High Precision on Charming  Matter Anti-matter Differences

September 7th 2013

Summary: New measurements of matter-antimatter asymmetries involving charm quarks have been presented at the CHARM 2013 workshop. The results provide the most stringent limits on these asymmetries to date.

Stringent limits on CP violation: Combination of existing measurements to a world average of the CP-violation parameters |q/p| and φ with the two latest LHCb results included. The black dot marks the no-CP violation point.

Image: .(Credit LHCb)

There are four neutral mesons which allow particle-antiparticle transitions---mixing. These are ideal laboratories for studies of matter-antimatter asymmetries (CP violation) and indeed for three of these mesons, neutral kaons as well as B0 and Bs0, such an asymmetry has been observed. For neutral charm mesons (D0) such searches have thus far not revealed a positive result. Being the only of the four mesons to contain up-type quarks, D0 mesons provide a unique access to effects from physics beyond the Standard Model.

Several new measurements are available from the LHCb experiment and have been presented at the CHARM 2013 workshop, held in Manchester from 31st August to 4th September 2013. The most important of these results is based on an analysis spearheaded by UK scientists from the Universities of Manchester, Glasgow, and Oxford. It makes use of several million decays into a pair of kaons or a pair of pions recorded in 2011. The asymmetry between the mean lifetimes measured in D0 and D0bar decays is called AΓ, which is a measurement of so-called indirect CP violation---the type of CP violation related to meson mixing. The results [1] for the two final states are
AΓ(KK) = (-0.04±0.06±0.01)%
AΓ(ππ) = (0.03±0.11±0.01)%.

Dr Marco Gersabeck of the University of Manchester a lead scientist on these measurements, and co-chair of the Charm2013 Workshop explained

“This is the first time that a search for indirect CP violation in charm mesons reaches a sensitivity of better than 0.1%. This measurement is a major milestone for LHCb’s charm physics programme.”

Dr Chris Thomas of the University of Oxford, who also led one of the measurements for this new result added that

“The LHCb results are now driving the world precision in this area, and these results are based on only one third of the data sample we have collected. The precision of these results allows us to search for  new physics beyond the Standard Model”.

The combination of previous measurements performed by the Heavy Flavor Averaging Group hinted at potentially non-zero values for the parameters of CP violation in D0 mixing, |q/p| and φ. Shown in the figure, the new results from LHCb do not support this indication and provide very stringent limits on the underlying parameters, thus constraining the room for physics beyond the Standard Model.

[1] LHCb-PAPER-2013-054, to be submitted to Phys. Rev. Lett.

Dr Marco Gersabeck, University of Manchester, marco.gersabeck ATNOSPAM
Dr Chris Thomas, University of Oxford c.thomas2 ATNOSPAM 
The UK participation in the international LHCb experiment is from eleven institutes.
University of Birmingham, University of Bristol, University of Cambridge, University of Edinburgh, University of Glasgow, Imperial College London, University of Liverpool, University of Manchester, University of Oxford, STFC Rutherford Appleton Laboratory, University of Warwick

UK participation in the experiment is funded by the Science and Technology Facillities Council (STFC), with contributions from the participating institutes, the Royal Society and European Union.