arXiv subjects

Graham G. Ross

Publications and source records attributed to Graham G. Ross.

54 records · Page 3Linked to original sources

arXiv2000

Solar neutrino oscillation from large extra dimensions

A plausible explanation for the existence of additional light sterile neutrinos is that they correspond to modulini, fermionic partners of moduli, which propagate in new large dimensions. We discuss the phenomenological implications of such states and show that solar neutrino oscillation is well described by small angle MSW oscillation to the tower of Kaluza Klein states associated with the modulini. In the optimal case the recoil electron energy spectrum agrees precisely with the measured one, in contrast to the single sterile neutrino case which is disfavoured. We also consider how all oscillation phenomena can be explained in a model including bulk neutrino states. In particular, we show that a naturally maximal mixing for atmospheric neutrinos can be easily obtained.

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arXiv2000

A note on the RG flow in (N=1, D=4) Supergravity and applications to Z_3 orbifold/orientifold compactification

We apply the standard approach of RG flow for the gauge couplings in N=1 D=4 Supergravity to show how to match its results with the heterotic $Z_3$ orbifold and Type IIB ${Z_3}$ orientifold-based models. Using only supergravity, anomaly cancellation and the requirement of unification we determine the part of the Kähler potential of the model invariant under the symmetries of the model. For heterotic orbifolds/type IIB orientifolds, this shows that the lowest order Kähler term of the dilaton has the structure $-\ln(S+{\bar S})$ in agreement with string calculations. The structure of the holomorphic coupling is also found from arguments of unification and anomaly cancellation under the conjectured $SL(2,Z)_{T_i}$ symmetries of the models. A consequence of the latter is that in the case of the $Z_3$ orientifold the holomorphic coupling necessarily contains a part with coefficient proportional to the one loop beta function, in agreement with string calculations which do not however assume this symmetry. This gives circumstantial evidence for the existence of this symmetry at string level in $Z_3$ orientifold. Finally, we comment on the values of the unification scale and examine the possibility of mirage unification in which the effective unification scale may be situated far above the string scale.

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arXiv2000

Multi-Brane Worlds and modification of gravity at large scales

We discuss the implications of multi-brane constructions involving combinations of positive and negative tension brane and show how anomalously light KK states emerge when negative tension ''-'' branes are sandwiched between ''+'' branes. We present a detailed study of a ''+--+'' brane assignment which interpolates between two models that have been previously proposed in which gravity is modified at large scales due to the anomalously light states. We show that it has the peculiar characteristic that gravity changes from four dimensional (4D) to 5D at large distances and returns to 4D at even larger scales. We also consider a crystalline universe which leads to a similar structure for gravity. The problems associated with intermediate negative tension branes are discussed and a possible resolution suggested.

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arXiv2000

Brane Universe and Multigravity: Modification of gravity at large and small distances

We consider a modification of gravity at large distances in a Brane Universe which was discussed recently. In these models the modification of gravity at large distances is ultimately connected to existence of negative tension brane(s) and exponentially small tunneling factor. We discuss a general model which interpolates between Bi-gravity model and GRS model. We also discuss the possible mechanism of stabilization for negative tension branes in AdS background. Finally we show that extra degrees of freedom of massive gravitons do not lead to disastrous contradiction with General Relativity if the stabilization condition $\int dy \sqrt{-G^{(5)}} (T^μ_μ-2T^5_5)=0$ is implemented.

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arXiv1999

Neutrino masses from U(1) symmetries and the Super-Kamiokande data

Motivated by the Super-Kamiokande data, we revisit models with U(1) symmetries and discuss the origin of neutrino masses and mixings in such theories. We show that, in models with just three light neutrinos and a hierarchy of neutrino masses, large (2-3) mixing fixes the lepton doublet U(1) charges and is thus related to the structure of the charged lepton mass matrix. We discuss the fermion mass structure that follows from the Abelian family symmetry with an extended gauge group. Requiring that the quark and lepton masses be ordered by the family symmetry, we identify the most promising scheme. This requires large, but not necessarily maximal, mixing in the mu-tau sector and gives small e-mu mixing in the range that is required for the small angle solution of the solar neutrino deficit.

