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G. G. Ross

Publications and source records attributed to G. G. Ross.

54 records · Page 3Linked to original sources
arXiv1998

The effect of Yukawa couplings on Unification Predictions and the non-perturbative limit

We investigate the effects of Yukawa couplings on the phenomenological predictions for a class of supersymmetric models which allows for the presence of complete SU(5) multiplets in addition to the Minimal Supersymmetric Standard Model spectrum. We develop a two loop analytical approach to quantify the predictions for gauge unification including Yukawa couplings. The effects of the heavy thresholds of the model are also included. In some cases accurate predictions can be made for the unification scale, irrespective of the initial (unknown) Yukawa couplings, so long as perturbation theory remains valid. We also consider the limit of a large number of extra states and compute the predictions in a resummed perturbation series approach to show that the results are stable in this limit. Finally we consider the possibility of making predictions for the case the gauge and Yukawa couplings enter the non-perturbative domain below the unification scale and estimate the errors which affect these predictions.

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arXiv1998

Unification predictions

The unification of gauge couplings suggests that there is an underlying (supersymmetric) unification of the strong, electromagnetic and weak interactions. The prediction of the unification scale may be the first quantitative indication that this unification may extend to unification with gravity. We make a precise determination of these predictions for a class of models which extend the multiplet structure of the Minimal Supersymmetric Standard Model to include the heavy states expected in many Grand Unified and/or superstring theories. We show that there is a strong cancellation between the 2-loop and threshold effects. As a result the net effect is smaller than previously thought, giving a small increase in both the unification scale and the value of the strong coupling at low energies.

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arXiv1998

D-term Inflation in Superstring Theories

An inflationary stage dominated by a $D$-term avoids the slow-roll problem of inflation in supergravity and may emerge in theories with a non-anomalous or anomalous U(1) gauge symmetry. The most intriguing and commonly invoked possibility is that the Fayet-Iliopoulos $D$-term triggering inflation is the one emerging in superstring theories. We discuss the complications one has to face when trying to build up a successful $D$-term inflationary scenario in superstring models. In particular, we show that the ``vacuum shifting'' phenomenon of string theories is usually very efficient even in the early Universe, thus preventing inflation from taking place. On the other hand, when $D$-term inflation is free to occur, the presence of a plethora of fields and several non-anomalous additional abelian symmetries in string theories may help in reconciling the value of the Fayet-Iliopoulos $D$-term required by the COBE normalization with the value predicted by string theories. We also show that in superstring $D$-term inflation gravitinos are likely to pose no cosmological problem.

hep-ph↗
arXiv1998

Soft Masses in Strong Unification

We examine the infra-red structure of soft supersymmetry breaking masses of the squarks and sleptons in supersymmetric (SUSY) strong unification schemes where $α_{GUT}\sim 0.2-1$. We show that combinations of soft masses approach fixed points which leads to very simple predictions for soft masses at the intermediate scale. The assumption that the high energy gaugino mass $M_{1/2}$ dominates over the soft scalar masses leads to a strong suppression of flavour changing neutral currents, and a low energy SUSY spectrum which is simply predicted in terms of two parameters, namely $α_{GUT}$ and $M_{1/2}$. Due to the quickly falling gauge couplings beneath the high energy scale, the low energy spectrum has a characteristic ``scalar dominated'' signature quite unlike the standard gaugino dominated MSSM where the right-handed sleptons are predicted to be rather light. We also examine the new sources of flavour changing expected in such models, and in particular show that the flavour violation coming from the D-term of a $U(1)_X$ gauged family symmetry may be reduced to an acceptable level.

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arXiv1997

Strong Unification

We investigate the possibility that unification occurs at strong coupling. We show that, despite the fact the couplings pass through a strong coupling regime, accurate predictions for their low energy values are possible because the couplings of the theory flow to infrared fixed points. We determine the low-energy QCD coupling in a favoured class of strong coupling models and find it is reduced from the weak coupling predictions, lying close to the experimentally measured value. We extend the analysis to the determination of quark and lepton masses and show that (even without Grand Unification) the infra-red fixed point structure may lead to good predictions for the top mass, the bottom to tau mass ratio and $\tan β$. Finally we discuss the implications for the unification scale finding it to be increased from the MSSM value and closer to the heterotic string prediction.

