arXiv subjects

John Ellis

Publications and source records attributed to John Ellis.

At least 415 records · Page 23Linked to original sources

arXiv1999

Particles and Cosmology: Learning from Cosmic Rays

The density budget of the Universe is reviewed, and then specific particle candidates for non-bayonic dark matter are introduced, with emphasis on the relevance of cosmic-ray physics. The sizes of the neutrino masses indicated by recent atmospheric and solar neutrino experiments may be too small to contribute much hot dark matter. My favoured candidate for the dominant cold dark matter is the lightest supersymmetric particle, which probably weighs between about 50 GeV and about 600 GeV. Strategies to search for it via cosmic rays due to annihilations in the halo, Sun and Earth, or via direct scattering experiments, are mentioned. Possible superheavy relic particles are also discussed, in particular metastable string- or M-theory cryptons, that may be responsible for the ultra-high-energy cosmic rays. Finally, it is speculated that a non-zero contribution to the cosmological vacuum energy might result from incomplete relaxation of the quantum-gravitational vacuum.

astro-ph↗

arXiv1999

Future of Heavy-Flavour Physics

The prospects for the coming Golden Age of heavy-flavour physics are discussed from the perspective of one who hopes it may provide a window onto physics beyond the Standard Model. Precise QCD calculations are necessary for accurate determinations of CKM parameters, and may be provided by the lattice and new perturbative techniques. The future of CKMology in the wake of the relatively small branching ratio for B_d -> pi pi are discussed, including alternative strategies for measuring alpha and gamma. The opportunities for measurements of rare B decays are reviewed briefly, as are possible K-decay windows on physics beyond the Standard Model, in the wake of the establishment of a relatively large value for epsilon'/epsilon. Finally, relations to other aspects of flavour physics are discussed, including the grand unification of quark and lepton masses in the context of neutrino-mass models motivated by the recent oscillation data from Super-Kamiokande and elsewhere.

hep-ph↗

arXiv1999

Probing Models of Quantum Space-Time Foam

We review the possibility that quantum fluctuations in the structure of space-time at the Planck scale might be subject to experimental probes. We study the effects of space-time foam in an approach inspired by string theory, in which solitonic D-brane excitations are taken into account when considering the ground state. We model the properties of this medium by analyzing the recoil of a D particle which is induced by the scattering of a closed-string state. We find that this recoil causes an energy-dependent perturbation of the background metric, which in turn induces an energy-dependent refractive index in vacuo, and stochastic fluctuations of the light cone. We show how distant astrophysical sources such as Gamma-Ray Bursters (GRBs) may be used to test this possibility, making an illustrative analysis of GRBs whose redshifts have been measured. Within this framework, we also discuss the propagation of massive particles and the possible appearance of cosmological vacuum energy that relaxes towards zero. We also discuss D-brane recoil in models with `large' extra dimensions.

gr-qc↗

arXiv1999

Logarithmic Operators Fold D branes into AdS_3

We use logarithmic conformal field theory techniques to describe recoil effects in the scattering of two Dirichlet branes in D dimensions. In the particular case that a D1 brane strikes a D3 brane perpendicularly, thereby folding it, we find that the recoil space-time is maximally symmetric, with AdS_3 x E_{D-3} geometry. We comment on the possible applications of this result to the study of transitions between different background metrics.

hep-th↗

arXiv1999

Elevating the Free-Fermion $Z_2\times Z_2$ Orbifold Model to a Compactification of F-Theory

We study the elliptic fibrations of some Calabi-Yau three-folds, including the $Z_2\times Z_2$ orbifold with $(h_{1,1},h_{2,1})=(27,3)$, which is equivalent to the common framework of realistic free-fermion models, as well as related orbifold models with $(h_{1,1},h_{2,1})=(51,3)$ and (31,7). However, two related puzzles arise when one considers the $(h_{1,1},h_{2,1})=(27,3)$ model as an F-theory compactification to six dimensions. The condition for the vanishing of the gravitational anomaly is not satisfied, suggesting that the F-theory compactification does not make sense, and the elliptic fibration is well defined everywhere except at four singular points in the base. We speculate on the possible existence of N=1 tensor and hypermultiplets at these points which would cancel the gravitational anomaly in this case.

hep-th↗

arXiv1999

Higgs Mass Textures in Flipped SU(5)

We analyze the Higgs doublet-triplet mass splitting problem in the version of flipped SU(5) derived from string theory. Analyzing non-renormalizable terms up to tenth order in the superpotential, we identify a pattern of field vev's that keeps one pair of electroweak Higgs doublets light, while all other Higgs doublets and all Higgs triplets are kept heavy, with the aid of the economical missing-doublet mechanism found in the field-theoretical version of flipped SU(5). The solution predicts that second-generation charge -1/3 quarks and charged leptons are much lighter than those in the third generation.