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arXiv1999

Nearly Degenerate Neutrinos and Broken Flavour Symmetry

Theories with non-Abelian flavour symmetries lead at zeroth order to neutrino degeneracy and massless charged fermions. The flavour symmetry is spontaneously broken sequentially to give hierarchies $Δm_{atm}^2 \gg Δm_{\odot}^2$ and $m_τ\gg m_μ\gg m_e$, and a misalignment of the vacuum between neutrino and charged sectors gives large $θ_{atm}$. Explicit models are given which determine $θ_{atm} = 45^\circ$. A similar construction gives vacuum misalignment for the lighter generations and gives a vanishing $ββ_{0 ν}$ rate, so that there is no laboratory constraint on the amount of neutrino hot dark matter in the universe, and the solar mixing angle is also maximal, $θ_\odot = 45^\circ$.

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arXiv1998

Flat directions, String Compactification and 3 Generation Models

We show how identification of absolutely flat directions allows the construction of a new class of compactified string theories with reduced gauge symmetry that may or may not be continuously connected to the original theory. We use this technique to construct a class of 3 generation models with just the Standard Model gauge group after compactification. We discuss the low-energy symmetries necessary for a phenomenologically viable low-energy model and construct an example in which these symmetries are identified with string symmetries which remain unbroken down to the supersymmetry breaking scale. Remarkably the same symmetry responsible for stabilising the nucleon is also responsible for ensuring one and only one pair of Higgs doublets is kept light. We show how the string symmetries also lead to textures in the quark and lepton mass matrices which can explain the hierarchy of fermion masses and mixing angles.

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arXiv1998

Aspects of String unification

We consider the phenomenological implications of a class of compactified string theories which naturally reproduces the flavour multiplet structure of the Standard Model. The implications for gauge unification depends on which of three possibilities is realised for obtaining light Higgs multiplets. The more conventional one leads to predictions for the gauge couplings close to that of the MSSM but with an increased value of the unification scale. The other two cases offer a mechanism for bringing the prediction for the strong coupling into agreement with the measured value while still increasing the unification scale. The various possibilities lead to different expectations for the structure of the quark masses.

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arXiv1998

Hierarchies of R-violating interactions from Family Symmetries

We investigate the possibility of constructing models of R-violating LQD Yukawa couplings using a single U(1) flavour-symmetry group and supermultiplet charge assignments that are compatible with the known hierarchies of quark and lepton masses. The mismatch of mass and current eigenstates inferred from the known charged-current mixing induces the propagation of strong phenomenological constraints on some R-violating couplings to many others. Applying these constraints, we look for flavour-symmetry models that are consistent with different squark-production hypotheses devised to explain the possible HERA large-Q^2 anomaly. The e^+ d -> stop interpretation of the HERA data is accommodated relatively easily, at the price of postulating an extra parity. The e^+ s -> stop interpretation of the events requires models to have only small (2,3) mixing in the down quark sector. The e^+ d -> scharm mechanism cannot be accommodated without large violations of squark-mass universality, due to the very strong experimental constraints on R-violating operators. We display a model in which baryon decay due to dangerous dimension-five operators is automatically suppressed.

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arXiv1997

Multiple inflation

Attempts at building an unified description of the strong, weak and electromagnetic interactions usually involve several stages of spontaneous symmetry breaking. We consider the effects of such symmetry breaking during an era of primordial inflation in supergravity models. In cases that these occur along flat directions at intermediate scales there will be a succession of short bursts of inflation which leave a distinctive signature in the spectrum of the generated scalar density perturbation. Thus measurements of the spectral index can directly probe the structure of unified theories at very high energy scales. An observed feature in the power spectrum of galaxy clustering from the APM survey may well be associated with such structure. If so, this implies a characteristic suppression of the secondary Doppler peaks in the angular power spectrum of temperature fluctuations in the cosmic microwave background.

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arXiv1996

Natural Supergravity inflation

We identify a new mechanism in supergravity theories which leads to successful inflation without any need for fine tuning. The simplest model yields a spectrum of density fluctuations tilted away from scale-invariance and negligible gravitational waves. We demonstrate that this is consistent with the observed large-scale structure for a cold dark matter dominated, critical density universe. The model can be tested through measurements of microwave background anisotropy on small angular scales.

