SEWM 24

Aug 25, 2024, 8:00 AM → Aug 30, 2024, 6:00 PM Europe/Berlin

HS 3 of Otto-Stern-Zentrum (Goethe-University, Frankfurt/Main Campus Riedberg )

Strong and Electro-Weak Matter 2024

26--30 August 2024

Institute for Theoretical Physics, Goethe-University, Frankfurt/Main Campus Riedberg

          

The focus of the conference is the theory of the Standard Model and beyond at finite temperature and density and in and out of equilibrium, including applications to cosmology, astrophysics and relativistic heavy-ion collision experiments. Among the topics to be discussed are:

• QCD in extreme conditions and dense nuclear matter
• Heavy-ion collisions and the Quark-Gluon Plasma
• Quantum fields in and out equilibrium and thermalisation
• Baryogenesis and leptogenesis
• Electroweak phase transition beyond the Standard Model
• Early Universe physics and sources for gravitational waves
• Quantum field dynamics and inflation
• Compact stars

 

The organisers can be contacted by email at SEWM24@itp.uni-frankfurt.de.

 

Previous editions of SEWM took place in:

• Saclay, France, 2022
• Saclay, France (online), 2021
• Barcelona, Spain, 2018
• Stavanger, Norway, 2016
• Lausanne, Switzerland, 2014
• Swansea, UK, 2012
• Montreal, Canada, 2010
• Amsterdam, the Netherlands, 2008
• Brookhaven, USA, 2006
• Helsinki, Finland, 2004
• Heidelberg, Germany, 2002
• Marseille, France, 2000
• Copenhagen, Denmark, 1998
• Eger, Hungary, 1997
• Sintra, Portugal, 1994

sewm.24.fun.pdf

sewm24.pdf

Sun, August 25

6:00 PM Reception and Registration

Mon, August 26

Registration

Introduction

Convener: Owe Philipsen (Goethe University Frankfurt)

sewm24 .pdf

Invited

Convener: Dirk Rischke (Goethe University)

1 Causal theories of fluid dynamics: from heavy-ion collisions to binary neutron star mergers

Speaker: Jorge Noronha

Noronha_SEWM_2024.pdf

2 The QCD phase structure and its signatures from functional approaches

Speaker: Fabian Rennecke (JLU Giessen)

SEWM2024_Rennecke.pdf

10:30 AM Break

Non Equilibrium Dynamics

Convener: Fabian Rennecke (JLU Giessen)

3 Bulk viscosity of dense quark matter from weak to strong coupling

The postmerger dynamics of binary neutron-star merger events is known to be sensitive not only to the equation of state of the constituent matter, but also to its transport properties. A quantity of particular importance here is the bulk viscosity, which arises in a subtle interplay between the strong and electroweak interactions. In this talk, I will review a recent determination of the bulk viscosity of dense unpaired quark matter, utilizing both perturbative and holographic tools. In particular, I will derive a simple analytic expression for the quantity that agrees with first-principles perturbative results at ultrahigh densities, but remains applicable in the astrophysically relevant regime, where QCD is strongly coupled.

Speaker: Aleksi Vuorinen (University of Helsinki)

SEWM_2024.pptx

4 Surface tension of neutron star matter

The phase transition from hadronic to quark matter may take place already during the early post-bounce stage of core collapse supernovae and in neutron star mergers. If the phase transition is of first order, the formation of the quark matter phase occurs via the nucleation of droplets. The timescales relevant for the phase conversion dynamics, as well as the possibility of mixed phases, are very sensitive to the value of the surface tension in this dense environment. We discuss the computation of the surface tension from the initial purely chiral models to a nucleon-meson model that describes nuclear matter in the low-density sector, with fully broken chiral symmetry, and the approximately chirally restored phase at high density within a unified effective potential. Finally, we incorporate parity doubling, which allows for stable static configurations of stars with a metastable matter core, enabling stars with masses higher than the expected minimum mass of a neutron star formed via core collapse supernova and around the value of the less massive observed neutron star which makes metastability related phenomena particularly relevant. In all cases, we find values of the surface tension that favor the nucleation of quark matter.

Speaker: Eduardo Fraga (Universidade Federal do Rio de Janeiro)

SEWM-fraga.pdf

5 Quantifying the degree of hydrodynamic behaviour in heavy-ion collisions

Exploiting the first measurements of the same ion species in O+O collisons at RHIC and LHC, we propose an experimentally accessible observable to distinguish whether collective behavior builds up through a hydrodynamic expansion of a strongly interacting QGP or through few final state rescatterings. Our procedure allows to disentangle the effects of the initial state geometry and the dynamical response mechanism on the total resulting anisotropic flow. We validate the ability of our proposed observable to discriminate between systems with different interaction rates using results from event-by-event simulations in RTA kinetic theory. As a proof of concept, we extract the degree of hydrodynamization for Pb+Pb collisions at LHC from experimental data.

