Standard Model Spectrum Research Articles

The effect of composite resonances on Higgs decay into two photons

In scenarios of strongly coupled electroweak symmetry breaking, heavy composite particles of different spin and parity may arise and cause observable effects on signals that appear at loop levels. The recently observed process of Higgs to $\gamma \gamma$ at the LHC is one of such signals. We study the new constraints that are imposed on composite models from $H\to \gamma\gamma$, together with the existing constraints from the high precision electroweak tests. We use an effective chiral Lagrangian to describe the effective theory that contains the Standard Model spectrum and the extra composites below the electroweak scale. Considering the effective theory cutoff at $\Lambda = 4\pi v \sim 3 $ TeV, consistency with the $T$ and $S$ parameters and the newly observed $H\to \gamma\gamma$ can be found for a rather restricted range of masses of vector and axial-vector composites from $1.5$ TeV to $1.7$ TeV and $1.8$ TeV to $1.9$ TeV, respectively, and only provided a non-standard kinetic mixing between the $W^{3}$ and $B^{0}$ fields is included.

Proton stability and lightZ′inspired by string derived models

Proton stability is one of the most perplexing puzzles in particle physics. While the renormalizable Standard Model forbids proton decay mediating operators due to accidental global symmetries, many of its extensions introduce such dimension four, five and six operators. Furthermore, it is, in general, expected that quantum gravity only respects local gauge, or discreet, symmetries. String theory provides the arena to study particle physics in a consistent framework of perturbative quantum gravity. An appealing proposition, in this context, is that the dangerous operators are suppressed by an Abelian gauge symmetry, which is broken near the TeV scale. A viable U(1) symmetry should also be anomaly free, be family universal, and allow the generation of fermion masses via the Higgs mechanism. We discuss such U(1) symmetries that arise in quasi--realistic free fermionic heterotic--string derived models. Ensuring that the U(1) symmetry is anomaly free at the low scale requires that the Standard Model spectrum is augmented by additional states that are compatible with the charge assignments in the string models. We construct such string--inspired models and discuss some of their phenomenological implications.

A composite Higgs model analysis of forward-backward asymmetries in the production of tops at Tevatron and bottoms at LEP and SLC

We perform a joint analysis of the prediction of composite Higgs models for the discrepancies of the forward-backward asymmetries of top and bottom quarks at Tevatron and LEP+SLC, respectively. We build a two sector model which can be thought as an effective low energy description of 5D warped models and choose the quantum numbers of the fermionic resonances to protect the Z-couplings of the partially composite Standard Model light quarks. We analyze the cross section, forward-backward asymmetry and invariant mass distribution of the top anti-top production at Tevatron, as well as the bottom forward-backward asymmetry and the Z-branching fraction into b-quarks at LEP and SLC, for the two sector model. In the region of the parameter space that naturally leads to the Standard Model spectrum and solves the bottom anomaly, the model improves the top forward-backward asymmetry, giving a prediction of up to 10%, and predicts a small decrease of the $t \bar t$ cross section. It also predicts non-universal corrections of the Z-couplings to the light quarks of order 0.001.

See-saw masses for quarks and leptons in SU(5)

We build on a recent paper by Grinstein, Redi and Villadoro, where a see-saw like mechanism for quark masses was derived in the context of spontaneously broken gauged flavour symmetries. The see-saw mechanism is induced by heavy Dirac fermions which are added to the Standard Model spectrum in order to render the flavour symmetries anomaly-free. In this letter we report on the embedding of these fermions into multiplets of an SU(5) grand unified theory and discuss a number of interesting consequences.

Complete gauge threshold corrections for intersecting fractional D6-branes: The [formula omitted] and [formula omitted] Standard Models

We perform a complete analysis of one-loop threshold corrections to the gauge couplings of fractional D6-branes. This includes besides SU ( N ) also symplectic, orthogonal and massless Abelian gauge factors and the full computation of contributions from discrete and continuous Wilson lines and brane displacements. Two classes of globally consistent supersymmetric compactifications with Standard Model spectra on T 6 / Z 6 and T 6 / Z 6 ′ are presented in detail with the latter exhibiting the potential of supersymmetry breaking via a hidden sector gaugino condensate. The T 6 / Z 6 ′ Standard Models are completely classified, and it turns out that out of 768 distinct D6-brane configurations only 16 different sets of massless spectra and ten distinct values of gauge couplings at one-loop arise. The gauge threshold corrections enhance the diversity to 196 nonequivalent models.

