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B. C. Allanach

Publications and source records attributed to B. C. Allanach.

125 records · Page 7Linked to original sources
arXiv1998

Renormalization Group Naturalness of GUT Higgs Potentials

We analyze the symmetry-breaking patterns of grand unified theories from the point of view of a recently-proposed criterion of renormalization-group naturalness. We perform the analysis on simple non-SUSY SU(5) and SO(10) and SUSY SU(5) GUTs. We find that the naturalness criterion can favor spontaneous-symmetry-breaking in the direction of the smallest of the maximal little groups. Some differences between theories with and without supersymmetry are also emphasized.

hep-ph↗
arXiv1997

Hybrid MSW + VO Solution of the Solar Neutrino Problem in String-Motivated Unified Theories

It is shown that the hybrid MSW + VO solution of the solar neutrino problem, according to which the solar nu_e undergo matter-enhanced transitions into nu_mu, nu_tau in the Sun followed by long wave length (about 1.5 10^8 km) nu_e -> nu_mu, nu_tau oscillations in vacuum between the Sun and the Earth, can occur naturally in string-motivated grand unified theories. We consider the supersymmetric version of a string-type SU(4)xSU(2)_LxSU(2)_R theory with U(1)_X family symmetry, which was shown to successfully describe the charged fermion masses and the quark mixing, and extend the earlier fermion mass analysis to the neutrino sector. We show that the four oscillation parameters Delta m_31^2, Delta m_21^2 and sin^2 2 theta_12, sin^2 2 theta_13, characterising the combined matter-enhanced transitions and vacuum oscillations of the solar nu_e, naturally get values in the ranges of the hybrid MSW + VO solutions found recently.

hep-ph↗
arXiv1997

Single Sneutrino/Slepton Production at LEP2 and the NLC

We propose a new method of detecting supersymmetry at LEP2 when R-parity is violated by an LLE operator. We consider the processes $eγ\ra\snu_j e_k$ and $eγ\ra \slep_k \barν_j$. These are sensitive to seven of the nine LLE-operators. A Monte-Carlo analysis is performed to investigate the sensitivity to the sneutrino signal, and the 5$σ$ discovery contours in the mass vs. coupling plane are presented. For an integrated luminosity of 100 pb^-1, sneutrinos with masses up to 170 GeV/c^2 could be discovered in the near future at LEP2. For the charged slepton production the cross-section is too low to be detectable.

hep-ph↗
arXiv1997

String Unification, Spaghetti Diagrams and Infra-Red Fixed Points

The minimal supersymmetric standard model (MSSM), is perhaps the leading candidate for new physics beyond the standard model, but it encounters difficulties with string gauge unification and in addition does not shed any light on the question of fermion masses. We consider a scenario in which the MSSM is valid up to an energy scale of 10^16 GeV, but that above this scale the theory is supplemented by extra vector-like representations of the gauge group, plus a gauged U(1)_X family symmetry. In our approach the extra heavy matter above the scale 10^16 GeV is used in two different ways: (1) to allow (two-loop) gauge coupling unification at the string scale; (2) to mix with quarks, leptons and Higgs fields via spaghetti diagrams and so lead to phenomenologically acceptable Yukawa textures. We determine the most economical models in which the extra matter can satisfy both constraints simultaneously. We then give a general discussion of the infra-red fixed points of such models, pointing out the conditions for infra-red stability, then discuss two semi-realistic examples: a Higgs mixing model, and a quark mixing model.

hep-ph↗
arXiv1997

Infra-Red Stable Fixed Points of Yukawa Couplings in Supersymmetric Models

We provide the solutions of the fixed point conditions of the Yukawa sector for a large class of N=1 supersymmetric theories including the minimal and next-to-minimal supersymmetric standard models and their grand unified and other extensions. We also introduce a test which can discriminate between infra-red stable, infra-red unstable and saddle point solutions, and illustrate our methods with the example of the next-to-minimal supersymmetric standard model. We show that in this case, the fixed point prediction of the top quark mass is equivalent to that of the minimal supersymmetric standard model, supporting previous numerical analyses.

