Search Results (6 total)

We report on the observation of rapid particle acceleration in numerical simulations of relativistic jet-plasma interactions and discuss the underlying mechanisms. The dynamics of a charge-neutral, narrow, electron-positron jet propagating through an unmagnetized electron-ion plasma was investigated using a three-dimensional, electromagnetic, particle-in-cell computer code. The interaction excited magnetic filamentation as well as electrostatic plasma instabilities. In some cases, the longitudinal electric fields generated inductively and electrostatically reached the cold plasma-wave-breaking limit, and the longitudinal momentum of about half the positrons increased by 50% with a maximum gain exceeding a factor of 2 during the simulation period. Particle acceleration via these mechanisms occurred when the criteria for Weibel instability were satisfied.

The status of the research on muon colliders is discussed and plans are outlined for future theoretical and experimental studies. Besides work on the parameters of a 3–4 and 0.5 TeV center-of-mass (COM) energy collider, many studies are now concentrating on a machine near 0.1 TeV (COM) that could be a factory for the s-channel production of Higgs particles. We discuss the research on the various components in such muon colliders, starting from the proton accelerator needed to generate pions from a heavy-Z target and proceeding through the phase rotation and decay (π→μν_μ) channel, muon cooling, acceleration, storage in a collider ring, and the collider detector. We also present theoretical and experimental R&D plans for the next several years that should lead to a better understanding of the design and feasibility issues for all of the components. This report is an update of the progress on the research and development since the feasibility study of muon colliders presented at the Snowmass '96 Workshop [R. B. Palmer, A. Sessler, and A. Tollestrup, Proceedings of the 1996 DPF/DPB Summer Study on High-Energy Physics (Stanford Linear Accelerator Center, Menlo Park, CA, 1997)].

We analytically study the generation of longitudinal plasma waves in an underdense plasma by two electromagnetic waves with frequency difference approximately equal to the plasma frequency, as envisioned in the plasma beat-wave accelerator concept of Tajima and Dawson [Phys. Rev. Lett. 43, 267 (1979)]. The relativistic electron fluid equations describing driven electron oscillations with phase velocities near the speed of light in a cold, collisionless plasma are reduced to a single, approximate ordinary differential equation of a parametrically excited nonlinear oscillator. We give amplitude-phase equations describing the asymptotic solutions to this equation valid for plasma-wave amplitudes below wave breaking. We numerically compare the behavior of the asymptotic equations with that of the original equation and with particle-simulation results.

We make a quantitative study of instanton-induced baryon- and lepton-number-violating processes in an SU(2)×U(1) electroweak gauge theory at zero and finite temperatures (in the "dilute-instanton-gas" approximation). As an example we consider a simplified model involving only the proton, neutron, electron, and electron neutrino. At zero temperature the total cross sections for p+n→e̅ +ν̅ and eleven other similar reactions are of order s×10^-195 cm², where s is the total center-of-momentum energy squared in GeV². The neutron decays via n→p̅ +e̅ +ν̅ with a lifetime of the order 10¹⁴⁶ years. The cross sections and neutron decay width decrease with temperature because color-electric-charge screening reduces the self-dual-instanton density at finite temperature. At high temperature the cross sections (for a given s) and neutron decay width fall off as T^{-47 / 3} in this simplified model. It is suggested that correctly treating the instanton gas as very dense (as discussed by Berg, Luscher, and Stehr) and including finite-energy tunneling solutions could increase the predicted reaction rates.

We study semiclassical methods for evaluating the canonical probability distribution function ρ(φ)=〈φ|exp(-βH)|φ〉 in field theory for systems in thermodynamic equilibrium. Field configurations which dominate the semiclassical distribution function are interpretable as "finite-temperature most-probable escape paths" (FTMPEP's) in field space and are related to the recently discovered caloron solutions, which are known to partially dominate the semiclassical partition function Z=∫Dφ ρ(φ). We present a semiclassical path-integral approximation for the distribution function and also discuss a Hartree self-consistent field approximation.

The Euclidean solutions (instantons) discovered in field theories seem to be interpretable as tunnelings between vacuums with different winding numbers. The associated transition rate for such tunnelings has been estimated. We study the effect of temperature on the transition rate between quantum-field configurations in thermodynamic equilibrium. We do this by extending the "most probable escape path" (MPEP) WKB vacuum-tunneling formalism of Bitar and Chang to finite temperatures. Our approach employs elementary results from quantum statistical mechanics. We believe the method offers an easily calculable approximation for quantum-field transition rates at finite temperatures.