Masses Of Fourth Generation Quarks Research Articles

Dispersive determination of fourth generation quark masses

We determine the masses of the sequential fourth generation quarks b′ and t′ in the extension of the Standard Model by solving the dispersion relations associated with the mixing between the neutral states Qq¯ and Q¯q, with Q (q) being a heavy (light) quark. The box diagrams responsible for the mixing, which provide the perturbative inputs to the dispersion relations, involve multiple intermediate channels, i.e., the ut and ct channels, u (c, t) being an up (charm, top) quark, in the b′ case, and the db′, sb′, and bb′ ones, d (s, b) being a down (strange, bottom) quark, in the t′ case. The common solutions for the above channels lead to the masses mb′=(2.7±0.1) and mt′≈200 TeV unambiguously. We show that these superheavy quarks, forming bound states in a Yukawa potential, barely contribute to Higgs boson production via gluon fusion and decay to photon pairs and bypass current experimental constraints. The mass of the b¯′b′ ground state is estimated to be about 3.2 TeV. It is thus worthwhile to continue the search for b′ quarks or b¯′b′ resonances at the (high-luminosity) Large Hadron Collider. Published by the American Physical Society 2024

Some new symmetric relations of quark masses and prediction of fourth-generation quark masses

In this study, we discover a mass space defined by generalized Koide relations, named here as k-relations, and achieve some new symmetric relations. These relations can be further used to predict the fourth-generation quark masses in terms of dilation magnitude and angular rotation ratios in the general mass space. Thus far, no theory has been proposed that can constrain the number of generations of quarks; this theory naturally limits the number of generations of quarks.

FOURTH GENERATION MAJORANA NEUTRINO, DARK MATTER AND HIGGS PHYSICS

We consider extensions of the standard model with fourth generation fermions (SM4) in which extra symmetries are introduced such that the transitions between the fourth generation fermions and the ones in the first three generations are forbidden. In these models, the stringent lower bounds on the masses of fourth generation quarks from direct searches can be relaxed, and the lightest fourth neutrino is allowed to be stable and light enough to trigger the Higgs boson invisible decay. In addition, the fourth Majorana neutrino can be a subdominant but highly detectable dark matter component. We perform a global analysis of the current Large Hadron Collider (LHC) data on the Higgs production and decay in this type of SM4. The results show that the mass of the lightest fourth Majorana neutrino is confined in the range ~41–59 GeV. Within the allowed parameter space, the predicted effective cross-section for spin-independent DM–nucleon scattering is ~3×10-48–6×10-46 cm 2, which is close to the current XENON100 upper limit and is within the reach of the XENON1T experiment in the near future. The predicted spin-dependent cross sections can also reach ~8×10-40 cm 2.

Status of the fourth fermion generation before ICHEP2012: Higgs data and electroweak precision observables

We perform a global fit of the parameters of the Standard Model with a sequential fourth generation (SM4) to LHC and Tevatron Higgs data and electroweak precision data. Using several likelihood ratio tests we compare the performance of the SM4 and SM3 at describing the measured data. Since the SM3 and SM4 are not nested (i.e. the SM3 can not be considered as a special case of the SM4 with some parameters fixed) the usual analytical formulae for p-values in likelihood ratio tests do not hold. We thus apply a new method to compute these p-values. For a Higgs mass of 126.5 GeV and fourth-generation quark masses above 600 GeV we find that the SM4 is excluded at 3.1 sigma.

