March 12th 2013

Summary: New LHCb measurements released on matter anti-matter differences using charm quarks show no significant evidence for an asymmetry

Image: Event Display of B meson (blue) decaying into a charm meson (green) and a muon (purple). The charm meson then decays into the two (light blue) final state tracks

(Credit LHCb)

New measurements have been released by the LHCb collaboration on the search for matter anti-matter differences (CP violation) in particles containing charm quarks.  A previous LHCb measurement gave a first hint of CP violation differences in the charm sector. This was further strengthened by results from two other experiments (CDF in the USA, and Belle in Japan). The new results, which are more precise, are compatible with the previous results but no longer provide significant evidence for CP Violation.

Two new measurements have been performed. Both are of the same quantity, known as ΔACP, the difference between CP violation in charm-quark containing D mesons decaying into two modes. Taking the difference between the two decay modes helps to cancel effects coming either from the detector response or from the original production that are not related to the underlying matter anti-matter differences. One of the new results is an update of the original measurement which is nearly twice as large as the original sample, and uses improved detector calibrations and reconstruction software. The second uses a different technique and event sample to access the same quantity. Both measurements show a smaller indication for CP violation than the original study.

Dr Matthew Charles, an STFC Advanced Fellow at the University of Oxford, who worked on the final data analysis said "it seems that the original result was an upward statistical fluctuation - the new results are compatible with the previous ones but no longer show as strong an effect for CP violation" and remarked "sometimes nature is kind and discoveries can be made, and sometimes things turn out to be more tricky, and one has to work a bit harder."

Dr Marco Gersabeck, an STFC Ernest Rutherford Fellow at the University of Manchester, and convenor of the LHCb charm working group said "We have collected an enormous sample of charm mesons from the first LHC running period, and these will give us many ways to search for matter anti-matter differences over the coming years. The charm sector is still largely unexplored and has many places for surprises to be hiding.".

The results will be presented for the first time at 11:00 CET (10:00 UK) on March 12th, at a CERN seminar and at the Recontres de Moriond QCD and high energy interactions conference.

Technical information:

The results are both obtained on 1fb-1 of data collected in 2011.

From the update of the original measurement using D mesons produced promptly from the proton-proton collision:

  ΔACP = (−0.34 ± 0.15 (stat.) ± 0.10 (syst.))%.

The result is statistically compatible with the original measurement. From the new measurement technique that uses D mesons produced from the decay of a B meson

  ∆ACP = (+0.49 ± 0.30 (stat.) ± 0.14 (syst.))%.

The result from the two techniques agree at the 2 sigma level.


Dr Matthew Charles, University of Oxford,matthew.john.charles ATNOSPAM

Dr Marco Gersabeck, University of Manchester, Marco.Gersabeck 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.

Full Results available on LHCb Winter Results Page

Previous Highlight Results

Ultra-rare Decay Spotted - November 2012

Mysterious Particle is ‘Exotic’ state - February 2013