V. Ayyar, T. DeGrand, D. Hackett, W. I. Jay, E. T. Neil, Y. Shamir, and B. SvetitskyFinite-temperature phase structure of SU(4) gauge theory with multiple fermion representations, Phys. Rev. D 97 (2018) 114502 (arXiv:1802.09644 [hep-lat], February 2018).
We investigate the phase structure of SU(4) gauge theory with the gauge field simultaneously coupled to two flavors of fermion in the fundamental representation and two flavors of fermion in the two-index antisymmetric representation. We find that the theory has only two phases, a low-temperature phase with both species of fermion confined and chirally broken, and a high-temperature phase with both species of fermion deconfined and chirally restored. The single phase transition in the theory appears to be first order, in agreement with theoretical predictions.
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V. Ayyar, T. DeGrand, D. Hackett, W. I. Jay, E. T. Neil, Y. Shamir, and B. SvetitskyBaryon spectrum of SU(4) composite Higgs theory with two distinct fermion representations, Phys. Rev. D 97 (2018) 114505 (arXiv:1801.05809 [hep-ph], January 2018).
We use lattice simulations to compute the baryon spectrum of SU(4) lattice gauge theory coupled to dynamical fermions in the fundamental and two-index antisymmetric (sextet) representations simultaneously. This model is closely related to a composite Higgs model in which the chimera baryon made up of fermions from both representations plays the role of a composite top-quark partner. The dependence of the baryon masses on each underlying fermion mass is found to be generally consistent with a quark-model description and large-Nc scaling. We combine our numerical results with experimental bounds on the scale of the new strong sector to derive a lower bound on the mass of the top partner.
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V. Ayyar, T. DeGrand, M. Golterman, D. Hackett, W. I. Jay, E. T. Neil, Y. Shamir, and B. SvetitskySpectroscopy of SU(4) composite Higgs theory with two distinct fermion representations, Phys. Rev. D 97 (2018) 074505 (arXiv:1710.00806 [hep-lat], October 2017).
We have simulated the SU(4) lattice gauge theory coupled to dynamical fermions in the fundamental and two-index antisymmetric (sextet) representations simultaneously.  Such theories arise naturally in the context of composite Higgs models that include a partially composite top quark. We describe the low-lying meson spectrum of the theory and fit the pseudoscalar masses and decay constants to chiral perturbation theory. We infer as well the mass and decay constant of the Goldstone boson corresponding to the non-anomalous U(1) symmetry of the model. Our results are broadly consistent with large-Nc scaling and vector-meson dominance.
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V. Ayyar, T. DeGrand, D. Hackett, W. I. Jay, E. T. Neil, Y. Shamir, and B. SvetitskyChiral Transition of SU(4) Gauge Theory with Fermions in Multiple Representations, talk given by D. Hackett at Lattice 2017, the 35th International Symposium on Lattice Field Theory, Granada, Spain, June 2017, published in proceedings, EPJ Web Conf. 175, 08026 (2018) (arXiv:1709.06190 [hep-lat], September 2017).
We report preliminary results on the finite temperature behavior of SU(4) gauge theory with dynamical quarks in both the fundamental and two-index antisymmetric representations. This system is a candidate to present scale separation behavior, where fermions in different representations condense at different temperature or coupling scales. Our simulations, however, reveal a single finite-temperature phase transition at which both representations deconfine and exhibit chiral restoration. It appears to be strongly first order. We compare our results to previous single-representation simulations. We also describe a Pisarski-Wilczek stability analysis, which suggests that the transition should be first order.
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B. SvetitskyLooking behind the Standard Model with lattice gauge theory, plenary lecture given at Lattice 2017, the 35th International Symposium on Lattice Field Theory, Granada, Spain, June 2017, published in proceedings, EPJ Web Conf. 175, 01017 (2018) (arXiv:1708.04840 [hep-lat], August 2017).
Models for what may lie behind the Standard Model often require non-perturbative calculations in strongly coupled field theory. This creates opportunities for lattice methods, to obtain quantities of phenomenological interest as well as to address fundamental dynamical questions. I survey recent work in this area.
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Earlier papers on Lattice Gauge Theory applied beyond the Standard Model

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