Physics Beyond the Standard Model

Lattice study on chiral dynamics of two-color six-flavors QCD
Masaaki Tomii, Masashi Hayakawa, Ken-Ichi Ishikawa, Yusuke Osaki, Shinji Takeda, Norikazu Yamada
Mon, 14:00, Seminar Room F -- Parallels 1F (Slides)

The electroweak symmetry breaking and origin of masses may be attributed to the breaking of chiral symmetry due to new strong gauge dynamics. Among several candidates of such gauge systems, we have focused on two-color QCD with \(N_F=6\) massless Dirac fermion in the fundamental representation, and study on whether this gauge dynamics triggers chiral symmetry breaking or not by simulating with Wilson fermions on lattices sized up to \(L/a=32\). We show the result obtained from analysis on quark mass and volume dependence of various quantities such as meson masses, decay constant, and by comparing these behaviors with the theoretical expectation in the presence of breakdown of chiral symmetry and that for the conformal dynamics in the infrared limit, respectively.

Back to Programme Back to Participants Back to Contributions

Two-Color Schrödinger Functional with Six-Flavors of Stout-Smeared Wilson Fermions
Gennady Voronov
Mon, 14:20, Seminar Room F -- Parallels 1F (Slides)

We study the Schrödinger functional running coupling in the two-color gauge theory with six-flavors of massless fermions. The aim is to determine whether the above theory has an infrared fixed point (IRFP). With the standard Wilson gauge action and the stout-smeared Wilson fermion action, we see clear evidence of the renormalized coupling flowing from perturbatively weak coupling to a strength where it is expected that chiral symmetry is broken without encountering an IRFP. We therefore conclude that the \(N_{f}=6\) two-color theory is outside the conformal window.

Back to Programme Back to Participants Back to Contributions

Walking signals in \(N_f=8\) QCD on the lattice
Kei-ichi Nagai
Mon, 14:40, Seminar Room F -- Parallels 1F (Slides)

We investigate walking signals of \(N_f=8\) QCD using HISQ action. Our data (the pion decay constant, the pion and \(\rho\) meson masses and the chiral condensate) for the \(N_f=8\) QCD are consistent with the spontaneously broken chiral symmetry in the chiral limit extrapolation of ChPT. Remarkably enough, while the \(N_f=8\) data near the chiral limit are well described by the ChPT, those for the relatively large fermion bare mass \(m_f\) away from the chiral limit actually exhibit a finite-size hyperscaling relation, suggesting a large anomalous dimension \(\gamma_m \sim 1\). This implies that there exists a remnant of the infrared conformality, and suggests that a typical technicolor (``one-family model'') as modeled by the \(N_f=8\) QCD can be a walking technicolor theory.

Back to Programme Back to Participants Back to Contributions

Exploring for a light composite scalar in eight flavor QCD
Hiroshi Ohki
Mon, 15:00, Seminar Room F -- Parallels 1F (Slides)

The eight flavor (\(N_f=8\)) QCD is very attractive to a candidate of the walking technicolor model, where the light scalar (technidilaton) is predicted as a pseudo Nambu-Goldstone boson of the spontaneous scale symmetry breaking. We study a composite scalar in the \(N_f=8\) QCD using the HISQ action on several lattice sizes with various fermion masses. Measuring both the disconnected and connected diagrams of the flavor singlet scalar correlation function, we present our preliminary results of the scalar mass and its fermion mass dependence. Through the analysis, we investigate the property of the scalar as a composite Higgs boson.

Back to Programme Back to Participants Back to Contributions

Eight light flavors on large lattice volumes
David Schaich
Mon, 15:20, Seminar Room F -- Parallels 1F (Slides)

I will present initial results from large-scale lattice investigations of SU(3) gauge theory with eight light flavors in the fundamental representation. This work, carried out using leadership computing resources provided to the United States lattice gauge theory community, considers large lattice volumes up to L=48 in order to permit controlled chiral extrapolations. We generate configurations with improved staggered fermions, and carry out some measurements with valence domain wall fermions. I will focus on analyses of the light hadron spectrum, Dirac eigenvalues, and low-energy constants of chiral perturbation theory, including the electroweak S parameter.

Back to Programme Back to Participants Back to Contributions

Gluonic observables and the scalar spectrum of twelve-flavour QCD
Enrico Rinaldi
Mon, 15:40, Seminar Room F -- Parallels 1F (Slides)

We measure glueball masses and the string tension in twelve-flavour QCD, aiming at comparing the emerging gluonic spectrum to the mesonic one. When approaching the critical surface at zero quark mass, the hierarchy of masses in the different sectors of the spectrum gives a new handle to determine the existence of an infrared fixed point. We describe the details of our gluonic measurements and the results obtained on a large number of gauge configurations generated with the HISQ action. In particular we focus on the scalar glueball and its mixing with a flavour-singlet fermionic state, which is lighter than the pseudoscalar (would-be pion) state. The results are interpreted in view of a light composite Higgs boson in walking technicolor theories.

Back to Programme Back to Participants Back to Contributions

Higgs Sector Spectroscopy
Axel Maas, Tajdar Mufti
Mon, 16:30, Seminar Room F -- Parallels 2F (Slides)

Just like the weakly-interacting QED the weak and Higgs interaction can sustain bound states. Field-theoretical considerations suggest that these are deeply-bound, relativistic bound states, at least for a sufficiently light Higgs. Thus, they cannot be described using quantum mechanics. In the here presented lattice results these considerations are confirmed, and the low-lying spectrum of bound states is determined. It is shown how these relate to the observed Higgs and W/Z signals, being in agreement with present experimental results. Furthermore, first estimates for masses for a range of quantum numbers and, using Luescher's method, for excited states are provided. With the obtained widths preliminary estimates can be made on the experimental accessibility at both LHC and ILC.

