Asymptotic behavior of amplitudes for longitudinal-leptoquark processes and structure of the scalar sector in the minimal model involving four-color symmetry

Within the minimal model based on the four-color symmetry of quarks and leptons of the Pati-Salam type, the asymptotic behavior of amplitudes for processes involving longitudinal leptoquarks (and W or Z′ bosons) is investigated, together with the mechanism according to which the growth of these amplitudes at high energies is suppressed by scalar fields. It is shown that, within the Higgs mechanism of mass generation and of the mass splitting of quarks and leptons, the four-color symmetry of quarks and leptons requires that scalar-leptoquark doublets, scalar-gluon doublets, and an extra color-singlet scalar doublet exist in addition to the standard Higgs doublet.     

Unitarity bounds on low scale quantum gravity

We study the unitarity of models with low scale quantum gravity both in four dimensions and in models with a large extra-dimensional volume. We find that models with low scale quantum gravity have problems with unitarity below the scale at which gravity becomes strong. An important consequence of our work is that their first signal at the Large Hadron Collider would not be of a gravitational nature such as graviton emission or small black holes, but rather would be linked to the mechanism which fixes the unitarity problem. We also study models with scalar fields with non-minimal couplings to the Ricci scalar. We consider the strength of gravity in these models and study the consequences for inflation models with non-minimally coupled scalar fields. We show that a single scalar field with a large non-minimal coupling can lower the Planck mass in the TeV region. In that model, it is possible to lower the scale at which gravity becomes strong down to 14 TeV without violating unitarity below that scale.     

Author index to volume 67B

A new class of leptons, with its own quantum number carried by the supersymmetry generator, includes a Goldstone-neutrino, a photon-neutrinos, gluon-neutrinos and heavy leptons. The Goldstone-and-photon-neutrinos can be produced in low-energy processes, but only in pairs. Ordinary leptons and quarks are associated with heavy scalar bosons under supersymmetry. The latter decay quickly into the corresponding leptons or quarks, emitting a Goldstone or a photon-neutrino. Some of the phenomenological implications are discussed.     

Can extra dimensions accessible to the SM explain the recent measurement of anomalous magnetic moment of the muon?

We investigate whether models with flat extra dimensions in which SM fields propagate can give a significant contribution to the anomalous magnetic moment of the muon (MMM). In models with only SM gauge and Higgs fields in the bulk, the contribution to the MMM from Kaluza–Klein (KK) excitations of gauge bosons is very small. This is due to the constraint on the size of the extra dimensions from tree-level effects of KK excitations of gauge bosons on precision electroweak observables such as Fermi constant. If the quarks and leptons are also allowed to propagate in the (same) bulk (“universal” extra dimensions), then there are no contributions to precision electroweak observables at tree-level. However, in this case, the constraint from one-loop contribution of KK excitations of (mainly) the top quark to T parameter again implies that the contribution to the MMM is small. We show that in models with leptons, electroweak gauge and Higgs fields propagating in the (same) bulk, but with quarks and gluon propagating in a sub-space of this bulk, both the above constraints can be relaxed. However, with only one Higgs doublet, the constraint from the process b→sγ requires the contribution to the MMM to be smaller than the SM electroweak correction. This constraint can be relaxed in models with more than one Higgs doublet.     

The Coupling Constants for an Electroweak Model with an SU(4)PS\otimes SU(4)EW Unification Symmetry

We introduce the sequence of spontaneous symmetry breaking of a coupling between Pati-Salam and electroweak symmetries SU(4)_PS\otimes SU(4)_EW in order to establish a mathematically consistent relation among the coupling constants at grand unification energy scale. With the values of baryon minus lepton quantum numbers of known quarks and leptons, by including right-handed neutrinos, we can find the mixing angle relations at different energy levels up to the electromagnetic U(1)_EM scale.     

Generations of massless composite quarks and leptons

We present a QCD-like composite model in which quarks, leptons and technifermions are three-body systems made out of three kinds of massless elementary fermions t, c and w, each carrying technicolor, color and weak gauge interactions, respectively. Discrete symmetries, remnants of the U(1)_A of the original lagrangian, are responsible for the masslessness of all the quarks and leptons and give the precise meaning of the generations. The model exhibits three generations for both quarks and leptons. Small but non-zero masses of the quarks and leptons are produced by the technicolor condensate of the composite technifermions, which thereby leads to the non-trivial Cabibbo mixing. Proton decays are all forbidden at the mass scale of the QCD-like theory.     