Speaker: Clemens Werthmann (University of Wroclaw)

SEWM-Werthmann.pdf

12:15 PM Lunch Break

Invited

Convener: Prof. Lorenz von Smekal (Justus-Liebig University Giessen)

6 Entanglement and quantum dynamics: a journey in lower dimensions

Speaker: Adrien Florio

florio.pdf

7 The Quark-Gluon Plasma through Energy Correlators

Speaker: Carlota Andres (LIP, Lisbon)

andresc_SEWM2024.pdf

3:30 PM Break

Hard Probes

Convener: Carlota Andres (LIP, Lisbon)

8 Minijet quenching in non-equilibrium quark-gluon plasma

We study the energy deposition and thermalisation of high-momentum on-shell partons (minijets) travelling through a non-equilibrium Quark-Gluon Plasma using QCD kinetic theory. For thermal backgrounds, we show that the parton energy first flows to the soft sector by collinear cascade and then isotropises via elastic scatterings. In contrast, the momentum deposition from a minijet reaches the equilibrium distribution directly. For expanding non-equilibrium QGP, we study the time for a minijet perturbation to lose memory of its initial conditions, namely, the hydrodynamisation time. We show that the minijet evolution scales well with the relaxation time τ_R∝η/s/T(τ), where T(τ) is the effective temperature and η/s is the viscosity over entropy ratio. This scaling allows to extract minijet response functions similarly as in KøMPøST, describing macroscopically how initial minijet perturbations deposit their energy and momentum in the medium.

Ref: Fabian Zhou, Jasmine Brewer, Aleksas Mazeliauskas
Minijet quenching in non-equilibrium quark-gluon plasma
arXiv:2402.09298

Speaker: Fabian Zhou (ITP Heidelberg)

SEWM2024_Fabian_Zhou.pdf

9 Jet momentum broadening during the initial stages in heavy-ion collisions from kinetic theory

Jet quenching measurements may provide an experimental opportunity to infer non-equilibrium properties of the quark-gluon plasma created in heavy-ion collisions. An important medium parameter determining the strength of jet-medium interactions is the jet quenching parameter $\hat q$. In this talk, I will present our results for this jet quenching parameter $\hat q$ in the initial pre-equilibrium stages of heavy-ion collisions using the QCD kinetic theory description of the anisotropic quark-gluon plasma. This allows us to smoothly close the gap in the literature between the early glasma stage of the collision and the onset of hydrodynamics. We find that the pre-hydrodynamic evolution of $\hat q$ during the bottom-up kinetic scenario shows little sensitivity to the initial conditions, jet energies and models of the transverse momentum cutoff. We also observe that, similarly to the glasma case, the jet quenching parameter is enhanced along the beam axis as compared to the transverse direction during most of the kinetic evolution.

This talk is based on Phys.Lett.B 850 (2024) 138525, and arXiv:2312.00447

Speaker: Florian Lindenbauer (TU Wien)

talk_lindenbauer_sewm_2024.pdf

10 Unraveling QGP and jet physics via perturbing attractors

The rapid longitudinal expansion characteristic of heavy-ion collisions leads to universal attractor behavior of the resulting drop of Quark-Gluon Plasma. Assuming approximate boost invariance, we incorporate transverse dynamics and parton evolution by linearizing the Israel-Stewart theory around the attractor. This yields a system of coupled ordinary differential equations which describe the proper-time evolution of perturbations encoding the transverse structure of the initial and jet energy deposition across a wide range of geometric configurations and parton energy loss scenarios. The late-time asymptotic behavior of the solutions is described by transseries which manifest the stability of the attractor against transverse perturbations, as well as a dominant power-law series attributed to the parton source. Although most of the physically relevant initial information resides in the exponentially suppressed transseries corrections to the evolution along the attractor, they are not yet negligible at freeze-out. These findings advocate for a simple numerical approach to QGP dynamics which accounts for the transverse dynamics and jet-medium interactions via a finite set of Fourier modes. Physical observables can be expressed in terms of the asymptotic data evaluated at freeze-out. We demonstrate the efficacy of this approach in describing key observables such as collectivity across various system scales, as well as the consequential effects of jet wakes.

Speaker: Xin An (National Center for Nuclear Research)

XA_SEWM24.pdf

Tue, August 27

Invited

Convener: Philipp Schicho (Goethe University Frankfurt)

11 CP-violating sources in Electroweak Baryogenesis

Speaker: Jorinde van de Vis (Leiden Univerisity)

ElectroweakBaryogenesisJvdVis.pdf

12 From theory to phenomenology for cosmological phase transitions

The prospect of gravitational wave detectors observing the remnant signal of a cosmological phase transition has inspired a flurry of developments in their theoretical description. Automating these developments is needed to improve predictions for phenomenological studies of BSM models. In this talk, I will describe some key theoretical developments and discuss progress and challenges for their automation. I will highlight recent and forthcoming computer programs for computing bubble determinants and for tackling the question: how fast does the bubble wall go?