Back to Programme Back to Participants Back to Contributions

The phase structure of a chirally-invariant Higgs-Yukawa model
Bastian Knippschild, Karl Jansen, Attila Nagy, C.-J. David Lin, Kei-ichi Nagai, Prasad Hegde, Kenji Ogawa
Mon, 16:50, Seminar Room F -- Parallels 2F (Slides)

We present new results of our ongoing project on the investigation of the phase structure of the Higgs-Yukawa model at small and large bare Yukawa couplings. The critical exponents of the second order bulk phase transitions of this model are determined from finite size analyses and compared to the pure O(4)-model to test for triviality and the possibility of having a non-Gaussian fixed point. In addition, we will present a first study of Higgs boson and fermion masses.

Back to Programme Back to Participants Back to Contributions

Stabilizing the electroweak vacuum by higher dimensional operators in a Higgs-Yukawa model
Attila Nagy, Prasad Hegde, Karl Jansen, C.-J. David Lin
Mon, 17:10, Seminar Room F -- Parallels 2F (Slides)

The Higgs boson-like discovery at the LHC with a mass of approximateley 126GeV suggests, that the electroweak vacuum of the standard model maybe metastable at high energies. However, any new physics beyond the standard model can change this picture. We want to adress this important question within a lattice Higgs-Yukawa model as the limit of the standard model (SM). In this framework we will probe the effect of a higher dimensional operator for which we take a (\(\phi^6\))-term. Such a term could easily originate as a remnant of physics beyond the SM at very large scales. As a first step we investigate the phase diagram of the model including such a (\(\phi^6\)) operator. Exploratory results suggest the existence of regions in parameter space where first order transitions turn to second order ones, suggesting the existence of a tri-critical point. We will address the phase structure and the consequences for the stability of the SM both, analytically by analyzing the constrained effective potential in lattice perturbation theory and by studying the system non-perturbatively by means of lattice simulations.

Back to Programme Back to Participants Back to Contributions

Multi-boson spectrum of the SU(2)-Higgs model
Mark Wurtz, Randy Lewis
Mon, 17:30, Seminar Room F -- Parallels 2F (Slides)

Lattice simulations are used to compute the spectrum of energy levels for all available angular momentum and parity quantum numbers in the SU(2)-Higgs model, with parameters chosen to match experimental data from the Higgs-W boson sector of the standard model. Creation operators are constructed for all lattice irreducible representations, and a correlation matrix is formed from which the spectrum is extracted using a variational analysis. Many multi-boson states are observed and careful analysis reveals that all are consistent with weakly-interacting Higgs and W bosons.

Back to Programme Back to Participants Back to Contributions

Progress in Gauge-Higgs Unification on the Lattice (I)
Kyoko Yoneyama, Nikos Irges, Francesco Knechtli, Peter Dziennik
Mon, 17:50, Seminar Room F -- Parallels 2F (Slides)

In Gauge-Higgs unification model, the Higgs field is identified with (some of) the extra dimensional components of the gauge field. Perturbative studies indicate that spontaneous symmetry breaking (SSB) occurs when fermions are included. By using a mean-field expansion, we show that SSB occurs in a 5-dimensional pure SU(2) gauge theory on the \(S^1/Z_2\) orbifold. We take the continuum limit in a regime on the phase diagram where the Higgs remains light and predict an excited state of the gauge boson at a mass of 1 Tev. We compare to results of the spectrum (Higgs, Z boson) obtained from Monte Carlo simulations.

Back to Programme Back to Participants Back to Contributions

Progress in Gauge-Higgs Unification on the Lattice (II)
Francesco Knechtli, Peter Dziennik, Nikos Irges, Kyoko Yoneyama
Mon, 18:10, Seminar Room F -- Parallels 2F (Slides)

We study a five-dimensional pure SU(2) gauge theory formulated on the orbifold and discretized on the lattice by means of Monte Carlo simulations. The gauge symmetry is explicitly broken to U(1) at the orbifold boundaries. The action is the Wilson plaquette action with a modified weight for the boundary U(1) plaquettes. We study the phase transition and present results for the shape of the static potential in the hyperplanes orthogonal to the extra dimension, which is sensitive to the presence of a massive Z-boson. We compare to results from the case of periodic boundary conditions and from the direct measurement of the Z-boson mass presented in part (I). The results may support an alternative view of the lattice orbifold (stemming from its mean-field study) as a 5d bosonic superconductor.

Back to Programme Back to Participants Back to Contributions

Can a light Higgs impostor hide in composite gauge models?
Chik Him Wong, Julius Kuti, Dániel Nógrádi, Kieran Holland, Zoltan Fodor
Mon, 18:30, Seminar Room F -- Parallels 2F (Slides)

Close to the conformal window, a gauge model model with a fermion doublet in the two-index symmetric (sextet) representation of the SU(3) gauge group has attracted considerable interest. Its flavor singlet scalar spectroscopy with Higgs quantum numbers is investigated in the report.