Constructing the supersymmetric Standard Model from intersecting D6-branes on the [formula omitted] orientifold

Intersecting stacks of supersymmetric fractional branes on the Z6′ orientifold may be used to construct the supersymmetric Standard Model. If a,b are the stacks that generate the SU(3)colour and SU(2)L gauge particles, then, in order to obtain just the chiral spectrum of the (supersymmetric) Standard Model (with non-zero Yukawa couplings to the Higgs multiplets), it is necessary that the number of intersections a∩b of the stacks a and b, and the number of intersections a∩b′ of a with the orientifold image b′ of b satisfy (a∩b,a∩b′)=(2,1) or (1,2). It is also necessary that there is no matter in symmetric representations of the gauge group, and not too much matter in antisymmetric representations, on either stack. Fractional branes having all of these properties may be constructed on the Z6′ orientifold. We provide a number of new examples having these properties, some of which may be extended to give the Standard Model spectrum. Specifically, we construct four-stack models with two further stacks, each with just a single brane, which have the matter spectrum of the supersymmetric Standard Model, including a single pair of Higgs doublets, plus three right-chiral neutrino singlets. Ramond–Ramond tadpole cancellation is achieved by the introduction of background H¯3 flux, the 3-form field strength associated with the Kalb–Ramond 2-form field B2. There remains a single unwanted gauged U(1)B−L.

Heterotic road to the MSSM withRparity

In a previous paper, referred to as a ``Mini-Landscape'' search, we explored a ``fertile patch'' of the heterotic landscape based on a ${\mathbb{Z}}_{6}\ensuremath{-}\mathrm{II}$ orbifold with SO(10) and ${\mathrm{E}}_{6}$ local grand unified theory structures. In the present paper we extend this analysis. We find many models with the minimal supersymmetric standard model spectra and an exact $R$ parity. In all of these models, the vectorlike exotics decouple along $D$-flat directions. We present two benchmark models which satisfy many of the constraints of a realistic supersymmetric model, including nontrivial Yukawa matrices for three families of quarks and leptons and Majorana masses for right-handed neutrinos with nontrivial seesaw masses for the three light neutrinos. In an appendix we comment on the important issue of string selection rules and, in particular, the so-called ``gamma-rule''.

Towards realistic standard model from D-brane configurations

Effective low energy models arising in the context of D-brane configurations with standard model (SM) gauge symmetry extended by several gauged Abelian factors are discussed. The models are classified according to their hypercharge embeddings consistent with the SM spectrum hypercharge assignment. Particular cases are analyzed according to their perspectives and viability as low energy effective field theory candidates. The resulting string scale is determined by means of a two-loop renormalization group calculation. Their implications in Yukawa couplings, neutrinos and flavor changing processes are also presented.

Configuration mixing in nucleon-induced pre-equilibrium reactions

We present a unified semiclassical model of nucleon-induced pre-equilibrium reactions that permits the simulation of varying degrees of internal configuration mixing. We can reproduce the limits of no intraclass transitions, assumed in the hybrid Monte Carlo simulation (HMS) model, or of intraclass equilibrium, assumed in exciton models. We compare the two limiting cases with a natural model, in which the intraclass transitions are treated on an equal footing with the others. Monte Carlo simulations of the models with no intraclass transitions and with the natural ones yield very similar emission spectra, which are quite different from the exciton model ones. To reproduce standard exciton model spectra, the model simulations require an intraclass transition rate a thousand times larger than the natural one.

Configuration mixing in pre-equilibrium reactions

We present a unified semiclassical model of nucleon-induced pre-equilibrium reactions that permits the simulation of varying degrees of internal configuration mixing. We use it to compare the case of no configuration mixing with that of complete mixing. We also compare the two with an intermediate case that we call the natural model. The no-mixing and natural models yield very similar results, which are quite different from the complete-mixing ones. To reproduce standard exciton model spectra, the model simulations require complete mixing and an intraclass transition rate that is a thousand times larger than the natural one.

