科研之友
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摘要
The experimental value for the isospin amplitudeReA(2) in K -> pi pi decays has been successfully explained within the standard model (SM), both within the large N approach to QCD and by QCD lattice calculations. On the other hand within the large N approach the value of ReA(0) is by at least 30% below the data. While this deficit could be the result of theoretical uncertainties in this approach and could be removed by future precise QCD lattice calculations, it cannot be excluded that the missing piece in ReA(0) comes from new physics (NP). We demonstrate that this deficit can be significantly softened by tree-level FCNC transitions mediated by a heavy colourless Z' gauge boson with a flavour-violating left-handed coupling Delta(sd)(L) (Z') and an approximately universal flavour diagonal right-handed coupling Delta(qq)(R) (Z') to the quarks. The approximate flavour universality of the latter coupling assures negligible NP contributions to ReA(2). This property, together with the breakdown of the GIM mechanisms at tree level, allows one to enhance significantly the contribution of the leading QCD-penguin operator Q(6) to ReA(0). Alarge fraction of the missing piece in the Delta I = 1/2 rule can be explained in this manner for M-Z' in the reach of the LHC, while satisfying the constraints from epsilon(K), epsilon'/epsilon, Delta M-K, LEP-II and the LHC. The presence of a small right-handed flavour-violating coupling Delta(sd)(R) (Z') << Delta(sd)(L) (Z') and of enhanced matrix elements of Delta S = 2 left-right operators allows one to satisfy simultaneously the constraints from ReA(0) and Delta M-K, although this requires some fine-tuning. We identify the quartic correlation between Z' contributions to ReA(0), epsilon'/epsilon, epsilon(K) and Delta M-K. The tests of this proposal will require much improved evaluations of ReA(0) and Delta M-K within the SM, of < Q(6)>(0) as well as precise tree-level determinations of vertical bar V-ub vertical bar and vertical bar V-cb vertical bar. We present correlations between epsilon/epsilon', K+ -> pi(+)nu nu(-). and K-L -> pi(0)nu nu(-). with and without the Delta I = 1/2 rule constraint and gener-alise the whole analysis to Z' with colour (G') and Z with FCNC couplings. In the latter case no improvement on ReA(0) can be achieved without destroying the agreement of the SM with the data on ReA(2). Moreover, this scenario is very tightly constrained by epsilon'/epsilon. On the other hand, in the context of the Delta I = 1/2 rule G' is even more effective than Z': it provides the missing piece in ReA(0) for M-G' = (3.5-4.0) TeV.
- 出版日期2014-7-11
