Measurement of the charm mixing parameter y CP − y K

A measurement of the ratios of the effective decay widths of D 0 → π − π þ and D 0 → K − K þ decays over that of D 0 → K − π þ decays is performed with the LHCb experiment using proton – proton collisions at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 6 fb − 1 . These observables give access to the charm mixing parameters y ππ CP − y K π CP and y KKCP − y K π CP , and are measured as y ππ CP − y K π CP ¼ ð 6 . 57 (cid:2) 0 . 53 (cid:2) 0 . 16 Þ × 10 − 3 , y KKCP − y K π CP ¼ ð 7 . 08 (cid:2) 0 . 30 (cid:2) 0 . 14 Þ × 10 − 3 , where the first uncertainties are statistical and the second systematic. The combination of the two measurements is y CP − y K π CP ¼ ð 6 . 96 (cid:2) 0 . 26 (cid:2) 0 . 13 Þ × 10 − 3 , which is four times more precise than the previous world average.