1293335
Probing technicolor theories with staggered fermions
One exciting possibility of new physics beyond the Standard Model is that the fundamental Higgs sector is replaced by a strongly-interacting gauge theory, known as technicolor. A viable theory must break chiral symmetry dynamically, like in QCD, to generate Goldstone bosons which become the longitudinal components of the W ± and Z. By measuring the eigenvalues of the Dirac operator, one can determine if chiral symmetry is in fact spontaneously broken. We simulate SU(3) gauge theory with ns = 2 and 3 staggered flavors in the fundamental representation, corresponding to Nf = 8 and 12 flavors in the continuum limit. Although our first findi ngs show that both theories are consistent with dynamically broken chiral symmetry and QCD-like behavior, flavor breaking effects in the spectrum may require further c larifications before final conclusions can be drawn. We also compare various improved staggered actions, to suppress this potentially large flavor breaking.
116984453
Equation of state in ( 2+1 )-flavor QCD
We present results for the equation of state in (2 + 1)-flavor QCD using the highly improved staggered quark action and lattices with temporal extent N tau = 6, 8, 10, and 12. We show that these data can be reliably extrapolated to the continuum limit and obtain a number of thermodynamic quantities and the speed of sound in the temperature range 130-400 MeV. We compare our results with previous calculations and provide an analytic parameterization of the pressure, from which other thermodynamic quantities can be calculated, for use in phenomenology. We show that the energy density in the crossover region, 145 MeV <= T <= 163 MeV, defined by the chiral transition, is epsilon(c) = (0.18-0.5) GeV/fm(3), i.e., (1.2-3.1) epsilon(nuclear). At high temperatures, we compare our results with resummed and dimensionally reduced perturbation theory calculations. As a byproduct of our analyses, we obtain the values of the scale parameters r(0) from the static quark potential and w(0) from the gradient flow.
125327431
The Chiral Phase Transition from Lattice QCD with Physical Pion Masses and Domain Wall Fermions
The HotQCD collaboration has extended its study of the finite temperature transition in 2+1 flavor QCD using domain wall fermions (DWF) to the physical point. With chiral fermions and truly physical pion masses, these calculations are arguably the most realistic of their kind to date, and not possible without a combination of algorithmic advances (chiefly the dislocation suppressing determinant ratio, DSDR), highly optimized BAGEL/CPS software, and access to the LLNL/IBM 20 Petaflops Sequoia Blue Gene/Q. All results are for a fixed temporal lattice size of 8, yet discretization effects, at least for the spectrum at zero temperature, are expected to be at the 5% level. Presented here are early results – in particular, the critical temperature and pion mass dependence of the light quark chiral susceptibility.
121271565
NEARLY CONFORMAL GAUGE THEORIES ON THE LATTICE
We present selected new results on chiral symmetry breaking in nearly conformal gauge theories with fermions in the fundamental representation of the SU(3) color gauge group. We found chiral symmetry breaking (χSB) for all flavors between Nf = 4 and Nf = 12 with most of the results discussed here for Nf = 4, 8, 12 as we approach the conformal window. To identify χSB we apply several methods which include, within the framework of chiral perturbation theory, the analysis of the Goldstone spectrum in the p-regime and the spectrum of the fermion Dirac operator with eigenvalue distributions of random matrix theory in the ϵ-regime. Chiral condensate enhancement is observed with increasing Nf when the electroweak symmetry breaking scale F is held fixed in technicolor language. Important finite-volume consistency checks from the theoretical understanding of the SU(Nf) rotator spectrum of the δ-regime are discussed. We also consider these gauge theories at Nf = 16 inside the conformal window. Our work on the running coupling is presented separately.1
121056309
Lattice QCD thermodynamics in the presence of the charm quark
Abstract We present an update on the 2 + 1 flavor QCD equation of state of the Wuppertal–Budapest Collaboration, extending our previous studies to finer lattice spacings and providing a continuum extrapolation of the trace anomaly. A Symanzik improved gauge and a stout-link improved staggered fermion action is utilized. We also present preliminary results for the fully dynamical charmed equation of state.