hep-ph↗
arXiv1997

Yukawa Textures from Family Symmetry and Unification

In this letter, we show how the combination of both a gauged U(1)_X family symmetry and an extended vertical gauge symmetry in a single model, allows for the presence of additional Clebsch texture zeroes in the fermion mass matrices. This leads to new structures for the textures, with increased predictivity, as compared to schemes with enhanced family symmetries only. We illustrate these ideas in the context of the Pati-Salam gauge group SU(4)xSU(2)_LxSU(2)_R$ supplemented by a U(1)_X gauged family symmetry. In the case of symmetric mass matrices, two of the solutions of Ramond, Roberts and Ross that may not be obtained by family symmetries only, are accurately reproduced. For non-symmetric textures, new structures arise in models of this type. To distinguish between the solutions in this latter case, we performed a numerical fit to the charged fermion mass and mixing data. The best solution we found allows a fit with a total chi^2 of 0.39, for three degrees of freedom.

hep-ph↗
arXiv1997

Yukawa Textures in String Unified Models With SU(4)xO(4) Symmetry

We discuss the origin of Yukawa textures in the string-inspired and string derived models based on the gauge group SU(4)xSU(2)_LxSU(2)_R supplemented by a U(1)_X gauged family symmetry. The gauge symmetries are broken down to those of the minimal supersymmetric standard model which is the effective theory below 10^16 GeV. The combination of the U(1)_X family symmetry and the Pati-Salam gauge group leads to a successful and predictive set of Yukawa textures involving two kinds of texture zeroes: horizontal and vertical texture zeroes. We discuss both symmetric and non-symmetric textures in models of this kind, and in the second case perform a detailed numerical fit to the charged fermion mass and mixing data. Two of the Yukawa textures allow a low energy fit to the data with a total chi^2 of 0.39 and 1.02 respectively, for three degrees of freedom. We also make a first attempt at deriving the non - renormalisable operators required for the Yukawa textures from string theory.

hep-ph↗
arXiv1996

Infra-red fixed point structure characterising SUSY SU(5) symmetry breaking

We analyze the one-loop renormalisation group equations for the parameters of the Higgs potential of a supersymmetric SU(5) model with first step of symmetry breaking involving an adjoint Higgs. In particular, we investigate the running of the parameters that decide the first step of symmetry breaking in an attempt to establish which symmetry-breaking scenarios would be most likely if the model is the effective low-energy description of some more fundamental theory. An infra-red fixed point is identified analytically. We show that it is located at the boundary between the region of Higgs parameter space corresponding to unbroken SU(5) and the region corresponding to the breaking of SU(5) to the Standard Model, and we elaborate on its implications. We also observe that certain forms of the Higgs potential discussed at tree level in the literature are not renormalisation group invariant.

hep-ph↗
arXiv1996

String Gauge Unification and Infra-red Fixed Points in MSSM+X Models

In order to achieve gauge unification at the string scale M_X sim 5.10^17 GeV in the minimal supersymmetric standard model (MSSM) it is necessary to add extra gauge non-singlet representations at an intermediate scale M_I<M_X, leading to a class of models which we refer to as MSSM+X models. We perform a detailed analysis of a large class of MSSM+X models and find that the number of (3,1) representations added must be greater than the total of the number of (3,2) and (1,2) representations. Predictions of M_I, M_X and alpha(M_X) are obtained for models with up to 5 extra vector representations than the MSSM\@. Upper bounds on the U(1) string gauge normalisation k_1 and the sum of the squares of the hypercharge assignments of the extra matter are also obtained for the models. We also study the infra-red fixed point behaviour of the top quark Yukawa coupling in a large class of MSSM+X models and find that the low energy MSSM quasi-fixed point prediction of the top quark mass is more likely to be realised in these theories than in the MSSM. In other words the top quark tends to be heavier in MSSM+X models than in the MSSM. The implementation of a U(1)_X family symmetry into MSSM+X models to account for the Standard Model fermion masses is discussed and a particular viable model is presented.

hep-ph↗
arXiv1996

Gauge Unification at the String Scale and Fermion Masses

In the context of the minimal supersymmetric standard model (MSSM), we discuss the introduction of exotic matter below the string scale $M_X$ in order to achieve gauge unification at $M_X$ (a constraint of a large class of string models). The possible types of exotic matter that can realise this are investigated and its effect on the top quark mass $m_t$ is presented. The implementation of a theory of fermion masses which utilises the exotic matter is briefly discussed.

