws of physics look identical. This is equivalent to probing matter-antimatter asymmetry and therefore studying CP violations could explain why there is much more matter than antimatter in the universe.
“Great place to look”
“CP violation can be accommodated within the Standard Model,” says Wilkinson, “but it can’t be explained. The reason that the charm system is a great place to look is that the level of CP violation you expect in the Standard Model is tiny. If there’s CP violation coming from some source outside the Standard Model, it should manifest itself much more clearly…”
LHCb did observe CP violation in a 2019 measurement of neutral charm mesons’ decay into other mesons. Whether this was consistent with the Standard Model, however, was disputed: “You had some theorists saying ‘This is remarkable: it’s much higher than we expect’ and many others saying ‘We can just about accommodate this’,” says Wilkinson; “If you look for CP violation in these mixing-related phenomena, people are much more confident of their calculations.”
It was unclear, however, whether the oscillation arising from the mass difference between the neutral charm mesons would be detectable. “It could have been way beyond our sensitivity, but it turns out that, though it’s a small parameter, it’s not ridiculously small,” says Wilkinson.
No firm evidence
The researchers’ observations to date show no firm evidence of CP violation. However, they are now installing an upgraded version of their experiment ready for the startup of the high-luminosity LHC next year. “If we see any CP-violation in the next 10 years it would be very difficult for a theorist to explain it in any other way but to say this is physics beyond the Standard Model,” Wilkinson concludes.
CERN physicists spot symmetry violation in charm mesons
“It’s very important,” says Tom Browder of the University of Hawaii, who was part of the Belle collaboration’s 2007 measurement of the lifetime parameter and now works on its successor Belle II. “I and many others have been working for decades to observe neutral charm mixing and LHCb has finally succeeded in measuring this mass parameter.” He hopes the results will be confirmed by other experiments such as his own and says that, if the researchers were to see any evidence of CP violation in the results at current levels of sensitivity, “that could very well be a smoking gun for new physics”.
“It certainly has a way to go before seeing CP violation in this process, unless the CP violation is much larger than the standard model predicts,” says Jonathan Rosner of the Enrico Fermi Institute in Chicago; “It’s just showing the versatility of the LHCb detector: that’s been a huge success story.”
The research is described in a preprint on arXiv.