Speaker: Oliver Gould (University of Nottingham)

gould.pdf

10:30 AM Break

Phase Transitions

Convener: Eduardo Fraga (Universidade Federal do Rio de Janeiro)

13 Gravitational waves from first-order phase transitions: Approaching reliable predictions

With the approval of the space-based LISA experiment, the hunt is on: A search for gravitational-wave remnants of the Electroweak phase transition; to probe the Higgs potential and perchance even explain the Baryon asymmetry problem. Yet the theoretical hurdles are great—theoretical predictions can misjudge the peak gravitational-wave spectrum by orders of magnitude.

In this talk I will illustrate the progress of using perturbation theory, together with Lattice Monte-Carlo simulations, to give robust predictions of the gravitational-wave spectrum from phase transitions. To that end I will compare state-of-the-art theoretical calculations with new Lattice simulations and discuss how remaining theoretical hurdles can be overcome.

Speaker: Andreas Ekstedt (Uppsala University)

sewm_Ekstedt.pdf

14 Gravitational waves induced by first-order QCD phase transitions

We calculated the gravitational waves induced by different QCD phase transitions including in a chirality imbalanced system, in pure gluon system, in PQM, Qm, and Friedberg-Lee model, and the gravitaitional waves can be detected by LISA, Taiji and DECIGO. We find that the values of inverse duration $\beta/H$ of these QCD phase transitions are of order $10^4$ or $10^3$, which means the phase transitions complete in an extreme short time. We also find that with larger chemical potential $\mu_B$, the values of inverse duration increase with lower nucleation temperatures.

Speaker: Jingdong Shao (University of Chinese Academy of Sciences)

SEWM2024.pptx

12:00 PM Lunch Break and IAC meeting

Invited

Convener: Jürgen Schaffner-Bielich (Uni Frankfurt)

15 Conformal matter in the core of neutron stars

Speaker: Yuki Fujimoto

YukiFujimoto_SEWM24.pdf

16 Quark matter equation of state for neutron stars from perturbative QCD

Speaker: Risto Paatelainen (University of Helsinki)

SEWM24paatelainenristo_final.pdf

3:30 PM Break

Astrophysics & Neutron Stars

Convener: Aleksi Vuorinen (University of Helsinki)

17 Astrophysical Equation-of-State Constraints on the Color-Superconducting Gap

I discuss how astrophysical constraints on the dense-matter equation of state place an upper bound on the color-superconducting gap in dense matter above the transition from nuclear matter to quark matter. Pairing effects in the color-flavor locked (CFL) quark matter phase increase the pressure at high density, and if this effect is sufficiently large then the requirements of causality and mechanical stability make it impossible to reach such a pressure in a way that is consistent with what is known at lower densities. The intermediate-density equation of state is inferred by considering extensions of chiral effective field theory (CEFT) to neutron star densities, and conditioning these using current astrophysical observations of neutron star radius, maximum mass, and tidal deformability (PSR J0348+0432, PSR J1624-2230, PSR J0740+6620, GW170817). At baryon number chemical potential μ=2.6 GeV we find a 95% upper limit on the CFL pairing gap Δ of 457 MeV using overly conservative assumptions and 216 MeV with more reasonable assumptions. This constraint may be strengthened by future astrophysical measurements as well as by future advances in high density QCD calculations.

Speaker: Aleksi Kurkela (University of Stavanger)

SEWM2024.pdf

18 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 QCD. 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 talk, I present results for the thermodynamic properties of two-flavor color-superconducting quark matter at high densities and zero temperature at next-to-leading order in the coupling in the presence of a finite gap. I will discuss findings of our work as well as comment on whether they should be expected to extend to the case of three-flavor quark matter of relevance to neutron stars.

Speaker: Tyler Gorda (Goethe University Frankfurt)

gorda.pdf

Poster session: Flashtalks

SEWM2024_FlashTalkSession1.pdf

SEWM2024_FlashTalkSession2.pdf

19 Baryon Number Violation and Constrained Instantons

Electroweak Theory (EWT) allows the existence of BNV processes mediated by topologically non-trivial field configurations. The most well-known such configuration is the instanton. However, due to the breaking of scale invariance by the Higgs mass, instantons are not allowed in EWT. Instead, one must use constrained instantons to obtain the BNV rate.

