Fourth Generation Of Quarks Research Articles

Could there be a fourth generation of quarks without more leptons?

We investigate the possibility of adding a fourth generation of quarks. We also extend the Standard Model gauge group by adding another SU(N) component. In order to cancel the contributions of the fourth generation of quarks to the gauge anomalies we must add a generation of fermions coupling to the SU(N) group. This model has many features similar to the Standard Model and, for example, includes a natural generalisation of the Standard Model charge quantisation rule. We discuss the phenomenology of this model and, in particular, show that the infrared quasi-fixed point values of the Yukawa coupling constants put upper limits on the new quark masses close to the present experimental lower bounds.

Constraining four generation Standard Model with b → sγ and b → sg

Using the experimental result on b → sγ and theoretical result on b → sg, a four generation Standard Model (SM) is analysed to constrain the rephasing-invariant combinations of the CKM matrix and masses of the fourth generation quarks.

Masses of Heavy Fermions and Higgs Boson in Four-Generation Fermion Condensate Scheme11The project partially supported by National Natural Science Foundation of China and by Grant No. LWTZ-1298 of The Chinese Academy of Sciences

The renormalization group analyses based on low-energy effective Lagrangian indicate that a model of electroweak symmetry breaking of Nambu-Jona-Lasinio(NJL)-type by full four-generation quark-lepton condensates could accommodate itself to the top-quark mass 174 GeV for the acceptable momentum cutoff GeV. The fourth generation of quarks will have masses in the region 228 366 GeV. The corresponding lep tons will have masses in the region 110 246 GeV and can be heavier than the top quark only if GeV. The mass of the Higgs boson is predicted to be in the region 287 481 GeV which could provide an important experimental test of the model.

Weak mixing in the case of a fourth quark generation

The possible existence of a fourth quark generation is considered within the context of the exponential parametrization of the Cabibbo-Kobayashi-Maskawa (CKM) matrix. A specific hypothesis for the quark flavour mixing hierarchy is suggested, leading to an interesting structure of the mixing matrix. The weak-interaction eigenstates are deduced in exact form.

Electric dipole moment of the neutron in the Kobayashi-Maskawa model with four generations of quarks

We show that the existence of a possible fourth heavy generation of quarks gives rise to a significant enhancement of the neutron electric dipole moment in comparison with the Standard Model prediction. The smaller degree of suppression in this case is linked to the presence of the operators of dimension ≤ 6 which enter into the effective Lagrangian with coefficients proportional to the square of the top quark mass. Numerically, the enhancement is mainly associated with the chromoelectric dipole moment of the s quark which appears at the three loop level, of the order α sα w 2m sm t 2 m w 4 from the CP-odd combination of mixing angles between second, third and fourth generations. Its value is calculated explicitly in the limit of large masses of the fourth generation of quarks. The corresponding contribution to the electric dipole moment of the neutron is 5·10 −30 e·cm in the most optimistic scenarios about the values of the Kobayashi-Maskawa matrix elements. The additive renormalization of the θ-term in this model is estimated as 10 −13.

Two-loop O( alpha sGFmt2) corrections to the fermionic decay rates of the standard-model Higgs boson.

Low- and intermediate-mass Higgs bosons decay preferably into fermion pairs. The one-loop electroweak corrections to the respective decay rates are dominated by a flavor-independent term of O(${\mathit{G}}_{\mathit{F}}$${\mathit{m}}_{\mathit{t}}^{2}$). We calculate the two-loop gluon correction to this term. It turns out that this correction screens the leading high-${\mathit{m}}_{\mathit{t}}$ behavior of the one-loop result by roughly 10%. We also present the two-loop QCD correction to the contribution induced by a pair of fourth-generation quarks with arbitrary masses. As expected, the inclusion of the QCD correction considerably reduces the renormalization-scheme dependence of the prediction.

Toponium tests of top quark Higgs bags

Recently it has been suggested that top quarks, or very massive fourth generation quarks, might surround themselves with a Higgs “bag” of deformation of the Higgs expectation value from its vacuum magnitude. In this paper we address the question of whether such non-linear Higgs-top interaction effects are subject to experimental test. We first note that if top quarks were necessarily accompanied by Higgs “bags”, then top quark weak decay would involve the sudden disruption of the Higgs “bag”, with copious production of physical Higgs particles accompanying the decay. We then examine the effects that such Higgs “bags” would produce on the spectrum of toponium, where the two bound top (anti) quarks, and their “bags”, overlap. We numerically evaluate the effects that the non-linear feedback in the Higgs-toponium system would have on the energy level splittings of the toponium bound states, and find that for allowed values of the top and Higgs mass the effect is negligible, thus indicating that even in this favourable circumstance Higgs “bag” formation around top quarks does not observably occur. Finally, we consider the case of a second Higgs doublet, allowing the possibility of enhanced couplings for one of the physical Higgs to top. Even in this non-standard scenario the effects are minimal, and we infer the general absence of observable effects at any level that might suggest the utility of considering top quarks to be accompanied by Higgs “bags”.

