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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Available from: Alexander Lenz
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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.
    Full-text · Article · Jun 2013 · Journal of High Energy Physics
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    • "In Figure 1 we display the resulting ratio í µí±Ÿ Ω = Ω 1 /Ω DM , where Ω DM is the observed total DM density [1], for various values of í µí±š 1 and the í µí¼’ 1 -í µí¼’ 2 mixing angle í µí¼ƒ, as the í µí¼’ 1 couplings to the Higgs, í µí±Š, and í µí± bosons depend on í µí¼ƒ, which varies from 0 to í µí¼‹/4. In the numerical work, Q ℓ is set to unity, the í µí±’ 4 mass to í µí±š í µí±’ 4 = í µí±š 1 + 50 GeV, consistent with constraints from electroweak precision data [22] [23], and the Higgs mass to í µí±š ℎ = 115 GeV, outside the 120–600 GeV zone excluded by LHC data at 95% CL [24] [25]. Hence in this model the stable fourthgeneration neutrino can constitute up to about half of the total DM density. "
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    ABSTRACT: Dark matter presents perhaps one of the most compelling direct indications of new physics beyond the standard model with three generations of fermions. In this paper, we survey several scenarios for dark matter in association with a fourth generation of chiral matter. The surveyed scenarios include stable heavy neutrino dark matter, composite dark matter consisting of stable heavy quarks, heavy quarks as mediators between the dark and visible sectors, and the four-generation standard model with the minimal addition of a stable real scalar field. We discuss the basic properties of the models, direct search constraints on their dark matter, and their collider phenomenology, as well as the possible effects of dark matter on the searches for a Higgs boson in the presence of four generations. We also comment on the potential implication of the recent observation of a Higgs-like new particle at the LHC.
    Full-text · Article · Mar 2013 · Advances in High Energy Physics
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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.
    Full-text · Article · Apr 2012 · Journal of High Energy Physics
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