arXiv subjects

Niklas Götz

Publications and source records attributed to Niklas Götz.

9 recordsLinked to original sources

arXiv2026

3D Initial-State Dynamics across scales: A Comparative Study of saturation and string-based descriptions

We compare the longitudinal deposition of various conserved quantities in the initial condition models of a string based (SMASH) and a saturation based (McDipper) approach. SMASH has been shown to work reasonably well at lower collision energies as an initial condition for the SMASH-vHLLE hybrid approach, while McDipper, based on the color-glass-condensate (CGC), works well in the regime of perturbative QCD. The two models are capable of providing longitudinally resolved initial conditions, which is essential for 3D hydrodynamical simulations. The goal of this study is to interface the different regions of applicability of the two models, to investigate the initial state dynamics in the intermediate energy regime. We analyze the deposition of transverse energy, charge and baryon number across a large range of collision energies (62.4 GeV to 5.02 TeV) and find that, while they are good agreement at lower energies, their energy and baryon deposition differs substantially at higher center of mass energies.

hep-ph↗

arXiv2026

Collective effects in O-O and Ne-Ne collisions at $\sqrt{s_{\mathrm{NN}}}$=5.36 TeV from a hybrid approach

Many features of heavy-ion collisions are well described by hybrid approaches, where the droplet of strongly coupled quark gluon plasma (QGP) is modeled by hydrodynamics and the subsequent dilute stage is performed with a hadronic transport model. Conventionally, the formation of a QGP is well established in larger collision systems like lead and gold. However, hints of collectivity were found even in proton-proton collisions, raising the question where the onset of QGP formation lays. This study aims at making predictions for the light-ions run at the CERN Large Hadron Collider in July 2025, in order to explore the applicability of hybrid approaches in smaller collision systems. We employ three different models: the SMASH-vHLLE hybrid approach, the pure hadronic cascade of SMASH, and Angantyr to simulate O-O collisions at a center-of-mass energy of $\sqrt{s_{\mathrm{NN}}}$=5.36 TeV. This setup allows us to compare evolutions with and without a hydrodynamic description on an equal basis, while Angantyr serves as a baseline for no collective effects.

nucl-th↗

arXiv2026

SPARKX: A Software Package for Analyzing Relativistic Kinematics in Collision Experiments

SPARKX is an open-source Python package developed to analyze simulation data from heavy-ion collision experiments. By offering a comprehensive suite of tools, SPARKX simplifies data analysis workflows, supports multiple formats such as OSCAR2013, and integrates seamlessly with SMASH and JETSCAPE/X-SCAPE. This paper describes SPARKX's architecture, features, and applications and demonstrates its effectiveness through detailed examples and performance benchmarks. SPARKX enhances productivity and precision in relativistic kinematics studies.

physics.data-an↗

arXiv2025

Bayesian analysis of a (3+1)D hybrid approach with initial conditions from hadronic transport

This study aims to apply statistical learning, specifically Bayesian inference, to the (3+1)D SMASH-vHLLE-hybrid model using initial conditions generated by the SMASH transport code itself, with the objective of constraining model parameters and gaining deeper insight on the temperature and baryochemical potential dependence of both the shear and the bulk viscosity. This study is performed in the hybrid approach SMASH-vHLLE, composed of the hadronic transport approach SMASH and the (3+1)D viscous hydrodynamic code vHLLE. A Bayesian framework is employed, utilizing Markov Chain Monte Carlo (MCMC) sampling to explore the parameter space. The analysis compares model predictions against experimental observables, including particle yields, momentum and flow coefficients both at midrapidity as well as in forward and backward direction. We find that the SMASH-vHLLE-hybrid framework, using hadronic initial conditions for Au+Au collisions at different beam energies, can reproduce a variety of experimental observables at midrapidity and forward/backward rapidities. Notably, the preferred posterior distribution suggests a near-vanishing specific shear viscosity in the high-temperature QGP phase, combined with moderate-to-large bulk viscosity around the phase transition region, although the constraints on baryochemical potential dependence are weak. Our findings reveal that a hadronic initial condition constrains the evolution more strictly at intermediate energies, making parameters such as the hydrodynamic onset time highly sensitive. Intriguingly, the extracted shear viscosity differs substantially from previous Bayesian analyses, motivating further systematic studies with higher-statistics data sets and refined modeling assumptions.

nucl-th↗

arXiv2024

Accelerating HEP simulations with Neural Importance Sampling

Many high-energy-physics (HEP) simulations for the LHC rely on Monte Carlo using importance sampling by means of the VEGAS algorithm. However, complex high-precision calculations have become a challenge for the standard toolbox, as this approach suffers from poor performance in complex cases. As a result, there has been keen interest in HEP for modern machine learning to power adaptive sampling. While previous studies have shown the potential of normalizing-flow-powered neural importance sampling (NIS) over VEGAS, there remains a gap in accessible tools tailored for non-experts. In response, we introduce ZüNIS, a fully automated NIS library designed to bridge this divide, while at the same time providing the infrastructure to customise the algorithm for dealing with challenging tasks. After a general introduction on NIS, we first show how to extend the original formulation of NIS to reuse samples over multiple gradient steps while guaranteeing a stable training, yielding a significant improvement for slow functions. Next, we introduce the structure of the library, which can be used by non-experts with minimal effort and is extensivly documented, which is crucial to become a mature tool for the wider HEP public. We present systematic benchmark results on both toy and physics examples, and stress the benefit of providing different survey strategies, which allows higher performance in challenging cases. We show that ZüNIS shows high performance on a range of problems with limited fine-tuning.