This poster will explain constrained instanton formalism and its application to the calculation of the tunnelling rate. It will also show the results of such calculation in a toy phi^4 model and explain how the formalism can be extended to EWT

Speaker: Kinga Gawrych (Imperial College London)

K Gawrych - SEWM24 - Poster flash talk.pdf

20 Progress on the QCD chiral phase transition for various numbers of flavors and imaginary chemical potential

The order of the thermal chiral phase transition in lattice QCD is strongly cutoff-dependent. A recent study from our group using mass-degenerate, unimproved staggered quarks on $N_\tau=\{4,6,8\}$ lattices found that the first-order regions shrink to zero for $N_\text{f}\in[2,6]$ as the continuum limit is approached for zero chemical potential. Here we present the progress of an analogous study at a fixed value of imaginary baryon chemical potential of $\mu_i=0.81\frac{\pi T}{3}$. The same qualitative behavior as for zero chemical potential is found: The first-order regions disappear with decreasing lattice spacing in tricritical points and they are bounded by $Z_2$-critical lines which exhibit tricritical scaling for sufficiently small quark masses. The results predict a second order chiral phase transition in the continuum limit for $N_\text{f}\in[2,6]$ for both cases, at zero and imaginary chemical potential. Additionally an effective Ginzburg-Landau theory is developed around the tricritical point in the chiral limit. The possibility to encode the dependence of the Landau potential on the parameters of the lattice theory is explored.

Speaker: Reinhold Kaiser (University of Frankfurt)

main.pdf

21 Shear viscosity from quenched to full lattice QCD

The shear viscosity of the quark-gluon plasma (QGP) is important ingredient for describing current measurements in heavy-ion collisions and are key inputs to hydrodynamical models. The interest in shear viscosity also lies in the fact that QGP is the most ideal fluid ever observed and has the shear viscosity to entropy ratio ($\eta / s$) close to the theoretical bound $\eta / s \geq 1/ 4 \pi$ in the strong coupling region within AdS/CFT formalism. We utilize the gradient flow method to renormalize the Energy-Momentum Tensor (EMT) and to suppress UV fluctuations in correlators on a lattice SU(3) gauge theory. We take the continuum limit and then the zero flow-time limit to extract shear viscosity in the pure-gauge theory. For spectral reconstruction from lattice correlators, we model the spectral function by combining perturbative UV part with the hydro-motivated infrared part. We also present the extension of quenched findings to full QCD, specifically the renormalization of EMT and determination of relevant coefficients for shear viscosity with $m_s / m_l = 20$ on the lattice.

Speaker: Pavan Pavan (Bielefeld University)

SEWM_FT_pavan.pdf

22 The force-force correlator at the hard thermal scale of hot QCD

Jets play an important role in heavy-ion collisions, interacting with and thereby probing the quark-gluon plasma (QGP) as they propagate to the detector. The asymptotic mass, $m_{\infty}$ and the transverse scattering rate, $\mathcal{C}(k_{\perp})$ control this interaction insofar as they serve as input to calculations of transverse momentum broadening and medium-induced radiation. Both $m_{\infty}$ and $\mathcal{C}(k_{\perp})$ receive classical corrections, arising from the exchange of gluons distributed on the IR tail of the Bose-Einstein distribution.

In this talk, we describe how recent advances have shown that these classical corrections can be obtained from the lattice in dimensionally reduced Electrostatic QCD (EQCD) [1], which has in turn paved the way for their non-perturbative determination [2,3]. Based on [4], we explain how said EQCD determination needs to be supplemented with a perturbative calculation in full thermal QCD at $\mathcal{O}(g^2)$ to render the classical contribution finite, ridding it of unphysical UV divergences. We furthermore report on our progress towards the calculation of the rest of the $\mathcal{O}(g^2)$ contribution and provide an outlook on the potential impact on medium-induced radiation rates.

References:
[1] S. Caron-Huot, 0811.1603
[2] G.D. Moore, N. Schlusser, 2009.06614
[3] J. Ghiglieri, G.D. Moore, P. Schicho, N. Schlusser, 2112.01407
[4] J. Ghiglieri, P. Schicho, N. Schlusser, E. Weitz, 2312.11731

Speaker: Dr Eamonn Weitz (Subatech, Nantes Université)

weitz_SEWM24.pdf

23 The temperature of the chiral phase transition in LQCD at its tricritical point

The nature of the QCD phase transition in the chiral limit constitutes a challenging problem for lattice QCD as it is not directly simulable. Its study, however, provides constraints on the phase diagram at the physical point. Recently, the thermal transition for massless fermions was shown to be of second order for all numbers of flavours $N_f \lesssim 7$. For this, the lattice chiral limit was approached by mapping out the chiral critical surface separating the first-order region from the crossover region in an enlarged parameter space which consists of the gauge coupling, a variable number of quark flavours, their masses, and the lattice spacing. Based on simulations of lattice QCD with standard staggered quarks, it was found that for all $N_f \lesssim 7$ there exists a tricritical lattice spacing $a^{tric}(N_f)$, where the chiral transition changes from first order (above) to second order (below). The first-order region thus constitutes a cutoff effect and the transition in the continuum chiral limit is of second order for all $N_f \lesssim 7$. In the current work we determine the associated temperatures $T(N_f^{tric},a ^{tric})$ at these tricritical points. We confirm an expected decrease in the tricritical temperature for increasing number of flavours. Running simulations on finer lattices and for larger $N_f$ will allow us to determine the location of the tricritical point in the continuum limit and let us resolve the question whether the conformal window is approached by a first or second order phase transition.