Back to Programme Back to Participants Back to Contributions

String tension vs critical temperature in walking regime
Kohtaroh Miura, Albert Deuzeman, Maria Paola Lombardo, Elisabetta Pallante, Tiago Nunes da Silva
Tue, 14:00, Seminar Room F -- Parallels 3F (Slides)

The conformal and walking dynamics associated with the infra-red fixed point (IRFP) in many flavor QCD have received lots of attentions both theoretically and phenomenologically. In the six and eight flavor QCD, we evaluate the critical temperature (\(T_c\)) for the chiral phase transition at finite temperature and the string tension at zero temperature. We argue that the \(T_c\) in the unit of the UV scale \(r_x\) determined by the string tension gives a prove of the walking dynamics. We show our preliminary results on (\(T_c * r_x\)) as a function of \(N_f\), and discuss the onset of the walking dynamics.

Back to Programme Back to Participants Back to Contributions

Finite size scaling and the effect of the gauge coupling in 12 flavor systems
Anna Hasenfratz, Anqi Cheng, Gregory Petropoulos, David Schaich
Tue, 14:20, Seminar Room F -- Parallels 3F (Slides)

We investigate finite size scaling in with 12 fundamental flavors at several gauge coupling values, from the weak coupling region to the edge of the bulk phase transition in the strong coupling. The data show strong scaling violations in the strong coupling where the predicted anomalous dimension depends on the observable considered and changes with the gauge coupling. We argue that the observed scaling violation effects are caused by the irrelevant but slowly running gauge coupling. Finite size scaling fits that take into account this effect predict a consistent picture with universal scaling curves with common anomalous dimension even in the strong coupling.

Back to Programme Back to Participants Back to Contributions

Step Scaling Study of SU(3) 12 Flavor Theory with Larger Lattice
Kenji Ogawa, C.-J. David Lin, Hiroshi Ohki, Eigo Shintani
Tue, 14:40, Seminar Room F -- Parallels 3F (Slides)

The existence of the infrared fixed point in the SU(3) gauge theory with 12 flavors is still controversial. In this talk, we present our improved study of running coupling constant with Twisted Polyakov Loop scheme including the data of \(24^4\) lattice. It allows us a more detailed study of the lattice artefacts.

Back to Programme Back to Participants Back to Contributions

Composite flavor-singlet scalar in twelve-flavor QCD
Takeshi Yamazaki
Tue, 15:00, Seminar Room F -- Parallels 3F (Slides)

We present our results of flavor-singlet scalar meson in twelve-flavor QCD with a HISQ type action at the several volumes and fermion masses. A large number of gauge configurations and a noise reduction method were employed to reduce statistical noise of the disconnected diagram. We found that the flavor-singlet scalar state is lighter than the pseudoscalar state, which is different from usual QCD. From the investigation of the fermion mass dependence, we show that the results are consistent with hyperscaling, which is expected in the conformal theory. Therefore we consider that the lightness of the flavor-singlet scalar state is due to the infrared conformality. This result shed some light on the possibility of a light composite Higgs boson in walking technicolor theories.

Back to Programme Back to Participants Back to Contributions

Fisher's zeros for SU(3) with Nf flavors and RG flows
Yuzhi Liu, Zechariah Gelzer, Yannick Meurice, Donald K. Sinclair
Tue, 15:20, Seminar Room F -- Parallels 3F (Slides)

We calculate the Fisher's zeros for SU(3) with \(N_f=4\), \(8\) and \(12\) flavors of staggered fermions for various values the fermion mass. We discuss the finite size scaling near the end point of the line of discontinuity of psibar-psi in the beta-mass plane and in the larger beta-lower mass region. We discuss possible interpretations of these results in terms of Wilsonian RG flows and their possible relevance to construct composite Higgs models.

Back to Programme Back to Participants Back to Contributions

Improved Lattice Renormalization Group Techniques
Gregory Petropoulos, Anqi Cheng, Anna Hasenfratz, David Schaich
Tue, 15:40, Seminar Room F -- Parallels 3F (Slides)

We compute the step scaling function for SU(3) lattice gauge theories with many fundamental fermions using a fully non-perturbative Wilson flow optimized Monte Carlo Renormalization Group technique. By using Wilson flow to approach the renormalized trajectory of a specific RG transformation we are able to determine the unique step scaling function of a fixed renormalization group scheme. We apply our Wilson Flow MCRG technique to SU(3) gauge theory with 12 flavors in the chiral limit and find an infrared fixed point. By choosing different renormalization group schemes we are able to move the fixed point but the existence of the fixed point is universal. Furthermore, different RG transformations predict a consistent value for the scaling dimension of the irrelevant gauge coupling.

Back to Programme Back to Participants Back to Contributions

Further studies of QCD with sextet quarks
Donald Sinclair, John Kogut
Tue, 16:20, Seminar Room F -- Parallels 4F (Slides)

QCD with 2 flavours of colour-sextet quarks is a candidate walking-technicolor theory. We are studying the position of the chiral-symmetry-restoration transition to determine whether it is a finite-temperature transition in the continuum limit or if it is a bulk transition. In the first case it would be a walking gauge theory; in the second case it would be a conformal gauge theory. The 3-flavour case is being studied for comparison. We simulate the lattice version of these theories with unimproved staggered quarks using the RHMC algorithm.

Back to Programme Back to Participants Back to Contributions

Gauge theories with fermions in two-index representations
Yigal Shamir, Benjamin Svetitsky, Thomas DeGrand
Tue, 16:40, Seminar Room F -- Parallels 4F (Slides)

After a brief discussion of the methodology I will report results obtained using the Schrodinger functional technique for two gauge theories that are believed to lie near the bottom of the conformal window: the SU(3) theory with \(N_f=2\) adjoint Dirac fermions, and the SU(4) theory with \(N_f=6\) Dirac fermions in the two-index antisymmetric representation. In both cases we find a small beta function in strong coupling. We cannot confirm or rule out an infrared fixed point, though the SU(4) theory shows hints of walking. In both theories the mass anomalous dimension levels off, staying well below 0.5, much like the theories with fermions in the two-index symmetric representation investigated earlier.

