More statistics, less surprise

The LHCb collaboration has recently announced new results for a parameter that measures the CP violation effect in particles containing charm quarks. The new values obtained with a larger data set and with a new independent method are showing that the effect is smaller than previous measurements had  suggested. The parameter is back into the Standard Model picture.

CP violation signals – in particles containing charm quarks, such as the D⁰ particle, is a powerful probe of new physics. Indeed, such effects could result in unexpected values of parameters whose expectation values in the Standard Model are known. Although less precise than similar approaches used in particles made of b quarks, the investigation of the charm system has proven  to be intriguing.

The LHCb collaboration has reported new measurements of ΔA_CP, the difference in CP violation between the D⁰→K⁺K^– and D⁰→π⁺π^– decays. The results are obtained with two independent analyses performed on the full 2011 data set. The two methods allow useful cross-checks, for example because background has different origins in the two analyses and can therefore be understood better.

Compared to LHCb previous value, the new averaged result, ΔA_CP = (−0.15 ± 0.16)%, is closer to zero and therefore does not confirm the previous evidence of CP violation in the charm system. Detailed investigations on the samples reveal that the change from the previous result is consistent with statistical fluctuations.

Theoretical work triggered by the initial surprising measurement had shown that although the non-zero value found by LHCb could be accommodated within the Standard Model picture, the room for new physics remained open. These new results constrain significantly the window still open to new physics. Further update of this and related measurements will be needed to discover if – and at what level – nature distinguishes between charm and anticharm.

by Antonella Del Rosso & the LHCb collaboration