Observation of CP violation in charm mesons

21 March 2019

The LHCb collaboration, of which Oxford is a founding institute, has today (21/3/19) announced a major new discovery in particle physics: the first observation of CP violation in decays of particles containing charm quarks.

CP violation can be considered the difference in behaviour between matter and antimatter. The fact that the universe is made overwhelmingly of matter is itself an observation of CP violation, but one not explained by the Standard Model of particle physics (SM). The SM accommodates CP violation in certain particle decays, but at a level far too small to explain this cosmological mystery. The LHCb experiment at CERN, which Oxford physicists co-proposed in 1995, has embarked on an extensive programme of measurements since the LHC inception in 2010, to study CP violation and probe for effects that may point to a deeper understanding of nature.

CP violation has been observed in mesons containing strange quarks (in 1964), and beauty quarks (in 2001), but never with charm. This is because CP violation in charm is known to be exceedingly small, and was feared to lie far beyond experimental sensitivity. To pursue this physics, the LHCb experiment has recorded many tens of millions of neutral charm meson decays to two kaons, and also to two pions, which are both considered promising processes to search for CP violation. A CP asymmetry, which quantifies the difference in decay rate between charm and anti-charm particles, has been calculated for both decay modes. The difference in these CP asymmetries, dubbed \Delta A_{CP}, has now been determined to unprecedented accuracy. The measurement of \Delta A_{CP} has many experimental advantages; many possible sources of experimental bias can be eliminated, whilst remaining sensitive to genuine CP violation, which is expected to be non-identical for the two categories of decay.

The result, announced today and submitted in a paper to Physics Review Letters, is \Delta A_{CP} = (-0.154\pm 0.029)\% is 5.3 standard deviations from zero and is the first observation of CP violation in charm. The effect, though extremely small, is at the higher end of theoretical expectations and opens up an exciting new field of study.

Oxford physicists have played a central role in charm physics studies on LHCb, as well as having a leading role in the construction and operation of the Ring Imaging Cherenkov Counters (RICH), which are essential for cleanly identifying the kaons and pions in the decay of the charm mesons. The Oxford group is now leading an upgrade of the RICH system, and other components of the experiment, which will allow for even more precise studies of this and other phenomena to be performed over the coming decade.

- CERN press release: home.cern/news/press-release/physics/lhcb-sees-new-flavour-matter-antimatter-asymmetry

- LHCb public page: lhcb-public.web.cern.ch/lhcb-public/Welcome.html

- Preprint: http://cds.cern.ch/record/2668357/files/LHCb-PAPER-2019-006.pdf