arXiv subjects

John March-Russell

Publications and source records attributed to John March-Russell.

100 records · Page 6Linked to original sources

arXiv1995

String Unification, Higher-Level Gauge Symmetries, and Exotic Hypercharge Normalizations

We explore the extent to which string theories with higher-level gauge symmetries and non-standard hypercharge normalizations can reconcile the discrepancy between the string unification scale and the GUT scale extrapolated from the Minimal Supersymmetric Standard Model (MSSM). We determine the phenomenologically allowed regions of (k_Y,k_2,k_3) parameter space, and investigate the proposal that there might exist string models with exotic hypercharge normalizations k_Y which are less than their usual value k_Y=5/3. For a broad class of heterotic string models (encompassing most realistic string models which have been constructed), we prove that k_Y >= 5/3. Beyond this class, however, we show that there exist consistent MSSM embeddings which lead to k_Y < 5/3. We also consider the constraints imposed on k_Y by demanding charge integrality of all unconfined string states, and show that only a limited set of hypercolor confining groups and corresponding values of k_Y are possible.

hep-th↗

arXiv1995

New Symmetries of Supersymmetric Effective Lagrangians

We consider the structure of effective lagrangians describing the low-energy dynamics of supersymmetric theories in which a global symmetry $G$ is spontaneously broken to a subgroup $H$ while supersymmetry is unbroken. In accordance with the supersymmetric Goldstone theorem, these lagrangians contain Nambu--Goldstone superfields associated with a coset space $G^c / \hat{H}$, where $G^c$ is the complexification of $G$ and $\hat{H}$ is the largest subgroup of $G^c$ that leaves the order parameter invariant. The lagrangian may also contain additional light matter fields. To analyze the effective lagrangian for the matter fields, we first consider the case where the effective lagrangian is obtained by integrating out heavy modes at weak coupling (but including non-perturbative effects such as instantons). We show that the superpotential of the matter fields is $\hat{H}$ invariant, which can give rise to non-trivial relations among independent $H$-invariants in the superpotential. We also show that the Kahler potential of the matter fields can be restricted by a remnant of $\hat{H}$ symmetry. These results are non-perturbative and have a simple group-theoretic interpretation. When we relax the weak-coupling constraint, there appear to be additional possibilities for the action of $\hat{H}$ on the matter fields, hinting that the constraints imposed by $\hat{H}$ may be even richer in strongly coupled theories.

hep-ph↗

arXiv1994

Baryon Magnetic Moments in a Simultaneous Expansion in $1/N$ and $m_s$

We consider the baryon octet and decuplet magnetic moments in a simultaneous expansion in $m_s$ and $1/N$ taking $N_F / N \sim 1$, where $N$ is the number of QCD colors and $N_F$ is the number of light quark flavors. At leading order in this expansion, the magnetic moments obey the non-relativistic quark-model relations. We compute corrections to these relations using an effective lagrangian formalism which respects chiral symmetry to all orders in the $1/N$ expansion. Including corrections up to order $m_s^{1/2}$, we find 8 relations among the 9 measured octet and decuplet magnetic moments; including corrections up to order $1/N$ and $m_s$, we find 4 remaining relations. The relations work well, and suggest that the expansion is under control. We give predictions for the unmeasured decuplet magnetic moments.

hep-ph↗

arXiv1993

Baryons from Quarks in the $1/N$ Expansion

We present a diagrammatic analysis of baryons in the $1/N$ expansion, where $N$ is the number of QCD colors. We use this method to show that there are an infinite number of degenerate baryon states in the large-$N$ limit. We also show that forward matrix elements of quark bilinear operators satisfy the static quark-model relations in this limit, and enumerate the corrections to these relations to all orders in $1/N$. These results hold for any number of light quark flavors, and the methods used can be extended to arbitrary operators. Our results imply that for two flavors, the quark-model relations for the axial currents and magnetic moments get corrections of order $1/N^2$. For three or more flavors, the results are more complicated, and corrections are generically of order $1/N$. We write an explicit effective lagrangian which can be used to carry out chiral perturbation theory calculations in the $1/N$ expansion. Finally, we compare our results to what is expected from a chiral constituent quark model.

hep-ph↗

arXiv1993

Radiatively-Induced First-Order Phase Transitions: The Necessity of the Renormalization Group