hep-ph↗

arXiv1999

Neutrino Physics: Theory and Phenomenology

Various issues in neutrino phenomenology are reviewed, including: the possibility of large mixing angles in various models for neutrino masses, difficulties for degenerate neutrinos as candidates for hot dark matter, strategies for discriminating between different oscillation interpretations of the atmospheric and solar neutrino anomalies, the programme of work for long-baseline neutrino experiments, and the possible future option of a muon storage ring as a neutrino factory.

hep-ph↗

arXiv1999

Light-Cone Broadening and TeV Scale Extra Dimensions

We examine the effect of light-cone broadening induced by quantum-gravity foam in the context of theories with ``large'' extra dimensions stretching between two parallel brane worlds. We consider the propagation of photon probes on one of the branes, including the response to graviton fluctuations, from both field- and string-theoretical viewpoints. In the latter approach, the dominant source of light-cone broadening may be the recoil of the D-brane, which scales linearly with the string coupling. Astrophysical constraints then place strong restrictions on consistent string models of macroscopic extra dimensions. The broadening we find in the field-theoretical picture seems to be close to the current sensitivity of gravity-wave interferometers, and therefore could perhaps be tested experimentally in the foreseeable future.

hep-th↗

arXiv1999

Supergravity and Supersymmetry Breaking in Four and Five Dimensions

We discuss supersymmetry breaking in the field-theoretical limit of the strongly-coupled heterotic string compactified on a Calabi-Yau manifold, from the different perspectives of four and five dimensions. The former applies to light degrees of freedom below the threshold for five-dimensional Kaluza-Klein excitations, whereas the five-dimensional perspective is also valid up to the Calabi-Yau scale. We show how, in the latter case, two gauge sectors separated in the fifth dimension are combined to form a consistent four-dimensional supergravity. In the lowest order of the $κ^{2/3}$ expansion, we show how a four-dimensional supergravity with gauge kinetic function $f_{1,2}=S$ is reproduced, and we show how higher-order terms give rise to four-dimensional operators that differ in the two gauge sectors. In the four-dimensional approach, supersymmetry is seen to be broken when condensates form on one or both walls, and the goldstino may have a non-zero dilatino component. As in the five-dimensional approach, the Lagrangian is not a perfect square, and we have not identified a vacuum with broken supersymmetry and zero vacuum energy. We derive soft supersymmetry-breaking terms for non-standard perturbative embeddings, that are relevant in more general situations such as type I/type IIB orientifold models.

hep-th↗

arXiv1999

A Microscopic Recoil Model for Light-Cone Fluctuations in Quantum Gravity

We present a microscopic model for light-cone fluctuations ``in vacuo'', which incorporates a treatment of quantum-gravitational recoil effects induced by energetic particles. Treating defects in space-time as solitons in string theory, we derive an energy-dependent refractive index and a stochastic spread in the arrival times of mono-energetic photons due to quantum diffusion through space-time foam, as found previously using an effective Born-Infeld action. Distant astrophysical sources provide sensitive tests of these possible quantum-gravitational phenomena.

gr-qc↗

arXiv1999

Theoretical Summary: 1999 Electroweak Session of the Rencontres de Moriond

The following aspects of the electroweak interactions are discussed, based on presentations here: the status of the Standard Model, CP violation, neutrino masses and oscillations, supersymmetry and models in extra dimensions, and future projects. Particular emphasis is laid on the tests of CP and CPT by KTeV and CPLEAR, on the problems of degenerate neutrinos, on supersymmetric dark matter, on future long-baseline neutrino beams, and on muon storage rings that may be used as neutrino factories.

hep-ph↗

arXiv1999

Quantum-Gravitational Diffusion and Stochastic Fluctuations in the Velocity of Light

We argue that quantum-gravitational fluctuations in the space-time background give the vacuum non-trivial optical properties that include diffusion and consequent uncertainties in the arrival times of photons, causing stochastic fluctuations in the velocity of light ``in vacuo''. Our proposal is motivated within a Liouville string formulation of quantum gravity that also suggests a frequency-dependent refractive index of the particle vacuum. We construct an explicit realization by treating photon propagation through quantum excitations of $D$-brane fluctuations in the space-time foam. These are described by higher-genus string effects, that lead to stochastic fluctuations in couplings, and hence in the velocity of light. We discuss the possibilities of constraining or measuring photon diffusion ``in vacuo'' via $γ$-ray observations of distant astrophysical sources.

gr-qc↗

arXiv1999

Can Neutrinos be Degenerate in Mass?