Three extra mirror or sequential families: case for a heavy Higgs boson and inert doublet

We study the possibility of the existence of extra fermion families and an extra Higgs doublet. We find that requiring the extra Higgs doublet to be inert leaves space for three extra families, allowing for mirror fermion families and a dark matter candidate at the same time. The emerging scenario is very predictive: It consists of a standard model Higgs boson, with a mass above 400 GeV, heavy new quarks between 340 and 500 GeV, light extra neutral leptons, and an inert scalar with a mass below M(Z).     

A REALISTIK MODEL WITH LOW METACOLOR SCALE SATISFYING COMPLEMENTARITY

A Realistic Chiral Tripreon Model with SU(3)×SO(5) metacolor group is obtained This model has a low metacolor energy scale and three generations of composite fermions with the quantum numbers of quarks and leptons of the standard model.This model also satisfies complementarity.     

Super-light baryo-photons,weak gravity conjecture and exotic instantons in neutron-antineutron transitions

In companion papers(A. Addazi, Nuovo Cim. C, 38(1): 21(2015); A. Addazi, Z. Berezhiani, and Y. Kamyshkov, ar Xiv:1607.00348), we have discussed current bounds on a new super-light baryo-photon, associated with a U(1)B-L gauge, from current neutron-antineutron data, which are competitive with E¨otv¨os-type experiments.Here, we discuss the implications of possible baryo-photon detection in string theory and quantum gravity. The discovery of a very light gauge boson should imply violation of the weak gravity conjecture, carrying deep consequences for our understanding of holography, quantum gravity and black holes. We also show how the detection of a baryophoton would exclude the generation of all B-L violating operators from exotic stringy instantons. We will argue against the common statement in the literature that neutron-antineutron data may indirectly test at least the 300-1000 Te V scale. Searches for baryo-photons can provide indirect information on the Planck(or string) scale(quantum black holes, holography and non-perturbative stringy effects). This strongly motivates new neutron-antineutron experiments with adjustable magnetic fields dedicated to the detection of super-light baryo-photons.     

ANOMALOUS WEAK CHARGED CURRENTS OF LEPTONS AND QUARKS IN A SUBSTRUCTURE MODEL

We present a general approach to get the anomalous weak charged current of leptons and quarks in a substructure mode1,in which quarks, leptons and W-bosons are composed of preons (fermions and scalar bosons). We have shown that the (V+A) current can be determined by the structure of leptons and quarks and the anomalous weak current can be very small. In particular, it can approach zero, if the wave function of leptons (or quarks) has aspinor structure (I-P/m_f)F.     

Massless gauge bosons other than the photon

Gauge bosons associated with unbroken gauge symmetries, under which all standard model fields are singlets, may interact with ordinary matter via higher-dimensional operators. A complete set of dimension-six operators involving a massless U(1) field, gamma('), and standard model fields is presented. The mu-->egamma(') decay, primordial nucleosynthesis, star cooling, and other phenomena set lower limits on the scale of chirality-flip operators in the 1-15 TeV range if the operators have coefficients given by the corresponding Yukawa couplings. Simple renormalizable models induce gamma(') interactions with leptons or quarks at two loops, and may provide a cold dark matter candidate.     

Search for composite and exotic fermions at LEP 2

A search for unstable heavy fermions with the DELPHI detector at LEP is reported. Sequential and non-canonical leptons, as well as excited leptons and quarks, are considered. The data analysed correspond to an integrated luminosity of about 48 pb at an centre-of-mass energy of 183 GeV and about 20 pb equally shared between the centre-of-mass energies of 172 GeV and 161 GeV. The search for pair-produced new leptons establishes 95% confidence level mass limits in the region between 70 GeV/ and 90 GeV/, depending on the channel. The search for singly produced excited leptons and quarks establishes upper limits on the ratio of the coupling of the excited fermion to its mass () as a function of the mass. Received: 30 October 1998 / Published online: 1 March 1999     

Using radiative corrections to bound substructure

Composite models predict the existence of excited quarks and leptons. We find bounds for the masses of possible excited leptons and for the substructure scale by using radiative corrections at theZ scale. A non-decoupling scenario arises naturally which induces upper bounds on these masses as a function of the substructure scale.     

