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Jens Braun

Publications and source records attributed to Jens Braun.

At least 19 recordsLinked to original sources
arXiv2026

Analysis of regulator and cutoff artifacts in the phase diagram of the quark-meson model

We study regulator and cutoff artifacts in the quark-meson model at finite temperature and quark chemical potential within the functional renormalization-group approach using the local potential approximation. To this end, we discuss the concept of renormalization-group consistency in effective models, which necessitates a nontrivial parameter-fixing procedure to enable a meaningful comparison of results obtained with different regulators and cutoffs. We employ a standard range of cutoff values used in phenomenological studies and regulators that differ significantly in their analytic properties as well as in their classification according to the principle of strongest singularity. We find that regulator and cutoff dependences are small at low temperatures and quark chemical potentials. At high temperatures and low quark chemical potentials, significant cutoff artifacts arise, whereas the properties of the regulator affect the dynamics in the regime governed by a chiral phase transition of first order at low temperatures and high quark chemical potentials.

hep-ph↗
arXiv2026

How to Trust Learned Loop Amplitudes

Higher-order theory predictions are crucial for the precision LHC program, but the time-consuming amplitude evaluation challenges the corresponding Monte-Carlo simulations. Machine-learned amplitude surrogates can resolve this problem, if we can guarantee their precision over the entire phase space. First, we show that our surrogates provide a calibrated learned uncertainty, even for non-Gaussian systematics; second, we describe how less accurate phase space regions can be identified; third, we demonstrate how the precision in these regions can be improved reliably.

hep-ph↗
arXiv2025

State-of-the-art electroweak Higgs boson pair production in association with two jets at the LHC in the Standard Model and beyond

We present a systematic comparison of two state-of-the-art tools for the simulation of Higgs boson pair production via vector boson fusion (VBF) as implemented in the Monte-Carlo tools GoSam+Whizard and the POWHEG-BOX. Cross sections and distributions are provided within the Standard Model and beyond, within scenarios typical for experimental physics analyses, and for a range of energies of relevance to the LHC and its upcoming high luminosity phase. We further perform a detailed study of the so-called VBF approximation, in particular in the presence of anomalous Higgs boson couplings.

hep-ph↗
arXiv2025

Nonperturbative fluctuation effects of charged bosonic fields: A quark-diquark model study at nonzero density

We study the renormalization group flow of the scale-dependent effective potential of a quark-diquark model with full field dependence at nonzero chemical potential. This includes a discussion of approximations in relation to complex bosonic fields and the Silver-Blaze property. The resulting flow equation for the scale-dependent effective potential can in principle be solved down to the infrared limit. For our quark-diquark model, which may serve as a low-energy model for dense strong-interaction matter, we find that a competition between the Bardeen-Cooper-Schrieffer singularity and bosonic fluctuations can trigger a first-order phase transition at low temperatures that turns into a second-order phase transition at a tricritical point as the temperature increases.

hep-ph↗
arXiv2025

Do large QCD corrections to di-Higgs decay survive parton showering? A study of $HH\to b\bar{b}γγ$

While significant effort has been devoted to precision calculations of the production of two Higgs bosons via gluon fusion, the treatment of their decays in this process has only recently begun to attract attention. It has been found that fixed-order QCD corrections to fiducial di-Higgs decay rates involving the $b\bar{b}$ decay channel can be substantial. Considering $HH\to b\bar{b}γγ$, we show that such corrections arise predominantly from sensitivity to soft and collinear QCD radiation at fixed order, and that they are largely washed out once parton showers are included.

hep-ph↗
arXiv2025

Double Higgs Production in Vector Boson Fusion at NLO QCD in HEFT

We present the next-to-leading order QCD corrections to Higgs boson pair production in vector boson fusion, including the leading operators in the framework of Higgs Effective Field Theory (HEFT). The corresponding calculation is based on an automated interface between the Monte Carlo event generator Whizard and the one-loop amplitude generator GoSam. The QCD corrections also include non-factorising diagrams and diagrams of Higgs-Strahlung type, thus going beyond the structure function approach. We find that some constellations of anomalous couplings, while being well within the current experimental constraints, can have a significant impact on the shape of typical observables for this process.

hep-ph↗
arXiv2025

Soft modes in hot QCD matter

The chiral crossover of QCD at finite temperature and vanishing baryon density turns into a second order phase transition if lighter than physical quark masses are considered. If this transition occurs sufficiently close to the physical point, its universal critical behaviour would largely control the physics of the QCD phase transition. We quantify the size of this region in QCD using functional approaches, both Dyson-Schwinger equations and the functional renormalisation group. The latter allows us to study both critical and non-critical effects on equal footing, facilitating a precise determination of the scaling regime. We find that the physical point is far away from the critical region. Importantly, we show that the physics of the chiral crossover is dominated by soft modes even far beyond the critical region. While scaling functions determine all thermodynamic properties of the system in the critical region, the order parameter potential is the relevant quantity away from it. We compute this potential in QCD using the functional renormalisation group and Dyson-Schwinger equations and provide a simple parametrisation for phenomenological applications.