Back to Programme Back to Participants Back to Contributions

Clover fermions in the adjoint representation and simulations of supersymmetric Yang-Mills theory
Stefano Piemonte
Tue, 17:00, Seminar Room F -- Parallels 4F (Slides)

Lattice simulations of supersymmetry are available since the last two decades, but only recently the algorithmic improvements have allowed investigations close enough to the continuum limit. The Monte Carlo studies of SUSY have many peculiarities with respect to standard QCD. The quarks are replaced by gluinos, which are majorana fermions in adjoint representation. The finite lattice spacing breaks SUSY explicitly, therefore, it is crucial to remove the O(a) discretization errors from the action. Our current research is focused on the N=1 supersymmetric Yang-Mills theory and the talk will present our recent algorithmic progresses. In particular, the removal of O(a) discretization errors from the Dirac operator with the clover term will be presented.

Back to Programme Back to Participants Back to Contributions

First results for SU(2) Yang-Mills with one adjoint Dirac Fermion
Andreas Athenodorou, Ed Bennett, Georg Bergner, Biagio Lucini, Agostino Patella
Tue, 17:20, Seminar Room F -- Parallels 4F (Slides)

We present the first exploratory results for the SU(2) gauge theory with one Dirac flavour in the adjoint representation. We provide initial results for the spectroscopy, static potential and the anomalous dimension for the fermionic condensate. Our results indicate that the theory is conformal with a fermionic anomalous dimension of order \(\sim O(1)\). These findings are relevant for walking technicolor scenarios.

Back to Programme Back to Participants Back to Contributions

Large volume results in SU(2) with adjoint fermions
Luigi Del Debbio, Biagio Lucini, Agostino Patella, Claudio Pica, Antonio Rago
Tue, 17:40, Seminar Room F -- Parallels 4F (Slides)

We present our latest results for the spectrum of SU(2) with two adjoint fermions, computed from numerical simulations on large lattices. We discuss the impact of finite volume effects on the scaling predicted from RG arguments, and review the determination of the mass anomalous dimension from measurements of the hadronic spectrum of the theory.

Back to Programme Back to Participants Back to Contributions

Conformality at large number of fermion flavors and composite Higgs
Terry Tomboulis
Wed, 08:30, Seminar Room F -- Parallels 5F (Slides)

It has recently been shown, both by direct MC studies and fermion hopping expansion resummation techniques, that SU(\(N_c\)) lattice gauge theories can be in a chirally symmetric phase even at infinite gauge coupling provided the number of fermion flavors \(N_f\) is large enough. We briefly review the resummation argument that provides some intuitive understanding of the phenomenon. We then discuss proposals for the complete \((g, m,(N_f/N_c))\)-phase diagram in light of the presently available information and open issues associated with it. Existence of a non-trivial conformal phase (non-trivial IR fixed point(s)) in (UV cutoff) gauge theories at large \(N_f/N_c\), if confirmed by further studies, provides a potentially new paradigm of composite Higgs models where the putative Higgs is not a GN boson. Instead, naturally light and weakly coupled composite scalar states appearing in the spectrum become unstable under mass generation by the CW mechanism upon coupling to electroweak gauge fields.

Back to Programme Back to Participants Back to Contributions

Search for the end point of first order phase transition in many-flavor lattice QCD
Norikazu Yamada, Shinji Ejiri
Wed, 08:50, Seminar Room F -- Parallels 5F (Slides)

As a first step to the feasibility study of the electroweak baryogenesis in technicolor scenario, we propose an easy-to-use approach to investigate the phase structure of \((2+N_f)\)-flavor QCD, where two light and \(N_f\) massive flavors exist. The approach is combining the reweighting and the histogram methods. By determining the location of the end point of the first order phase transition, we demonstrate that the approach works. We perform two lattice calculations, each of which uses staggered and Wilson fermion. The light quark mass dependence of the location of the end point is studied. Possible applications are also discussed.

Back to Programme Back to Participants Back to Contributions

More results on theories inside the conformal window
Tiago Jose Nunes da Silva, Albert Deuzeman, Maria Paola Lombardo, Kohtaroh Miura, Elisabetta Pallante
Wed, 09:10, Seminar Room F -- Parallels 5F (Slides)

We report on the study of theories inside the conformal window that is being carried on within our collaboration. We are studying the SU(3) gauge theory with twelve and sixteen flavors of fundamental flavors and we present results that highlight the properties of such theories. We also comment on the role of improvement away from the perturbative limit.

Back to Programme Back to Participants Back to Contributions

Vector and axialvector currents in multi-species staggered fermions
Yasumichi Aoki
Wed, 09:30, Seminar Room F -- Parallels 5F

Flavor non-singlet vector current can be constructed for staggered fermions when multiple fermion fields are involved. The axial vector current which forms an exact multiplet with the vector current under the exact axial transformation can be made. The difference of the vacuum polarization functions with the vector and axial vector currents is free from power divergence. Applications of these currents include the Peskin-Takeuchi S parameter and vector meson decay constant. A practical way to calculate these quantities in multi-species HISQ formulation is discussed.

