Measurement of the Charm-Mixing Parameter

Neutral charm mesons can change their flavor and turn into antimesons, and vice versa, before they decay. This phenomenon, known as flavor oscillation or D–D mixing, occurs because the eigenstates of the Hamiltonian governing the time evolution of the neutral D system are superpositions of the flavor eigenstates, jD1;2i 1⁄4 pjD0i qjD0i, where p and q are complex parameters satisfying jpj2 þ jqj2 1⁄4 1. In the limit of charge-parity (CP) symmetry, q equals p and the oscillations are characterized by only two dimensionless parameters, x≡ ðm1 −m2Þ=Γ and y≡ ðΓ1 − Γ2Þ=2Γ, where m1ð2Þ and Γ1ð2Þ are the mass and decay width of the CP-even (odd) eigenstate D1ð2Þ, respectively, and Γ≡ ðΓ1 þ Γ2Þ=2 is the average decay width [1]. The values of x and y are of the order of 1% or smaller [2]. In the presence of CP violation, the mixing rates for mesons produced as D and D differ, further enriching the phenomenology. Because of D–D mixing, the effective decay width ΓCPþ of decays to CP-even final states, such as hþh− (h 1⁄4 K, π), differs from the average width Γ. The latter can be measured in decays that involve an equal mixture of CPeven and CP-odd states, such asD → K−πþ. (Throughout this Letter, the inclusion of the charge-conjugate decay mode is implied unless otherwise stated.) The quantity