arXiv subjects

C. P. Burgess

Publications and source records attributed to C. P. Burgess.

At least 217 records · Page 12Linked to original sources

arXiv1997

NRQED and Next-to-Leading Hyperfine Splitting in Positronium

We compute the next-to-leading, O(m alpha^5), contribution to the hyperfine splitting in positronium within the framework of NRQED. When applied to the ground state, our calculation reproduces known results, providing a further test of NRQED techniques. Besides providing a very simple method of calculation of the standard result, we also obtain new expressions for excited states of positronium with negligible additional effort. Our calculation requires the complete next-to-leading matching of the lowest-dimension NRQED four-fermi couplings, which we publish here for the first time.

hep-ph↗

arXiv1997

SO(5) Invariance and Effective Field Theory for High-Tc Superconductors

We set up the effective field theories which describe the SO(5)-invariant picture of the high-Tc cuprates in various regimes. We use these to get quantitative conclusions concerning the size of SO(5)-breaking effects. We consider two applications in detail: (i) the thermodynamic free energy, which describe the phase diagram and critical behaviour, and (ii) the Lagrangian governing the interactions of the pseudo-Goldstone bosons with each other and with the electron quasiparticles deep within the ordered phases. We use these effective theories to obtain predictions for the critical behaviour near the possible bicritical point and the pseudo-Goldstone boson dispersion relations, as well as some preliminary results concerning their contribution to response functions. We systematically identify which predictions are independent of the microscopic details of the underlying electron dynamics, and which depend on more model-dependent assumptions.

cond-mat.supr-con↗

arXiv1997

Four Dimensional Black Holes and Duality in Superstring Theory

Some recent results on the applications of duality (and related) transformations to general four-dimensional, spherically symmetric, asymptotically flat and time-independent string configurations are summarized. Two classes of results have been obtained. First, these transformations are used to generate the general such solution to the lowest-order field equations in the alpha' expansion. Second, the action and implications of duality (based on time-translation) on the general configuration is determined. It is found to interchange two pairs of the six parameters which label these configurations, namely: (1) the mass with the dilaton charge, and (2) the axion charge with the Taub-NUT parameter. For the special case of the Schwarzshild black hole this implies the relation M -> - k/M, where k is a known, positive, quantity. It is argued that, in some circumstances, dual theories need not be equivalent in the simplest sense.

hep-th↗

arXiv1996

Neutrino Propagation Through a Fluctuating Sun (The Talk)

We summarize a general formulation of particle propagation in fluctuating media, as applied to the description of neutrino propagation through the sun. It contains the familiar MSW effective hamiltonian, plus corrections which describe neutrino interactions with fluctuations in the medium. An estimate of the size of these corrections for a simple model of solar fluctuations is made, with the conclusion that they can produce surprisingly large effects since they grow with the correlation length of the fluctuation. For MSW oscillations, the leading effect for small fluctuations is to diminish the quality of the resonance, making the suppression of the ${}^7$Be neutrinos an experimental probe of fluctuations deep within the sun. Fluctuations can also provide an energy-independent suppression factor of $\hf$, away from the resonant region, even for small vacuum mixing angles. To be even potentially detectable, density fluctuations must be correlated on scales of hundreds of kilometres, and have amplitudes of a few percent or larger.

hep-ph↗

arXiv1996

One-Dimensional Flows in the Quantum Hall System

We construct the c-function whose gradient determines the RG flow of the conductivities (sigma_xy and sigma_xx) for a quantum Hall system, subject to two assumptions. (1) We take the flow to be invariant with respect to the infinite discrete symmetry group, recently proposed by several workers to explain the `superuniversality' of the delocalization exponents in these systems. (2) We also suppose the flow to be `quasi-holomorphic' (which we make precise) in the sense that it is as close as possible to a one-dimensional flow in the complex parameter sigma_xy +i sigma_xx. These assumptions together with the known asymptotic behaviour for large sigma_xx, completely determine the c-function, and so the phase diagram, for these systems. A complete description of the RG flow also requires a metric in addition to the c-function, and we identify the features which are required for this by the RG. A similar construction produces the c-function for other systems enjoying an infinite discrete symmetry, such as for supersymmetric QED.

