arXiv subjects

B. B. Brandt

Publications and source records attributed to B. B. Brandt.

16 recordsLinked to original sources

arXiv2026

Testing strong isospin breaking effects in QCD thermodynamics

Most modern lattice QCD calculations take the masses of the light quarks to be degenerate, setting $m_u=m_d$, a choice that, while not physical, increases computational efficiency and lowers the complexity of simulations and analysis. For many observables, the systematic error associated with this simplification is expected to be negligible, as $Δm_{ud}=m_d-m_u$ is significantly below the QCD scale. So far lattice studies have examined the role of strong isospin breaking (SIB) in hadron mass splittings, but SIB effects in thermodynamic observables have only been studied in terms of the crossover temperature with unphysically heavy quarks. Motivated by the recent observation of an unexpectedly large ratio between the abundances of charged versus neutral kaons in heavy-ion collision experiments, we investigate the effects of SIB in QCD thermodynamics by computing observables for three non-degenerate flavors ($N_f=1+1+1$) with physical quark masses using dynamical improved staggered fermions. In particular, we investigate the difference between the $u$- and $d$-quark condensates and extract the pseudocritical temperature, the equation of state, and conserved-charge fluctuations, comparing results to standard $N_f=2+1$ simulations.

hep-lat↗

arXiv2026

Equation of state of dense magnetized QCD matter via a robust analytic continuation

We compute the equation of state of dense and magnetized strongly interacting matter using lattice QCD at a non-zero value of the lattice spacing. Our simulations employ $2+1+1$ dynamical flavors of rooted staggered quarks with masses tuned to the physical point. To conform with experimental constraints, we impose strangeness neutrality and isospin asymmetry on our observables. We circumvent the sign problem by simulating imaginary values of the chemical potential and applying the $T'$-expansion scheme to carry out the analytic continuation of our observables to the real chemical potential axis. This novel scheme, adapted to the setup with nonzero magnetic fields, is demonstrated to produce significantly smaller systematic errors in the final observables, compared to the standard analytical continuation at fixed temperature, allowing for better-controlled extrapolations. Our results for the pressure, the entropy density and the interaction measure are tabulated for direct use in heavy-ion phenomenology applications.

hep-lat↗

arXiv2026

Spectral reconstruction techniques, their shortcomings and relevance to the electric conductivity coefficient

Spectral reconstruction is a well studied numerically ill-posed problem which arises due to the relation of the Euclidean correlator to the spectral function via an inhomogeneous Fredholm equation of the first kind. Several different methods are on the market to resolve this issue, each taking different approaches and assumptions. In this proceedings we focus on implementing and testing a machine learning framework for spectral reconstruction, as well as implementing a novel method of estimating the behavior of the spectral function in the vicinity of vanishing frequency, which we denote as multipoint method, and compare these methods to well established spectral reconstruction techniques from the literature using mock data. As a physics application, we apply the reconstruction techniques to quenched lattice data for the correlation function in the vector channel at non-zero external magnetic field to extract the spectral function and the electric conductivity through its behaviour at vanishing frequency via a Kubo formula.

hep-lat↗

arXiv2025

Localized Chiral Magnetic Effect in equilibrium QCD

We study the impact of a non-uniform magnetic background field on the Chiral Magnetic Effect (CME) in equilibrium QCD using lattice simulations with 2+1 flavors of dynamical staggered quarks at the physical point. We show that in the presence of a non-uniform magnetic field the CME manifests itself via a localized electromagnetic current density along the direction of the field, which integrates to zero over the full volume. Our primary observable is the leading-order coefficient of the current in a chiral chemical potential expansion, which we compute for various lattice spacings and extrapolate to the continuum limit. Our findings demonstrate that, even though the global spatial average of the CME conductivity vanishes in equilibrium, steady currents still exist locally. Thus, spatially modulated magnetic fields provide a possible way of generating a non-trivial CME signal in equilibrium.

hep-lat↗

arXiv2025

Out-of-equilibrium Chiral Magnetic Effect via Kubo formulas

In this proceedings article, we present the first steps towards the determination of the out-of-equilibrium conductivity of the Chiral Magnetic Effect (CME) in the presence of strong interactions. Using linear response theory, we obtain an analytical expression for the spectral function associated with this effect at one-loop in perturbation theory. In addition, we provide a first estimate of the CME conductivity by calculating the associated Euclidean correlator using quenched Wilson fermions and dynamical staggered fermions in physical Quantum Chromodynamics (QCD) simulations. In particular, we focus on the midpoint of the correlator, which can be used as a proxy of the full conductivity. We present results in a wide range of temperatures, showing how this observable is suppressed at low temperatures, while at high temperatures it approaches the perturbation theory prediction.

