Closing the light gluino window

The running of the strong coupling constant, $R_{e^+e^-},R_Z$ and $R_\tau$ is studied on the three-loop level. Based on experimental data of $R_{e^+e^-},R_Z$ and $R_\tau$ and the LEP multijet analysis, the light gluino scenario is excluded to 99.97% CL (window I) and 99.89% CL (window III).Comment: 7 pages, 6 figures, talk at HEP97, Jerusale

The QCD phase diagram at finite density

We study the density of states method to explore the phase diagram of the chiral transition on the tempeature and quark chemical potential plane. Four quark flavours are used in the analysis. Though the method is quite expensive small lattices show an indication for a triple-point connecting three different phases on the phase diagram.Comment: Talk presented on Lattice 2005 (non-zero temperature and density), 6 pages, 6 figure

New Higgs physics from the lattice

We report the first results from our comprehensive lattice tool 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 non-zero interactions. The vacuum instability problem is investigated and the lattice approach is compared with the traditional renormalization group procedure which sets similar goals to correlate 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 approach is extended to full Top-Higgs-QCD dynamics. We also report results from our large $N_F$ 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 heavy Higgs particle scenarios is illustrated in extensions of the Standard Model.Comment: 28 pages, based on contributions from K. Holland, J. Kuti, D. Nogradi, and C. Schroeder at The XXV International Symposium on Lattice Field Theory, July 30 - August 4 2007, Regensburg, German

Chiral symmetry breaking in fundamental and sextet fermion representations of SU(3) color

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.Comment: 14 pages, 12 figures. Based on talks presented by J.Kuti and K.Holland at the XXVIII International Symposium on Lattice Field Theory, Lattice2010, June 14-19, 2010, Villasimius, Ital

Chiral symmetry breaking in nearly conformal gauge theories

We present new results on chiral symmetry breaking in nearly conformal gauge theories with fermions in the fundamental representation of the SU(3) color gauge group. The number of fermion flavors is varied in an extended range below the conformal window with chiral symmetry breaking ($\chi{\rm SB}$) for all flavors between $N_f=4$ and $N_f=12$. To identify $\chi{\rm SB}$ 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 \eps-regime. Chiral condensate enhancement is observed with increasing $N_f$ when the electroweak symmetry breaking scale $F$ is held fixed in technicolor language. Important finite-volume consistency checks from the theoretical understanding of the $SU(N_f)$ rotator spectrum of the $\delta$-regime are discussed. We also consider these gauge theories at $N_f=16$ inside the conformal window. The importance of understanding finite volume, zero momentum gauge field dynamics inside the conformal window is pointed out. Staggered lattice fermions with supressed taste breaking are used throughout the simulations.Comment: 21 pages, 9 figures. Presented at the XXVII International Symposium on Lattice Field Theory, July 26-31, 2009, Peking University, Beijin

Calculating the running coupling in strong electroweak models

One possibility for Beyond Standard Model physics is a new strongly-interacting gauge theory. One way to determine if a non-abelian gauge theory is QCD-like or conformal is to measure the running of the renormalized gauge coupling. We define the renormalized coupling from Wilson loop ratios, and measure these ratios via lattice simulations. We test this method in SU(3) pure gauge theory and show some first results for simulations with dynamical fermions in the fundamental representation.Comment: 7 pages, 5 figures. Presented at the XXVII International Symposium on Lattice Field Theory, July 26-31, 2009, Peking University, Beijin

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 n_s=2 and 3 staggered flavors in the fundamental representation, corresponding to N_f=8 and 12 flavors in the continuum limit. Although our first findings show that both theories are consistent with dynamically broken chiral symmetry and QCD-like behavior, flavor breaking effects in the spectrum may require further clarifications before final conclusions can be drawn. We also compare various improved staggered actions, to suppress this potentially large flavor breaking.Comment: 7 pages, 5 figures, talk presented at The XXVI International Symposium on Lattice Field Theory, July 14 - 19, 2008, Williamsburg, Virginia, US

Nearly conformal electroweak sector with chiral fermions

SU(3) gauge theory with dynamical overlap fermions in the 2-index symmetric (sextet) representation is considered. This model may be a viable model of the electroweak symmetry breaking sector along the lines of the walking technicolor paradigm. The number of fermion species is chosen such that the theory is expected to be below the conformal window. We will discuss how the epsilon-regime and random matrix theory can be used to test whether at any given set of parameters (N_c, N_f, representation) the theory is in the conformal phase or indeed just below it. Quenched Monte Carlo results are included in the fundamental representation and also preliminary dynamical ones in the 2-index symmetric representation.Comment: 7 pages, 2 figures, talk presented at The XXVI International Symposium on Lattice Field Theory, July 14 - 19, 2008, Williamsburg, Virginia, US