Speaker: Jan Philipp Klinger (Goethe University Frankfurt)

SEWM_Slide.pdf

24 QCD Anderson transition with overlap valence quarks on a twisted-mass sea

In this work we probe the QCD Anderson transition by studying spectral distributions of the massless overlap operator on gauge configurations created by the twisted mass at finite temperature collaboration (tmfT) with 2+1+1 flavors of dynamical quarks and the Iwasaki gauge action. We assess finite-size and discretization effects by considering two different lattice spacings and several physical volumes, and mimic the approach to the continuum limit through stereographic projection. Fitting the inflection points of the participation ratios of the overlap Dirac eigenmodes, we obtain estimates of the temperature dependence of the mobility edge, below which quark modes are localized. We observe that it is well described by a quadratic polynomial and systematically vanishes at temperatures below the pseudocritical one of the chiral transition. In fact, our best estimates within errors overlap with that of the chiral phase transition temperature of QCD in the chiral limit.

Speaker: Robin Kehr

QCD_Anderson_Flashtalk_SEWM2024.pdf

QCD_Anderson_Poster_SEWM2024.pdf

25 The chiral transition at finite baryon and isospin densities at strong coupling

The Hamiltonian formulation of lattice QCD based on an effective theory (the so-called dual formulation) can be extended in the strong coupling limit to two flavors. It has no sign problem at both non-zero baryon and isospin densities and allows for Quantum Monte Carlo simulations. We present results on the phase boundary for the chiral transition in the $\mu_B - \mu_I$ plane in the chiral limit. We will also comment on the prospects to extend the framework beyond strong coupling.

Speaker: Dr Wolfgang Unger (Deutsch)

Flashtalk_Unger.pdf

26 Universal dynamics at the chiral phase transition of QCD

In this talk, I will give an overview of the universal critical dynamics at the chiral phase transition of two-flavor QCD in the chiral limit. I will review the general argument about the dynamic universality class by explicitly constructing the stochastic equations of motion in the vicinity of the second-order phase transition. To extract dynamic universal quantities associated with these stochastic equations, we use a novel formulation of the functional renormalization group for dynamical systems with "reversible mode couplings". I will show results for dynamic universal quantities like the non-trivial value $z=d/2$ of the dynamic critical exponent at the strong-scaling fixed point (where $d$ is the number of spatial dimensions) and the dynamic universal scaling function for the isovector and isoaxial-vector charge diffusion coefficient. Beyond universality, I will show preliminary results about the size of the dynamic scaling region along the temperature axis.

Speaker: Johannes Roth (JLU Gießen)

Roth_flashtalk.pdf

27 Color-superconductivity in the two-flavor quark-meson-diquark model

We investigate the 2SC phase at medium baryon density using the quark-meson-diquark model. Our analysis involves calculating the thermodynamic potential in the mean field approximation to one fermion loop. The model includes the following effective degrees of freedom: the sigma, pions, quarks, and diquarks. Parameters are expressed with the physical values of the sigma mass, pion mass, and pion decay constant. We introduce five new free parameters through the coupling of the diquark with the sigma, pion, and quark, as well as the diquark potential. The new beta functions are derived in the same approximation and compared to those of the established parameters in the quark-meson model. We use the model to map out the phase diagram in the $\mu_B-T$ plane.

Speaker: Mathias Pavely Nødtvedt (NTNU (Norwegian University of Science and Technology))

Slide_SEWM2024.pdf

28 Chiral density wave in dense matter under neutron star conditions

Employing a phenomenological nucleon-meson model, we study the possible existence of a chiral density wave in neutron star matter. We impose charge neutrality and β-equilibrium, extending previous works that studied isospin-symmetric nuclear matter. We find that the chiral density wave is energetically preferred in a region of our parameter space that predict a very soft equation of state. In this parameter regime, the maximum neutron star mass does not meet astrophysical constraints. Therefore, the model predicts that such a phase is unlikely to be found in the interior of neutron stars.

