arXiv subjects

David E. Brahm

Publications and source records attributed to David E. Brahm.

7 recordsLinked to original sources

arXiv1996

Z -> b b-bar Excess from R-Parity Violation

The $\Zbb$ excess, $\Zcc$ deficit, and low left-right asymmetry $A_b$ may be explained by a single term $\lam C^c B^c B^{\prime c}$ in the superpotential. This operator violates R-parity and requires a sequential 4th generation. 1-loop diagrams involving squark exchange interfere with the tree-level processes to give an excess of right-handed $b$ quarks, and a deficit of right-handed $c$ quarks. Though the coupling must be large ($\lam\approx 2$ or $3$), the model is phenomenologically and cosmologically acceptable.

hep-ph↗

arXiv1996

Large-N_c Relations Among Isgur-Wise Functions

We investigate the relations that must hold among baryonic Isgur-Wise functions $η_i$ in the large-N_c limit from unitarity constraints, and compare to those found by Chow using the Skyrme model [or SU(4)]. Given the exponential dropoff of the $η_i$ away from threshold, unitarity requires only that the usual normalization conditions hold at w=1, and that $η=η_1$ near threshold. Our results are consistent with, but less powerful than, the Skyrme model relations.

hep-ph↗

arXiv1995

Thermal Activation Rates in the Chirally Asymmetric Gross-Neveu Model

We address the problem of how to incorporate quantum effects into the calculation of finite-temperature decay rates for a metastable state of a quantum field theory. To do this, we consider the Gross-Neveu model with an explicit chiral symmetry breaking term, which allows for a metastable state. This theory can be shown to have a "critical bubble" which is a solution to the *exact* equations of motions (i.e. to all orders in perturbation theory, including all higher derivative, quantum and thermal corrections). This configuration mediates the thermal activation of the metastable vacuum to the true ground state, with a decay rate $Γ\propto \exp(-F_c/T)$, where $F_c$ is the free energy of the critical bubble. We then compare this exact calculation to various approximations that have been used in previous work. We find that these approximations all *overestimate* the activation rate. Furthermore, we study the effect of finite baryon number upon the bubble profile and the activation barriers. We find that beyond a critical baryon number the activation barriers disappear altogether.

hep-ph↗

arXiv1995

Domain Walls in a FRW Universe

We solve the equations of motion for a scalar field with domain wall boundary conditions in a Friedmann-Robertson-Walker (FRW) spacetime. We find (in agreement with Basu and Vilenkin) that no domain wall solutions exist in de Sitter spacetime for h = H/m >= 1/2, where H is the Hubble parameter and m is the scalar mass. In the general FRW case we develop a systematic perturbative expansion in h to arrive at an approximate solution to the field equations. We calculate the energy momentum tensor of the domain wall configuration, and show that the energy density can become negative at the core of the defect for some values of the non-minimal coupling parameter xi. We develop a translationally invariant theory for fluctuations of the wall, obtain the effective Lagrangian for these fluctuations, and quantize them using the Bunch-Davies vacuum in the de Sitter case. Unlike previous analyses, we find that the fluctuations act as zero-mass (as opposed to tachyonic) modes. This allows us to calculate the distortion and the normal-normal correlators for the surface. The normal-normal correlator decreases logarithmically with the distance between points for large times and distances, indicating that the interface becomes rougher than in Minkowski spacetime.

hep-ph↗

arXiv1994

The Lattice Cutoff for $λϕ^4_4$ and $λϕ^6_3$

We analyze the critical line of $λϕ^4_4$ perturbatively in the bare coupling $λ_0$, by setting the daisy-improved renormalized mass to zero. By comparing to lattice data, we can then quantify the relation between the continuum cutoff and the lattice spacing; for the 4-dimensional hypercubic lattice we find $(Λa)_{C4} = 4.893$. We perform a similar analysis for $λϕ^6_3$, and find in 3 dimensions $(Λa)_{C3} = 4.67$. We present two theoretical predictions for $(Λa)$. For small $λ_0$, both the critical line and the renormalized mass near criticality are easily and accurately calculated from the lattice input parameters.

hep-lat↗

arXiv1993

Resummation Methods at Finite Temperature: The Tadpole Way

We examine several resummation methods for computing higher order corrections to the finite temperature effective potential, in the context of a scalar $ϕ^4$ theory. We show by explicit calculation to four loops that dressing the propagator, not the vertex, of the one-loop tadpole correctly counts ``daisy'' and ``super-daisy'' diagrams.

hep-ph↗

arXiv1992

Complex Effective Potentials and Critical Bubbles

The Higgs contribution to the effective potential appears to be complex. How do we interpret this, and how should we modify the calculation to calculate physical quantities such as the critical bubble free energy?

hep-ph↗