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arXiv1995

Infra-red fixed point structure of soft supersymmetry breaking mass terms

We show that the soft SUSY breaking mass terms may have infra-red stable fixed points at which they are related to the gaugino masses and argue that in a generic unification these masses should lie close to their fixed points. We consider the implications for the family dependence of squark and slepton masses and the related flavour changing neutral currents and determine conditions under which models with flavour changing couplings and masses at a high scale may lead to a family independent effective theory at low scales. The analysis is illustrated for a variety of models for which we compute both the fixed point structure and determine the rate of approach to the fixed point.

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arXiv1995

Fermion mass prediction from Infra-red fixed points

We argue that in a wide class of theories the fermion and soft supersymmetry breaking mass structure is largely determined by the infra-red fixed point structure of the theory lying beyond the \smp We show how knowlege of the symmetries and multiplet content of this theory is sufficient to determine the infra-red structure, illustrating the idea for the case of a simple abelian family symetry. The resulting structure determines the fermion masses and mixing angles in terms of a restricted number of parameters.

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arXiv1995

Infra-red fixed points revisited

We reconsider how Yukawa couplings may be determined in terms of a gauge coupling through the infra-red fixed point structure paying particular regard to the rate of approach to the fixed point. Using this we determine whether the fixed point structure of an underlying unified theory may play a significant role in fixing the couplings at the gauge unification scale. We argue that, particularly in the case of compactified theories, this is likely to be the case and illustrate this by a consideration of phenomenologically interesting theories. We discuss in particular what the infra-red fixed point structure implies for the top quark mass.

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arXiv1994

Neutrino Masses from Gauge Symmetries

A very simple extension of the standard model to include an Abelian family symmetry is able to describe the hierarchy of quark and lepton masses and their mixing angles together with the unification of gauge couplings. We consider the implications of this model for neutrino masses and mixing angles and show that they are determined up to a discrete ambiguity corresponding to the representation content of the Higgs sector responsible for the Majorana mass matrix.

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arXiv1993

Improved QCD sum rule estimates of the higher twist contributions to polarised and unpolarised nucleon structure functions

We re-examine the estimates of the higher twist contributions to the integral of $g_1$, the polarised structure function of the nucleon, based on QCD sum rules. By including corrections both to the perturbative contribution and to the low energy contribution we find that the matrix elements of the relevant operators are more stable to variations of the Borel parameter $M^2$, allowing for a meaningful estimate of the matrix elements. We find that these matrix elements are typically twice as large as previous estimates. However, inserting these new estimates into the recently corrected expressions for the first moments of $g_1$ leads to corrections too small to affect the phenomenological analysis. For the unpolarised case the higher twist corrections to the GLS and Bjorken sum rules are substantial and bring the estimate of $Λ_{QCD}$ from the former into good agreement with that obtained from the $Q^2$ dependence of deep inelastic data.

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arXiv1993

Constraints on discrete symmetries from anomaly cancellation in compactified superstring theories

Compactified string theories give rise to discrete symmetries which are essential if they are to provide a realistic low energy theory. We find that in a class of four dimensional string theories these symmetries are constrained by similar conditions to those discrete anomaly cancellation conditions found in the case the discrete symmetry is a residue of a spontaneously broken gauge symmetry. Such conditions strongly constrain the allowed form of the low energy effective theory.

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arXiv1992

Sphaleron Erasure of Primordial Baryogenesis

If the present baryon-asymmetry is due to a Planck or GUT-scale matter asymmetry then baryon- or lepton-number violating processes are constrained by the condition that they do not subsequently erase this asymmetry. We present a revision of the analysis of sphaleron baryon-number violating processes in the standard model including lepton-mass effects. We find the surprising result that a GUT-scale matter-asymmetry can survive the $B$ and $L$ violating sphaleron interactions even though ($B- L$) is conserved and equals zero for all temperatures. We extend the analysis to cover the minimal supersymmetric standard model (MSSM) and also derive the constraints on the R-parity violating couplings in extensions of the MSSM. In the case of the baryon number violating dimension 4 operators we find, contrary to current wisdom, that the resulting bounds can be avoided completely because of a residual lepton-flavour number conservation; in the case of lepton number violating operators we find the bounds are flavour dependent and can be avoided completely in definite flavour channels. We also consider how the bounds are modified in the case there is a Grand Unified extension of the supersymmetric model which introduces new lepton flavour violating couplings.

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