hep-lat↗

arXiv2025

Impact of extreme magnetic fields on the QCD topological susceptibility in the vicinity of the crossover region

We present the first determination of the topological susceptibility from lattice QCD in the presence of strong background magnetic fields. Our simulations employ 2+1 flavours of stout improved staggered quarks with physical masses and cover a broad range of temperatures and magnetic field values. The results are extrapolated to the continuum limit using four different lattice spacings and an eigenvalue reweighting technique to reduce discretisation errors. For low temperatures, our calculations show an enhancement of the topological susceptibility due to the magnetic field, compatible with predictions from chiral perturbation theory. At high temperatures, we observe the impact of inverse magnetic catalysis on the susceptibility.

hep-lat↗

arXiv2024

Steady electric currents in magnetized QCD and their use for the equation of state

In this paper we study the emergence of steady electric currents in QCD as a response to a non-uniform magnetic background using lattice simulations with 2 + 1 quark flavors at the physical point, as well as leading-order chiral perturbation theory. Using these currents, we develop a novel method to determine the leading-order coefficient of the equation of state in a magnetic field expansion: the magnetic susceptibility of the QCD medium. We decompose the current expectation value into valence- and sea-quark contributions and demonstrate that the dominant contribution to the electric current is captured by the valence term alone, allowing for a comparably cheap determination of the susceptibility. Our continuum extrapolated lattice results for the equation of state confirm the findings of some of the existing studies in the literature, namely that the QCD medium behaves diamagnetically at low and paramagnetically at high temperatures.

hep-lat↗

arXiv2024

QCD equation of state in the presence of magnetic fields at low density

Peripheral heavy-ion collisions are expected to exhibit magnetic fields with magnitudes comparable to the QCD scale, as well as non-zero baryon densities. Whereas QCD at finite magnetic fields can be simulated directly with standard lattice algorithms, the implementation of real chemical potentials is hindered by the infamous sign problem. Aiming to shed light on the QCD transition and on the equation of state in that regime, we carry out lattice QCD simulations with 2+1+1 flavors of staggered quarks with physical masses at finite magnetic fields and employ a Taylor expansion scheme to circumvent the sign problem. We present the leading-order coefficient of the expansion calculated at non-zero magnetic fields and discuss the impact of the field on the strangeness neutrality condition.

hep-lat↗

arXiv2024

Thermal QCD in a non-uniform magnetic background

Off-central heavy-ion collisions are known to feature magnetic fields with magnitudes and characteristic gradients corresponding to the scale of the strong interactions. In this work, we employ equilibrium lattice simulations of the underlying theory, QCD, involving similar inhomogeneous magnetic field profiles to achieve a better understanding of this system. We simulate three flavors of dynamical staggered quarks with physical masses at a range of magnetic fields and temperatures, and extrapolate the results to the continuum limit. Analyzing the impact of the field on the quark condensate and the Polyakov loop, we find non-trivial spatial features that render the QCD medium qualitatively different as in the homogeneous setup, especially at temperatures around the transition. In addition, we construct leading-order chiral perturbation theory for the inhomogeneous background and compare its prediction to our lattice results at low temperature. Our findings will be useful to benchmark effective theories and low-energy models of QCD for a better description of peripheral heavy-ion collisions.

hep-lat↗

arXiv2021

Lattice QCD with an inhomogeneous magnetic field background

The magnetic fields generated in non-central heavy-ion collisions are among the strongest fields produced in the Universe, reaching magnitudes comparable to the scale of the strong interactions. Backed by model simulations, the resulting field is expected to be spatially modulated, deviating significantly from the commonly considered uniform profile. To improve our understanding of the physics of quarks and gluons under such extreme conditions, we use lattice QCD simulations with $2+1$ staggered fermion flavors with physical quark masses and an inhomogeneous magnetic background for a range of temperatures covering the QCD phase transition. We assume a $1/\cosh^2$ function to model the field profile and vary its strength to analyze the impact on the computed observables and on the transition. We calculate local chiral condensates, local Polyakov loops and estimate the size of lattice artifacts. We find that both observables show non-trivial spatial features due to the interplay between the sea and the valence effects.