Z′→gggdecay in left-right symmetric models with three and four fermion families

We study the ${Z}^{\ensuremath{'}}\ensuremath{\rightarrow}\overline{q}q,ggg$ decays in the context of a manifest left-right symmetric gauge theory with three and four generations. The ${Z}^{\ensuremath{'}}$ couplings to quarks are fixed essentially by the parameters of the standard model and we obtain $\ensuremath{\Gamma}({Z}^{\ensuremath{'}}\ensuremath{\rightarrow}q\overline{q})\ensuremath{\approx}14\text{ }\text{ }\mathrm{GeV}$ for ${M}_{{Z}^{\ensuremath{'}}}\ensuremath{\approx}1\text{ }\text{ }\mathrm{TeV}$. For the ${Z}^{\ensuremath{'}}\ensuremath{\rightarrow}ggg$ decay and three families we obtain a branching ratio $\mathrm{BR}({Z}^{\ensuremath{'}}\ensuremath{\rightarrow}ggg)=\frac{\ensuremath{\Gamma}({Z}^{\ensuremath{'}}\ensuremath{\rightarrow}ggg)}{\ensuremath{\Gamma}({Z}^{\ensuremath{'}}\ensuremath{\rightarrow}q\overline{q})}=1.2\ensuremath{-}2.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ for ${m}_{{Z}^{\ensuremath{'}}}=700--1500\text{ }\text{ }\mathrm{GeV}$. The fourth generation produces an enhancement in the branching ratio for ${Z}^{\ensuremath{'}}$ masses close to the ${\overline{b}}^{\ensuremath{'}}{b}^{\ensuremath{'}}$ threshold and a dip for ${Z}^{\ensuremath{'}}$ masses close to the ${\overline{t}}^{\ensuremath{'}}{t}^{\ensuremath{'}}$ threshold. Using the values of the fourth-generation quark masses allowed by electroweak precision data, we obtain a branching ratio $\mathrm{BR}({Z}^{\ensuremath{'}}\ensuremath{\rightarrow}ggg)=(1\ensuremath{-}6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ for ${m}_{{Z}^{\ensuremath{'}}}=(700--1500)\text{ }\text{ }\mathrm{GeV}$.

Breaking local baryon and lepton number at the TeV scale

Simple models are proposed where the baryon and lepton number are gauged and spontaneously broken near the weak scale. The models use a fourth generation that is vector-like with respect to the strong, weak and electromagnetic interactions to cancel anomalies. One does not need large Yukawa couplings to be consistent with the experimental limits on fourth generation quark masses and hence the models are free of coupling constants with Landau poles near the weak scale. We discuss the main features of simple non-supersymmetric and supersymmetric models. In these models the light neutrino masses are generated through the seesaw mechanism and proton decay is forbidden even though B and L are broken near the weak scale. For some values of the parameters in these models baryon and/or lepton number violation can be observed at the Large Hadron Collider.

Fourth-generation quark mass limits in CKM-element space

We present a reanalysis of CDF data to extend limits on individual fourth-generation quark masses from particular flavor-mixing rates to the entire space of possible mixing values. Measurements from CDF have set individual limits on masses, $m_{b'}$ and $m_{t'}$, at the level of $335$--$385$ GeV assuming specific and favorable flavor-mixing rates. We consider the space of possible values for the mixing rates and find that the CDF data imply limits of $290$ GeV and greater over a wide range of mixing scenarios. We also analyze the limits from the perspective of a four-generation CKM matrix. We find that present experimental constraints on CKM elements do not suggest further constraints on fourth-generation quark masses.

Fourth generation bound states

We investigate the spectrum and wave functions of {\bar q}'q' bound states for heavy fourth generation quarks (q') that have a very small mixing with the three observed generations of standard model quarks. Such bound states come with different color, spin and flavor quantum numbers. Since the fourth generation Yukawa coupling, \lambda_{q'}, is large we include all perturbative corrections to the potential between the heavy quark and antiquark of order \lambda_{q'}^2N_c/16\pi^2 where N_c is the number of colors, as well as relativistic corrections suppressed by (v/c)^2. We find that the lightest fourth generation quark masses for which a bound state exists for color octet states. For the the color singlet states, which always have a bound state, we analyze the influence that the Higgs couplings have on the size and binding energy of the bound states.

Leading effect ofCPviolation with four generations

In the standard model with a fourth generation of quarks, we study the relation between the Jarlskog invariants and the triangle areas in the $4\ifmmode\times\else\texttimes\fi{}4$ Cabibbo-Kobayashi-Maskawa matrix. To identify the leading effects that may probe the $CP$ violation in processes involving quarks, we invoke small mass and small angle expansions, and show that these leading effects are enhanced considerably compared to the three-generation case by the large masses of fourth-generation quarks. We discuss the leading effect in several cases, in particular, the possibility of large $CP$ violation in $b\ensuremath{\rightarrow}s$ processes, which echoes the heightened recent interest because of experimental hints.

