Strong dynamics, composite Higgs and the conformal window

We review recent progress in the lattice investigations of near-conformal non-abelian gauge theories relevant for dynamical symmetry breaking and model building of composite Higgs models. The emphasis is placed on the mass spectrum and the running renormalized coupling. The role of a light composite scalar isosinglet particle as a composite Higgs particle is highlighted.Comment: 28 pages, 1 figure. Invited review for IJMPA special issue "Lattice gauge theories beyond QCD.

Multi-calorons and their moduli

Pure Yang-Mills instantons are considered on S^1 x R^3 -- so-called calorons. The holonomy -- or Polyakov loop around the thermal S^1 at spatial infinity -- is assumed to be a non-centre element of the gauge group SU(n) as most appropriate for QCD applications in the confined phase. It is shown that a charge k caloron can be seen as a collection of nk massive magnetic monopoles each carrying fractional topological charge. This interpretation offers a physically appealing way of introducing monopole degrees of freedom into pure gluodynamics: as constituents of finite temperature instantons. New and exact solutions are found along with the fermionic zero-modes of the Dirac operator. The properties of the zero-modes are analysed as well as the hyperkahler and twistor geometry of the caloron moduli space. Lattice gauge theoretic applications are also mentioned.Comment: PhD thesis, 109 pages, 24 figure

SU(3) gauge theory with sextet fermions

SU(3) gauge theory coupled to N_f = 2 fermions in the sextet representation is a promising candidate for a technicolor inspired Standard Model extension. In this note the progress in the past few years aimed at understanding the non-perturbative properties of the model is reviewed. The main difficulties lying ahead in order to make robust conclusions from lattice simulations are outlined.Comment: Contribution to the XXIX International Symposium on Lattice Field Theory - Lattice 2011 (references added

Computational efficiency of staggered Wilson fermions: A first look

Results on the computational efficiency of 2-flavor staggered Wilson fermions compared to usual Wilson fermions in a quenched lattice QCD simulation on $16^3\times32$ lattice at $\beta=6$ are reported. We compare the cost of inverting the Dirac matrix on a source by the conjugate gradient (CG) method for both of these fermion formulations, at the same pion masses, and without preconditioning. We find that the number of CG iterations required for convergence, averaged over the ensemble, is less by a factor of almost 2 for staggered Wilson fermions, with only a mild dependence on the pion mass. We also compute the condition number of the fermion matrix and find that it is less by a factor of 4 for staggered Wilson fermions. The cost per CG iteration, dominated by the cost of matrix-vector multiplication for the Dirac matrix, is known from previous work to be less by a factor 2-3 for staggered Wilson compared to usual Wilson fermions. Thus we conclude that staggered Wilson fermions are 4-6 times cheaper for inverting the Dirac matrix on a source in the quenched backgrounds of our study.Comment: v2: Major correction and revisions: we had overlooked a factor 1/4 in the cost estimate for matrix-vector multiplication with the staggered Wilson Dirac matrix. This gives an increased speed-up by a factor 4 for the overall computation cost. 7 pages, 3 figures, presented at the 31st International Symposium on Lattice Field Theory (Lattice 2013), 29 July - 3 August 2013, Mainz, 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

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

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