Less space for a new family of fermions

Physical review D: Particles and fields (Impact Factor: 4.86). 05/2010; 82(9). DOI: 10.1103/PhysRevD.82.095006
Source: arXiv


We investigate the experimentally allowed parameter space of an extension of the standard model (SM3) by one additional family of fermions. Therefore we extend our previous study of the CKM like mixing constraints of a fourth generation of quarks. In addition to the bounds from tree-level determinations of the 3$\times$3 CKM elements and FCNC processes ($K$-, $D$-, $B_d$-, $B_s$-mixing and the decay $b \to s \gamma$) we also investigate the electroweak $S$, $T$, $U$ parameters, the angle $\gamma$ of the unitarity triangle and the rare decay $B_s \to \mu^+ \mu^-$. Moreover we improve our treatment of the QCD corrections compared to our previous analysis. We also take leptonic contributions into account, but we neglect the mixing among leptons. As a result we find that typically small mixing with the fourth family is favored, but still some sizeable deviations from the SM3 results are not yet excluded. The minimal possible value of $V_{tb}$ is 0.93. Also very large CP-violating effects in $B_s$ mixing seem to be impossible within an extension of the SM3 that consists of an additional fermion family alone. We find a delicate interplay of electroweak and flavor observables, which strongly suggests that a separate treatment of the two sectors is not feasible. In particular we show that the inclusion of the full CKM dependence of the $S$ and $T$ parameters in principle allows the existence of a degenerate fourth generation of quarks. Comment: 38 pages, 26 figures; references added

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    • "A heavy fourth sequential family, for example, is now disfavored on these grounds [3] [4]. Precision electroweak data constrain the mass splitting between the fourth family quarks, while data from B-meson physics restrict their mixing pattern [5] [6] [7] [8] [9] [10] [11]. Direct searches performed by the ATLAS and CMS collaborations rule out mass-degenerate fourth generation quarks with masses below 685 GeV [12]. "
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    ABSTRACT: We present a new model based on the SU(3)xSU(2)xU(1) symmetry, in which there is a new consistent set of chiral fermion fields that renders the model free from anomalies. The new fermions do not share the usual family structure of the Standard Model and some of them have exotic electric charges, as the quarks X and Y with electric charge 5/3 and -4/3, respectively. Interestingly, the model contains a new heavy neutral lepton which may be a dark matter candidate. Two Higgs doublets are present in our construction, so that two CP even scalars are present in the model particle spectrum. One of them is similar to Standard Model Higgs boson, while the other one couples mainly with the new exotic fermions. We performed a discovery analysis showing that the 8 TeV LHC can find the Y quark from single and pair production with masses from 300 GeV up to ~ 750 GeV. We also show that the new spectrum does not contribute significantly to the oblique EW parameters, and that dangerous flavor changing neutral currents are suppressed. Characteristic signatures from the other new fermions in the model are also commented.
    Journal of High Energy Physics 06/2013; 2013(7). DOI:10.1007/JHEP07(2013)129 · 6.11 Impact Factor
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    • "As we neglect the 1-4 and 2-4 CKM mixing, the electroweak precision observables are found to give more severe constraints on the maximally allowed values for |V t b |. However, both the electroweak precision and the flavour constraints are known to favour small mixing with a heavy fourth family [42]. "
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    ABSTRACT: A perturbative new family of fermions is now severely constrained, though not excluded yet. We reconsider the current bounds (i.e., direct and from Higgs searches, R_b, oblique parameters) on the fourth generation parameter space assuming the case of a small CKM mixing with the third generation. We identify viable scenarios featuring either a light or a heavy Higgs boson. A set of representative benchmark points targeted for LHC searches is proposed with a normal (inverted) quark mass hierarchy where t' -> b'W (b' -> tW) decays are sizable. In the case where the fourth generation couplings to the lighter quark families are small, we suggest that search strategies at the LHC should include both pair (strong) and single (weak) production with bb+nW (n=2,...,6) final state signatures.
    Journal of High Energy Physics 04/2012; 2012(6). DOI:10.1007/JHEP06(2012)135 · 6.11 Impact Factor
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    • "For a fourth-generation up-type quark, the mass splitting between the t quark and the corresponding down-type b quark is favoured to be smaller than the mass of the W boson [11] [12] [13]. "
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    ABSTRACT: The results of a search for the pair production of a fourth-generation up-type quark (t') in proton-proton collisions at sqrt(s) = 7 TeV are presented, using data corresponding to an integrated luminosity of about 5.0 inverse femtobarns collected by the Compact Muon Solenoid experiment at the LHC. The t' quark is assumed to decay exclusively to a W boson and a b quark. Events with a single isolated electron or muon, missing transverse momentum, and at least four hadronic jets, of which at least one must be identified as a b jet, are selected. No significant excess of events over standard model expectations is observed. Upper limits for the t' anti-t' production cross section at 95% confidence level are set as a function of t' mass, and t'-quark production for masses below 570 GeV is excluded. The search is equally sensitive to nonchiral heavy quarks decaying to Wb. In this case, the results can be interpreted as upper limits on the production cross section times the branching fraction to Wb.
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