The Impact of a 4th Generation on Mixing and CP Violation in the Charm System

TL;DR

This paper investigates how a hypothetical fourth generation of quarks could significantly influence mixing and CP violation in the charm system, predicting observable effects beyond the Standard Model's capabilities.

Contribution

It provides the first detailed analysis of CP violation and mixing in the charm system within the Standard Model with four generations (SM4), highlighting unique correlations and potential experimental signatures.

Findings

Large CP violation effects are possible in the charm system with SM4.

Correlations between charm and other mesonic observables are identified.

Large effects in B_s system are feasible unless certain CP asymmetries are extreme.

Abstract

We study D0-D0 mixing in the presence of a fourth generation of quarks. In particular, we calculate the size of the allowed CP violation which is found at the observable level well beyond anything possible with CKM dynamics. We calculate the semileptonic asymmetry a_SL and the mixing induced CP asymmetry eta_fS_f which are correlated with each other. We also investigate the correlation of eta_fS_f with a number of prominent observables in other mesonic systems like epsilon'/epsilon, Br(K_L -> pi0 nu nu), Br(K+ -> pi+ nu nu), Br(B_s ->mu+ mu-), Br(B_d -> mu+ mu-) and finally S_psi phi in the B_s system. We identify a clear pattern of flavour and CP violation predicted by the SM4 model: While simultaneous large 4G effects in the K and D systems are possible, accompanying large NP effects in the B_d system are disfavoured. However this behaviour is not as pronounced as found for the LHT and RSc models. In contrast to this, sizeable CP violating effects in the B_s system are possible unless extreme effects in eta_fS_f are found, and Br(B_s ->mu+ mu-) can be strongly enhanced regardless of the situation in the D system. We find that, on the other hand, S_psi phi > 0.2 combined with the measured epsilon'/epsilon significantly diminishes 4G effects within the D system.