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arXiv1996

QCD Sum Rule Analysis of Leading Twist Non-Singlet Operator Matrix Elements

We use QCD sum rules to determine the difference between moments of the non-singlet structure functions. This combination decouples from the singular behaviour of the structure functions near x=1 as calculated in the quark-gluon basis and thus should lead to improved sum rule predictions. However, we find there are still very large errors due to higher order corrections. In order to refine the error analysis, we study the effect of renormalon ambiguities on our QCD sum rules results.

hep-ph↗
arXiv1996

Determination of Leading Twist Non-Singlet Operator Matrix Elements by QCD Sum Rules

We use QCD sum rules to determine the twist-two non-singlet operator matrix elements and fixed x structure functions paying particular regard to the estimate of the errors. Particularly for the matrix element determination, we find large uncertainties due to radiative and higher dimension contributions. We consider the origin of these large corrections and comment on their consequences for other operator matrix element determinations.

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arXiv1996

Successful Supersymmetric Inflation

We reconsider the problems of cosmological inflation in effective supergravity theories. A singlet field in a hidden sector is demonstrated to yield an acceptable inflationary potential, without fine tuning. In the simplest such model, the requirement of generating the microwave background anisotropy measured by COBE fixes the inflationary scale to be about $10^{14}$ GeV, implying a reheat temperature of order $10^{5}$ GeV. This is low enough to solve the gravitino problem but high enough to allow baryogenesis after inflation. Such consistency requires that the generation of gravitational waves be negligible and that the spectrum of scalar density perturbations depart significantly from scale-invariance, thus improving the fit to large-scale structure in an universe dominated by cold dark matter. We also consider the problems associated with gravitino production through inflaton decay and with other weakly coupled fields such as the moduli encountered in (compactified) string theories.

hep-ph↗
arXiv1996

Quark Model Description of Polarised Deep Inelastic Scattering and the prediction of $g_2$

We show how the operator product expansion evaluated in the approximation of ignoring gluons leads to the covariant formulation of the quark parton model. We discuss the connection with other formulations and show how the free quark model prediction, $g_2=0$, changes smoothly into the Wandzura-Wilczek (WW) relation for quark masses small relative to the nucleon mass. Previous contradictory parton model predictions are shown to follow from an inconsistent treatment of the mass shell conditions. The description is extended to include quark mass corrections.

hep-ph↗
arXiv1995

Heavy neutrino threshold effects in low energy phenomenology

Right handed neutrinos with mass of ${\cal O} \; (10^{12}-10^{13})$ GeV are required to implement the see-saw mechanism and generate neutrino masses capable of playing a role in structure formation. Moreover models of fermion masses often relate the Yukawa couplings involving these neutrinos to the up-quark Yukawa couplings. Here we study the effects of such couplings on the radiative corrections to quark masses. We find that $b-τ$ equality at $M_{GUT}$ may still give the correct $m_b/m_τ$-- ratio at low energies, but only if there is large $μ-τ$ mixing in the charged leptonic sector. We propose specific mass matrix ``textures'' dictated by a $U(1)$ family symmetry whose structure preserves $m_b=m_τ$ at $M_{GUT}$. In these schemes, due to the large $ν_μ - ν_τ$ mixing, it is possible to give a simultaneous solution to the solar neutrino deficit and the atmospheric neutrino problem.

hep-ph↗
arXiv1995

An analytic solution of the BFKL equation with momentum cutoffs

We outline a general method for obtaining the solution to the ($t=0$) BFKL equation in the presence of transverse momentum cutoffs. A lower cutoff allows one to avoid integration over nonperturbative momenta and an upper one is needed from energy-momentum conservation. Our method allows for the inclusion of an arbitrary number of poles in the kernel and is applicable to any input distribution. Taking Mellin transforms, we discuss the effect of introducing cutoffs by considering the singularity structure in the transform space. We present an improved calculation of the small-$x$ slope of the gluon density.