Back to Programme Back to Participants Back to Contributions

The Gradient Flow Coupling in Minimal Walking Technicolor
Jarno Rantaharju
Wed, 09:50, Seminar Room F -- Parallels 5F (Slides)

We present a measurement of the running coupling in SU(2) with two adjoint fermions in the Yang-Mills gradient flow scheme. The simulations are performed with Schrödinger Functional boundary conditions using an improved HEX-smeared Wilson fermion action. We obtain a step scaling function by defining the coupling at a scale relative to the finite size of the lattice. We find a continuum limit with a non-trivial infrared fixed point.

Back to Programme Back to Participants Back to Contributions

Toward the Global Structure of Conformal Theories in the SU(3) Gauge Theory
Yoichi Iwasaki
Wed, 10:10, Seminar Room F -- Parallels 5F (Slides)

We clarify the vacuum structure and properties of correlation functions in QCD with \(N_f\) flavors in fundamental representation from the viewpoint of the ``conformal theories with an IR cutoff''. We claim that the large \(N_f\) QCD (referred as Conformal QCD) with an IR cutoff and small \(N_f\) QCD at \(T/T_c >1\) (referred as High Temperature QCD) shows the common feature as the ``conformal theories with an IR cutoff''. We argue from our theoretical analysis based on the RG flow and our numerical simulations that there is a precise correspondence between the two in the correlation functions and phase structure under the change of the parameters \(N_f\) and \(T/T_c\) respectively: the one boundary is close to meson states and the other is close to free quark states. In particular, we find the correspondence between Conformal QCD with \(N_f = 7\) and High Temperature QCD with \(N_f=2\) at \(T\sim 2\, T_c\) being in close relation to a meson unparticle model. From this we estimate the anomalous mass dimension \(\gamma^* = 1.2 (1)\) for \(N_f=7\). We also show that the asymptotic state in the limit \(T/T_c \rightarrow \infty\) is a free quark state in the Z(3) twisted vacuum. The approach to a free quark state is very slow; even at \(T/T_c \sim 10^5\), the state is affected by non-perturbative effects. This explains the slow approach of the free energy to the Stefan-Boltzmann ideal gas limit. Our finding above is consistent with our conjecture that the lower critical flavor number \(N_f^{c}=7\) for the conformal window in the continuum limit.

Back to Programme Back to Participants Back to Contributions

Anomalous dimensions of four-fermion operators from conformal EWSB dynamics
Carlos Pena, Luigi Del Debbio, Liam Keegan
Wed, 11:00, Seminar Room F -- Parallels 6F (Slides)

(Quasi)conformal scaling of composite operators from a strongly coupled EWSB dynamics helps to produce the characteristic hierarchies exhibited by the flavour couplings of the SM. It is however crucial to ensure that specific models satisfy bounds on Higgs and flavour dynamics; this in turn requires to control not only the anomalous dimensions of bilinears, but also those of higher-dimensional operators. We report on an ongoing effort to determine four-fermion operator anomalous dimensions, via Schrödinger Functional techniques, in the benchmark scenario of Minimal Walking Technicolour.

Back to Programme Back to Participants Back to Contributions

The anomalous mass dimension from the techniquark propagator in Minimal Walking Technicolor
Daniel August, Axel Maas
Wed, 11:20, Seminar Room F -- Parallels 6F (Slides)

Minimal Walking Technicolor is a 2-color QCD-like theory with two quark flavors in the adjoint representation. We calculated the techniquark propagator in minimal Landau gauge, using lattice simulations of the standard Wilson action. This data enabled us to derive the anomalous mass dimension from a single lattice setup. We present results for a range of quark masses, and give estimate of the systematic errors. The results are compatible with previous determinations.

Back to Programme Back to Participants Back to Contributions

Determining the anomalous dimension through the eigenmodes of Dirac operator
Anqi Cheng, Anna Hasenfratz, Gregory Petropoulos, David Schaich
Wed, 11:40, Seminar Room F -- Parallels 6F (Slides)

We extract the scale-dependent mass anomalous dimension from the eigenmode number of the massless Dirac operator, for SU(3) lattice gauge theories with many fundamental fermions. A stochastic algorithm allows us to efficiently measure the mode number up to the cutoff scale on lattices as large as L=32. By combining simulations on multiple lattice volumes, and when possible several gauge couplings, we are able to measure the anomalous dimension across a wide range of energy scales. The 4-flavor model behaves as expected for a QCD-like system, while our 12-flavor results suggest the existence of an infrared fixed point with the anomalous dimension 0.32(3). For the 8-flavor model we observe a large anomalous dimension across a wide range of scales. We also investigate the 16-flavor system which is known to be infrared conformal. Our method is universal and can be applied to any lattice model of interest.

Back to Programme Back to Participants Back to Contributions

The chiral condensate from the Dirac spectrum in BSM gauge theories
Kieran Holland, Zoltan Fodor, Julius Kuti, Dániel Nógrádi, Chik Him Wong
Wed, 12:00, Seminar Room F -- Parallels 6F (Slides)

Chiral symmetry breaking in near-conformal gauge theories can be masked by the steep mass dependence of the chiral condensate. We discuss the renormalization group invariant mode number of the Dirac operator for the 2-flavor SU(3) sextet and the 12-flavor SU(3) fundamental models, and consistency with direct measurements of the chiral condensate.

Back to Programme Back to Participants Back to Contributions

SU(2) Adjoint MWT in the chirally rotated Schrödinger functional scheme
Claudio Pica, Rudy Arthur, Luigi Del Debbio, Biagio Lucini, Agostino Patella, Antonio Rago, Stefan Sint, Pol Vilaseca
Wed, 12:20, Seminar Room F -- Parallels 6F (Slides)

I will present preliminary results for our ongoing simulations for the determination of the non-perturbative beta-function of the SU(2) Adjoint Minimal Walking Technicolor (MWT) model in the chirally rotated Schrödinger functional scheme.

