Warren B. Perkins

MHV-Vertices for Gravity Amplitudes

We obtain a CSW-style formalism for calculating graviton scattering amplitudes and prove its validity through the use of a special type of BCFW-like parameter shift. The procedure is illustrated with explicit examples.

One-loop gluon scattering amplitudes in theories with N < 4 supersymmetries

Abstract Generalised unitarity techniques are used to calculate the coefficients of box and triangle integral functions of one-loop gluon scattering amplitudes in gauge theories with N 4 supersymmetries. We show that the box coefficients in N = 1 and N = 0 theories inherit the same coplanar and collinear constraints as the corresponding N = 4 coefficients. We use triple cuts to determine the coefficients of the triangle integral functions and present, as an example, the full expression for the one-loop amplitude A N = 1 ( 1 − , 2 − , 3 − , 4 + , … , n + ) .

Twistor space structure of the box coefficients of N=1 one-loop amplitudes

Abstract We examine the coefficients of the box functions in N = 1 supersymmetric one-loop amplitudes. We present the box coefficients for all six point N = 1 amplitudes and certain all n example coefficients. We find for “next-to MHV” amplitudes that these box coefficients have coplanar support in twistor space.

Supersymmetric ward identities and NMHV amplitudes involving gluinos

We show how Supersymmetric Ward Identities can be used to obtain amplitudes involving gluinos or adjoint scalars from purely gluonic amplitudes. We obtain results for all one-loop six-point NMHV amplitudes in = 4 Super Yang-Mills theory which involve two gluinos or two scalar particles. More general cases are also discussed.

One-loop NMHV amplitudes involving gluinos and scalars in Script N = 4 gauge theory

We use Supersymmetric Ward Identities and quadruple cuts to generate n-pt NMHV amplitudes involving gluinos and adjoint scalars from purely gluonic amplitudes. We present a set of factors that can be used to generate one-loop NMHV amplitudes involving gluinos or adjoint scalars in N = 4 Super Yang-Mills from the corresponding purely gluonic amplitude.

Cosmological phase transitions in a brane world

In brane world scenarios the Friedmann equation is modified, resulting in an increased expansion at early times. This has important effects on cosmological phase transitions which we investigate, elucidating significant differences to the standard case. First order phase transitions require a higher nucleation rate to complete; baryogenesis and particle abundances could be suppressed. Topological defect evolution is also affected, though the current defect densities are largely unchanged. In particular, the increased expansion does not solve the usual monopole and domain wall problems.

Colliding bubble worlds

Abstract We consider a cosmological model in which our Universe is a spherically symmetric bubble wall in 5-dimensional anti-de Sitter space–time. We argue that the bubble on which we live will undergo collisions with other similar bubbles and estimate the spectrum of such collisions. The collision rate is found to be independent of the age of our Universe. Collisions with small bubbles provide an experimental signature of this scenario, while collisions with larger bubbles would be catastrophic.

Brane world cosmology without the Z2 symmetry

The Friedmann equation for a positive tension brane situated between two bulk spacetimes that posses the same 5D cosmological constant, but which does not posses a Z2 symmetry of the metric itself is derived, and the possible effects of dropping the Z2 symmetry on the expansion of our Universe are examined; cosmological constraints are discussed. We show the effect of this is an inflation-like period at very early times. The global solutions for the metric in the infinite extra dimension case are found and comparison with the symmetric case is made. We show that any brane world senario of this type must revert to a Z2 symmetric form at late times, and hence rule out certain proposed scenarios.

Cosmic String Zero Modes and Multiple Phase Transitions

The zero modes and current carrying capability of a cosmic string formed at one phase transition can be modified at subsequent phase transitions. A new, generalised index theorem is derived that is applicable to theories with multiple phase transitions. This enables us to investigate the fate of string zero modes during sequences of phase transitions in a variety of models. Depending on the couplings that the breaking introduces, the zero modes may be destroyed and the superconductivity of the string removed, and thus vortons dissipate. We discuss the features of the theory that are required to produce this behaviour and consider the implications of spectral flow.

Stability of an electroweak string with a fermion condensate

Abstract A solution of the standard electroweak theory with a single lepton family is constructed, consisting of a cosmic string and a fermion condensate within its core. The stability of this system to small perturbations is examined, and it is found that stability is not enhanced relative to the bare electroweak string. The presence of quark zero modes is shown to violate the existence criteria for embedded defects.