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A. Hasenfratz

Publications and source records attributed to A. Hasenfratz.

At least 19 recordsLinked to original sources
arXiv2023

Hidden Conformal Symmetry from the Lattice

We analyze newly expanded and refined data from lattice studies of an SU(3) gauge theory with eight Dirac fermions in the fundamental representation. We focus on the light composite states emerging from these studies, consisting of a set of pseudoscalars and a single light scalar. We first consider the view that this theory is just outside the conformal window. In this case, the pseudoscalars arise from spontaneous breaking of chiral symmetry. Identifying the scalar in this case as an approximate dilaton, we fit the lattice data to a dilaton effective field theory, finding that it yields a good fit even at lowest order. For comparison, we then consider the possibility that the theory is inside the conformal window. The fermion mass provides a deformation, triggering confinement. We employ simple scaling laws to fit the lattice data, and find that it is of lesser quality.

hep-lat↗
arXiv2023

Light Scalar Meson and Decay Constant in SU(3) Gauge Theory with Eight Dynamical Flavors

The SU(3) gauge theory with $N_f=8$ nearly massless Dirac fermions has long been of theoretical and phenomenological interest due to the near-conformality arising from its proximity to the conformal window. One particularly interesting feature is the emergence of a relatively light, stable flavor-singlet scalar meson $σ$ $(J^{PC}=0^{++})$ in contrast to the $N_f=2$ theory QCD. In this work, we study the finite-volume dependence of the $σ$ meson correlation function computed in lattice gauge theory and determine the $σ$ meson mass and decay constant extrapolated to the infinite-volume limit. We also determine the infinite volume mass and decay constant of the flavor-nonsinglet scalar meson $a_0$.

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arXiv2022

Goldstone Boson Scattering with a Light Composite Scalar

The appearance of a light composite $0^+$ scalar resonance in nearly conformal gauge-fermion theories motivates further study of the low energy structure of these theories. To this end, we present a nonperturbative lattice calculation of s-wave scattering of Goldstone bosons in the maximal-isospin channel in SU(3) gauge theory with $N_f=8$ light, degenerate flavors. The scattering phase shift is measured both for different values of the underlying fermion mass and for different values of the scattering momentum. We examine the effect of a light flavor-singlet scalar (reported in earlier studies) on Goldstone boson scattering, employing a dilaton effective field theory (EFT) at the tree level. The EFT gives a good description of the scattering data, insofar as the magnitude of deviations between EFT and lattice data are no larger than the expected size of next-to-leading order corrections in the EFT.

hep-ph↗
arXiv2021

Stealth dark matter confinement transition and gravitational waves

We use non-perturbative lattice calculations to investigate the finite-temperature confinement transition of stealth dark matter, focusing on the regime in which this early-universe transition is first order and would generate a stochastic background of gravitational waves. Stealth dark matter extends the standard model with a new strongly coupled SU(4) gauge sector with four massive fermions in the fundamental representation, producing a stable spin-0 'dark baryon' as a viable composite dark matter candidate. Future searches for stochastic gravitational waves will provide a new way to discover or constrain stealth dark matter, in addition to previously investigated direct-detection and collider experiments. As a first step to enabling this phenomenology, we determine how heavy the dark fermions need to be in order to produce a first-order stealth dark matter confinement transition.

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arXiv2019

Nonperturbative determination of $β$ functions for SU(3) gauge theories with 10 and 12 fundamental flavors using domain wall fermions

Nonperturbative lattice field theory simulations provide a systematic framework to investigate properties of conformal systems at strong couplings. These simulations can be performed using different lattice discretizations. Here we present numerical results for the step scaling beta function in SU(3) gauge theories with ten and twelve fundamental flavors. We calculate the renormalized $β$ function in the finite volume gradient flow renormalization scheme. Using Möbius domain wall fermions with Symanzik gauge action, Zeuthen gradient flow, and perturbative tree-level improvement, we implement a fully $O(a^2)$ Symanzik improved set-up and demonstrate its advantages. We compare our findings to existing results in the literature. For the ten flavor system we observe excellent agreement with the domain wall step-scaling function calculated by Chiu for the range in $g_c^2$ where our data overlap. In the case of the twelve flavor system, our $O(a^2)$ Symanzik improved set-up predicts a conformal infrared fixed point around $g_c^2 \sim 5.5$ in the $c=0.25$ scheme, which is presently in tension with staggered fermion results in the literature. We consider possible reasons for the discrepancy.