We advocate a (Wilson) renormalization-group (RG) treatment of finite-temperature first-order phase transitions, in particular those driven by radiative corrections such as occur in the standard model, and other spontaneously-broken gauge theories. We introduce the scale-dependent coarse-grained free energy $S_\La[ϕ]$ which we explicitly calculate, using the Wilson RG and a $(4-\ep)$-expansion, for a scalar toy model that shares many features of the gauged case. As argued by Langer and others, the dynamics of the phase transition are described by $S_\La[ϕ]$ with $\La$ of order the bubble wall thickness, and {\it not} by the usual (RG-improved) finite-temperature effective action which is reproduced by $S_\La[ϕ]$ for $\La\to 0$. We argue that for weakly first-order transitions (such as that in the standard model) the $(4-\ep)$-expansion is necessary to control an inevitable growth of the effective scale-dependent coupling towards the strong-coupling regime, and that diagrammatic resummation techniques are unlikely to be appropriate.

hep-ph↗

arXiv1992

On the Possibility of Second-Order Phase Transitions in Spontaneously Broken Gauge Theories

In the ``Type-II'' regime, $m_{\rm Higgs}\gap m_{\rm gauge}$, the finite-temperature phase transition in spontaneously-broken gauge theories (including the standard model) must be be studied using a renormalization group treatment. Previous studies within the $(4-\ep)$-expansion suggest a 1st-order transition in this regime. We use analogies with experimentally accessible phase transitions in liquid crystals, and theoretical investigations of superconductor phase transitions to argue that, in this range, the critical behavior of a large class of gauge-Higgs-fermion systems changes from 1st to 2nd-order as a function of Higgs mass. We identify a set of models which, within the $(2+\ep)$-expansion, possess fixed points that can describe this 2nd-order behavior. As usual, a definitive demonstration that the claimed critical behavior occurs (and a reliable estimate of $m_{\rm Higgs}$ at the tricritical point) will probably require numerical simulations. This paper uses the ``phyzzx'' macro package. To obtain figure contact the author.

hep-ph↗

arXiv1992

Planck-Scale Physics and the Peccei-Quinn Mechanism

Global-symmetry violating higher-dimension operators, expected to be induced by Planck-scale physics, in general drastically alter the properties of the axion field associated with the Peccei-Quinn solution to the strong-CP problem, and render this solution unnatural. The particle physics and cosmology associated with other global symmetries can also be significantly changed.

hep-th↗

arXiv1992

Are Textures Natural?

We make the simple observation that, because of global symmetry violating higher-dimension operators expected to be induced by Planck-scale physics, textures are generically much too short-lived to be of use for large-scale structure formation.

hep-th↗

arXiv1991

Internal Frame Dragging and a Global Analog of the Aharonov-Bohm Effect

It is shown that the breakdown of a {\it global} symmetry group to a discrete subgroup can lead to analogues of the Aharonov-Bohm effect. At sufficiently low momentum, the cross-section for scattering of a particle with nontrivial $\Z_2$ charge off a global vortex is almost equal to (but definitely different from) maximal Aharonov-Bohm scattering; the effect goes away at large momentum. The scattering of a spin-1/2 particle off a magnetic vortex provides an amusing experimentally realizable example.

hep-th↗

arXiv1991

Quantum Field Theory of Nonabelian Strings and Vortices

We develop an operator formalism for investigating the properties of nonabelian cosmic strings (and vortices) in quantum field theory. Operators are constructed that introduce classical string sources and that create dynamical string loops. The operator construction in lattice gauge theory is explicitly described, and correlation functions are computed in the strong--coupling and weak--coupling limits. These correlation functions are used to study the long--range interactions of nonabelian strings, taking account of charge--screening effects due to virtual particles. Among the phenomena investigated are the Aharonov--Bohm interactions of strings with charged particles, holonomy interactions between string loops, string entanglement, the transfer of ``Cheshire charge'' to a string loop, and domain wall decay via spontaneous string nucleation. We also analyze the Aharonov--Bohm interactions of magnetic monopoles with electric flux tubes in a confining gauge theory. We propose that the Aharonov--Bohm effect can be invoked to distinguish among various phases of a nonabelian gauge theory coupled to matter.

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