Back to Programme Back to Participants Back to Contributions

The spectrum of supersymmetric Yang Mills theory -- new results and recent measurements
Gernot Münster, Georg Bergner
Thu, 14:00, Seminar Room F -- Parallels 7F (Slides)

Numerical simulations of supersymmetric theories on the lattice are intricate and challenging with respect to their theoretical foundations and algorithmic realization. Nevertheless, the simulations of a fourdimensional supersymmetric gauge theory have made considerable improvements over the recent years. In this talk I summarize the results of our collaboration concerning the mass spectrum of this theory. The investigation of systematic errors allows now a more precise estimate concerning the expected formation of supersymmetric multiplets of the lightest particles. These multiplets contain flavour singlet mesons, gluballs, and an additional fermonic state.

Back to Programme Back to Participants Back to Contributions

Loop formulation for the non-linear supersymmetric O(N) sigma-model
Kyle Steinhauer, Urs Wenger
Thu, 14:20, Seminar Room F -- Parallels 7F (Slides)

In this contribution we present the fermion loop formulation of the Non-linear supersymmetric O(N) model on the lattice using Wilson fermions. The formulation provides an efficient way to simulate the fermionic degrees of freedom for arbitrary boundary conditions and at arbitrary mass values. We show in detail how the symmetry constraints induce flavour changing interactions between the bosons and fermions which may potentially lead to a sign problem. For N=2 first results such as the determination of the critical point and the spectrum of the bosons and fermions will be presented.

Back to Programme Back to Participants Back to Contributions

Eigenvalue spectrum of lattice N=4 super Yang-Mills
David Weir, Simon Catterall, Paul Damgaard, Thomas DeGrand, Dhagash Mehta
Thu, 14:40, Seminar Room F -- Parallels 7F (Slides)

We present preliminary results for the eigenvalue spectrum of four-dimensional N=4 super Yang-Mills theory on the lattice. In particular, by studying the the spectral density a measurement of the anomalous dimension is made and found to be consistent with zero. Properties of the Pfaffian are also explored numerically to support the claim that the theory has no sign problem.

Back to Programme Back to Participants Back to Contributions

Euclidean 4D quantum gravity with a non-trivial measure term
Andrzej Görlich, Jan Ambjørn, Lisa Glaser, Jerzy Jurkiewicz
Thu, 15:00, Seminar Room F -- Parallels 7F (Slides)

Dynamical Triangulations provide us with a lattice regularization of four-dimensional Euclidean quantum gravity within the realm of ordinary quantum field theory. We add a local measure term, which can also serve as a generalized higher curvature term, and explore an extended coupling constant space. We determine the phase diagram of this model using non-degenerated triangulations. A first order phase transition line is observed, but no second order transition point is located. As a consequence we cannot attribute any continuum physics interpretation to the so-called crinkled phase of 4D dynamical triangulations.

Back to Programme Back to Participants Back to Contributions

Gauge theory of Lorentz group on the lattice
Mikhail Zubkov
Thu, 15:20, Seminar Room F -- Parallels 7F (Slides)

The model with the fermions coupled in the nonminiaml way to the gauge theory of Lorentz group is considered. The lattice regularization is suggested. It is argued that this model may exist in the phase with broken chiral symmetry and without confinement. We speculate about the possibility that this construction may serve as an origin of the dynamical electroweak symmetry breaking.

Back to Programme Back to Participants Back to Contributions

Euclidean Dynamical Triangulation revisited: is the phase transition really 1st order?
Tobias Rindlisbacher, Philippe de Forcrand
Thu, 15:40, Seminar Room F -- Parallels 7F (Slides)

The transition between the two phases of 4D Euclidean Dynamical Triangulation is believed to be of first order [1, 2]. However, doubts on this conclusion can be raised because of the enormous critical slowing down observed in the simulations, the use of a potential term to control the volume fluctuations, and the definition of a sweep as N4 accepted moves. We re-investigate the order of the phase transition by using an optimized parallel tempering method as described in [3] with up to 48 replicas in order to decrease the auto-correlation time. A sweep we define as usual as N4 attempted moves in order to satisfy detailed balance and instead of using an additional potential, we allow the volume to fluctuate within a fixed interval while 4 is tuned to its pseudo-critical value. [1] P. Bialas, Z. Burda, A. Krzywicki, B. Petersson: Focusing on the Fixed Point of 4D Simplicial Gravity, hep-lat/9601024 [2] B. V. de Bakker: Further evidence that the transition of 4D dynamical triangulation is 1st order, hep-lat/9603024 [3] H. G. Katzgraber, S. Trebst, D. A. Huse, M. Troyer: Feedback-optimized parallel tempering Monte Carlo, cond-mat/0602085

Back to Programme Back to Participants Back to Contributions

1st order phase transition in the MSSM and baryogenesis
Kari Rummukainen, Germano Nardini, Mikko Laine
Thu, 16:30, Seminar Room F -- Parallels 8F (Slides)

We study the cosmological electroweak phase transition in a MSSM-like theory using lattice simulations of a dimensionally reduced effective theory. The theory has a physical Higgs mass \(m_H \sim 126\) GeV and a relatively light SU(3) coloured SU(2) singlet, right handed stop. The theory is observed to have a strong first order phase transition, sufficient for preventing baryon number erasure in electroweak baryogenesis scenarios

Back to Programme Back to Participants Back to Contributions

Predictions for LHC from SO(4) MWT
Ari Hietanen, Claudio Pica, Francesco Sannino, Ulrik Søndegaard
Thu, 16:50, Seminar Room F -- Parallels 8F (Slides)

Among the relevant models of dynamical electroweak symmetry breaking is SO(4) MWT. It is an SO(4)-gauge theory with two fermions in vector representation, and it features dark matter candidates. We present preliminary results of calculation of the masses of vector and axial vector meson using Wilson fermions. The values of the masses are directly relevant for technicolor searches at the LHC.