hep-lat↗

arXiv2019

Dirac spectrum and the BEC-BCS crossover in QCD at nonzero isospin asymmetry

For large isospin asymmetries, perturbation theory predicts the QCD ground state to be a superfluid phase of $u$ and $\bar{d}$ Cooper pairs. This phase, which is denoted as the BCS phase, is expected to be smoothly connected to the standard phase with Bose-Einstein condensation (BEC) of charged pions at $μ_I\ge m_π/2$ by an analytic crossover. A first hint for the existence of the BCS phase, which is likely characterised by the presence of both, deconfinement and charged pion condensation, is coming from the lattice observation that the deconfinement crossover smoothly penetrates into the BEC phase. To further scrutinize the existence of the BCS phase, in this proceedings article we investigate the complex spectrum of the massive Dirac operator in 2+1-flavor QCD at nonzero temperature and isospin chemical potential. The spectral density near the origin is related to the BCS gap via a generalization of the Banks-Casher relation to the case of complex Dirac eigenvalues (derived for the zero-temperature, high-density limits of QCD at nonzero isospin chemical potential).

hep-lat↗

arXiv2018

QCD phase diagram for nonzero isospin-asymmetry

The QCD phase diagram is studied in the presence of an isospin asymmetry using continuum extrapolated staggered quarks with physical masses. In particular, we investigate the phase boundary between the normal and the pion condensation phases and the chiral/deconfinement transition. The simulations are performed with a small explicit breaking parameter in order to avoid the accumulation of zero modes and thereby stabilize the algorithm. The limit of vanishing explicit breaking is obtained by means of an extrapolation, which is facilitated by a novel improvement program employing the singular value representation of the Dirac operator. Our findings indicate that no pion condensation takes place above $T\approx 160$ MeV and also suggest that the deconfinement crossover continuously connects to the BEC-BCS crossover at high isospin asymmetries. The results may be directly compared to effective theories and model approaches to QCD.

hep-lat↗

arXiv2014

Vector screening masses in the quark-gluon plasma and their physical significance

Static and non-static thermal screening states that couple to the conserved vector current are investigated in the high-temperature phase of QCD. Their masses and couplings to the current are determined at weak coupling, as well as using two-flavor lattice QCD simulations. A consistent picture emerges from the comparison, providing evidence that non-static Matsubara modes can indeed be treated perturbatively. We elaborate on the physical significance of the screening masses.

hep-lat↗

arXiv2014

A relation between screening masses and real-time rates

Thermal screening masses related to the conserved vector current are determined for the case that the current carries a non-zero Matsubara frequency, both in a weak-coupling approach and through lattice QCD. We point out that such screening masses are sensitive to the same infrared physics as light-cone real-time rates. In particular, on the perturbative side, the inhomogeneous Schrodinger equation determining screening correlators is shown to have the same general form as the equation implementing LPM resummation for the soft-dilepton and photon production rates from a hot QCD plasma. The static potential appearing in the equation is identical to that whose soft part has been determined up to NLO and on the lattice in the context of jet quenching. Numerical results based on this potential suggest that screening masses overshoot the free results (multiples of 2piT) more strongly than at zero Matsubara frequency. Four-dimensional lattice simulations in two-flavour QCD at temperatures of 250 and 340 MeV confirm the non-static screening masses at the 10% level. Overall our results lend support to studies of jet quenching based on the same potential at T > 250 MeV.

hep-ph↗

arXiv2011

Form factors in lattice QCD

Lattice simulations of QCD have produced precise estimates for the masses of the lowest-lying hadrons which show excellent agreement with experiment. By contrast, lattice results for the vector and axial vector form factors of the nucleon show significant deviations from their experimental determination. We present results from our ongoing project to compute a variety of form factors with control over all systematic uncertainties. In the case of the pion electromagnetic form factor we employ partially twisted boundary conditions to extract the pion charge radius directly from the linear slope of the form factor near vanishing momentum transfer. In the nucleon sector we focus specifically on the possible contamination from contributions of higher excited states. We argue that summed correlation functions offer the possibility of eliminating this source of systematic error. As an illustration of the method we discuss our results for the axial charge, gA, of the nucleon.

hep-lat↗

arXiv2010

Wilson fermions at fine lattice spacings: scale setting, pion form factors and (g-2)_mu

We present an update on our on-going project to compute hadronic observables for Nf=2 flavours of O(a) improved Wilson fermions at small lattice spacings. The procedure to determine the lattice scale via the mass of the Omega baryon is described. Furthermore we present preliminary results for the pion form factor computed using twisted boundary conditions, and report on the implementation of a novel approach to determine the contribution of the hadronic vacuum polarisation to the anomalous magnetic moment of the muon.

hep-lat↗