A charming measurement of CP violation

Although the violation of CP symmetry in the weak decays of some hadrons was initially a surprise to those in the field, the standard model of particle physics allows for and, in fact, predicts it (see Physics Today, December 2008, page 16 ). Over the past 55 years, researchers have detected subtle differences in the decay rates of K and anti-K mesons and of B and anti-B mesons (see Physics Today, May 2001, page 17 ). Now researchers at the Large Hadron Collider (LHC) have found strong evidence for CP violation in the decays of D mesons, which contain charm quarks. It’s the first time such asymmetry has been observed in particles with an up-type (+2/3 charge) quark.

Researchers with the LHCb detector , a spectrometer designed for studying hadrons with bottom or charm quarks, measured the decay rates of both the neutral D meson, which is composed of a charm quark bound to an up antiquark, and its antimatter partner. To distinguish between matter and antimatter, the researchers relied on particles produced alongside the mesons that served as tags. For example, the detection of a positively charged pion indicated the presence of a D⁰, whereas a positively charged muon accompanied an anti-D⁰. By clocking both the tags and subsequent decays into either K⁺K^– or π⁺π^–, the researchers developed a census of the decays, which they used to hunt for differences in the decay rates. Exploiting all the LHCb data collected from 2011 to 2018—9 femtobarn^–1 worth of particle-collision events in all—they measured a discrepancy of about 0.15%, a value 5.3 standard deviations from zero.

The measured degree of CP violation is consistent with the standard model, but it’s on the high end of theoretical expectations. Further probing hadrons with up-type quarks could expose potential discrepancies that go beyond the standard model. The LHC is shut down for upgrades and will reopen in 2021. Other experiments, including the Belle II detector in Japan , may be able to chime in as well. (R. Aaij et al., LHCb collaboration, Phys. Rev. Lett., in press ; first reported at Rencontres de Moriond conference , 21 March 2019.)