Patterns of flavour violation in models with vector-like quarks

Abstract

We study the patterns of flavour violation in renormalisable extensions of the Standard Model (SM) that contain vector-like quarks (VLQs) in a single complex representation of either the SM gauge group G SM or G SM ≡ G SM ⊗ U(1) Lµ−Lτ . We first decouple VLQs in the M = (1 − 10) TeV range and then at the electroweak scale also Z, Z gauge bosons and additional scalars to study the phenomenology. The results depend on the relative size of Z-and Z -induced flavour-changing neutral currents, as well as the size of |∆F | = 2 contributions including the effects of renormalisation group Yukawa evolution from M to the electroweak scale that turn out to be very important for models with right-handed currents through the generation of left-right operators. In addition to rare decays like P → ¯ , P → P ¯ , P → P νν with P = K, B s , B d and |∆F | = 2 observables we analyze the ratio ε /ε which appears in the SM to be significantly below the data. We study patterns and correlations between these observables which taken together should in the future allow for differentiating between VLQ models. In particular the patterns in models with left-handed and right-handed currents are markedly different from each other. Among the highlights are large Z-mediated new physics effects in Kaon observables in some of the models and significant effects in B s,d -observables. ε /ε can easily be made consistent with the data, implying then uniquely the suppression of K L → π 0 νν. Significant enhancements of Br(K + → π + νν) are still possible. We point out that the combination of NP effects to |∆F | = 2 and |∆F | = 1 observables in a given meson system generally allows to determine the masses of VLQs in a given representation independently of the size of VLQ couplings.

Direct CP Violation in K→μ+μ−

“…The following is a specific example of new physics: we focus on a modified Z-coupling model [41][42][43][44][45], which can easily explain a 2.8σ-2.9σ discrepancy in K / K between the measured values and the predicted one at next-toleading order [7][8][9]. In this model, after the electroweak symmetry breaking, the following flavor-changing Z interactions emerge:…”

Flavourful Z ′ models for R K ∗ $$ {R}_{K^{\left(\ast \right)}} $$

“…One of the key challenges faced by these models is the requirement that they be anomaly free. This has motivated the phenomenological analysis of Z ′ models based on gauged L µ − L τ , possibly combined with vector-like quarks [38]. Without a Z ′ , vector-like quarks directly mixing with ordinary quarks via the Higgs Yukawa couplings can lead to FCNCs [39,40].…”

“…Without a Z ′ , vector-like quarks directly mixing with ordinary quarks via the Higgs Yukawa couplings can lead to FCNCs [39,40]. However, vector-like quarks with a gauged U(1) ′ typically forbids the Higgs coupling of vector-like quarks to ordinary quarks, but allows new possibilities [38]. For example, a simple idea is to have a dark U(1) X under which the SM quarks and leptons are neutral, but which is felt by vector-like fermions with the SM quantum numbers of the doublets Q L and L L , leading to a dark matter candidate and flavour-changing Z ′ operators after the vector-like fermion mass terms mix with SM fermions [41,42].…”

Standard Model and New physics for ε′k/εk

“…Such interference contributions between the NP and the SM have been overlooked in the literature. References [35][36][37] have revisited the modified Z-coupling scenario including the interference contributions, and found the parameter regions allowed by the indirect CP violation change significantly.…”