Speaker: Orestis Papadopoulos (University of Southampton)

SEWM 2024 Presentation OP V4.pdf

29 Deconfinement transition within the Curci-Ferrari model - Renormalization scale and scheme dependences

I will present perturbative results[1] on the confinement/deconfinement transition of Yang-Mills theories found in a center-symmetric Landau gauge with a Curci-Ferrari mass term to account for IR Gribov copies. We obtained one-loop predictions for the SU(2) and SU(3) transition temperatures close to lattice values. I will show that our results are consistent across different renormalization schemes and relatively insensitive to scale variations in a wide range. This underscores the efficacy of the Curci-Ferrari model in probing Yang-Mills theories and confinement and paves the way for including quarks.

[1] V.T. Mari Surkau, U. Reinosa, Deconfinement transition within the Curci-Ferrari model - Renormalization scale and scheme dependences, [arXiv:2401.17869 [hep-ph]]

Speaker: Tomas Mari Surkau (CPHT (École Polytechnique, CNRS))

FlashTalkSEWM24.pdf

30 No-quenching baseline for energy-loss signals in small system collisions

In this work, we perform computations of inclusive jet and semi-inclusive jet-hadron cross sections for minimum-bias oxygen-oxygen collisions. We compute the no-quenching baseline for the jet nuclear modification factor $R_{AA}$ and jet-, and hadron-triggered semi-inclusive nuclear modification factors $I_{AA}$. We do this with state-of-the-art nuclear parton distribution functions, NLO matrix elements and parton shower. We show significant deviations from unity due to cold-nuclear effects even in the absence of quenching. We demonstrate that the nPDF uncertainties constitute a major limitation in detecting potentially small energy loss effects in small collision systems. Hadron-triggered observables are in particular sensitive to uncertainties due to the non-trivial correlation of the trigger hadron and analyzed particles. For jet-triggered $I_{AA}$, there exist kinematic regions in which errors cancel down to 2\%, overcoming the main limitation of small-system energy loss measurements.

Ref.: Jannis Gebhard, Aleksas Mazeliauskas and Adam Takacs, No-quenching baseline for energy-loss signals in small system collisions, in preparation.

Speaker: Mr Jannis Gebhard (Institute for Theoretical Physics, Heidelberg University)

SEWM24_FT_JannisGebhard.pdf

31 Evidence for a transport peak affecting heavy-quark diffusion in 2+1D gluonic plasmas

The early stages of relativistic heavy-ion collisions involve a non-thermal gluonic plasma with high occupancy. We examine how the broad quasiparticle excitation characteristics found in an earlier study for such a 2+1 dimensional system affect the heavy quark diffusion coefficient. We show that essential transport properties can be extracted from gauge-fixed quantities, accurately reproducing the time evolution of the gauge-invariant coefficient. Notably, we identify a novel transport peak in the low-frequency spectrum that is crucial for the correct reproduction. Additionally, we draw parallels to features observed in the Glasma, indicating significant implications for its transport properties.

Speaker: Paul Hotzy (TU Wien)

hotzy_flashtalk_sewm24.pdf

32 Generating ultra-compact neutron stars with bosonic dark matter

Neutron stars with admixed dark matter enable new possibilities in the description of unusal mass and radius measurements, such as e.g. HESS J1731-347. We are including bosonic, self-interacting dark matter with a sufficiently stiff self-interaction potential in the form of $V \propto \phi^n$ and find that these neutron stars become ultra-compact ($C \geq 1/3$). They are compact enough to have a stable photon orbit at their surface or even above, a property that is otherwise exclusively attributed to black holes or hypothetical boson stars. These ultra-compact neutron stars are characterized by small radii of the ordinary matter ($R < 7$ km) and masses of approximately $1.5 \, M_\odot$. We furthermore study the stability of these configurations by investigating the onset of unstable radial modes.

Speaker: Sarah Louisa Pitz (Goethe Universität Frankfurt)

ShortPresentationSarahPitz.pdf

33 Disordering of exotic phases through bosonic fluctuations in finite density QCD

In the intermediate density region of the QCD phase diagram, there have been conjectures about inhomogeneous, chiral phases, i.e., phases with translational symmetry breaking through the chiral condensate. A precursor phenomenon, the so-called Moat regime, where mesonic dispersion relations favor non-vanishing momenta, was found in a recent FRG study of the QCD phase diagram in the vicinity of the critical point.
In his talk, we use a QCD-inspired model describing the scalar mesonic modes with focus on the investigation of inhomogeneous phases, moat regimes and alternative scenarios. The stability of these different phases with respect to bosonic fluctuations are investigated comparing analytical findings and lattice field theory simulations. We demonstrate how bosonic quantum fluctuations disorder inhomogeneous condensates. The mesonic correlation functions are described by an oscillatory function times the usual exponential decay, a regime termed `quantum pion liquid', in certain parameter regions. First results regarding an infinite-volume phase diagram are presented.