Can the rho parameter allow for the existence of a nondegenerate fourth family?

The question of mass splitting within a doublet of heavy fermions, in particular a fourth generation of quarks and leptons, is reexamined with emphasis on the contribution of possible heavy-fermion ``vector-meson'' bound states to the \ensuremath{\rho} parameter. The heavy-fermion bound states are formed by the exchange of the Higgs boson. We have found that, in the minimal model, the constraint on mass splitting which now also depends on the Higgs boson mass gives rise to some interesting phenomena, including the possibility of having a top-quark mass greater than 180 GeV.

SIGNATURES OF DECAYS OF ELECTROWEAK NON-TOPOLOGICAL SOLITONS

It is not totally excluded that there exists the fourth generation of quarks with masses in the TeV region. In that case they may form non-topological solitons. We discuss experimental signatures of their decays which differ them from conventional perturbative decays of very heavy fermions. These signatures are the number of the Higgs bosons produced and their spectrum.

Heavy-fermion effects in e+e--->ZH and Z-->H nu nu -bar.

We consider the effects of the one-loop radiative corrections due to virtual heavy fermions on the process {ital e}{sup +}{ital e{minus}}{r arrow}{ital ZH} and on the related process {ital Z}{r arrow}{ital H}{nu}{bar {nu}}. We show how to compute the one-loop corrections for asymptotically large fermion masses in a very simple manner, using the equivalence theorem where the on-shell {ital Z} is replaced by the corresponding Goldstone scalar. Since heavy fermions do not decouple from low-energy phenomena involving the production of external Higgs bosons, the corrections lead to a significant reduction in the rate if there is a heavy fourth generation of quarks and/or leptons.

Electroweak symmetry breaking by fourth-generation condensates and the neutrino spectrum.

We consider a new mechanism for dynamical symmetry breaking of the electroweak symmetries involving condensates of fourth-generation quarks and leptons. A dynamical generalization of the seesaw mechanism is proposed based upon the BCS theory in which a neutrino condensate gives rise to right-handed-neutrino Majorana masses and all associated spin-zero bosons are composite. The fourth-generation neutrino is naturally heavier than $\frac{{M}_{Z}}{2}$ and the scale of new physics is bounded above. The renormalization-group equations for the effective Lagrangian of this model are derived and used to solve the model. Implications for neutrino masses are discussed.

Properties of heavy quarkonia and related states

Mass levels and widths of very heavy quarkonia and similar onium resonances of squarks, color octet and sextet constituents are studied in the mass range of 50 to 1000 GeV. The heavy quark potential with QCD two-loop improved short-distance behavior that gives a good description of the charmonium and bottomonium spectra is used, and the results are given for several values of the QCD scale parameter Λ MS (4) in the range between 0.1 and 0.4 GeV. We present parametrizations for the first ten S-wave quarkonium resonances, which are relevant if the top quark is relatively light ( m t≲125 GeV) or if a heavy fourth-generation quark with suppressed weak decay rates exists. We also discuss the onset of coulombic behavior and the possible effects of Higgs boson exchange on superheavy quarkonia.

Quark masses using Barut-like mass formula

An extension of Barut's (1979) lepton mass formula to quark masses, using lepton-quark symmetry is applied to obtain the masses of the third and fourth generation quarks. The t-quark mass is found to be quite a bit smaller than present estimates.

Search for the top quark in UA2

Data from the 1988 and 1989 runs of the CERN pp̄ Collider have been analysed to look for charged current decays of the top quark. A lower limit of 67(70) GeV at the 95(90)% confidence level has been set for the top mass. A similar analysis for a light fourth generation quark (the b′) gives a lower limit of 53(56) GeV at the 95(90)% confidence level.

ρ PARAMETER CONSTRAINTS ON FOURTH GENERATION QUARK MASSES

Constraints on the masses of possible fourth generation quarks (a, υ) are obtained from measurements of the ρ parameter and the elements of the quark mixing matrix. Stringent mass limits are found when the off-diagonal elements Vtυ and Vab are large. For example, with mt=90 GeV and |Vtυ|≃0.5 we find ma,v≤ 300 GeV. Stronger constraints are obtained as mt or |Vtυ| increase.

