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.

title: Lattice simulations with eight flavors of domain wall fermions in SU(3) gauge theory
abstract: We study an SU(3) gauge theory with Nf=8 degenerate flavors of light fermions in the fundamental representation. Using the domain wall fermion formulation, we investigate the light hadron spectrum, chiral condensate and electroweak S parameter. We consider a range of light fermion masses on two lattice volumes at a single gauge coupling chosen so that IR scales approximately match those from our previous studies of the two- and six-flavor systems. Our results for the Nf=8 spectrum suggest spontaneous chiral symmetry breaking, though fits to the fermion mass dependence of spectral quantities do not strongly disfavor the hypothesis of mass-deformed infrared conformality. Compared to Nf=2 we observe a significant enhancement of the chiral condensate relative to the symmetry breaking scale F, similar to the situation for Nf=6. The reduction of the S parameter, related to parity doubling in the vector and axial-vector channels, is also comparable to our six-flavor results.

title: Probing technicolor theories with staggered fermions
abstract: 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.

title: Chiral symmetry breaking in fundamental and sextet fermion representations of SU(3) color
abstract: We report new results for lattice gauge theories with twelve fermion flavors in the fundamental representation and two fermion flavors in the two-index symmetric (sextet) representation of the SU(3) color gauge group. Both models are important in searching for a viable composite Higgs mechanism in the Beyond the Standard Model (BSM) paradigm. We subject both models to opposite hypotheses inside and outside of the conformal window. In the first hypothesis we test chiral symmetry breaking ($\chi{\rm SB}$) with its Goldstone spectrum, $F_\pi$, the $\chi{\rm SB}$ condensate, and several composite hadron states as the fermion mass is varied in a limited range with our best effort to control finite volume effects. Supporting results for $\chi{\rm SB}$ from the running coupling based on the force between static sources is 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 very low level of confidence in the conformal scenario.

title: Conformal finite size scaling of twelve fermion flavors
abstract: 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.

title: Topology and higher dimensional representations
abstract: SU(3) gauge theory in the 2-index symmetric (sextet) and fundamental representations is considered in symmetric and periodic boxes. Using the overlap formulation in the quenched approximation it is shown that the topological charge obtained from the sextet index theorem always leads to an integer value and agrees with the charge obtained from the fundamental index theorem in the continuum. At larger lattice spacing configurations exist with fractional topological charge if the sextet index is used but these are lattice artifacts and the probability of finding such a configuration rapidly approaches zero. By considering the decomposition of the sextet representation with respect to an SU(2) subgroup it is shown that the SU(2) adjoint index theorem leads to integer charge as well. We conclude that the non-zero value of the bilinear gaugino condensate in = 1 super-Yang-Mills theory cannot be attributed to configurations with fractional topological charge once periodic boundary conditions are imposed.

title: Accurate Modeling of Laser‐Plasma Accelerators with Particle‐In‐Cell Codes
abstract: Particle‐In‐Cell (PIC) codes are often used to study systems where the details of phase‐space are important; for example, self trapping or optical injection in laser‐plasma accelerators. Here we investigate the numerical heating and macro‐particle trajectory errors in 2D PIC simulations of laser‐plasma accelerators. The effects of grid resolution and laser polarization on the momentum spread and on subsequent spurious trapping in a plasma wave is studied. It is shown that when the laser is polarized in the plane of the simulation, which mimics the 3D behavior, the macro‐particles are subject to trajectory errors resulting in a high momentum spread. The phase‐space error associated with this momentum spread results in unphysical trapping. Smoother particle shapes are considered to reduce the interpolation errors. With these schemes, the macro‐particle trajectories are more accurately modeled and unphysical momentum spread is reduced.

title: Terahertz Radiation as a Bunch Diagnostic for Laser‐Wakefield‐Accelerated Electron Bunches
abstract: Experimental results are reported on the temporal and spectral characterization of coherent terahertz (THz) radiation. The radiation is emitted by relativistic electron bunches, which in turn are produced by a laser‐driven plasma‐based accelerator. As the femtosecond electron bunches exit the plasma‐vacuum interface, coherent transition radiation (at THz frequencies) is emitted. Measuring the properties of this radiation allows characterization of the electron bunches. Theoretical work on the emission mechanism is presented, including a model that calculates the THz waveform from a given bunch profile. It is found that the spectrum of the THz pulse is coherent up to the 200‐μm‐thick ZnTe detection limit of 4 THz, which corresponds to the production of sub‐50 fs (root‐mean‐square) electron bunch structure. The measurements demonstrate both the shot‐to‐shot stability of bunch parameters that are critical to THz emission (such as total charge and bunch length), as well as femtosecond synchronization between ...

title: Injection and staging for laser plasma accelerators
abstract: Controlled injection and staging are critical elements to the improvement of the stability, quality, and tunability of laser plasma accelerators (LPAs) [1, 2]. This paper will discuss techniques such as colliding pulse , density tailoring [4, 5], and ionization injection [6, 7, 8], which have recently allowed for the generation of high-energy femtosecond electron beams with percent-level energy spread and stability, and state-of-the-art emittance. Progress on staged acceleration will also be presented, which promises to further increase beam quality, allow for mapping of the wakefield, and overcome the limit of pump depletion .

title: Undulator radiation from laser-plasma-accelerated electron beams
abstract: Recent experiments coupled electron beams from the LOASIS TREX laser plasma accelerator (LPA) [1, 2, 3] to the Tapered Hybrid Undulator (THUNDER). Using the 1.5m, 66 period undulator, followed by an XUV spectrometer, spontaneous radiation was observed at photon energies extending to 100 eV. Previous experiments have reported visible  and soft-x-ray  radiation. The purpose of our experiments is to do highly precise, single shot diagnostics of the energy spread and emittance for each electron beam. We present recent results including measurements of electron beam transport through the undulator with and without the use of permanent magnetic quadrapoles, and measurements of XUV spectra up to 100 eV from LPA produced e-beams.

title: Modeling a 10 GeV Laser-Plasma Accelerator with INF&RNO
abstract: The numerical modeling code INF&RNO (INtegrated Fluid & paRticle simulatioN cOde, pronounced “inferno”) is an efficient 2D cylindrical code to model the interaction of a short laser pulse with an underdense plasma. The code is based on an envelope model for the laser while either a particle-in-cell (PIC) or a fluid description can be used for the plasma. The effect of the laser pulse on the plasma is modeled with the time-averaged ponderomotive force. These and other features allow for a significant speedup compared to standard full PIC simulations while still retaining physical fidelity. A boosted Lorentz frame (BLF) modeling capability has been introduced within the fluid framework enhancing the performance of the code. An example of a 10 GeV laser-plasma accelerator modeled using INF&RNO in the BLF is presented.