rpp2010-rev-cp-violation - 12 CP violation in meson decays...

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12. CP violation in meson decays 1 12. VIOLATION IN MESON DECAYS Revised August 2009 by D. Kirkby (UC Irvine) and Y. Nir (Weizmann Institute). The transformation combines charge conjugation C with parity P . Under C , particles and antiparticles are interchanged, by conjugating all internal quantum numbers, e.g. , Q →− Q for electromagnetic charge. Under P , the handedness of space is reversed, ±x . Thus, for example, a left-handed electron e L is transformed under into a right-handed positron, e + R . If were an exact symmetry, the laws of Nature would be the same for matter and for antimatter. We observe that most phenomena are C -and P -symmetric, and therefore, also -symmetric. In particular, these symmetries are respected by the gravitational, electromagnetic, and strong interactions. The weak interactions, on the other hand, violate C and P in the strongest possible way. For example, the charged W bosons couple to left-handed electrons, e L , and to their -conjugate right-handed positrons, e + R , but to neither their C -conjugate left-handed positrons, e + L ,northe ir P -conjugate right-handed electrons, e R . While weak interactions violate C and P separately, is still preserved in most weak interaction processes. The symmetry is, however, violated in certain rare processes, as discovered in neutral K decays in 1964 [1], and observed in recent years in B decays. A K L meson decays more often to π e + ν e than to π + e ν e ,thu s allowing electrons and positrons to be unambiguously distinguished, but the decay-rate asymmetry is only at the 0.003 level. The -violating e±ects observed in B decays are larger: the asymmetry in B 0 / B 0 meson decays to eigenstates like J/ψK S is about 0.70 [2,3]. These e±ects are related to K 0 K 0 and B 0 B 0 mixing, but violation arising solely from decay amplitudes has also been observed, ²rst in K ππ decays [4–6] and more recently in various neutral [7,8] and charged [9,10] B decays. violation has not yet been observed in D or B s meson decays, or in the lepton sector. In addition to parity and to continuous Lorentz transformations, there is one other spacetime operation that could be a symmetry of the interactions: time reversal T , t t . Violations of T symmetry have been observed in neutral K decays [11], and are expected as a corollary of violation if the combined CPT transformation is a fundamental symmetry of Nature [12]. All observations indicate that is indeed a symmetry of Nature. Furthermore, one cannot build a Lorentz-invariant quantum ²eld theory with a Hermitian Hamiltonian that violates .(A ts ev e r a lpo in t sinou r discussion, we avoid assumptions about , in order to identify cases where evidence for violation relies on assumptions about .) Within the Standard Model, symmetry is broken by complex phases in the Yukawa couplings (that is, the couplings of the Higgs scalar to quarks). When all manipulations to remove unphysical phases in this model are exhausted, one ²nds that there is a single -violating parameter [13]. In the basis of mass eigenstates, this single phase appears in the 3 ×
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rpp2010-rev-cp-violation - 12 CP violation in meson decays...

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