arXiv subjects

Reinhold Kaiser

Publications and source records attributed to Reinhold Kaiser.

8 recordsLinked to original sources

arXiv2026

Testing machine-learned distributions against Monte Carlo data for the QCD chiral phase transition

We demonstrate that conditional Masked Autoregressive Flows constitute a flexible interpolation tool for lattice QCD observables, conditioned on bare lattice parameters. As a benchmark, we use the chiral phase structure of QCD with five degenerate light quark flavours, which on coarse lattices exhibits a region of first-order chiral transitions terminating in a critical quark mass. The method successfully reproduces standard reweighting in the gauge coupling, and naturally extends to interpolation in quark mass and spatial volume, for which reweighting is computationally prohibitive or inapplicable, respectively. Once trained, the model generates samples across the full parameter space in minutes, which can be used to obtain consistent first estimates of the critical quark mass without simulating all intermediate parameter values. This offers a concrete reduction in the number of lattice ensembles required. Precision on the critical mass from learned distributions is so far prohibited by the mode-covering effect inherent to maximum-likelihood-based training, which introduces a systematic bias near first-order transitions. At the current stage, the method is well-suited for a range of practical applications: localising phase boundaries, identifying the universal scaling axes at a critical point, and accelerating informed determinations of parameter values ahead of high-precision Monte Carlo campaigns.

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arXiv2026

On the phase structure of massless many-flavour QCD with staggered fermions

When the number of massless fermions exceeds a critical value $N_f^*$, QCD enters the conformal window and becomes chirally symmetric already in the vacuum. Determining $N_f^*$ from lattice simulations is challenging, since calculations are performed at finite lattice spacing, quark mass, and temporal lattice size, where both a thermal transition and an unphysical bulk transition obscure the conformal behaviour. In this work, we present results on the chiral phase boundaries in the bare lattice parameter space $(N_\tau,\;\beta,\;am,\;N_f)$ of unimproved staggered fermions. Our analysis indicates that the chiral transition in continuum QCD is of second order for all $N_f$ up to the onset of the conformal window. By systematically studying the thermal chiral transition and its interplay with the bulk transition, we obtain a coherent picture of the lattice phase structure and suggest how the onset of the conformal window can be identified from simulations performed away from the chiral and continuum limits.

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arXiv2025

On the nature of the QCD chiral phase transition with imaginary chemical potential

The order of the thermal chiral phase transition in lattice QCD is known to be strongly cutoff-dependent. A previous study using $N_\mathrm{f}\in[2,6]$ mass-degenerate, unimproved staggered quark flavours on $N_\tau\in\{4,6,8\}$ lattices found that the bare mass regions displaying explicit first-order transitions shrink to zero, with their critical boundary line terminating in a tricritical point before the continuum limit is reached. Here we perform an analogous study for fixed imaginary baryon chemical potential and find the same behaviour: first-order regions observed on coarse lattices disappear in tricritical points with diminishing lattice spacing. These observations are consistent with currently available results from improved staggered discretisations, both at zero and non-zero imaginary chemical potential. Unless additional first-order transitions are found on finer lattices or with chiral lattice actions, this implies a second-order transition in the continuum chiral limit for all these cases, at zero and imaginary chemical potential. Implications for the $N_\mathrm{f}=2+1$ QCD phase diagram at the physical point are discussed.

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arXiv2025

The order of the chiral phase transition in massless many-flavour lattice QCD

The nature of the QCD phase transition in the chiral limit presents a challenging problem for lattice QCD. However, its study provides constraints on the phase diagram at the physical point. In this work, we investigate how the order of the chiral phase transition depends on the number of light quark flavours. To approach the lattice chiral limit, we map out and extrapolate the chiral critical surface that separates the first-order region from the crossover region in an extended parameter space, which includes the gauge coupling, the number of quark flavours, their masses, and the lattice spacing. Lattice simulations with standard staggered quarks reveal that for each $N_f < 8$, there exists a tricritical lattice spacing $a^\text{tric}(N_f)$, at which the chiral transition changes from first order ($a>a^\text{tric}$) to second order ($a<a^\text{tric}$). Thus, the first-order region is merely a lattice artifact and not connected to the continuum. By determining the associated temperatures $T(N_f^\text{tric},a ^\text{tric})$ at these tricritical points, we confirm the expected decrease in the critical temperature as the number of flavours increases. The obtained temperatures define a tricritical line which is connected to the continuum and terminates at a physical $ N_f^\text{tric}(a=0) $. Our data is compatible with a vanishing temperature at that point, $T(N_f^\text{tric}(a=0))=0 $.