hep-ph↗
arXiv2025

Color superconductivity under neutron-star conditions at next-to-leading order

The equation of state of deconfined strongly interacting matter at high densities remains an open question, with effects from quark pairing in the preferred color-flavor-locked (CFL) ground state possibly playing an important role. Recent studies suggest that at least large pairing gaps in the CFL phase are incompatible with current astrophysical observations of neutron stars. At the same time, it has recently been shown that in two-flavor quark matter, subleading corrections from pairing effects can be much larger than would be na\"ively expected, even for comparatively small gaps. In the present Letter, we compute next-to-leading-order corrections to the pressure of quark matter in the CFL phase arising from the gap and the strong coupling constant, incorporating neutron-star equilibrium conditions and current state-of-the-art perturbative QCD results. We find that the corrections are again quite sizable, and they allow us to constrain the CFL gap in the quark energy spectrum to $\Delta_{\rm CFL} \lesssim 140~{\rm MeV}$ at a baryon chemical potential $\mu_{\rm B} = 2.6~{\rm GeV}$, even when allowing for a wide range of possible behaviors for the dependence of the gap on the chemical potential.

hep-ph↗
arXiv2025

Lattice Monte Carlo meets lattice functional Renormalization Group: A quantitative comparison

Lattice Monte Carlo (MC) simulations and the functional Renormalization Group (RG) are powerful approaches that allow for quantitative studies of non-perturbative phenomena such as bound-state formation, spontaneous symmetry breaking and phase transitions. While results from both methods have recently shown remarkable agreement for many observables, e.g., in Quantum Chromodynamics, an analysis of deviations in certain quantities turns out to be challenging. This is because calculations with the two methods are based on different approximations, regularizations and scale fixing procedures. In the present work, we present a framework for a more direct comparison by formulating the functional RG approach on a finite spacetime lattice. This removes all ambiguities of regularization, finite size and scale fixing procedures in concrete studies. By investigating the emergence of spontaneous symmetry breaking and phase transitions in a $Z(2)$ scalar theory in $d=1,2,3$ spacetime dimensions, we demonstrate at the example of the local potential approximation how this framework can be used to evaluate and compare the systematic errors of both approaches.

hep-lat↗
arXiv2025

Juggling with Tensor Bases in Functional Approaches

Systematic expansion schemes in functional approaches require the inclusion of higher order vertices. These vertices are expanded in independent tensor bases with a rapidly increasing number of basis elements. Amongst the related tasks are the construction of bases and projection operators, the importance ordering of their elements, and the optimisation of such tensor bases, as well as an analysis of their regularity in momentum space. We present progress in all these directions and introduce the Mathematica package TensorBases designed for the aforementioned tasks.

hep-th↗
arXiv2025

Subtleties in the calculation of correlation functions for hot and dense systems

We discuss subtleties in the calculation of loop integrals in studies of hot and dense systems as they appear in both perturbative and non-perturbative approaches. To be specific, we address subtleties which appear in situations where the order of integration, differentiation, and limit processes plays a crucial role. For example, this applies to computations of the effective action and the computation of momentum-dependent correlation functions. In particular, the zero-temperature limit is delicate in systems with fermions because of the presence of discontinuities at the Fermi surface. We provide a general discussion of scenarios where the computation and evaluation of loop integrals in the context of relativistic theories requires particular attention as a change of the order of the involved mathematical operations may lead to a different result. Our general considerations are then illustrated with the aid of concrete examples, namely by the computation of masses from fully momentum-dependent correlation functions in the context of the Gross-Neveu-Yukawa model and quantum electrodynamics.

nucl-th↗
arXiv2024

Functional Renormalization Group meets Computational Fluid Dynamics: RG flows in a multi-dimensional field space

Within the Functional Renormalisation Group (FRG) approach, we present a fluid-dynamical approach to solving flow equations for models living in a multi-dimensional field space. To this end, the underlying exact flow equation of the effective potential is reformulated as a set of nonlinear advection-diffusion-type equations which can be solved using the Kurganov-Tadmor central scheme, a modern finite-volume discretization from computational fluid dynamics (CFD). We demonstrate the effectiveness of our approach by performing explicit benchmark tests using zero-dimensional models with two discretized field space directions or two symmetry invariants. Our techniques can be directly applied to flow equations of effective potentials of general (fermion-)boson systems with multiple invariants or condensates, as we also demonstrate for two concrete examples in three spacetime dimensions.

cond-mat.stat-mech↗
arXiv2024

Phase structure of quark matter and in-medium properties of mesons from Callan-Symanzik flows