cond-mat.mes-hall↗

arXiv1996

On the SO(5) Effective Field Theory of High T_c Superconductors

We construct the low-energy effective theory for the SO(5) model of high-T_c superconductivity, recently proposed by S.C. Zhang (cond-mat/9610140). This permits us to develop a systematic expansion for low-energy observables in powers of the small symmetry-breaking interactions. The approximate SO(5) symmetry predicts relations amongst these observables, which are model-independent consequences of Zhang's proposed symmetry-breaking pattern.

cond-mat.supr-con↗

arXiv1996

Neutrino Propagation in a Fluctuating Sun

We adapt to neutrino physics a general formulation for particle propagation in fluctuating media, initially developed for applications to electromagnetism and neutron optics. In leading approximation this formalism leads to the usual MSW effective hamiltonian governing neutrino propagation through a medium. Next-to-leading contributions describe deviations from this description, which arise due to neutrino interactions with fluctuations in the medium. We compute these corrections for two types of fluctuations: ($i$) microscopic thermal fluctuations, and ($ii$) macroscopic fluctuations in the medium's density. While the first of these reproduces standard estimates, which are negligible for applications to solar neutrinos, we find the second can be quite large, since it grows in size with the correlation length of the fluctuation. We consider two models in some detail. For fluctuations whose correlations are extend only over a local region in space of length $l$, appreciable effects for MSW oscillations arise if $(δn/n)^2 l\gsim 100$ m or so. Alternatively, a crude model of helioseismic $p$-waves gives appreciable effects only when $(δn/n) \gsim 1%$. In general the dominant effect is to diminish the quality of the resonance, making the suppression of the ${}^7$Be neutrinos a good experimental probe of fluctuations deep within the sun. Fluctuations can also provide a new mechanism for reducing the solar neutrino flux, giving an energy-independent suppression factor of $1/2$, away from the resonant region, even for small vacuum mixing angles.

hep-ph↗

arXiv1996

R_b and New Physics: A Comprehensive Analysis

We survey the implications for new physics of the discrepancy between the LEP measurement of $R_b$ and its Standard Model prediction. Two broad classes of models are considered: ($i$) those in which new $Z\bbar b$ couplings arise at tree level, through $Z$ or $b$-quark mixing with new particles, and ($ii$) those in which new scalars and fermions alter the $Z \bbar b$ vertex at one loop. We keep our analysis as general as possible in order to systematically determine what kinds of features can produce corrections to $R_b$ of the right sign and magnitude. We are able to identify several successful mechanisms, which include most of those which have been recently been proposed in the literature, as well as some earlier proposals (\eg\ supersymmetric models). By seeing how such models appear as special cases of our general treatment we are able to shed light on the reason for, and the robustness of, their ability to explain $R_b$.

hep-ph↗

arXiv1995

Hawking Radiation and Ultraviolet Regulators

Polchinski has argued that the prediction of Hawking radiation must be independent of the details of unknown high-energy physics because the calculation may be performed using `nice slices', for which the adiabatic theorem may be used. If this is so, then any calculation using a manifestly covariant --- and so slice-independent --- ultraviolet regularization must reproduce the standard Hawking result. We investigate the dependence of the Hawking radiation on such a short-distance regulator by calculating it using a Pauli--Villars regularization scheme. We find that the regulator scale, $Λ$, only contributes to the Hawking flux by an amount that is exponentially small in the large variable $Λ/{T_\ssh} \gg 1$, where $T_\ssh$ is the Hawking temperature; in agreement with Polchinski's arguments. We also solve a technical puzzle concerning the relation between the short-distance singularities of the propagator and the Hawking effect.