hep-ph↗
arXiv1994

Reconciling the LEP and Slac Measurements of Sin^2(Θ_W)

We consider whether a discrepancy between the SLAC and LEP measurements of $\Sw$ can be explained by new physics. We find that only the contribution of a new neutral gauge boson, $Z^\prime$, nearly degenerate with the Z can affect the SLAC measurement while leaving the LEP observables almost unaffected. We briefly discuss possible signals for this new gauge boson, including changes in the $Z$ lineshape when measured with polarised electrons, small changes in $R_b$, $A^e_{FB}$, and larger changes in two jet and $t\bar{t}$ production at hadron colliders.

hep-ph↗
arXiv1994

Large scale structure from biased non-equilibrium phase transitions - percolation theory picture

We give an analytical description of the spatial distribution of domain walls produced during a biased nonequilibrium phase transition in the vacuum state of a light scalar field. We discuss in detail the spectrum of the associated cosmological energy density perturbations. It is shown that the contribution coming from domain walls can enhance the standard cold dark matter spectrum in such a way as to account for the whole range of IRAS data and for the COBE measurement of the microwave background anisotropy. We also demonstrate that in case of a biased phase transition which allows a percolative description, the number of large size domain walls is strongly suppressed. This offers a way of avoiding excessive microwave background distorsions due to the gravitational field of domain walls present after decoupling.

hep-ph↗
arXiv1994

Fermion masses and mixing angles from gauge symmetries

The structure of the quark and lepton masses and mixing angles provides one of the few windows we have on the underlying physics generating the \sm. In an attempt to identify the underlying symmetry group we look for the simplest gauge extension of the SUSY standard model capable of generating the observed structure. We show that the texture structure and hierarchical form found in the (symmetric) quark and lepton mass matrices follows if one extends the gauge group of the standard model to include an horizontal $U(1)$ gauge factor, constrained by the need for anomaly cancellation. This $U(1)$ symmetry is spontaneously broken slightly below the unification/string scale leaving as its only remnant the observed structure of masses and mixings. Anomaly cancellation is possible only in the context of superstring theories via the Green Schwarz mechanism with $sin^2(θ_W)=3/8$.

hep-ph↗
arXiv1993

Stitching the Yukawa Quilt

We develop a systematic analysis of quark mass matrices which, starting with the measured values of quark masses and mixing angles, allows for a model independent search for all possible (symmetric or hermitian) mass matrices having texture zeroes at the unification scale. A survey of all six and five texture zero structures yields a total of five possible solutions which may be distinguished by improved measurements of the CKM matrix elements and which may readily be extended to include lepton masses with the Georgi-Jarlskog texture. The solutions naturally suggest a parameterisation for the mass matrices based on a perturbative expansion and we present some speculations concerning the origin of such structure.

hep-ph↗
arXiv1992

Gaugino condensation in 4-D superstring models

We study the possibility that supersymmetry is broken via a gaugino condensate in four dimensional string theories. We derive an effective low-energy theory describing the Goldstone mode associated with the R-symmetry breaking driven by gaugino condensation and show that this gives a description of gaugino condensation equivalent, at tree level, to other effective Lagrangian approaches based on the symmetries of the theory. A study of the mass gap equation shows that non-perturbative effects favour the appearance of a gaugino condensate. By minimizing the effective potential describing both the condensate and the moduli determining the gauge coupling constant and radius of compactification we show that large hierarchy may occur between the gravitino mass and the Planck mass with the existence of only one gaugino condensate. Associated with this is a determination of the fine structure constant and the compactification scale.

hep-ph↗
arXiv1992

Electroweak Breaking in Supersymmetric Models

We discuss the mechanism for electroweak symmetry breaking in supersymmetric versions of the standard model. After briefly reviewing the possible sources of supersymmetry breaking, we show how the required pattern of symmetry breaking can automatically result from the structure of quantum corrections in the theory. We demonstrate that this radiative breaking mechanism works well for a heavy top quark and can be combined in unified versions of the theory with excellent predictions for the running couplings of the model. (To be published in ``Perspectives in Higgs Physics'', G. Kane editor.)

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