Split Supersymmetry and Magnetic Fluxes

Type I string theory in the presence of internal magnetic fields provides a concrete realization of split supersymmetry. To lowest order, gauginos are massless while squarks and sleptons are superheavy. For weak magnetic fields, the correct Standard Model spectrum guarantees gauge coupling unification with sin2 θW = 3/8 at the compactification scale of MGUT ≃ 2 × 1016 GeV. I discuss mechanisms for generating gaugino and higgsino masses at the TeV scale, as well as generalizations to models with split extended supersymmetry in the gauge sector.

Split supersymmetry in brane models

Type-I string theory in the presence of internal magnetic fields provides a concrete realization of split supersymmetry. To lowest order, gauginos are massless while squarks and sleptons are superheavy. For weak magnetic fields, the correct Standard Model spectrum guarantees gauge coupling unification with sin2 ϑW=3/8 at the compactification scale of M GUT ⋍ 2 × 1016 GeV. I discuss mechanisms for generating gaugino and higgsino masses at the TeV scale, as well as generalizations to models with split extended supersymmetry in the gauge sector.

Dual models of gauge unification in various dimensions

We construct a compactification of the heterotic string on an orbifold T 6 / Z 6 leading to the standard model spectrum plus vector-like matter. The standard model gauge group is obtained as an intersection of three SO ( 10 ) subgroups of E 8 . Three families of SO ( 10 ) 16 -plets are localized at three equivalent fixed points. Gauge coupling unification favours existence of an intermediate GUT which can have any dimension between five and ten. Various GUT gauge groups occur. For example, in six dimensions one can have E 6 × SU ( 3 ) , SU ( 4 ) × SU ( 4 ) × U ( 1 ) 2 or SO ( 8 ) × SO ( 8 ) , depending on which of the compact dimensions are large. The different higher-dimensional GUTs are ‘dual’ to each other. They represent different points in moduli space, with the same massless spectrum and ultraviolet completion.

Chiral supersymmetric Standard Model spectra from orientifolds of Gepner models

We construct d=4, N=1 orientifolds of Gepner models with just the chiral spectrum of the Standard Model. We consider all simple current modular invariants of c=9 tensor products of N=2 minimal models. For some very specific tensor combinations, and very specific modular invariants and orientifold projections, we find a large number of such spectra. We allow for Standard Model singlet (dark) matter and non-chiral exotics. The Chan–Paton gauge group is either U(3)×Sp(2)×U(1)×U(1) or U(3)×U(2)×U(1)×U(1). In many cases the Standard Model hypercharge U(1) has no coupling to RR 2-forms and hence remains massless; in some of those models the B–L gauge boson does acquire a mass.

Supersymmetric standard model spectra from RCFT orientifolds

We present supersymmetric, tadpole-free d = 4 , N = 1 orientifold vacua with a three family chiral fermion spectrum that is identical to that of the standard model. Starting with all simple current orientifolds of all Gepner models we perform a systematic search for such spectra. We consider several variations of the standard four-stack intersecting brane realization of the standard model, with all quarks and leptons realized as bifundamentals and perturbatively exact baryon and lepton number symmetries, and with a U ( 1 ) Y vector boson that does not acquire a mass from Green–Schwarz terms. The number of supersymmetric Higgs pairs H 1 + H 2 is left free. In order to cancel all tadpoles, we allow a “hidden” gauge group, which must be chirally decoupled from the standard model. We also allow for non-chiral mirror-pairs of quarks and leptons, non-chiral exotics and (possibly chiral) hidden, standard model singlet matter, as well as a massless B–L vector boson. All of these less desirable features are absent in some cases, although not simultaneously. In particular, we found cases with massless Chan–Paton gauge bosons generating nothing more than SU ( 3 ) × SU ( 2 ) × U ( 1 ) . We obtain almost 180 000 rationally distinct solutions (not counting hidden sector degrees of freedom), and present distributions of various quantities. We analyse the tree level gauge couplings, and find a large range of values, remarkably centered around the unification point.

Nonlinearly realized local scale invariance: gravity and matter

That the scalar field theories with no dimensional couplings possess local scale invariance (LSI) via the curvature gauging is utilized to show that the Goldstone boson, released by the spontaneous LSI breakdown, is swallowed by the spacetime curvature in order to generate Newton's constant in the same spirit as the induction of vector boson masses via spontaneous gauge symmetry breaking. For Einstein gravity to be reproduced correctly, the Goldstone boson of spontaneous LSI breaking must be endowed with ghost dynamics. The matter sector, taken to be the standard model spectrum, gains full LSI property with the physical Higgs boson acting as the Goldstone boson released by LSI breakdown at the weak scale. The pattern of particle masses is identical to that of the standard model. There are unitary LSI gauges in which either the Goldstone ghost from gravity sector or the Higgs boson from matter sector is eliminated from the spectrum. The heavy right-handed neutrinos as well as softly broken supersymmetry naturally fit into the nonlinearly realized LSI framework.

