A U-spin anomaly in charm CP violation

Abstract

Recent LHCb data shows that the direct CP asymmetries of the decay modes D0 → π+π− and D0 → K+K− have the same sign, violating an improved U-spin limit sum rule in an unexpected way at 2.1σ. From the new data, we determine for the first time the imaginary part of the CKM-subleading, U-spin breaking ∆U = 1 correction to the U-spin limit ∆U = 0 amplitude. The imaginary part of the ∆U = 0 amplitude is determined by $$ \Delta {a}_{\textrm{CP}}^{\textrm{dir}} $$ ∆ a CP dir . The corresponding strong phases are yet unknown and could be extracted in the future from time-dependent measurements. Assuming $$ \mathcal{O}(1) $$ O 1 strong phases due to non-perturbative rescattering, we find the ratio of U-spin breaking to U-spin limit contributions to the CKM-subleading amplitudes to be $$ \left({173}_{\hbox{--} 74}^{+85}\right)\% $$ 173 – 74 + 85 % . This highly exceeds the Standard Model (SM) expectation of ~ 30% U -spin breaking, with a significance of 1.95σ. If this puzzle is confirmed with more data in the future, in the SM it would imply the breakdown of the U -spin expansion in CKM-subleading amplitudes of charm decays. The other solution are new physics models that generate an additional ∆U = 1 operator, leaving the U-spin power expansion intact. Examples for the latter option are an extended scalar sector or flavorful Z′ models.