CP asymmetry in the decays B --> (X_s, X_d) mu+ mu- with four generations

TL;DR

This paper investigates how the addition of a fourth quark generation could significantly enhance CP asymmetry in certain B meson decays, especially at high momentum transfer, compared to the Standard Model predictions.

Contribution

It provides a comprehensive analysis of CP asymmetry in B decays within a four-generation model, exploring parameter space and constraining it with current data.

Findings

CP asymmetry can be enhanced by over an order of magnitude at high q^2 for B --> X_s mu+ mu-

CP asymmetry in B --> X_d mu+ mu- can be increased up to six times the SM prediction

Saturation of asymmetry enhancement occurs at high t' quark masses, up to 25 times the SM value

Abstract

We estimate the CP asymmetry A_{CP}(q^2) in the decays B --> X_s mu+ mu- and B --> X_d mu+ mu- in the standard model (SM) with an additional fourth generation. We use a parametrization that allows us to explore the complete parameter space of the 4X4 quark mixing matrix, and constrain these parameters from the current data on B decays. We find that the enhancement in A_{CP}(q^2) depends strongly on the mass of the t', the up-type quark in the fourth generation. For m_t' around 400 GeV, the CP asymmetry in the high-q^2 region (q^2 > 14.4 GeV^2) can be enhanced by more than an order of magnitude for B --> X_s mu+ mu- and up to a factor of 6 for B --> X_d mu+ mu-. There is no enhancement in the low-q^2 region (1< q^2 <6 GeV^2). With increasing m_t', the A_{CP}(q^2) in the high-q^2 (low-q^2) region first decreases (increases) and then saturates at a value a few times the SM prediction. In the high-q^2 region of B --> X_s mu+ mu-, this saturation value may be up to 25 times the SM expectation.