Yukawa couplings in intersecting D-brane models

We compute the Yukawa couplings among chiral fields in toroidal Type II compactifications with wrapping D6-branes intersecting at angles. Those models can yield realistic standard model spectrum living at the intersections. The Yukawa couplings depend both on the Kahler and open string moduli but not on the complex structure. They arise from worldsheet instanton corrections and are found to be given by products of complex Jacobi theta functions with characteristics. The Yukawa couplings for a particular intersecting brane configuration yielding the chiral spectrum of the MSSM are computed as an example. We also show how our methods can be extended to compute Yukawa couplings on certain classes of elliptically fibered CY manifolds which are mirror to complex cones over del Pezzo surfaces. We find that the Yukawa couplings in intersecting D6-brane models have a mathematical interpretation in the context of homological mirror symmetry. In particular, the computation of such Yukawa couplings is related to the construction of Fukaya's category in a generic symplectic manifold.

Preserving the lepton asymmetry in the brane world

In models where the Standard Model spectrum is localized on a brane embedded in a higher-dimensional spacetime, we discuss the lepton number violation induced by the emission of right-handed neutrinos from the brane. We show that the presence of the right-handed neutrinos in the bulk may lead to rapid lepton number violating processes which above the electroweak scale would wash away any prior lepton or baryon asymmetry. We derive constraints on the Yukawa couplings of these states in order to preserve the lepton asymmetry. We show that this has a natural interpretation in the brane world.

New Standard Model Vacua from Intersecting Branes

We construct new D6-brane model vacua (non-supersymmetric) that have at low energy exactly the standard model spectrum (with right handed neutrinos). The minimal version of these models requires five stacks of branes. and the construction is based on D6-branes intersecting at angles in $D = 4$ type toroidal orientifolds of type I strings. Three U(1)'s become massive through their couplings to RR couplings and from the two surviving anomaly free U(1)'s, one is the standard model hypercharge generator while the extra anomaly free U(1) could be broken from its non-zero couplings to RR fields and also by triggering a vev to previously massive particles. We suggest that extra massless U(1)'s should be broken by requiring some intersection to respect N=1 supersymmetry thus supporting the appearance of massless charged singlets at the supersymmetric intersection. Proton is stable as baryon number is gauged and its anomalies are cancelled through a generalized Green-Schwarz mechanism. Neutrinos are of Dirac type with small masses, as in the four stack standard models of hep-th/0105155, as a result of the existence of a similar PQ like-symmetry. The models are unique in the sense that they predict the existence of only one supersymmetric particle, the superpartner of $\nu_R$.

GUT Model Hierarchies from Intersecting Branes

By employing D6-branes intersecting at angles in $D = 4$ type I strings, we construct the first examples of three generation string GUT models (PS-A class), that contain at low energy exactly the standard model spectrum with no extra matter and/or extra gauge group factors. They are based on the group $SU(4)_C \times SU(2)_L \times SU(2)_R$. The models are non-supersymmetric, even though SUSY is unbroken in the bulk. Baryon number is gauged and its anomalies are cancelled through a generalized Green-Schwarz mechanism. We also discuss models (PS-B class) which at low energy have the standard model augmented by an anomaly free U(1) symmetry and show that multibrane wrappings correspond to a trivial redefinition of the surviving global U(1) at low energies. There are no colour triplet couplings to mediate proton decay and proton is stable. The models are compatible with a low string scale of energy less that 650 GeV and are directly testable at present or future accelerators as they predict the existence of light left handed weak fermion doublets at energies between 90 and 246 GeV. The neutrinos get a mass through an unconventional see-saw mechanism. The mass relation $m_e = m_d$ at the GUT scale is recovered. Imposing supersymmetry at particular intersections generates non-zero Majorana masses for right handed neutrinos as well providing the necessary singlets needed to break the surviving anomaly free U(1), thus suggesting a gauge symmetry breaking method that can be applied in general left-right symmetric models.