We reconsider the possibility that the masses of the three light neutrinos of the Standard Model might be almost degenerate and close to the present upper limits from Tritium beta decay and cosmology. In such a scenario, the cancellations required by the latest upper limit on neutrinoless double-beta decay enforce near-maximal mixing that may be compatible only with the vacuum-oscillation scenario for solar neutrinos. We argue that the mixing angles yielded by degenerate neutrino mass-matrix textures are not in general stable under small perturbations. We evaluate within the MSSM the generation-dependent one-loop renormalization of neutrino mass-matrix textures that yielded degenerate masses and large mixing at the tree level. We find that m_{nu_e} > m_{nu_mu} > m_{nu_tau} after renormalization, excluding MSW effects on solar neutrinos. We verify that bimaximal mixing is not stable, and show that the renormalized masses and mixing angles are not compatible with all the experimental constraints, even for tanbeta as low as unity. These results hold whether the neutrino masses are generated by a see-saw mechanism with heavy neutrinos weighing approx. 10^{13} GeV or by non-renormalizable interactions at a scale approx. 10^5 GeV. We also comment on the corresponding renormalization effects in the minimal Standard Model, in which m_{nu_e} < m_{nu_mu} < m_{nu_tau}. Although a solar MSW effect is now possible, the perturbed neutrino masses and mixings are still not compatible with atmospheric- and solar-neutrino data.

hep-ph↗

arXiv1999

Comments on CP, T and CPT Violation in Neutral Kaon Decays

We comment on CP, T and CPT violation in the light of interesting new data from the CPLEAR and KTeV Collaborations on neutral kaon decay asymmetries. Other recent data from the CPLEAR experiment, constraining possible violations of CPT and the $ΔS = ΔQ$ rule, exclude the possibility that the semileptonic-decay asymmetry $A_{T}$ measured by CPLEAR could be solely due to CPT violation, confirming that their data constitute direct evidence for T violation. The CP-violating asymmetry in $K_L \to e^-e^+π^-π^+$ recently measured by the KTeV Collaboration does not by itself provide direct evidence for T violation, but we use it to place new bounds on CPT violation.

hep-ph↗

arXiv1999

Limits on Sparticle Dark Matter

Arguments are given that the lightest supersymmetric particle should be a neutralino $χ$. Minimizing the fine tuning of the gauge hierarchy favours $Ω_χ h^2 \sim 0.1$. There are important constraints on the parameter space of the MSSM from the stability of the electroweak vacuum. Co-annihilation with the next-to-lightest supersymmetric particle is potentially significant. Incorporating the latest accelerator constraints from LEP and elsewhere, we find that 50 GeV $\lappeq m_χ\lappeq$ 600 GeV and $\tanβ\gappeq$ 2.5, if soft supersymmetry breaking parameters are assumed to be universal.

astro-ph↗

arXiv1999

On the Thermodynamics of a Gas of AdS Black Holes and the Quark-Hadron Phase Transition

We discuss the thermodynamics of a gas of black holes in five-dimensional anti-de-Sitter (AdS) space, showing that they are described by a van der Waals equation of state. Motivated by the Maldacena conjecture, we relate the energy density and pressure of this non-ideal AdS black-hole gas to those of four-dimensional gauge theory in the unconfined phase. We find that the energy density rises rapidly above the deconfinement transition temperature, whilst the pressure rises more slowly towards its asymptotic high-temperature value, in qualitative agreement with lattice simulations.

hep-th↗

arXiv1999

Heavy-Ion Collisions and Black Holes in Anti-de-Sitter Space

Recent developments linking non-perturbative quantum gauge theories in Minkowski space to classical gravity theories in anti-de-Sitter space are reviewed at a simple level. It is suggested how these spectacular advances may be extended to discuss the quark-gluon phase transition in terms of black holes in anti-de-Sitter space, with possible relevance to heavy-ion collisions.

nucl-th↗

arXiv1999

Learning Physics from the Cosmic Microwave Background

The Cosmic Microwave Background (CMB) provides a precious window on fundamental physics at very high energy scales, possibly including quantum gravity, GUTs and supersymmetry. The CMB has already enabled defect-based rivals to inflation to be discarded, and will be able to falsify many inflationary models. In combination with other cosmological observations, including those of high-redshift supernovae and large-scale structure, the CMB is on the way to providing a detailed budget for the density of the Universe, to be compared with particle-physics calculations for neutrinos and cold dark matter. Thus CMB measurements complement experiments with the LHC and long-baseline neutrino beams.

astro-ph↗