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arXiv2019

Linear Sigma EFT for Nearly Conformal Gauge Theories

We construct a generalized linear sigma model as an effective field theory (EFT) to describe nearly conformal gauge theories at low energies. The work is motivated by recent lattice studies of gauge theories near the conformal window, which have shown that the lightest flavor-singlet scalar state in the spectrum ($σ$) can be much lighter than the vector state ($ρ$) and nearly degenerate with the PNGBs ($π$) over a large range of quark masses. The EFT incorporates this feature. We highlight the crucial role played by the terms in the potential that explicitly break chiral symmetry. The explicit breaking can be large enough so that a limited set of additional terms in the potential can no longer be neglected, with the EFT still weakly coupled in this new range. The additional terms contribute importantly to the scalar and pion masses. In particular, they relax the inequality $M_σ^2 \ge 3 M_π^2$, allowing for consistency with current lattice data.

hep-ph↗
arXiv2016

Composite Higgs model at a conformal fixed point

We propose to construct a chirally broken model based on the infrared fixed point of a conformal system by raising the mass of some flavors while keeping the others massless. In the infrared limit the massive fermions decouple and the massless fermions break chiral symmetry. The running coupling of this system "walks" and the energy range of walking can be tuned by the mass of the heavy flavors. Renormalization group considerations predict that the spectrum of such a system shows hyperscaling. We have studied a model with four light and eight heavy flavors coupled to SU(3) gauge fields and verified the above expectations. We determined the mass of several hadronic states and found that some of them are in the 2-3 TeV range if the scale is set by the pseudoscalar decay constant $F_π\approx 250$ GeV. The $0^{++}$ scalar state behaves very differently from the other hadronic states. In most of our simulations it is nearly degenerate with the pion and we estimate its mass to be less than half of the vector resonance mass.

hep-ph↗
arXiv2015

Strongly coupled gauge theories: What can lattice calculations teach us?

The dynamical origin of electroweak symmetry breaking is an open question with many possible theoretical explanations. Strongly coupled systems predicting the Higgs boson as a bound state of a new gauge-fermion interaction form one class of candidate models. Due to increased statistics, LHC run II will further constrain the phenomenologically viable models in the near future. In the meanwhile it is important to understand the general properties and specific features of the different competing models. In this work we discuss many-flavor gauge-fermion systems that contain both massless (light) and massive fermions. The former provide Goldstone bosons and trigger electroweak symmetry breaking, while the latter indirectly influence the infrared dynamics. Numerical results reveal that such systems can exhibit a light $0^{++}$ isosinglet scalar, well separated from the rest of the spectrum. Further, when we set the scale via the $vev$ of electroweak symmetry breaking, we predict a 2 TeV vector resonance which could be a generic feature of SU(3) gauge theories.

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arXiv2010

Overlap Valence on 2+1 Flavor Domain Wall Fermion Configurations with Deflation and Low-mode Substitution

The overlap fermion propagator is calculated on 2+1 flavor domain wall fermion gauge configurations on 16^3 x 32, 24^3 x 64 and 32^3 x 64 lattices. With HYP smearing and low eigenmode deflation, it is shown that the inversion of the overlap operator can be expedited by ~ 20 times for the 16^3 x 32 lattice and ~ 80 times for the 32^3 x 64 lattice. Through the study of hyperfine splitting, we found that the O(m^2a^2) error is small and these dynamical fermion lattices can adequately accommodate quark mass up to the charm quark. The low energy constant Δ_{mix} which characterizes the discretization error of the pion made up of a pair of sea and valence quarks in this mixed action approach is calculated via the scalar correlator with periodic and anti-periodic boundary conditions. It is found to be small which shifts a 300 MeV pion mass by ~ 10 to 19 MeV on these sets of lattices. We have studied the signal-to-noise issue of the noise source for the meson and baryon. It is found that the many-to-all meson and baryon correlators with Z_3 grid source and low eigenmode substitution is efficient in reducing errors for the correlators of both mesons and baryons. With 64-point Z_3 grid source and low-mode substitution, it can reduce the statistical errors of the light quark (m_π ~ 200 - 300 MeV) meson and nucleon correlators by a factor of ~ 3-4 as compared to the point source. The Z_3 grid source itself can reduce the errors of the charmonium correlators by a factor of ~ 3.

hep-lat↗
arXiv2003

Topological susceptibility with the improved Asqtad action

Chiral perturbation theory predicts that in quantum chromodymamics light dynamical quarks suppress the topological (instanton) susceptibility. We investigate this suppression through direct numerical simulation using the Asqtad improved lattice fermion action. This action holds promise for carrying out nonperturbative simulations over a range of quark masses for which chiral perturbation theory is expected to converge. To test the effectiveness of the action in capturing instanton physics, we measure the topological susceptibility as a function of quark masses with 2+1 dynamical flavors. Our results, when extrapolated to zero lattice spacing, are consistent with predictions of leading order chiral perturbation theory. Included in our study is a comparison of three methods for analyzing the topological susceptibility: (1) the Boulder hypercubic blocking technique with the Boulder topological charge operator, (2) the more traditional Wilson cooling method with the twisted plaquette topological charge operator and (3) the improved cooling method of de Forcrand, Perez, and Stamatescu and their improved topological charge operator.

