Families Of Quarks Research Articles

Probing Higgs bosons with large bottom Yukawa coupling at hadron colliders

The small mass of the bottom quark, relative to its weak isospin partner, the top quark, makes the bottom an effective probe of new physics in Higgs and top sectors. We study the Higgs boson production associated with bottom quarks, ppbar/pp to phi+b+bbar to 4b, at the Fermilab Tevatron and the CERN LHC. We find that strong and model-independent constraints on the size of the phi-b-bbar coupling can be obtained for a wide range of Higgs boson masses. Their implications for the composite Higgs models with strong dynamics associated with the third family quarks (such as the top-condensate/topcolor models with naturally large bottom Yukawa couplings), and for the supersymmetric models with large tan(beta), are analyzed. We conclude that the Tevatron and the LHC can put stringent bounds on these models, if the phi+b+bbar signal is not found.

Model for particle masses, flavor mixing, andCPviolation, based on spontaneously broken discrete chiral symmetry as the origin of families

We construct extensions of the standard model based on the hypothesis that Higgs bosons also exhibit a family structure and that the flavor weak eigenstates in the three families are distinguished by a discrete ${Z}_{6}$ chiral symmetry that is spontaneously broken by the Higgs sector. We study in detail at the tree level models with three Higgs doublets and with six Higgs doublets comprising two weakly coupled sets of three. In a leading approximation of ${S}_{3}$ cyclic permutation symmetry the three-Higgs-doublet model gives a ``democratic'' mass matrix of rank 1, while the six-Higgs-doublet model gives either a rank-1 mass matrix or, in the case when it spontaneously violates CP, a rank-2 mass matrix corresponding to nonzero second family masses. In both models, the CKM matrix is exactly unity in the leading approximation. Allowing small explicit violations of cyclic permutation symmetry generates small first family masses in the six-Higgs-doublet model, and first and second family masses in the three-Higgs-doublet model, and gives a nontrivial CKM matrix in which the mixings of the first and second family quarks are naturally larger than mixings involving the third family. Complete numerical fits are given for both models, flavor-changing neutral current constraints are discussed in detail, and the issues of unification of couplings and neutrino masses are addressed. On a technical level, our analysis uses the theory of circulant and retrocirculant matrices, the relevant parts of which are reviewed.

Symmetries of the standard model without and with a right-handed neutrino

Given the particle content of the standard model without and with a right-handed neutrino, the requirement that all anomalies cancel singles out a set of possible global symmetries which can be gauged. I review this topic and propose a new gauge symmetry B - 3L_tau in the context of the minimal standard model consisting of the usual three families of quarks and leptons plus just one nu_R. The many interesting phenomenological consequences of this hypothesis are briefly discussed.

CP violation from new quarks in the chiral limit

We characterize CP violation in the SU(2) × U(1) model due to an extra vector-like quark or sequential family, giving special emphasis to the chiral limit m u, d, s = 0. In this limit, CP is conserved in the three-generation Standard Model (SM), thus implying that all CP violation is due to the two new CP violating phases whose effects may manifest either at high energy in processes involving the new quark or as deviations from SM unitarity equalities among imaginary parts of invariant quartets (or, equivalently, areas of unitarity triangles). In our analysis we use an invariant formulation, independent of the choice of weak quark basis or the phase convention in the generalized Cabibbo-Kobayashi-Maskawa matrix. We identify the three weak-basis invariants, as well as the three imaginary parts of quartets B 1–3 which, in the chiral limit, give the strength of CP violation beyond the SM. We find that for an extra vector-like quark ¦B i¦⩽ 10 −4 , whereas for an extra sequential family ¦B i¦⩽ 10 −2 . © 1998 Elsevier Science B.V.