We compute meson spectral functions at finite temperature and density in the quark-meson model, supplemented with a computation of the phase diagram. In particular, we provide a detailed analysis of the non-analytic structure of the meson two-point functions which is of great relevance for phenomenological applications, such as moat regimes and inhomogeneous phases. Furthermore, it is also relevant from a field-theoretical standpoint as it provides an insight into the applicability of derivative expansions of the effective action to studies of general fermion-boson models, both at zero and finite chemical potential. Our computation is based on a functional renormalization group setup that preserves causality, all spacetime symmetries, and the Silver-Blaze property. The combination of these properties can only be achieved by a Callan-Symanzik regulator. Instead of momentum shell integrations, renormalization group flows generated by such a regulator describe the change of the theory induced by a change of the masses of the mesons and quarks. A particular focus of our work lies on the construction of controlled Callan-Symanzik flows in the presence of spontaneous and explicit chiral symmetry breaking by means of chiral Ward-Takahashi identities.

hep-ph↗
arXiv2024

Optimization and Stabilization of Functional Renormalization Group Flows

We revisit optimization of functional renormalization group flows by analyzing regularized loop integrals. This leads us to a principle, the Principle of Strongest Singularity, and a corresponding order relation which allows to order existing regularization schemes with respect to the stability of renormalization group flows. Moreover, the order relation can be used to construct new regulators in a systematic fashion. For studies of critical behavior, which require to follow renormalization group flows down to the deep infrared regime, such new regulators may turn out to be particularly useful. The general application of this principle is demonstrated with the aid of a scalar field theory which is solved over a wide range of scales with novel methods borrowed from numerical fluid dynamics.

hep-ph↗
arXiv2024

Pressure and speed of sound in two-flavor color-superconducting quark matter at next-to-leading order

Deconfined quark matter at asymptotically high densities is weakly coupled, due to the asymptotic freedom of Quantum Chromodynamics. In this weak-coupling regime, bulk thermodynamic properties of quark matter, assuming a trivial ground state, are currently known to partial next-to-next-to-next-to-leading order. However, the ground state at high densities is expected to be a color superconductor, in which the excitation spectrum of (at least some) quarks exhibit a gap with a non-perturbative dependence on the strong coupling. In this work, we calculate the thermodynamic properties of color-superconducting quark matter at high densities and zero temperature at next-to-leading order (NLO) in the coupling in the presence of a finite gap. We work in the limit of two massless quark flavors, which corresponds to deconfined symmetric nuclear matter, and further assume that the gap is small compared to the quark chemical potential. In these limits, we find that the NLO corrections to the pressure and speed of sound are comparable in size to the leading-order effects of the gap, and further increase both quantities above their values for non-superconducting quark matter. We also provide a parameterization of the NLO speed of sound to guide phenomenology in the high-density region, and we furthermore comment on whether our findings should be expected to extend to the case of three-flavor quark matter of relevance to neutron stars.

hep-ph↗
arXiv2024

Speed of sound in dense strong-interaction matter

We study the speed of sound in strong-interaction matter at zero temperature and in density regimes which are expected to be governed by the presence of a color-superconducting gap. At (very) high densities, our analysis indicates that the speed of sound approaches its asymptotic value associated with the non-interacting quark gas from below, in agreement with first-principles studies which do not take the presence of a color-superconducting gap into account. Towards lower densities, however, the presence of a gap induces an increase of the speed of sound above its asymptotic value. Importantly, even if gap-induced corrections to the pressure may appear small, we find that derivatives of the gap with respect to the chemical potential can still be sizeable and lead to a qualitative change of the density dependence of the speed of sound. Taking into account constraints on the density dependence of the speed of sound at low densities, our general considerations suggest the existence of a maximum in the speed of sound. Interestingly, we also observe that specific properties of the gap can be related to characteristic properties of the speed of sound which are indirectly constrained by observations.

nucl-th↗
arXiv2023

A lattice pairing-field approach to ultracold Fermi gases

We develop a pairing-field formalism for ab initio studies of non-relativistic two-component fermions on a $(d\!+\!1)$-dimensional spacetime lattice. More specifically, we focus on theories where the interaction between the two components can be described by the exchange of a corresponding pairing field. The introduction of a pairing field may indeed be convenient for studies of, e.g., the finite-temperature phase structure and critical behavior of, e.g., ultracold atomic Fermi gases. Moreover, such a formalism allows to directly compute the momentum and frequency dependence of the pair propagator, from which the pair-correlation function can be extracted. For a first illustration of the application of our formalism, we compute the density equation of state and the superfluid order parameter for a gas of unpolarized fermions in $(0\!+\!1)$ dimensions by employing the complex Langevin approach to surmount the sign problem.

cond-mat.quant-gas↗