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.
118432510
Conformal finite size scaling of twelve fermion flavors
Extended simulation results and their analysis are reported in a strongly coupled gauge theory with twelve fermion flavors in the fundamental SU(3) color representation. The conformality of the model is probed using mass deformed conformal finite size scaling (FSS) theory driven by the fermion mass anomalous dimension. Two independent conformal FSS fitting procedures are used in the analysis. The first one deploys physics motivated scaling functions, complemented by a second fitting procedure with spline based general B-forms for the scaling functions. The results at fixed gauge coupling show unresolved problems with the conformal hypothesis.
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.
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.
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.
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.
55416075
Free-electron laser driven by the LBNL laser-plasma accelerator
A design of a compact free-electron laser (FEL), generating ultra-fast, high-peak flux, XUV pulses is presented. The FEL is driven by a high-current, 0.5 GeV electron beam from the Lawrence Berkeley National Laboratory (LBNL) laser-plasma accelerator, whose active acceleration length is only a few centimeters. The proposed ultra-fast source (~;;10 fs) would be intrinsically temporally synchronized to the drive laser pulse, enabling pump-probe studies in ultra-fast science. Owing to the high current (>10 kA) of the laser-plasma-accelerated electron beams, saturated output fluxes are potentially greater than 10^13 photons/pulse. Devices based both on self-amplified spontaneous emission and high-harmonic generated input seeds, to reduce undulator length and fluctuations, are considered.
119927263
Self-modulation of long electron beams in plasma at PITZ
The Photo Injector Test facility at DESY, Zeuthen site (PITZ), offers the unique possibility to study and demonstrate the self-modulation of long electron bunches in plasma. A set of numerical simulations with the particle-in-cell code OSIRIS has been carried out for a better understanding of the process. Of particular interest is the measurement of the energy modulation induced to the beam itself by means of the generated wakefields in plasma. It will reflect the key properties of the accelerating electric fields such as their magnitude and their phase velocity, both of significant importance in the design of experiments relying on this technique.
5535468
Terahertz radiation from laser accelerated electron bunches
Coherent terahertz and millimeter wave radiation from laser accelerated electron bunches has been measured. The bunches were produced by tightly focusing (spot diameter ≈6 μm) a high peak power (up to 10 TW), ultra-short (⩾50 fs) laser pulse from a high repetition rate (10 Hz) laser system (0.8 μm), onto a high density (>1019 cm−3) pulsed gas jet of length ≈1.5 mm. As the electrons exit the plasma, coherent transition radiation is generated at the plasma-vacuum boundary for wavelengths long compared to the bunch length. Radiation in the 0.3–19 THz range and at 94 GHz has been measured and found to depend quadratically on the bunch charge. The measured radiated energy for two different collection angles is in good agreement with theory. Modeling indicates that optimization of this table-top source could provide more than 100 μJ/pulse. Together with intrinsic synchronization to the laser pulse, this will enable numerous applications requiring intense terahertz radiation. This radiation can also be used as a...
31820310
GeV electron beams from a centimeter-scale laser-driven plasma accelerator
Results are presented on the generation of quasi- monoenergetic electron beams with energy up to lGeV using a 40 TW laser and a 3.3 cm-long hydrogen-filled capillary discharge waveguide [1,2]. Electron beams were not observed without a plasma channel, indicating that self- focusing alone could not be relied upon for effective guiding of the laser pulse. Results are presented of the electron beam spectra, and the dependence of the reliability of producing electron beams as a function of laser and plasma parameters.