Abnormal structure of fermion mixings in a seesaw quark mass matrix model

It is pointed out that in a seesaw quark mass matrix model which yields a singular enhancement of the top-quark mass, the right-handed fermion-mixing matrix U_R^u for the up-quark sector has a peculiar structure in contrast to the left-handed one U_L^u. As an example of the explicit structures of U_L^u and U_R^u, a case in which the heavy fermion mass matrix M_F is given by a form [(unit matrix)+(rank-one matrix)] is investigated. As a consequence, one finds observable signatures at projected high energy accelerators like the production of a fourth heavy quark family.

Comments on the high-2 HERA anomaly

Taking the reported high- Q 2 anomaly at HERA as a signal of new physics, we show that several independent considerations point towards s -channel leptoquark production as the most attractive interpretation. We argue that even this option is highly constrained by flavor-changing processes: the couplings must be accurately diagonal in the quark and lepton mass eigenstate basis and should preserve individual quark and lepton family numbers. We propose a dynamical mechanism that might produce this pattern; it has distinctive experimental consequences.

Constraint on the scale-unifying supersymmetric preon model from - mixing

We study the flavour changing neutral current process in - mixing in the supersymmetric preon model (SPM). Compared to the minimal supersymmetric standard model (MSSM), one distinguishing feature of - mixing in the SPM is that there are new contributions from box diagrams involving two vector-like families of quarks and their supersymmetric (SUSY) partners with masses of order 1 TeV. Another special feature of the process in the SPM, in contrast to the MSSM, is that left - right squark - mixing is highly suppressed owing to symmetries of the underlying model. We calculate the SUSY box diagrams for - mixing for the case of the SPM, using observed - mass splitting, and obtain the constraint of on the squark mass degeneracy in the model. This upper limit is smaller by about a factor of 3.6 than the corresponding limit for the MSSM. The difference arises because of the absence of - mixing in the the preon model.

Invariant formulation of CP violation for four quark families

We find a minimal set of constraints which are independent of the choice of weak quark basis and necessary and sufficient for CP conservation for four quark families, including also the case of degenerate quark masses. These invariant conditions are written in the mass eigenstate basis as a function of the fermion masses and charged current mixings. CP violation is then related to the areas of three unitarity quadrangles and the CP violating effects of the fourth family are discussed in the case of small mixings.

The problems of unification-mismatch and low α3: A solution with light vector-like matter

The commonly accepted notion of a weak unified coupling αX ≈ 0.04, based on the assumption of the MSSM-spectrum, is questioned. It is suggested that the four-dimensional unified string coupling should very likely have an intermediate value (∼ 0.2–0.3, say) so that it may be large enough to stabilize the dilaton but not so large as to disturb the coupling-unification relations. Bearing this in mind, as well as the smallness of the MSSM unification scale MX compared to the string scale, the consequences of a previously suggested extension of the MSSM spectrum are explored. The extension contains two vector-like families of quarks and leptons with relatively light masses of order 1 TeV, having the quantum numbers of 16+16 of SO(10). It is observed that such an extension provides certain unique advantages. These include: (a) removing the stated mismatch between MSSM and string unifications with regard to αX and to some extent MX as well, (b) achieving coupling unification with a relatively low value of α3(mZ), in accord with its world average value, and (c) following earlier works, providing a simple explanation of the observed inter-family mass-hierarchy. The extension provides scope for exciting new discoveries, beyond those of SUSY and Higgs particles, at future colliders, including the LHC and the NLC.

Are four-fermion operators relevant for the fermion mass problem?

Four-fermion operators may have large anomalous scaling and become relevant operators in some strongly interacting gauge theories. We present a detailed model which illustrates some of the implications of such operators for the generation of quark and lepton masses. Such operators would originate at high scales where quarks and leptons experience a new strong interaction, but no unbroken technicolor interaction is required. The breakdown of both the new gauge symmetry and electroweak gauge symmetry is associated with a dynamical TeV mass for fourth family quarks. Among the new physics signatures are anomalous (chromo)magnetic moments and their flavor-changing counterparts.

The emergence of a heavy quark family on a lattice

Within the framework of the “Rome approach” for a lattice chiral gauge theory, the four-quark interaction with flavour symmetry is included. We analyse spontaneous symmetry breaking and compute composite modes and their contributions to the ground state energy. As a result, it is shown that the emergence of a heavy quark family is the energetically favoured solution.