118648086
Twelve massless flavors and three colors below the conformal window
We report new results for a frequently discussed gauge theory with twelve fermion flavors in the fundamental representation of the SU(3) color gauge group. The model, controversial with respect to its conformality, is important in non-perturbative studies searching for a viable composite Higgs mechanism Beyond the Standard Model (BSM). To resolve the controversy, we subject the model to opposite hypotheses inside and outside of the conformal window. In the first hypothesis we test chiral symmetry breaking ( SB) with its Goldstone spectrum, F , the SB condensate, and several composite hadron states as the fermion mass is varied in a limited range with our best e ort to control finite volume e ects and extrapolation to the massless chiral limit. Supporting results for SB from the running coupling based on the force between static sources and some preliminary evidence for the finite temperature transition are also presented. In the second test for the alternate hypothesis we probe conformal behavior driven by a single anomalous mass dimension under the assumption of unbroken chiral symmetry. Our results show a very low level of confidence in the conformal scenario. Staggered lattice fermions with stout-suppressed taste breaking are used throughout the simulations.
122417761
Laser wakefield simulations towards development of compact particle accelerators
Laser driven wakefield accelerators produce accelerating fields thousands of times those achievable in conventional radio-frequency accelerators, offering compactness and ultrafast bunches to potentially extend the frontiers of high energy physics and enable laboratory scale ultrafast radiation sources. Realization of this potential requires understanding of accelerator physics to advance beam performance and stability, and particle simulations model the highly nonlinear, kinetic physics required. One-to-one simulations of experiments provide new insight for optimization and development of 100 MeV to GeV and beyond laser accelerator stages, and on production of reproducible and controllable low energy spread beams with improved emittance (focusability) and energy through control of injection.
12579112
Theory of coherent transition radiation generated at a plasma-vacuum interface.
Transition radiation generated by an electron beam, produced by a laser wakefield accelerator operating in the self-modulated regime, crossing the plasma-vacuum boundary is considered. The angular distributions and spectra are calculated for both the incoherent and the coherent radiation. The effects of the longitudinal and transverse momentum distributions on the differential energy spectra are examined. Diffraction radiation from the finite transverse extent of the plasma is considered and shown to strongly modify the spectra and energy radiated for long-wavelength radiation. This method of transition radiation generation has the capability of producing high peak power terahertz radiation, of order 100 microJ/pulse at the plasma-vacuum interface, which is several orders of magnitude beyond current state-of-the-art terahertz sources.
20413332
Generation of ultrashort electron bunches by colliding laser pulses.
A proposed laser-plasma-based relativistic electron source [E. Esarey et al., Phys. Rev. Lett. 79, 2682 (1997)] using laser-triggered injection of electrons is investigated. The source generates ultrashort electron bunches by dephasing and trapping background plasma electrons undergoing fluid oscillations in an excited plasma wake. The plasma electrons are dephased by colliding two counterpropagating laser pulses which generate a slow phase velocity beat wave. Laser pulse intensity thresholds for trapping and the optimal wake phase for injection are calculated. Numerical simulations of test particles, with prescribed plasma and laser fields, are used to verify analytic predictions and to study the longitudinal and transverse dynamics of the trapped plasma electrons. Simulations indicate that the colliding laser pulse injection scheme has the capability to produce relativistic femtosecond electron bunches with fractional energy spread of order a few percent and normalized transverse emittance less than 1 mm mrad using 1 TW injection laser pulses.
9545680
Long-range persistence of femtosecond modulations on laser-plasma-accelerated electron beams.
Laser plasma accelerators have produced femtosecond electron bunches with a relative energy spread ranging from 100% to a few percent. Simulations indicate that the measured energy spread can be dominated by a correlated spread, with the slice spread significantly lower. Measurements of coherent optical transition radiation are presented for broad-energy-spread beams with laser-induced density and momentum modulations. The long-range (meter-scale) observation of coherent optical transition radiation indicates that the slice energy spread is below the percent level to preserve the modulations.
125816478
Optimization of an X-ray SASE-FEL
Abstract The most important characteristics of an X-ray SASE-FEL are determined by the electron beam energy, transverse and longitudinal emittance, and by choice of the undulator period, field, and gap. Among them are the gain and saturation length, the amount and spectral characteristics of the spontaneous radiation, the wake fields due to the vacuum pipe. The spontaneous radiation intensity is very large in all X-ray SASE-FELs now being designed, and it contributes to the final electron beam energy spread, thus affecting the gain. It also produces a large background for the beam and radiation diagnostics instrumentation. The wake fields due to the resistivity and roughness of the beam pipe through the undulator, also affects the beam 6-dimensional phase space volume, and thus the gain and the line width. In this paper, we discuss ways to optimize the FEL when considering all these effects. In particular we consider and discuss the use of a hybrid iron-permanent magnet helical undulator to minimize some of these effects, and thus optimize the FEL design.