hep-lat↗
arXiv2002

Topological susceptibility with the improved Asqtad action

As a test of the chiral properties of the improved Asqtad (staggered fermion) action, we have been measuring the topological susceptibility as a function of quark masses for 2 + 1 dynamical flavors. We report preliminary results, which show reasonable agreement with leading order chiral perturbation theory for lattice spacing less than 0.1 fm. The total topological charge, however, shows strong persistence over Monte Carlo time.

hep-lat↗
arXiv2002

Simulation of dynamical fermions with smeared links

Smearing the gauge links of dynamical configurations removes small scale unphysical vacuum fluctuations und thus improves the chiral properties of lattice fermions. We present a new algorithm for the simulation of dynamical fermions coupled via smeared links based on the standard pure gauge overrelaxation and heatbath updatings. Smeared links play a fundamental role in making this algorithm effective. At fixed lattice spacing the computational cost of the algorithm has an extra volume factor due to the finite volume of the lattice region which can be updated. As the continuum limit is approached the physical volume of the updated region remains constant. We simulated four flavors of staggered fermions coupled via hypercubic (HYP) smeared links. The simulation cost of the new algorithm on 10 fm^4 volumes is a factor 2-8 larger than with the standard Hybrid Monte Carlo but the improved properties of the HYP action allow to gain a factor 2 in the lattice spacing. The new algorithm could be applicable to simulations of more complicated chiral fermionic actions, like overlap or perfect actions.

hep-lat↗
arXiv2001

The Static Potential with Hypercubic Blocking

We measure the static potential from Wilson loops constructed using hypercubic blocked (HYP) links. The HYP potential agrees with the potential measured using thin links for distances r/a>=2. We calculated the lowest order perturbative expansion of the lattice Coulomb potential of HYP links. These results are used in analyzing the static potential both on quenched and dynamical lattices. The statistical accuracy of the potential with HYP links improves by about an order of magnitude, giving a reliable scale even with limited statistics both on quenched and dynamical lattices.

hep-lat↗
arXiv2001

Simulating dynamical fermions with smeared links

Smearing the gauge links of dynamical configurations removes small scale unphysical vacuum fluctuations and thus improves the chiral properties of lattice fermions. Recently we proposed the hypercubic smearing (HYP) that improves the flavor symmetry of staggered fermions by an order of magnitude with only minimal distortions at small distances. We describe a new algorithm to simulate dynamical HYP fermions based on the standard pure gauge overrelaxation and heatbath updates. The algorithm has been used to simulate four and two flavors of staggered fermions. Unlike standard dynamical simulation techniques, this algorithm does not loose efficiency at small quark masses.

hep-lat↗
arXiv2001

Four Flavor Finite Temperature Phase Transition with HYP Action: Where is the First Order Phase Transition Line?

We study the finite temperature phase transition of four flavor staggered fermions with hypercubic fat link actions on N_t=4 and N_t=6 temporal lattices. Our fat links are constructed with hypercubic blocking (HYP) and therefore are very compact. We present a new algorithm for simulating fermions coupled to HYP fat links. The algorithm has a simple form based on the standard overrelaxation and heatbath updatings for the pure gauge action. We observe that as we increase the smoothness of the gauge fields by changing the parameters of the blocking the very pronounced first order phase transition of the thin link action becomes weaker and moves to physically uninteresting values of the gauge coupling. With our smoothest HYP action we do not find any indication of a phase transition in the accessible temperature range on the N_t=4 or 6 lattices even at quark masses close to the physical light quark mass. We argue that the observed difference in the phase diagram is due to the improved flavor symmetry of the smeared link actions.

hep-lat↗
arXiv2001

Flavor Symmetry and the Static Potential with Hypercubic Blocking

We introduce a new smearing transformation, the hypercubic (HYP) fat link. The hypercubic fat link mixes gauge links within hypercubes attached to the original link only. Using quenched lattices at beta = 5.7 and 6.0 we show that HYP fat links improve flavor symmetry by an order of magnitude relative to the thin link staggered action. The static potential measured on HYP smeared lattices agrees with the thin link potential at distances r/a >= 2 and has greatly reduced statistical errors. These quenched results will be used in forthcoming dynamical simulations of HYP staggered fermions.

hep-lat↗
arXiv2001

Dynamical Fermions with Fat Links

We present and test a new method for simulating dynamical fermions with fat links. Our construction is based on the introduction of auxiliary but dynamical gauge fields and works with any fermionic action and can be combined with any fermionic updating. In our simulation we use an over-relaxation step which makes it effective. For four flavors of staggered fermions first results indicate that flavor symmetry at a lattice spacing a~0.2 fm is restored to a few percent. With the standard action this amount of flavor symmetry restoration is achieved at a~0.07 fm. We estimate that the overall computational cost is reduced by at least a factor 10.

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