Four-Generation Condensate Scheme Without the Fourth Generation of Leptons

The renormalization group (RG) analyses show that in the four-generation fermion condensate scheme of electroweak symmetry breaking without the extra fourth generation of leptons thelimitation to the compositeness scale Λ could be greatly loosened and up to Λ<10¹⁰ GeV if the masses of the extra fourth generation of quarks are demanded to be bigger than the topquark mass m_t = 180 GeV. However, the mass constraints 2(m_Q)_minh⁰<2(m_Q)_max between the Higgs boson h⁰ and its constituent Q-fermions are no longer totally valid for Λ>10⁵ GeV. The ~redicted masses of the fourth generation of quarks and the Higgs boson will be larger than the corresponding ones in the four-generation quark-lepton scheme. The stability of the results for variation of the compositeness boundary conditions could be explained more clearly.     

Algebraic construction of static axially symmetric self-dual gauge fields for an arbitrary group

It is suggested that the Higgs and the Yukawa couplings of the standard model possess two global U(1)-symmetries: independent chiral rotations of quarks and leptons. The model requires the existence of two axions, one of which (massive) does not interact with leptons, whereas the second one interacts both with leptons and quarks.     

New U(1) gauge extension of the supersymmetric standard model

In extending the minimal standard model of quarks and leptons to include supersymmetry, the conservation of baryon and lepton numbers is no longer automatic. I show how the latter may be achieved with a new U(1) gauge symmetry and new supermultiplets at the TeV scale. Neutrino masses and a solution of the mu problem are essential features of this proposed extension.     

Search for scalar fermions and long-lived scalar leptons at centre-of-mass energies of 130 GeV to 172 GeV

Data taken by DELPHI during the 1995 and 1996 LEP runs have been used to search for the supersymmetric partners of electron, muon and tau leptons and of top and bottom quarks. The observations are in agreement with standard model predictions. Limits are set on sfermion masses. Searches for long lived scalar leptons from low scale supersymmetry breaking models exclude stau masses below 55 GeV/c at the 95% confidence level, irrespective of the gravitino mass. Received: 13 July 1998 / Published online: 19 November 1998     

The Weak Gravity Conjecture and emergence from an ultraviolet cutoff

We study ultraviolet cutoffs associated with the Weak Gravity Conjecture (WGC) and Sublattice Weak Gravity Conjecture (sLWGC). There is a magnetic WGC cutoff at the energy scale \(e G_N^{-1/2}\) with an associated sLWGC tower of charged particles. A more fundamental cutoff is the scale at which gravity becomes strong and field theory breaks down entirely. By clarifying the nature of the sLWGC for nonabelian gauge groups we derive a parametric upper bound on this strong gravity scale for arbitrary gauge theories. Intriguingly, we show that in theories approximately saturating the sLWGC, the scales at which loop corrections from the tower of charged particles to the gauge boson and graviton propagators become important are parametrically identical. This suggests a picture in which gauge fields emerge from the quantum gravity scale by integrating out a tower of charged matter fields. We derive a converse statement: if a gauge theory becomes strongly coupled at or below the quantum gravity scale, the WGC follows. We sketch some phenomenological consequences of the UV cutoffs we derive.     

Operational definition of (brane-induced) space–time and constraints on the fundamental parameters

First we contemplate the operational definition of space–time in four dimensions in light of basic principles of quantum mechanics and general relativity and consider some of its phenomenological consequences. The quantum gravitational fluctuations of the background metric that comes through the operational definition of space–time are controlled by the Planck scale and are therefore strongly suppressed. Then we extend our analysis to the braneworld setup with low fundamental scale of gravity. It is observed that in this case the quantum gravitational fluctuations on the brane may become unacceptably large. The magnification of fluctuations is not linked directly to the low quantum gravity scale but rather to the higher-dimensional modification of Newton's inverse square law at relatively large distances. For models with compact extra dimensions the shape modulus of extra space can be used as a most natural and safe stabilization mechanism against these fluctuations.     

Quarks and leptons beyond the third generation

The possibility of additional quarks and leptons beyond the three generations already established is discussed. The make-up of this Report is (1) Introduction: the motivations for believing that the present litany of elementary fermions is not complete; (2) quantum numbers: possible assignments for additional fermions; (3) masses and mixing angles: mass limits from precision electroweak data, vacuum stability and perturbative gauge unification; empirical constraints on mixing angles; (4) lifetimes and decay modes: their dependence on the mass spectrum and mixing angles of the additional quarks and leptons; the possibility of exceptionally long lifetimes; (5) dynamical symmetry breaking: the significance of the top quark and other heavy fermions for alternatives to the elementary Higgs Boson; (6) CP violation: extensions to more generations and how strong CP may be solved by additional quarks; (7) experimental searches: present status and future prospects; (8) conclusions.