After the recent high precision determinations of Vus and V ud , the first row of the CKM matrix shows more than 4σ deviation from unitarity. Two possible scenarios beyond the Standard Model can be investigated in order to fill the gap. If a 4th quark b participates in the mixing, with |V ub | ∼ 0.04, then its mass should be no more than 6 TeV or so. A different solution can come from the introduction of the gauge horizontal family symmetry acting between the lepton families and spontaneously broken at the scale of about 6 TeV. Since the gauge bosons of this symmetry contribute to muon decay in interference with Standard Model, the Fermi constant is slightly smaller than the muon decay constant so that unitarity is recovered. Also the neutron lifetime problem, that is about 4σ discrepancy between the neutron lifetimes measured in beam and trap experiments, is discussed in the light of the these determinations of the CKM matrix elements.1. The Standard Model (SM) contains three fermion families in the identical representations of the gauge symmetry SU (3)×SU (2)×U (1) of strong and electroweak interactions. One of its fundamental predictions is the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix of quark mixing in charged current(1)Deviation from the CKM unitarity can be a signal of new physics beyond the Standard Model (BSM). The experimental precision and control of theoretical uncertainties in the determination of the elements in the first row of V CKM are becoming sufficient for testing the conditionSince |V ub | 0.004 is very small, its contribution is negligible and (2) reduces essentially to the check of the Cabibbo mixing: |V us | = sin θ C , |V ud | = cos θ C and |V us /V ud | = tan θ C . In essence, this is the universality test for the W -boson coupling (g/ √ 2)W + µ J µ L + h.c. to the relevant part of the charged left-handed currentFor energies smaller than W -boson mass this coupling gives rise to the effective current × current interactionswhich are responsible for leptonic decays of the neutron, pions, kaons etc., as well as to the interaction − 4G F √ 2 e L γ µ ν e ν µ γ µ µ L (5) * responsible for the muon decay. All these couplings contain the Fermi constant G F / √ 2 = g 2 /8M 2 W . Precision experimental data on kaon decays, in combination with the lattice QCD calculations of the decay constants and form-factors, provide accurate information about |V us |. On the other hand, recent calculations of short-distance radiative corrections in the neutron decay allow to determine |V ud | with a remarkable precision.In this paper we analyze the present individual determinations of V ud and V us and find significant (more than 4σ) deviation from the CKM unitarity (2). We discuss two possible BSM scenarios which can explain this deviation. In the first one the three-family unitarity is extended to four species, by introducing the 4th down-type quark b with mass of few TeV. The second scenario assumes the existence of horizontal gauge symmetry between the lepton families which is spontaneously br...

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.