THE DECAY b→sγ IN THE 3–3–1 MODEL

The 3–3–1 model, based on the gauge group SU (3)c× SU (3)L× U (1)X, makes a natural prediction of three generations based on anomaly cancellation. Since this is accomplished by incorporating the third family of quarks differently from the other two, it leads to potentially large flavor-changing neutral currents. A sensitive place to look for such effects is the flavor-changing b→sγ decay, which has recently been measured at CLEO. We compute this decay rate in the 3–3–1 model and compare it with that of the two-Higgs-doublet model, a subset of the full 3–3–1 model. We find that the additional 3–3–1 physics weakens the bound on the charged Higgs mass from MH+>290 GeV to MH+≳120 GeV .

THE FERMION GENERATION PROBLEM IN THE GUST’S OF THE FREE WORLD SHEET FERMION FORMULATION

In the framework of the four-dimensional heterotic superstring with free fermions, we investigate the rank 8 grand unified string theories (GUST’s) which contain the SU (3)H gauge family symmetry, and develop some methods for building corresponding string models. We explicitly construct GUST’s with gauge symmetry G= SU (5)× U (1) × [ SU (3)× U (1)]H and G= SO (10)×[ SU (3)× U (1)]H ⊂ SO (16) or E(6)× SU (3)H ⊂ E(8), and consider the full massless spectrum for our string models. As the GUST’s originating from level 1 Kac–Moody algebra (KMA) contain only low-dimensional representations, we consider for the observable gauge symmetry the diagonal subgroup G sym of the rank 16 group G×G ⊂ SO (16)× SO (16) or ⊂ E(8)×E(8). We discuss the possible fermion matter and Higgs sectors in these theories. There has to exist “superweak” light chiral matter [Formula: see text] in the GUST’s under consideration. The understanding of quark and lepton mass spectra and family mixing leaves a possibility for the existence of an unusually low mass breaking scale of the SU (3)H family gauge symmetry (some TeV).

Extended technicolor model with QCD-like symmetry breaking.

We present a one-doublet extended technicolor model, with all fermions in fundamental representations. The bare lagrangian has no explicit mass terms but generates masses through gauge symmetry breaking by purely QCD-like dynamics. The model generates three families of quarks and leptons and can accommodate the observed third family mass spectrum (including a large top mass and light neutrinos). In addition, we show how the model may be extended to incorporate a top color driven top mass without the need for a strong U(1) interaction. We discuss the compatiblity of the model with experimental constraints and its possible predicitive power with respect to first and second family masses.

Possible effects of [formula omitted] mixing in weak B decays

D 0- D 0 mixing at a detectable level requires the presence of new physics and may lead to some non-trivial effects in weak decays of B mesons. We show that CP violation of the order 10 −3 – 10 −2 in D 0- D 0 mixing can give rise to an observable decay-rate asymmetry between B u + → D l( h) l + ν 1 and B u − → D l(h)l − ν l . A rephasing-invariant generalization of the Gronau-Wyler approach is made to determine the weak phase shift in B u ± → D l( h) K ±, which might deviate significantly from the CP-violating phase γ due to the underlying new physics in D 0- D 0 mixing. We also examine the possible effect of D 0- D 0 mixing on CP violation in decay modes of the type B d → D l( h) + ( π 0, ϱ 0, etc.). Finally the model of four quark families is taken as an example to illustrate how the new physics affects D 0- D 0 mixing, B d 0- B d 0 mixing and CP asymmetries in the relevant B decays.

The emergence of a heavy quark family on a lattice

Within the framework of the “Rome approach” for a lattice chiral gauge theory, the four-quark interaction with flavour symmetry is included. We analyse spontaneous symmetry breaking and compute composite modes and their contributions to the ground state energy. As a result, it is shown that the emergence of a heavy quark family is the energetically favoured solution.

