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.
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.
56420210
Generation of single pulse particle beams in a plasma channel by laser injection in laser wakefield accelerators
A laser injection mechanism that generates a single pulse, high quality beam is described. In the past, different all-optical injection schemes for the generation of ultrashort electron bunches were proposed. A very promising scheme proposed by E. Esarey [E. Esarey, R.F. Hubbard, W.P. Leemans A. Ting and P. Sprangle," Phys. Rev. Lett 79, 2682 (1997)] uses the collision of two counterpropagation laser pulses to inject particles. However, the results of ours 2D particle-in-cell (PIC) simulations have shown that in addition to the bunch created by pulse collision, a train of pulses is also generated by a different mechanism. In this paper we show that is possible to achieve a single pulse, high quality beam if the generation of the plasma wakefield takes place in a channel and particles are injected by a phase-kick mechanism [J. Cary at this conference]. We have performed 2D PIC simulations of our proposed injection scheme using the code VORPAL [C. Nieter at this conference]. We have obtained a high quality 11 MeV., 6 fs. single electron bunch with energy spread of 4% and emittance of 0.01/spl pi/ - mm - mrad.
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.
118397655
High Quality Electron Bunches up to 1 GeV from Laser Wakefield Acceleration at LBNL
Experiments at the LOASIS laboratory of LBNL have demonstrated production of 100 MeV to 1 GeV electron bunches with low energy spread and low divergence from laser wakefield acceleration. The radiation pressure of a 10 TW laser pulse, guided over 10 diffraction ranges by a few‐mm long plasma density channel, was used to drive an intense plasma wave (wakefield), producing electron bunches with energies on the order of 100 MeV and acceleration gradients on the order of 100 GV/m. Beam energy was increased from 100 MeV to 1 GeV by using a few‐cm long guiding channel at lower density, driven by a 40 TW laser, demonstrating the anticipated scaling to higher beam energies. Particle simulations indicate that the low energy spread beams were produced from self‐trapped electrons through the interplay of trapping, loading, and dephasing. Other experiments and simulations are also underway to control injection of particles into the wake, and hence improve beam quality and stability further.
55045125
Design considerations for a laser‐plasma linear collider
Design considerations for a next‐generation electron‐positron linear collider based on laser‐plasma‐accelerators are discussed. Several of the advantages and challenges of laser‐plasma‐based accelerator technology are addressed. An example of the parameters for a 1 TeV laser‐plasma‐based collider is presented.
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.
54818881
Approaching Conformality with Ten Flavors
We present first results for lattice simulations, on a single volume, of the low-lying spectrum of an SU(3) Yang-Mills gauge theory with N{sub f} = 10 light fermions in the fundamental representation. Fits to the fermion mass dependence of various observables are found to be globally consistent with the hypothesis that this theory is within or just outside the strongly-coupled edge of the conformal window, with mass anomalous dimension {gamma}* {approx} 1 over the range of scales simulated. We stress that we cannot rule out the possibility of spontaneous chiral-symmetry breaking at scales well below our infrared cutoff. We discuss important systematic effects, including finite-volume corrections, and consider directions for future improvement.
16605823
New Higgs physics from the lattice
We report the first results from our comprehensive lattice to ol set to explore non-perturbative aspects of Higgs physics in the Standard Model. We demonstrate in Higgs-Yukawa models that Higgs mass lower bounds and upper bounds can be determined in lattice simulations when triviality requires the necessity of a finite cutoff to maintain n on-zero interactions. The vacuum instability problem is investigated and the lattice approa ch is compared with the traditional renormalization group procedure which sets similar goals to corr elate lower and upper Higgs mass bounds with the scale of new physics. A novel feature of our lattice simulations is the use of Ginsparg-Wilson fermions to represent the effects of Top quark loops in Higgs dynamics. The need for chiral lattice fermions is discussed and the approa ch is extended to full Top-Higgs-QCD dynamics. We also report results from our large NF analysis of Top-Higgs Yukawa models to gain analytic insight and to verify our new lattice tool set which is deployed in the simulations. The role of non-perturbative lattice studies to investigate he avy Higgs particle scenarios is illustrated in extensions of the Standard Model.
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.
14044966
Nearly conformal gauge theories in finite volume
Abstract We report new results on nearly conformal gauge theories with fermions in the fundamental representation of the SU ( 3 ) color gauge group as the number of fermion flavors is varied in the N f = 4 – 16 range. To unambiguously identify the chirally broken phase below the conformal window we apply a comprehensive lattice tool set in finite volumes which includes the test of Goldstone pion dynamics, the spectrum of the fermion Dirac operator, and eigenvalue distributions of random matrix theory. We also discuss the theory inside the conformal window and present our first results on the running of the renormalized gauge coupling and the renormalization group beta function. The importance of understanding finite volume zero momentum gauge field dynamics inside the conformal window is illustrated. Staggered lattice fermions are used throughout the calculations.