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arXiv2023

Approaching the Continuum Limit of the Deconfinement Critical Point for $N_\text{f}=2$ Staggered Fermions

Quenched QCD at zero baryonic chemical potential undergoes a first-order deconfinement phase transition at a critical temperature $T_c$, which is related to the spontaneous breaking of the global center symmetry. The center symmetry is broken explicitly by including dynamical quarks, which weaken the first-order phase transition for decreasing quark masses. At a certain critical quark mass, which corresponds to the $Z(2)$-critical point, the first-order phase transition turns into a smooth crossover. We investigate the $Z(2)$-critical quark mass for $N_\text{f}=2$ staggered fermions on $N_τ=8, 10$ lattices, where larger $N_τ$ correspond to finer lattices. Monte-Carlo simulations are performed for several quark mass values and aspect ratios in order to extrapolate to the thermodynamic limit. We present final results for $N_τ=8$ and preliminary results for $N_τ=10$ for the critical mass, which are obtained from fitting to a kurtosis finite size scaling formula of the absolute value of the Polyakov loop.

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arXiv2023

Progress on the QCD deconfinement critical point for $N_\text{f}=2$ staggered fermions

The global center symmetry of quenched QCD at zero baryonic chemical potential is broken spontaneously at a critical temperature $T_c$ leading to a first-order phase transition. Including heavy dynamical quarks breaks the center symmetry explicitly and weakens the first-order phase transition for decreasing quark masses until it turns into a smooth crossover at a $Z(2)$-critical point. We investigate the $Z(2)$-critical quark mass value towards the continuum limit for $N_\text{f}=2$ flavors using lattice QCD in the staggered formulation. As part of a continued study, we present results from Monte-Carlo simulations on $N_τ=8,10$ lattices. Several aspect ratios and quark mass values were simulated in order to obtain the critical mass from a fit of the Polyakov loop to a kurtosis finite size scaling formula. Moreover, the possibility to develop a Ginzburg-Landau effective theory around the $Z(2)$-critical point is explored.

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arXiv2022

The chiral phase transition at non-zero imaginary baryon chemical potential for different numbers of quark flavours

The so-called Columbia plot summarises the order of the QCD thermal transition as a function of the number of quark flavours and their masses. Recently, it was demonstrated that the first-order chiral transition region, as seen for $N_f \in [ 3,6 ]$ on coarse lattices, exhibits tricritical scaling while extrapolating to zero on sufficiently fine lattices. Here we extend these studies to imaginary baryon chemical potential. A similar shrinking of the first-order region is observed with decreasing lattice spacing, which again appears compatible with a tricritical extrapolation to zero.

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arXiv2021

The QCD Deconfinement Critical Point for $N_\text{f}=2$ Flavors of Staggered Fermions

Quenched QCD at zero baryonic chemical potential undergoes a first-order deconfinement phase transition at a critical temperature $T_c$, which is related to the spontaneous breaking of the global center symmetry. Including heavy, dynamical quarks breaks the center symmetry explicitly and weakens the first-order phase transition. For decreasing quark masses the first-order phase transition turns into a smooth crossover at a $Z_2$-critical point. The critical quark mass corresponding to this point has been examined with $N_\text{f} = 2$ Wilson fermions for several $N_τ$ in a recent study within our group. For comparison, we also locate the critical point with $N_\text{f} = 2$ staggered fermions on $N_τ= 8$ lattices. For this purpose we perform Monte Carlo simulations for several quark mass values and various aspect ratios in order to extrapolate to the thermodynamic limit. The critical mass is obtained by fitting to a finite size scaling formula of the kurtosis of the Polyakov loop. Our results indicate large discretization effects, requiring simulations on lattices with $N_τ> 8$.

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