Physicists working on the LHCb experiment at the CERN particle-physics lab have released the best evidence yet for direct charge–parity (CP) violation in charm mesons. Speaking at the Hadron Collider Physics Symposium in Paris, Mat Charles of the University of Oxford in the UK presented an analysis of collision data that suggests a larger-than-expected CP violation in the decay of charm and anticharm mesons. While more data must be analysed to confirm the result, the work could point to new physics beyond the Standard Model and help physicists understand why there is more matter than antimatter in the universe.
One of the smaller experiments on the Large Hadron Collider (LHC), LHCb is designed to study the physics of B-mesons – particles that contain a bottom quark or an antibottom quark. However, when protons smash together in the LHC, they also produce other mesons, including the D0 – a charm quark and an anti-up quark – and its antiparticle. In this latest analysis, LHCb physicists looked at the decay of D0 into either a kaon/antikaon or pion/antipion pair.
CP violation refers to a process whereby CP symmetry does not occur. CP symmetry says that a process involving a particle and a process involving the mirror image of its antiparticle should be identical. This means that D0 and anti-D0 particles should decay at exactly the same rate, which can be tested by creating the particles and watching them decay. Such a test can reveal “direct” CP violation, which has already been seen in kaons and B-mesons. In contrast, “indirect” CP violation – which was first seen in 1964 – involves such neutral mesons transforming into their antiparticles.
Dealing with asymmetries
One challenge in testing CP violation at the LHC is that the proton-proton collisions produce about 1% more anti-D0 particles than D0 particles and this asymmetry must be taken into account when searching for CP violation. Fortunately, the LHCb physicists have found a clever way around this problem. They measure the asymmetry in the decay of both D0 and anti-D0 to kaon/antikaon pairs and the asymmetry of these D-mesons in the decay to pion/antipion pairs. Then they subtract one from the other, which eliminates the production asymmetry and enhances the asymmetry caused by CP violation.
Speaking in Paris, Charles presented an analysis of about half of the collision data gathered so far, which reveals an asymmetry value of –0.82% with statistical and systematic uncertainties of 0.21% and 0.11%, respectively. The significance of the result is 3.5σ, which means that there is about a 0.05% possibility that the result is not real but rather a fluke. While this might seem like good odds, particle physicists usually require a 5σ significance to avoid being caught out by unknown systematic errors.
Beyond the Standard Model
While physicists had expected a negative value – previous analysis of data from Fermilab’s CDF experiment and theory point in that direction – the magnitude of the asymmetry comes as a surprise. In fact, the Standard Model suggests that the effect of direct CP violation on any decays involving charm quarks should be no larger than about 0.1%.
According to LHCb physicist Tim Gershon of the UK’s University of Warwick, one possibility for the discrepancy is that the CP violation involves new physics beyond the Standard Model. “But we also have to ask if there could be subtle effects that make the Standard Model CP violation larger than previously thought,” he cautions.
Alex Kagan of the University of Cincinnati, who works on the theory of direct CP violation, agrees that the new finding is “on the large side of what would be expected”. However, he points out that calculations of phenomena in charm-quark decays are notoriously difficult. “In the case of this measurement, I would say that the experimentalists are ahead of the theorists,” he says.
But with LHCb physicists only half way through analysing their 2011 data, Gershon believes that there may already be enough collisions lurking within the remaining data to bring the significance up to 5σ – making this the first important discovery of the LHC. As Charles told delegates in Paris, “Watch this space.”