Constraints on the masses of fourth generation quarks

We study the one-loop contribution of the down-type quark of the standard model-like fourth generation (b') on the top quark electric dipole moment. Using the known limits on the top quark electric dipole moments, we place limits on the b' mass. Then, from the estimated ratio for the masses of the fourth generation of quarks from other studies and the achieved bound from top quark electric dipole moments on m b ', we obtain a limit for the up-type quark of fourth generation (t') mass.

Experimental constraints on fourth generation quark masses

The existing bounds from CDF on the masses of the fourth generation quarks, ${t}^{\ensuremath{'}}$ and ${b}^{\ensuremath{'}}$, are reexamined. The bound of 256 GeV on the ${t}^{\ensuremath{'}}$ mass assumes that the primary decay of the ${t}^{\ensuremath{'}}$ is into $q+W$, which is not the case for a substantial region of parameter space. The bound of 268 GeV on the ${b}^{\ensuremath{'}}$ mass assumes that the branching ratio for ${b}^{\ensuremath{'}}\ensuremath{\rightarrow}b+Z$ is very large, which is not only not true for much of parameter space, but is never true for ${b}^{\ensuremath{'}}$ masses above 255 GeV. In addition, it is assumed that the heavy quarks decay within the silicon vertex detector, and for small mixing angles this will not be the case. The experimental bounds, including all of these effects, are found as a function of the other heavy quark mass and the mixing angle.

Horizontal symmetry and the fourth generation.

A systematic investigation of all possible horizontal symmetries acting on four generations of quarks in a minimal left-right-symmetric model is carried out. There are only two consistent models with realistic constraints on the quark masses and mixing angles. It is shown that Z/sub 4/ is the unique symmetry group leading to these models. The fourth-generation quark masses m/sub b//sub '/ and m/sub t//sub '/ are constrained to be (A) m/sub b//m/sub t/approx. =m/sub b//sub '//m/sub t//sub '/, (B) m/sub s//m/sub c/approx. =(m/sub b/+m/sub b//sub '/)/(m/sub t/+m/sub t//sub '/). Thus, model (A) predicts m/sub t/,m/sub b//sub '/less than or equal to43 GeV whereas model (B) has m/sub b//sub '/less than or equal to58 GeV. The two models differ in the mixing of the fourth generation into the first three. A crucial test which can distinguish the two models is a direct measurement of the mixing matrix element chemically bondV/sub u//sub b/chemically bond. Model (A) predicts it to be approx.10/sup -4/, an order of magnitude smaller than the prediction of model (B).

Fourth-generation quark masses and Fritzsch mass matrices from Abelian family symmetry.

We examine possible mass relations resulting from Fritzsch-type mass matrices for quarks in left-right-symmetric models with an Abelian family symmetry. While the three-generation model predicts no realistic mass relation, six possible schemes exist in the presence of a fourth generation of quarks (t',b'), with realistic constraints on the quark masses. The most attractive scheme predicts (${m}_{b\mathcal{'}}$-${m}_{b}$)(${m}_{t\mathcal{'}}$-${m}_{t}$)\ensuremath{\simeq}(${m}_{\mathrm{sb}}$${m}_{b}$${\mathcal{'}}_{\mathrm{ct}\mathcal{'}}^{13}$@B. When combined with the experimental constraints on the \ensuremath{\rho} parameter, this leads to ${m}_{t}$\ensuremath{\le}68 GeV, ${m}_{b\mathcal{'}}$\ensuremath{\le}46 GeV, and 145 GeV\ensuremath{\le}${m}_{t\mathcal{'}}$\ensuremath{\le}310 GeV.

No future for the fourth generation?

We discuss bounds on and implications of a possible fourth generation of quarks and leptons. We derive lower bounds on the masses of fourth generation quarks from the recent experimental bound mτ' ⩾ 41 GeV, for both nonsupersymmetric and supersymmetric models. We also derive upper bounds on these masses from the requirement of a perturbative unification, and briefly discuss the non-perturbative case. We then show that a fourth generation renders the conventional mechanism of cosmological baryogenesis via Higgs-decay problematic, and tends to result in a large supersymmetry breaking scale in minimal supergravity models. We finally show that the incorporation of a fourth generation in superstring models can only work if the superpotential fulfills certain specific conditions, which appear to be in conflict with cosmological requirements.