Speaker: Marc Winstel (Goethe University Frankfurt)

Winstel.pdf

34 Electric field effects on hot and dense media

In this work we calculate the electric and magnetic susceptibilities of a hot and dense medium in equilibrium up to order $\mathcal{O}(\frac{m^4}{T^4})$ $\mathcal{O}(\frac{m^2}{T^2})$, respectively. These susceptibilities are associated with $\mathcal{O}(k^2)$ terms (power corrections) of the photon polarization tensor, which are computed here for a hot and dense medium of fermions with a small but nonzero mass, i.e., $0< m\ll T, \mu$. Our calculations are performed within the hard thermal loop approximation in the real-time formalism. In the high temperature and small chemical potential limit, our results are compatible with previous calculations and lattice simulations.

Speaker: Osvaldo Ferreira Neto (Federal University of Rio de Janeiro and Goethe University)

flash_talk_Ferreira.pdf

35 Scattering of dark pions in an Sp(4)-gauge theory

Analyzes of astrophysical data provide hints on the self-interactions of dark matter at low energies. Lattice calculations of strongly interacting dark matter (SIMP) theories are needed for motivating these models also from first principles. Sp(4) gauge theory with two fundamental fermions is a candidate SIMP theory. We compute the scattering phase shift for the scattering of two dark pions and determine the parameters of the effective range expansion. Our exploratory results in the supposedly most common interaction channel provide a lower limit for the dark matter mass when compared to astrophysical data. We also provide first benchmarks of velocity-weighted cross-sections in the relevant non-relativistic domain.

Speaker: Yannick Dengler (University of Graz)

One_Slide_SEWM.pdf

Poster_SEWM_final.pdf

36 Non-perturbative constraints on perturbation theory at finite temperature

Perturbative approaches to quantum field theory at finite temperature require theoretical constraints to be addressed which are not present in the vacuum theory. In this talk I will present evidence from lattice simulations of massive $\phi^4$ theory which shows that the standard perturbative predictions break down, even at relatively low temperatures. This suggests that non-perturbative thermal effects must be incorporated in order to define a consistent perturbative formulation at finite temperature.

Speaker: Peter Lowdon (Goethe University Frankfurt)

SEWM_Lowdon.pdf

37 Perturbative evaluation of adjoint correlators of chromoelectric fields at NLO in finite T

Heavy-ion collisions are an essential tool to better understand the deconfined phase of Quark-Gluon Plasma . Probes such as heavy particles are used indirectly to study its properties. In describing the propagation of heavy particles in a medium, non-relativistic effective field theories are employed. In doing so, correlators of chromoelectric fields appear and are important in the evolution of the probes inside the plasma; for example correlators in the adjoint representation are associated with heavy quarkonium physics. Moreover, such correlators are vital in determining some of the transport coefficients that characterise the propagation of heavy probes.

I will show results from a perturbative evaluation of chromoelectric correlators in adjoint representation at NLO in Imaginary-Time (Euclidean) QCD at finite temperatures. Euclidean space is extremely useful as it makes direct contact with Lattice QCD. We make use of recent advancements in thermal integration by parts methods to tackle the loop integrals present. This results in analytically more tractable integrals, allows automation and provides a clear path for potential extension to higher orders in perturbation theory. Finally, I will also discuss the steps needed to extract transport coefficients from these correlators and discuss their physical meaning.

Speaker: Panayiotis Panayiotou (Technical University of Munich)

SEWM_Panayiotou_Flashtalk.pdf

38 Determining proto-neutron stars' minimal mass with a chirally constrained nuclear equation of state

We study the minimal masses and radii of proto-neutron stars during different stages of their evolution.
The main focus lies on the stages directly before the supernova explosion where neutrinos are captured in the core
and the lepton per baryon ratio is approximately $Y_L = 0.4$ and a few seconds after the supernova,
when all neutrinos have left the star.
All equations of state used for this purpose fulfill $\textit{Chiral Effective Field Theory}$ constraints for the binding energy at $T = 0$.
We find for the neutrino-trapped cases higher minimal masses than for the cases when neutrinos are not in the system anymore.
Thermal effects, characterized by a higher constant entropy per baryon, further elevate the minimal mass. We additionally study the minimal mass dependence on the lepton fraction, which follows a similar curve for all equations of state studied.
The masses for the cases studied all lie between $M_{min}$ = $0.11$$M_{\odot}$ and $0.85$ $M_{\odot}$. Hence, proto-neutron stars can exhibit higher minimal masses than their cold, catalyzed form during all stages of their evolution.

