Measuring CP asymmetries in the decay of B_d⁰ mesons produced in proton-proton interactions

Apart from statistics, which is a major consideration in the study of CP violation, certain systematic effects occur which give rise to false asymmetries. One of these arises because of the difference in production cross section of, for example, B⁰ and B⁰ mesons, because of the unequal associated production of baryon and antibaryon containing respectively a b and b quark, despite the overall equality in production of b and b quarks. Unlike K⁰ decays, where the long-lived (K_L) mass eigenstate is self-selecting, no certain identification of the B⁰(B⁰) CP eigenstate is possible from the single decay, and it is necessary to tag the B⁰ via the decay of an accompanying B particle. A second, false, asymmetry may then arise from misidentified tags, in addition to a dilution of the signal which most certainly does occur. The authors concentrate on these two systematic effects, illustrated for the very important decay B_d⁰ to psi K_S, where the psi is identified from its decay into two leptons ( mu ⁺ mu ^- and e⁺e^-), and the K_S from its decay into pi ⁺ pi ^-.