hep-ph↗

arXiv2023

Role of initial transverse momentum in a hybrid approach

The purpose of this work is to study the effect of exchanging initial condition models in a modular hybrid approach. The focus lies on the event-by-event correlations of elliptic and triangular flow. This study is performed in the hybrid approach SMASH-vHLLE, composed of the hadronic transport approach SMASH and the (3+1)d viscous hydrodynamic code vHLLE. The initial condition models investigated are SMASH IC, Trento and IP-Glasma. Correlations are calculated on an event-by-event basis between the eccentricities and momentum anisotropies of the initial state as well as the momentum anisotropies in the final state, both for ultra-central and off-central collisions for AuAu collisions at $\sqrt{s_{NN}} = 200$ GeV. This work demonstrates that, although averaged values for the eccentricities of these models are very similar, substantial differences exist both in the distributions of eccentricities, the correlations amongst the initial state properties as well as in the correlations between initial state and final state properties. Notably, whereas initial state momentum anisotropy is shown to not affect the final state flow, the presence of radial flow affects the emergence of final state momentum anisotropies. Inclusion of radial flow in the linear fit improves the prediction of final state flow from initial state properties. The presence of momentum in the initial state has an effect on the emergence of flow and is therefore a relevant part of initial state models, challenging the common understanding of final state momentum anisotropies being a linear response to initial state eccentricity only.

nucl-th↗

arXiv2023

Momentum anisotropy generation in a hybrid approach

Anisotropic flow emerges in all three of hybrid approaches: initial conditions, viscous relativistic hydrodynamics as well as hadronic transport. Previous works focus mainly on a constant or temperature dependent shear viscosity $η/s$. Here instead, we study qualitatively the effect of a generalized $η/s(T,μ_B)$ in the hybrid approach SMASH-vHLLE-hybrid. The parameterization takes into account the constraints of matching to the transport coefficients in the hadronic phase, as well as of recent Bayesian analysis results. We compare the effect of the different parameterizations in the intermediate energy region of $\sqrt{s_{NN}}$=7.7 - 39.0 GeV. We observe that using the energy density dependent parameterization decreases the effect of the point of particlization. In addition, we quantify the uncertainty due to different initial state profiles, including the SMASH initial conditions as well as TrENTo and IP-Glasma profiles. It can be shown that the initial state transverse momentum impacts final state momentum anisotropy.

hep-ph↗

arXiv2023

Photon momentum anisotropies from the late stages of relativistic heavy-ion collisions

The photon emission from the late stages of the dynamical evolution of heavy-ion reactions at the highest RHIC and LHC energies is investigated. A comparison between a calculation from hadronic rates from a fluid dynamic evolution down to temperatures of 120 MeV and a full non-equilibrium hadronic transport approach is performed. The photon yields are very similar in both cases while the elliptic flow is slightly smaller in the non-equilibrium scenario. This study is important, since it is crucial to apply the same dynamical evolution model for hadronic and electromagnetic observables.

nucl-th↗

arXiv2022

Temperature and net baryochemical potential dependence of $η/s$ in a hybrid approach

In this work, the qualitative impact of the net baryochemical potential dependence of the shear viscosity to entropy density ratio $η/s$ in hydrodynamical simulations is studied. The effect of a predicted non-constant $η/s$($μ_B$) is largely unexplored in hydrodynamic simulations. Previous studies focus only on a temperature dependence or even only a constant effective shear viscosity. This work addresses this issue by studying qualitatively the effect of a generalized $η/s$($T,μ_B$) in the hybrid approach SMASH-vHLLE, composed of the hadronic transport approach SMASH and the (3+1)d viscous hydrodynamic code vHLLE. In order to reduce the bias of the result on the equation of state used in the hydrodynamic part of the model, $η/s$ is parameterized directly in the energy density and net baryon number density. The parameterization takes into account the constraints of matching to the transport coefficients in the hadronic phase, as well as pQCD results. This work compares the impact of the density dependence for different system sizes and energies and compares the observables with experimental results in the RHIC - BES region $\sqrt{s_{NN}}$ =7.7 - 39.0 GeV, as the effect of this generalization is especially relevant for intermediate collision energies, for which the system is in equilibrium for a relevant amount of time, but the net baryochemical potential does not vanish. It is shown that the effect of an explicit net baryon number dependence on the elliptic flow is negligible and only relevant in the early stages of the collision. Additionally, we find that the proposed parameterization could be a good proxy for the shear viscosity in the non-equilibrium hadronic transport stage.

hep-ph↗