Non-Abelian gauge family symmetry in rank-8 and -16 grand-unified string theories

One of the main points of the investigations in high-energy physics is to study the next chain: a law of the quark and lepton mass spectra→the puzzles of the quark and lepton family mixing→a possible new family dynamics. The new family symmetry dynamics might be connected to the existence of some exotic gauge or matter fields or something else. For this it will better to study the possibilities of the appearance of this gauge symmetry in the framework of the Grand-Unified String Theories. In the framework of four-dimensional heterotic superstring with free fermions we investigate the rank-eight Grand-Unified String Theories (GUST) which contain theSU(3)H-gauge family symmetry. We explicitly construct GUST with gauge symmetryG=SU(5)×U(1)×(SU(3)×U(1))H andG=SO(10)×(SU(3)×U(1))H⊃SO(16) orE(6)×SU(3)H⊃E(8) in free complex fermion formulation. As the GUSTs originating from Kac-Moody algebras (KMA) contain only low-dimensional representations it is usually difficult to break the gauge symmetry. We solve this problem taking for the observable gauge symmetry the diagonal subgroupG symm of rank-16 groupG×G⊃SO(16)×SO(16) or (E(6)×SU(3)H)2 ⊃E(8)×E(8). We discuss the possible fermion matter and Higgs sectors in these models. In these GUST there has to exist «superweak» light chiral matterm H <M W). The understanding of quark and lepton mass spectra and family mixing leaves a possibility for the existence of an unusually low-mass breaking scale of theSU(3)H family gauge symmetry (some TeV).

Third-family flavor physics in an SU(3)3 x SU(2)L x U(1)Y model.

We consider a model in which each family transforms under a different SU(3) color group. The low-energy effective theory is an extension of the Standard Model, with additional color octet gauge bosons $G_H$ with mass $M$ that couple preferentially to the third family quarks. We show that there are two distinct regions of the model's parameter space in which we can simultaneously evade all the current experimental constraints, one with $M \approx 250$ GeV and the other with $M \gtrsim 600$ GeV. Within each allowed region, we can obtain a correction to the $Zb\overline{b}$ vertex that is consistent with the slightly high value of $R_b$ observed at LEP. We show that there are $\Delta B = 1$ operators in our model that can suppress the $B$-meson semileptonic branching ratio $B_{SL}$ and the charm multiplicity per decay $n_c$ by enough to reconcile the spectator parton model predictions with the experimental data. In the non-supersymmetric version of our model, we can only obtain the desired corrections to $R_b$, $B_{SL}$ and $n_c$ in different regions of the allowed parameter space, while in the supersymmetric version, we can obtain all three corrections simultaneously. We also discuss a strong-coupling limit of our model in which the third-family quarks become composite.

On the overlap formulation of chiral gauge theory

The overlap formula proposed by Narayanan and Neuberger in chiral gauge theories is examined. The free chiral and Dirac Green's functions are constructed in this formalism. Four dimensional anomalies are calculated and the usual anomaly cancellation for one standard family of quarks and leptons is verified.

Generation nonuniversality and flavor-changing neutral currents in the SU(3)c x SU(3)L x U(1)X model.

The SU(3${)}_{\mathit{c}}$\ifmmode\times\else\texttimes\fi{}SU(3${)}_{\mathit{L}}$\ifmmode\times\else\texttimes\fi{}U(1${)}_{\mathit{X}}$ (3-3-1) model, an extension of the standard electroweak theory to SU(3${)}_{\mathit{L}}$\ifmmode\times\else\texttimes\fi{}U(1${)}_{\mathit{X}}$, naturally predicts three families of quarks and leptons via the requirement of anomaly cancellation. This is accomplished by making one of the quark families transform differently from the other two, thus leading to flavor-changing neutral currents. Using experimental input on neutral meson mixing, we show that the third family must be the one that is singled out, at least up to small family mixing. We additionally describe a convenient way to parametrize the new mixing matrix that plays a role in the gauge interactions of the ordinary quarks with the new 3-3-1 quarks.