Density of states techniques for fermion worldlines

Worldline representations were established as a powerful tool for studying bosonic lattice field theories at finite density. For fermions, however, the worldlines still may carry signs that originate from the Dirac algebra and from the Grassmann nature of the fermion fields. We show that a density of states approach can be set up to deal with this remaining sign problem, where finite density is implemented in a canonical approach by working with a fixed winding number of the fermion worldlines. We discuss the approach in detail and show first results of a numerical implementation in 2 dimensions.Comment: Contribution to the proceedings of the 39th International Symposium on Lattice Field Theory - Lattice2022, August 8 - 13, 2022, Bonn, German

Inelastic X-ray scattering in correlated (Mott) insulators

We calculate the inelastic light scattering from X-rays, which allows the photon to transfer both energy and momentum to the strongly correlated charge excitations. We find that the charge transfer peak and the low energy peak both broaden and disperse through the Brillouin zone similar to what is seen in experiments in materials like Ca_2 Cu O_2 Cl_2.Comment: 5 pages Revtex4, 6 figure

Developing and testing the density of states FFA method in the SU(3) spin model

The Density of States Functional Fit Approach (DoS FFA) is a recently proposed modern density of states technique suitable for calculations in lattice field theories with a complex action problem. In this article we present an exploratory implementation of DoS FFA for the SU(3) spin system at finite chemical potential $\mu$ - an effective theory for the Polyakov loop. This model has a complex action problem similar to the one of QCD but also allows for a dual simulation in terms of worldlines where the complex action problem is solved. Thus we can compare the DoS FFA results to the reference data from the dual simulation and assess the performance of the new approach. We find that the method reproduces the observables from the dual simulation for a large range of $\mu$ values, including also phase transitions, illustrating that DoS FFA is an interesting approach for exploring phase diagrams of lattice field theories with a complex action problem.Comment: Plot, reference and comments added. Final version to appear in Nucl. Phys.

Approaches to the sign problem in lattice field theory

Quantum field theories (QFTs) at finite densities of matter generically involve complex actions. Standard Monte-Carlo simulations based upon importance sampling, which have been producing quantitative first principle results in particle physics for almost fourty years, cannot be applied in this case. Various strategies to overcome this so-called Sign Problem or Complex Action Problem were proposed during the last thirty years. We here review the sign problem in lattice field theories, focussing on two more recent methods: Dualization to world-line type of representations and the density-of-states approach.Comment: mini-review, 20 pages, 4 figure

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

Selected Papers from “Theory of Hadronic Matter under Extreme Conditions”

The book is devoted to the discussion of modern aspects of the theory of hadronic matter under extreme conditions. It consists of 12 selected contributions to the second international workshop on this topic held in fall 2019 at JINR Dubna, Russia. Of particular value are the contributions to lattice gauge theory studies attacking the problem of simulating QCD at finite baryon densities, one of the major challenges at the present time in this field. Another unique aspect is provided by the discussion of puzzling effects that appear in the poduction of hadrons in nuclear collisions, like the horn in the K+/pi+ ratio, which are subject to hydrodynamic and reaction-kinetic modeling of these nonequilibrium phenomena