Back to Programme Back to Participants Back to Contributions

Composite Dark Matter Exclusions from the Lattice
Michael Buchoff
Thu, 17:10, Seminar Room F -- Parallels 8F (Slides)

One intriguing coincidence in cosmology is how the observed dark matter and baryonic densities are within a factor of 5 in magnitude. A natural explanation for such a coincidence is that the origin of the dark matter density is intimately related to the early universe processes that led to the baryon asymmetry. The majority of these ``asymmetric'' dark matter scenarios favor a strongly coupled composite sector ala QCD, where neutral, long-lived composites can survive to be observed today, but still have charged constituents to interact with early universe baryogenesis. As a result, these neutral composites are expected have non-zero electromagnetic properties, such as magnetic moments, charge radii, and polarizabilties, which can be observed in direct detection experiments. The values of these properties are inherent to the dynamics of these strongly coupled theories, where non-perturbative lattice methods allow for a reliable exploration with controlled systematics. In this talk, I will present some initial results for a three-color, QCD-like theory with two and six light fermion flavors, including implications on the latest exclusions from the Xenon100 collaboration.

Back to Programme Back to Participants Back to Contributions

Searching for a continuum 4D field theory arising from a 5D non-abelian gauge
Eliana Lambrou, Luigi Del Debbio, Richard Kenway, Enrico Rinaldi
Thu, 17:30, Seminar Room F -- Parallels 8F (Slides)

The anisotropic 5D SU(2) Yang-Mills model has been widely investigated on the lattice during the last decade. In the case where all dimensions are large in size, it was previously claimed that a second order phase transition is present in the regime where the lattice spacing in the fifth dimension is much larger than in the other four dimensions. At a second order phase transition a 4D continuum theory could be defined. I will present the extension of the previous work to large lattices, at which we found a first order phase transition. This leaves the scenario that a 5D theory can be dimensionally reduced to a continuum 4D field theory, doubtful.

Back to Programme Back to Participants Back to Contributions

Dynamical gauge symmetry breaking in strongly coupled fermion theories
Aarti Veernala, Simon Catterall
Thu, 17:50, Seminar Room F -- Parallels 8F (Slides)

We show how a strongly coupled lattice theory consisting of just fermions and gauge fields can exhibit a dynamical Higgs mechanism through the formation of a gauge invariant four fermion condensate. Furthermore, we show evidence that this lattice Higgs phase survives into the continuum limit where the broken symmetries are to be interpreted as chiral gauge symmetries.

Back to Programme Back to Participants Back to Contributions

Single site model of large N gauge theories coupled to adjoint fermions
Rajamani Narayanan, Robert Lohmayer
Fri, 16:30, Seminar Room F -- Parallels 10F (Slides)

We consider a single-site large-N gauge theory coupled to adjoint fermions at weak coupling. We study using a four-dimensional density function for the distribution of the eigenvalues of the link variables. We show that it is possible to recover the infinite-volume continuum limit for a certain range of fermion flavors if we use fermions with a bare mass of zero. We discuss the consequences of previous numerical work based on the conclusions reached in our study.

Back to Programme Back to Participants Back to Contributions

Twisted reduction in large N QCD with adjoint Wilson fermions
Masanori Okawa, Antonio Gonzalez-Arroyo
Fri, 16:50, Seminar Room F -- Parallels 10F (Slides)

The twisted space-time reduced model of large N QCD with adjoint Wilson fermions is constructed applying the symmetric twist boundary conditions with flux k for various number of flavors. With two flavors, the string tension calculated at N=289 approaches zero as we decrease quark mass. On the contrary, the string tension for the case of a single adjoint Wilson fermion seems to remain finite as the quark mass decreases to zero. A preliminary result for the supersymmetric 1/2 flavor theory is also presented.

Back to Programme Back to Participants Back to Contributions

Mass anomalous dimension from large N twisted volume reduction
Liam Keegan, Margarita Garcia-Perez, Antonio Gonzalez-Arroyo, Masanori Okawa
Fri, 17:10, Seminar Room F -- Parallels 10F (Slides)

In this work we consider the SU(N) gauge theory with two Dirac fermions in the adjoint representation, in the limit of large N. Taking advantage of large N twisted volume reduction we do this on a single site lattice, but we should still get infinite-volume physics in the large N limit. We describe our progress in extracting the mass anomalous dimension from the eigenvalue distribution of the adjoint Dirac operator, using data for N up to 289.

Back to Programme Back to Participants Back to Contributions

Large-N mesons
Gunnar Bali, Luca Castagnini, Biagio Lucini, Marco Panero
Fri, 17:30, Seminar Room F -- Parallels 10F (Slides)

We compute the meson spectrum in SU(N) gauge theory, up to N = 17, and extrapolate this to predict the QCD meson spectrum in the limit of infinite N. Related quantities like decay constants are obtained as well and non-perturbatively renormalized. Implications on N=3 QCD as well as on models inspired by the gauge-gravity duality are discussed.