Superallowed 0 + → 0 + nuclear β-decays: A critical survey with tests of CVC and the standard model

A complete and critical survey is presented of all lifetime, decay-energy and branching-ratio measurements relating to the superallowed 0 + → 0 + decays of 14O, 26mAl, 34Cl, 38mK, 42Sc, 46V, 50Mn and 54Co; no measurements are ignored, though some are rejected and others are updated. The associated calculations and theoretical corrections needed to convert these results into Ft values are also described and critically assessed. The resulting Ft values are then presented with careful attention to the origin and magnitude of their uncertainties. As an exacting confirmation of the conserved-vector-current hypothesis, the Ft values are seen to be constant to 4 parts in 10 4. In addition, the effective vector coupling constant of nuclear β decay is determined to be G' v/( h ̵ c) 3 = (1.14939 ± 0.00065) × 10 −5 GeV −2 ; the up-down quark-mixing element of the Kobayashi-Maskawa matrix is V ud = 0.9740 ± 0.0010; and the unitary test on the top row of the matrix becomes |V ud | 2+|V us | 2+¦V ub | 2 = 0.9970 ± 0.0021 , which sets a useful limit on the possible participation of a fourth generation of quarks. The data are also used to set a new limit for the Fierz interference term, b F ≤ 0.004, and for possible mixing of massive neutrinos. Finally, the results are shown to be consistent with, though more precise than, the current data for neutron and pion decay.

Box diagrams and mixings with heavy external quarks.

We present, within the standard model, an exact calculation of the effective Hamiltonian generated by the box diagrams when both the external and internal quarks are arbitrarily heavy. We use this result to calculate the mixings of the neutral hadrons involving one fourth-generation quark. Large ${B}_{b}^{\ensuremath{'}}\ensuremath{-}{\overline{B}}_{b}^{\ensuremath{'}} ({B}_{b}^{\ensuremath{'}}\ensuremath{\equiv}{b}^{\ensuremath{'}}\overline{b})$ mixing is predicted for a wide range of ${b}^{\ensuremath{'}}$-, $t$-, and ${t}^{\ensuremath{'}}$-quark masses.

Charm, Beauty, and Beyond

To search for the possible existence of the fourth generation of quarks, it is best to begin what we know for sure, which then can serve as a base for making guesses at what is coming. Here I shall report mainly the recent work done with Hai-Yang Cheng analyzing the recent data of exclusive two-body charm decays, and extracting their theoretical implications as well as pointing out future interesting experiments, based upon the quark diagram scheme. Using the same approach, we also classified the rare charmless beauty particle decays and estimated their branching ratios. We also pointed out decay channels of B meson decays with large partial decay rate differences for experimental search. Finally we emphasize again that it is possible for charm particle decays to have large partial decay rate differences if the fourth generation of quark exists. Of special interest are those partial decay rate differences coming solely from the tree-diagram interferences, which depends only on the existence of the fourth generation of quarks but not on their masses. It is therefore important to search for such charm, partial decay rate differences.

B Physics at ARGUS

The study of B mesons gives access to fundamental parameters of the Standard Model. Four of the nine elements of the Kobayashi‐Maskawa (KM) mass mixing matrix1 for three generations of quarks can be determined by measuring B meson properties. A precise determination of these matrix elements may also lead beyond the Standard Model. Any deviation from unitarity of the KM‐matrix for three generations requires the presence of new physics, e.g. a fourth generation of quarks.

Mixing of the Fourth Family Neutral Hadrons

In the standard model, the mixing of neutral hadrons (like Ko—Ko and Bo—Bo systems)1,2 takes place through the box-diagrams. For the Ko—Ko case, this was first studied by Gaillard and Lee.3 They calculated the effective Hamiltonian generated by the box-diagrams in the cases when all the external momenta are zero and the quark masses exchanged in the loop were very small compared with mW. Inami and Lim4 extended their calculation to the case of heavy quarks (mq±mW) in the internal loop. The calculation involving nonzero external momentum was done in the case of one heavy and one light external quark.5 However, a general calculation of the box-diagrams when all the external quarks are heavy (MQ1, MQ2± mW) does not exist. The purpose of this paper is twofold: i) to present the calculation of the effective Hamiltonian generated by the box-diagrams when all external quarks are heavy; and ii) to apply this general result to make an estimate for the mixings of neutral hadrons involving heavy quarks, in particular the fourth generation quarks, like (b'b, t't).