Are the CKM anomalies induced by vector-like quarks? Limits from flavor changing and Standard Model precision tests

Abstract

Recent high precision determinations of Vus and Vud indicate towards anomalies in the first row of the CKM matrix. Namely, determination of Vud from beta decays and of Vus from kaon decays imply a violation of first row unitarity at about 3σ level. Moreover, there is tension between determinations of Vus obtained from leptonic Kμ2 and semileptonic Kℓ3 kaon decays. These discrepancies can be explained if there exist extra vector-like quarks at the TeV scale, which have large enough mixings with the lighter quarks. In particular, extra vector-like weak singlets quarks can be thought as a solution to the CKM unitarity problem and an extra vector-like weak doublet can in principle resolve all tensions. The implications of this kind of mixings are examined against the flavour changing phenomena and SM precision tests. We consider separately the effects of an extra down-type isosinglet, up-type isosinglet and an isodoublet containing extra quarks of both up and down type, and determine available parameter spaces for each case. We find that the experimental constraints on flavor changing phenomena become more stringent with larger masses, so that the extra species should have masses no more than few TeV. Moreover, only one type of extra multiplet cannot entirely explain all the discrepancies, and some their combination is required, e.g. two species of isodoublet, or one isodoublet and one (up or down type) isosinglet. We show that these scenarios are testable with future experiments. Namely, if extra vector-like quarks are responsible for CKM anomalies, then at least one of them should be found at scale of few TeV, and anomalous weak isospin violating Z-boson couplings with light quarks should be detected if the experimental precision on Z hadronic decay rate is improved by a factor of 2 or so.

First-generation new physics in simplified models: from low-energy parity violation to the LHC

“…We first note that only the three representations U , D, or Q 1 (with couplings to both up and down quarks) can explain the CAA [1,[63][64][65] due to their mixing with the SM quarks, while, on the other hand, T 1 or T 2 worsen the CAA. EWPO places multi-TeV mass limits on the VLQs U , D, and Q 1 (for couplings fixed to unity), which come close to ruling out the best-fit regions from the CAA for U and D, while there is actually a small (1σ) preference for the other representations (Q 5,7 and T 1,2 ) driven by R 0 e and R 0 µ .…”

“…While the anomalies in semi-leptonic B decays and the anomalous magnetic moment of the muon point towards NP related to second-and third-generation fermions, the CAA and the CMS di-lepton excess can be related to first-generation quarks and leptons, with simultaneous explanations possible in terms of the effective dimension-6 operator Q (3) q [62]. Similarly, explanations of the CAA via modified W -u-d couplings also require NP related to first generation-quarks [1,[63][64][65]. In this paper, we take the large array of complementary measurements sensitive to first-generation NP, together with hints for potential NP effects, as motivation to perform a combined analysis, concentrating on possible correlations among the processes listed above.…”

Correlating nonresonant di-electron searches at the LHC to the Cabibbo-angle anomaly and lepton flavor universality violation

“…There are several possibilities to account for the CAA [52]. For instance, via modified W -quark couplings [34,53], a modified W − µν µ coupling [37,38,[54][55][56][57][58][59], a tree-level contribution to the muon decay [34,60,61] or by a tree-level effect in beta decays [62]. Since we aim at connecting the CAA to the CMS measurement we will focus on the latter possibility in which case only one operator, in the basis of Ref.…”

Explaining the Cabibbo Angle Anomaly

“…Solutions to the anomaly with vector-like quarks (VLQ) and vector-like leptons (VLL) are discussed in Ref. [1,2,31,[34][35][36]. They proceed via a modification of the gauge boson couplings with quarks and leptons respectively, therefore a global fit to a large set of observables has to be performed.…”