hep-th↗

arXiv1995

ON GAUGINO CONDENSATION WITH FIELD-DEPENDENT GAUGE COUPLINGS

We study in detail gaugino condensation in globally and locally supersymmetric Yang-Mills theories. We focus on models for which gauge-neutral matter couples to the gauge bosons only through nonminimal gauge kinetic terms, for the cases of one and several condensing gauge groups. Using only symmetry arguments, the low-energy expansion, and general properties of supersymmetry, we compute the low energy Wilson action, as well as the (2PI) effective action for the composite {\it classical} superfield $U\equiv\langle \Tr\WW \rangle$, with $W_α$ the supersymmetric gauge field strength. The 2PI effective action provides a firmer foundation for the approach of Veneziano and Yankielowicz, who treated the composite superfield, $U$, as a quantum degree of freedom. We show how to rederive the Wilson action by minimizing the 2PI action with respect to $U$. We determine, in both formulations and for global and local supersymmetry, the effective superpotential, $W$, the non-perturbative contributions to the low-energy Kähler potential $K$, and the leading higher supercovariant derivative terms in an expansion in inverse powers of the condensation scale. As an application of our results we include the string moduli dependence of the super- and Kähler potentials for simple orbifold models.

hep-th↗

arXiv1995

SCALAR-EMITTING MODES IN DOUBLE-BETA DECAY

The sum-energy spectrum of electrons emitted in double-beta decay is a well-known diagnostic for the nature of the physics which is responsible for the decay. Three types of spectra are usually considered when these experiments are analysed: one each for the standard two-neutrino decay, the neutrinoless process and a scalar-emitting decay. It has recently been shown that two other electron spectra can be possible for scalar-emitting modes, in addition to these traditional three. One of these is softer than the Standard-Model two-neutrino decay, while the other is intermediate between this and the usual majoron-emitting spectrum. The models which predict these new spectra are generically more natural than those which predict the traditional majoron-emitting spectrum, in that they can accomodate the constraints following from the steadily improving limits on neutrinoless decay without requiring the fine-tuning that is endemic to the usual models. This article reviews the properties of the physics which can produce the new kinds of electron spectra. (Invited Talk presented to the Workshop on Double Beta Decay and Related Topics, Trento Italy, April 1995.)

hep-ph↗

arXiv1995

NEW PHYSICS AND RECENT HIGH PRECISION ELECTROWEAK MEASUREMENTS

We analyze LEP and SLC data from the 1995 Winter Conferences for signals of new physics. We compare the data with the Standard Model (SM) as well as a number of test hypotheses concerning the nature of new physics: (i) nonstandard Zbb couplings, (ii) nonstandard Zff couplings for the entire third generation, (iii) nonstandard oblique corrections, (iv) nonstandard lepton couplings, (v) general nonstandard W and Z couplings to all fermions, as well as combinations of the above. In most of our analyses, we leave the SM variables $α_s$ and $m_t$ as free parameters to see how the various types of new physics can affect their inferred values. We find that the best fit ($χ^2/d.o.f. = 8.4/10$) is obtained for the nonstandard Zbb couplings, which also give a `low' value (0.112) for $α_s$. The SM also gives a good description of the Z data, having $χ^2/d.o.f. = 12.4/12$. If $α_s$ is held fixed to the low-energy value 0.112, then we find that a combination of the nonstandard Zbb couplings is fit to lie more than four standard deviations away from zero.

hep-ph↗

arXiv1995

THE SENSITIVITY TO NEW PHYSICS OF A LEP SCAN IN 1995

We study the implications of possible off-peak measurements in the 1995 LEP run, in regard to probing physics beyond the Standard Model. To do so, we determine the accuracy with which various nonstandard couplings can be expected to be measured in the three different scan scenarios recently discussed by Clarke and Wyatt. We find that each scan scenario allows greater sensitivity to a different set of new physics couplings. Oblique parameters are best measured with the longest scan, while nonstandard fermion couplings to the Z tend to be better constrained (albeit only marginally) if all of the 1995 LEP measurements are taken on the Z peak.