Back to Programme Back to Participants Back to Contributions

Phase structure and Hosotani mechanism in QCD-like theory with compact dimensions
Kouji Kashiwa
Fri, 17:50, Seminar Room F -- Parallels 10F (Slides)

We investigated the phase diagram of SU(3) gauge theory in four and five dimensions with one compact dimension by using perturbative one-loop and PNJL-model-based effective potentials. The effect of the adjoint and fundamental fermion is investigated and then the rich phase structure in the quark-mass and compact-size space is realized. The chiral properties in these cases are also discussed. Our results are qualitatively consistent with the recent lattice calculations and clearly show that the calculations can be understood from Hosotani mechanism. Moreover, we will show the result obtained by using the flavor twisted boundary condition for fundamental fermion.

Back to Programme Back to Participants Back to Contributions

Lattice investigations of the Hosotani Mechanism of spontaneous symmetry breaking
James Hetrick
Fri, 18:10, Seminar Room F -- Parallels 10F (Slides)

The Hosotani Mechanism is a method of dynamical gauge symmetry breaking which can occur in gauge theories with compact extra dimensions. Under certain conditions (fermion content and boundary conditions) the gauge field component in the compact dimension develops a VEV that is outside the center of the gauge group. Investigations of this phenomenon in 3+1 dimensions are reported, particularly the behavior of gauge correlators.

Back to Programme Back to Participants Back to Contributions

Hosotani mechanism on the lattice
Guido Cossu, Yutaka Hosotani, Etsuko Itou, Junichi Noaki, Hisaki Hatanaka
Fri, 18:30, Seminar Room F -- Parallels 10F (Slides)

We present the first steps in understanding the non perturbative realization of the Hosotani mechanism. The Hosotani mechanism (Hosotani Phys.Lett.B126 -1983- 309) is a mechanism to get massive gauge bosons without an ad-hoc Higgs particle but embedding the gauge field in a multiply-connected space where the path-ordered line integrals along non-contractible loops become dynamical degrees of freedom. The simplest realization is in a flat space plus a space-like extra dimension \(S^1\) where the fermions are periodic in the extra dimension. The one-loop effective potential shows that the lowest energy states can have a mass gap in some cases, e.g. gauge theory with adjoint fermions. The dynamical degrees of freedom that describe the phases are the eigenvalues of the Polyakov loop. We investigate non perturbatively the phase space of two flavor adjoint fermions, with periodic boundary conditions in the compact direction, to understand if the perturbative results persist in the strong coupling regime.

Back to Programme Back to Participants Back to Contributions

The anomalous dimension at the infrared fixed point of \(N_f = 12\) SU(3) theory
Etsuko Itou
Poster Session

We propose a novel renormalization scheme for the wave function renormalization for the hadronic operators. We show the numerical simulation result for the anomalous dimension of the pseudo scalar operator. It is related with the mass renormalization factor of the fermion thought the partially conserved axial-vector current (PCAC) relation. We derive the mass anomalous dimension at the Infrared fixed point (IRFP) in the SU(3) gauge theory coupled to \(N_f = 12\) fundamental fermions. Our numerical result is smaller than one of the previous works. We discuss a possible reason of the discrepancy.

Back to Programme Back to Participants Back to Contributions

N=1 supersymmetric Yang-Mills theory on the lattice
Gernot Münster, Georg Bergner, Istvan Montvay, Umut D. ÿzugurel, Stefano Piemonte, Dirk Sandbrink
Poster Session

N=1 supersymmetric Yang-Mills theory describes interacting gauge fields and their superpartners, the spin 1/2 gluinos. A gluino mass term breaks supersymmetry softly. In the supersymmetric limit, the physical particles are expected to form supermultiplets. In our recent numerical simulations we made progress in the study and reduction of systematic effects. The results are consistent with the formation of degenerate supermultiplets. This poster gives an overview of the project and its status.

Back to Programme Back to Participants Back to Contributions

Finetuning the continuum limit in low-dimensional supersymmetric theories
Daniel Körner, Raphael Flore, Björn Wellegehausen, Andreas Wipf
Poster Session

Supersymmetry is a prominent candidate for beyond the standard model physics. Experiments however have not confirmed the existence of superpartner particles that are predicted by susy models. In order to compute the spectrum of such theories, we employ nonperturbative lattice QFT techniques which due to the discretisation of spacetime violate supersymmetry at finite lattice spacing. Care has to be taken then to restore supersymmetry in the continuum limit. We discuss a discretisation of the supersymmetric Nonlinear O(N) Sigma model in two dimensions and argue that supersymmetry may be restored by finetuning of a single parameter. Furthermore, we show results for the spectrum of N=2 Super-Yang-Mills theory in 3 dimensions with emphasis on the Veneziano-Yankielowicz multiplet.

Back to Programme Back to Participants Back to Contributions

Lattice simulations with eight flavors of domain wall fermions in SU(3) gauge theory
Meifeng Lin
Poster Session

With the discovery of a Higgs-like boson at the Large Hadron Collider, the imminent task for the study of the beyond Standard Model theories is to find the candidate theories that may produce a light scalar particle to be consistent with the experimental observation. In the context of non-perturbative lattice gauge theory simulations, one of the first steps is to find possible non-QCD like behaviors in these theories. Over the past few years, the Lattice Strong Dynamics (LSD) Collaboration has worked extensively on the SU(3) gauge theories with many flavors of degenerate fermions, and found some interesting behaviors in theories of 6 and 10 flavors. Here we will present some latest results by the LSD collaboration from lattice simulations with 8 flavors of domain wall fermions in the SU(3) fundamental representation.

Back to Programme Back to Participants Back to Contributions