hep-ph↗
arXiv1995

Fermion Masses in a Supersymmetric SU(4)xSU(2)_LxSU(2)_R Model

We calculate quark and lepton masses and quark mixing angles in the framework of a supersymmetric SU(4)$\otimes$SU(2)$_L\otimes$SU(2)$_R$ model where the gauge group is broken at 10$^{16}$ GeV. The model predicts third family top-bottom-tau Yukawa unification, as in minimal SO(10). The other smaller Yukawa couplings are assumed to arise from non-renormalisable operators suppressed by powers of some heavy scale. We perform a systematic operator analysis of the model in order to find the minimum set of operators which describe the low energy quark and lepton masses, and quark mixing angles consistent with low-energy phenomenology. A novel feature of the model is the possibility of asymmetric texture zeroes in the Yukawa matrices at the scale of the new physics. Successful predictions are obtained for $m_t$, $\tan β$, $m_s/m_μ$, $m_d/m_e$ and $V_{ub}$ in terms of a CP violating phase $ϕ$. For example, we predict $\tan β= 35-65$, $m_t = 130 - 190$ GeV, and $|V_{ub}|>0.0040$.

hep-ph↗
arXiv1995

Neutrino Masses and Mixing Angles in a Supersymmetric SU(4)$\otimes$SU(2)$_L\otimes$SU(2)$_R$ Model

We consider the problem of neutrino masses and mixing angles in a supersymmetric model based on the gauge group SU(4)$\otimes$SU(2)$_L\otimes$SU(2)$_R$ broken at the scale $M_X\approx 10^{16}$ GeV. We extend a previous operator analysis of the charged lepton and quark masses and mixing angles in this model to include the neutrino sector, assuming a universal Majorana mass $M$ for the right-handed neutrinos. The Dirac part of the neutrino matrix is then fixed and the physical neutrino masses and magnitudes of all of the elements of the leptonic mixing matrix are then predicted in terms of the single additional parameter $M$. The successful ansatze predict a tau neutrino mass in the relevant range for the dark matter problem and structure formation, muon and electron neutrinos consistent with the MSW solution to the solar neutrino problem, and tau-muon neutrino mixing at a level which should soon be observed by the CHORUS and NOMAD experiments.

hep-ph↗
arXiv1995

Fermion Masses in SUSY GUTs

We discuss the fermion mass problem in SUSY GUTs, including such ideas as texture zeroes, and Georgi-Jarlskog textures. We focus on a specific supersymmetric model based on the gauge group $SU(4)\otimes SU(2)_L \otimes SU(2)_R$. In this model the gauge group is broken to that of the standard model at $10^{16}$ GeV, and supersymmetry is broken at low-energy. The model may be regarded as a ``surrogate SUSY GUT'', and has several advantages over the $SU(5)$ and $SO(10)$ SUSY GUTs, such as the absence of the doublet-triplet splitting problem, and a simpler Higgs sector which does not involve any large Higgs representations. The model predicts full quadruple Yukawa unification (top-bottom-tau-tau neutrino), leading to the prediction of large top mass and $\tan β$. An operator analysis leads to schemes where the strange and down mass are predicted, and $|V_{ub}|>0.004$.

hep-ph↗
arXiv1995

Quadruple Yukawa Unification in the Minimal Supersymmetric Model

Predictions for m_t, tan beta, m_nu_tau are calculated for quadruple third family t-b-tau-nu_tau Yukawa unified models in the MSSM. The renormalisation group equations for the 3 families of the MSSM, including the right handed neutrino, are presented. For right handed tau neutrino Majorana masses that are bigger than 10^11 GeV, the tau neutrino mass is consistent with present cosmological bounds. The m_t, tan beta predictions are approximately equivalent to those in triple third family Yukawa unified models.

hep-ph↗
arXiv1994

Supersymmetric Grand Unified Theories And Yukawa Unification

Talk given at Frontiers in Particle Physics Conference, Cargese. In this paper, I provide some motivation for supersymmetric grand unified theories, briefly explain an extension of the standard model based on them and present a calculation performed using certain properties of some SUSY GUTs to constrain the available parameter space.

hep-ph↗
arXiv1994

Bottom-Tau Yukawa Unification in the Next-to-Minimal Supersymmetric Standard Model

We discuss the unification of the bottom quark and tau lepton Yukawa couplings within the framework of the next-to-minimal supersymmetric standard model. We compare the allowed regions of the $m_t$-$\tan β$ plane to those in the minimal supersymmetric standard model, and find that over much of the parameter space the deviation between the predictions of two models is small, and nearly always much less than the effect of current theoretical and experimental uncertainties in the bottom quark mass and the strong coupling constant. However over some regions of parameter space top-bottom Yukawa unification cannot be achieved. We also discuss the scaling of the light fermion masses and mixing angles, and show that to within current uncertainties the results of recent texture analyses performed for the minimal model also apply to the next-to-minimal model.

hep-ph↗