hep-ph↗

arXiv1995

Duality and Four-Dimensional Black Holes

We consider the effects of abelian duality transformations on static, spherically-symmetric, asymptotically flat string spacetimes in four dimensions, where the dilaton, axion, metric, and gauge fields are allowed to be nonzero. Independent of the alpha' expansion, there is a six-parameter family of such configurations, labelled by the charges characterizing the asymptotic behaviour of the various fields: ie their mass, dilaton charge, axion charge, electric charge, magnetic charge, and Taub-NUT parameter. We show that duality, based on time-translation invariance, maps these solutions amongst themselves, with the effect of interchanging two pairs of these six labels, namely: (1) the mass and dilaton charge, and (2) the axion charge and the Taub-NUT parameter. We consider in detail the special case of the purely Schwarzschild black hole, for which the mass of the dual configuration vanishes to leading order in alpha'. Working to next-to-leading order in alpha' for the bosonic and heterotic strings, we find that duality takes a black hole of mass M to a (singular) solution having mass - 1/(alpha' M). Finally, we argue that two solutions which are related by duality based on a noncompact symmetry are {\it not} always physically equivalent.

hep-th↗

arXiv1995

GAUGINO CONDENSATES AND CHIRAL-LINEAR DUALITY: AN EFFECTIVE LAGRANGIAN ANALYSIS

We show how to formulate the phenomenon of gaugino condensation in a super-Yang-Mills theory with a field-dependent gauge coupling described with a linear multiplet. We prove the duality equivalence of this approach with the more familiar formulation using a chiral superfield. In so doing, we resolve a longstanding puzzle as to how a linear-multiplet formulation can be consistent with the dynamical breaking of the Peccei-Quinn symmetry which is thought to occur once the gauginos condense. In our approach, the composite gauge degrees of freedom are described by a real vector superfield, $V$, rather than the chiral superfield that is obtained in the traditional dual formulation. Our dualization, when applied to the case of several condensing gauge groups, provides strong evidence that this duality survives strong-coupling effects in string theory.

hep-th↗

arXiv1995

Multi-Majoron Modes for Neutrinoless Double-Beta Decay

We construct two new classes of models for double beta decay, each of which leads to an electron spectrum which differs from the decays which are usually considered. One of the classes has a spectrum which has not been considered to date, and which is softer than the usual two-neutrino decay of the Standard Model. We construct illustrative models to show how other phenomenological bounds can be accommodated. We typically find that, although all other bounds can be satisfied, the predicted double-beta decay rate only in one class of these models is at best large enough to be just detectable in current experiments.

hep-ph↗

arXiv1994

On Spherically Symmetric String Solutions in Four Dimensions

We reconsider here the problem of finding the general 4D spherically symmetric, asymptotically flat and time-independent solutions to the lowest-order string equations in the $\ap$ expansion. Our construction includes earlier work, but differs from it in three ways. (1) We work with general background metric, dilaton, axion and $U(1)$ gauge fields. (2) Much of the original solutions were required to be nonsingular at the apparent horizon, motivated by an interest in finding string corrections to black hole spacetimes. We relax this condition throughout, motivated by the realization that string theory has a less restrictive notion of what constitutes a singular field configuration than do point particle theories. (3) We can construct the general solution from a particularly simple one, by generating it from successive applications of the {\it noncommuting} \sltwor\ and \ooneone\ symmetries of the low-energy string equations containing $S$ and target--space dualities respectively. This allows its construction using relatively simple, purely algebraic, techniques. The general solution is determined by the asymptotic behaviour of the various fields: \ie\ by the mass, dilaton charge, axion charge, electric charge, magnetic charge, and Taub-NUT parameter.

hep-th↗

arXiv1994

The Effective Use of Precision Electroweak Measurements

Several reasonably model-independent formulations of the implications of new physics for precision electroweak measurements have been developed over the past years, most notably by Peskin and Takeuchi, and by Altarelli \etal. These formulations work by identifying a small, but useful, set of parameters through which new physics often enters into well-measured physical observables. For the theories to which such an analysis applies, this approach greatly streamlines the confrontation with the data. Since the experimentally-allowed range for these parameters has been determined from global fits to the data, theorists need only compute their predictions for these parameters to constrain their models. These methods are summarized here, together with several recent generalizations which permit applications to wider classes of new physics, and which include the original approaches as special cases.

hep-ph↗