118175505
Neutron-antineutron oscillations on the lattice
One possible low energy process due to beyond the Standard Model (BSM) physics is the neutron-antineutron transition, where baryon number changes by two units. In addition to providing a source of baryon number violation in the early universe, interactions of this kind are natural in grand unified theories (GUTs) with Majorana neutrinos that violate lepton number. Bounds on these oscillations can greatly restrict a variety of GUTs, while a non-zero signal would be a "smoking gun" for new physics; however, to make a reliable prediction, the six-quark nucleon-antinucleon matrix elements must first be calculated non-perturbatively via lattice QCD. We review the current understanding of this quantity, describe the lattice formalism, and present preliminary results from $32^3\times256$ clover-Wilson lattices with a pion mass of 390 MeV.
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.
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
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.
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.
118693393
The sextet gauge model, light Higgs, and the dilaton
The frequently discussed strongly interacting gauge theory with a fermion flavor doublet in the two-index symmetric (sextet) representation of the SU(3) color gauge group is investigated \cite{Fodor:2012ty}. The chiral condensate and the mass spectrum are consistent with chiral symmetry breaking at vanishing fermion mass. In contrast, sextet fermion mass deformations of spectral properties are not consistent with leading conformal scaling behavior near the critical surface of a conformal theory. A recent paper \cite{DeGrand:2012yq} which could not resolve the conformal fixed point of the gauge coupling from the slowly walking scenario of a very small nearly vanishing \beta -function is not in conflict with chiral symmetry breaking reported here. A light Higgs impostor could emerge as the dilaton from spontaneous symmetry breaking of scale invariance or, without the dilaton mechanism, as a composite state.
21248438
Mono-Energetic Beams from Laser Plasma Interactions
A laser driven wakefield accelerator has been tuned to produce high energy electron bunches with low emittance and energy spread by extending the interaction length using a plasma channel. Wakefield accelerators support gradients thousands of times those achievable in RF accelerators, but short acceleration distance, limited by diffraction, has resulted in low energy beams with 100% electron energy spread. In the present experiments on the L’OASIS laser, the relativistically intense drive pulse was guided over 10 diffraction ranges by a plasma channel. At a drive pulse power of 9 TW, electrons were trapped from the plasma and beams of percent energy spread containing > 200 pC charge above 80 MeV and with normalized emittance estimated at < 2 π-mm-mrad were produced. Data and simulations (VORPAL code) show the high quality bunch was formed when beam loading turned off injection after initial trapping, and when the particles were extracted as they dephased from the wake. Up to 4 TW was guided without trapping, potentially providing a platform for controlled injection. The plasma channel technique forms the basis of a new class of accelerators, with high gradients and high beam quality.
55971983
Scaled simulations of a 10 GeV accelerator
Laser plasma accelerators are able to produce high quality electron beams from 1 MeV to 1 GeV. The next generation of plasma accelerator experiments will likely use a multi-stage approach where a high quality electron bunch is first produced and then injected into an accelerating structure. In this paper we present scaled particle-in-cell simulations of a 10 GeV stage in the quasi-linear regime. We show that physical parameters can be scaled to be able to perform these simulations at reasonable computational cost. Beam loading properties and electron bunch energy gain are calculated. A range of parameter regimes are studied to optimize the quality of the electron bunch at the output of the stage.
13209862
Electron-beam conditioning by thomson scattering.
A method is proposed for conditioning electron beams via Thomson scattering. The conditioning provides a quadratic correlation between the electron energy deviation and the betatron amplitude of the electrons, which results in enhanced gain in free-electron lasers. Quantum effects imply conditioning must occur at high laser fluence and moderate electron energy. Conditioning of x-ray free-electron lasers should be achievable with present laser technology, leading to significant size and cost reductions of these large-scale facilities.
12096746
Radiative damping and electron beam dynamics in plasma-based accelerators.
The effects of radiation reaction on electron beam dynamics are studied in the context of plasma-based accelerators. Electrons accelerated in a plasma channel undergo transverse betatron oscillations due to strong focusing forces. These oscillations lead to emission by the electrons of synchrotron radiation, with a corresponding energy loss that affects the beam properties. An analytical model for the single particle orbits and beam moments including the classical radiation reaction force is derived and compared to the results of a particle transport code. Since the betatron amplitude depends on the initial transverse position of the electron, the resulting radiation can increase the relative energy spread of the beam to significant levels (e.g., several percent). This effect can be diminished by matching the beam into the channel, which could require micron sized beam radii for typical values of the beam emittance and plasma density.
122503946
Efficient electron injection into plasma waves using higher-order laser modes
Using higher-order transverse laser modes as drivers for plasma wave excitation, and, in particular, using ring laser beams with maximum intensities off-axis, results in reversal of the focusing and defocusing phase regions in a laser wakefield accelerator. This results in improved performance of self-trapping and laser injection schemes. Specifically, the trapping threshold required for optical injection is decreased and the maximum energy gain of the trapped electrons is increased. This method could also be of interest for the generation of ring electron beams or for beam conditioning.