Speaker: Selina Kunkel

slide_selina_kunkel.pdf

39 Hybrid star phenomenology from the properties of the special point

We study the properties of hybrid stars containing a color superconducting quark matter phase in their cores, which is described by the chirally symmetric formulation of the confining relativistic density functional approach [1]. It is shown that depending on the dimensionless vector and diquark couplings of quark matter, the characteristics of the deconfinement phase transition are varied, allowing us to study the relation between those characteristics and mass-radius relations of hybrid stars. Moreover, we show that the quark matter equation of state (EoS) can be nicely fitted by the Alford-Braby-Paris-Reddy model that gives a simple functional dependence between the most important parameters of the EoS and microscopic parameters of the initial Lagrangian. The developed approach is utilized for analyzing spinodal instability of quark matter and constructing hybrid quark-hadron EoS. Based on it, we analyze the special points of the mass-radius diagram in which several mass-radius curves intersect. Using the found empirical relation between the mass of the special point, the maximum mass of the mass-radius curve, and the onset mass of quark deconfinement, we constrain the range of values of the vector and diquark couplings of the quark matter model.With this constraint, we construct a family of mass-radius curves, which allow us to describe the black widow pulsar PSR J0952-0607 with a mass of as a hybrid star with a color superconducting quark matter core.

[1] C. Gärtlein, O. Ivanytskyi, V. Sagun, D. Blaschke, Phys. Rev. D 108, 114028 (2023).

Speaker: Christoph Gärtlein

Chris_Gärtlein.pdf

Poster session

SEWM2024_FlashTalkSession1.pdf

SEWM2024_FlashTalkSession2.pdf

Wed, August 28

Invited

Convener: Guy Moore (TU Darmstadt)

40 Real-time dynamics of lattice gauge theory via Complex Langevin

Speaker: Kirill Boguslavski (TU Wien)

Boguslavski_SEWM2024.pdf

41 Probing the photon emissivity of the quark-gluon plasma with lattice QCD

Speaker: Harvey Meyer

meyerhSEWM2024.pdf

10:30 AM Break

Lattice & Strong Fields

Convener: Kari Rummukainen (University of Helsinki)

42 Reconstructing the Thermal Photon Rate from Lattice QCD

The thermal photon rate is an import observable for understanding the quark-gluon plasma, and can be calculated from lattice QCD by reconstructing the spectral function of the vector current correlator. Since this correlator receives a large ultraviolet contribution from the vacuum, only the thermal part of the correlator is considered, which significantly simplifies the reconstruction. Since this reconstruction is still ill-conditioned, we estimate the systematic error by comparing different methods, namely model fits, Backus-Gilbert and Gaussian Process Regression. This is performed with lattice data for quenched QCD at $T\sim1.5~T_{c}$ and 2+1 flavor QCD at $T\sim1.2~T_{pc}~$.

Speaker: Jonas Turnwald (TU Darmstadt)

2024_08_28_PhotonRate_JonasTurnwald.pdf

43 Quarkonia spectral function in QGP from 2+1 Flavor Lattice QCD

Quarkonia, the bound states of heavy quark-antiquark pairs, have proven to be crucial probes for studying quark-gluon plasma (QGP). The color screening properties of the QGP weaken the interaction between quark-antiquark pairs, leading to the suppression of quarkonia yields within the QGP. We will present results on the fate of quarkonia bound states in the QGP by performing spectral reconstruction from lattice correlators in 2+1 flavor QCD at temperatures of 1.2 Tc, 1.4 Tc, and 1.62 Tc. The spectral function is reconstructed by combining the perturbative vacuum part, which is valid at large energy, with the one obtained from the thermal potential near the threshold. The spectral function near the threshold region has been obtained by using a non-perturbative thermal potential.
We observe that this spectral function, obtained in this way, can effectively describe the lattice QCD correlator in the pseudoscalar channel; however, for the vector channel, one needs to include an additional transport peak. Our findings indicate that the thermal interaction shifts the bound state mass and results in a significantly larger thermal width. In the charmonium system, the width is much larger than in the bottomonium system.

Speaker: Dibyendu Bala (Bielefeld University)

SEWM_Dibyendu_Updated.pdf

44 Nucleon screening masses in QCD at high temperature

Screening masses of states with nucleon quantum numbers are determined, for the first time, in QCD for temperatures ranging from $\sim 1$ GeV up to $\sim 160$ GeV. The non-perturbative data show that the bulk of the screening mass is given by the tree level value $3\pi T$, while interactions induce a $4 -8 \%$ positive deviation for all temperatures considered. While our results are compatible with the $\mathcal{O}(g^2)$ perturbative prediction at very high temperatures, higher powers of the QCD coupling $g$ are required to explain the data's behaviour over the whole temperature range. Restoration of chiral symmetry is manifest in the degeneracy of the positive and negative parity state's screening mass, which differ by hundreds of MeV at zero temperature.
The non-perturbative data is obtained from Lattice QCD, employing a recently proposed strategy to reach very high temperatures. We complement this study with a NLO computation of the screening mass in the 3d effective theory describing QCD at high temperatures to obtain the $\mathcal{O}(g^2)$ perturbative result.

Speaker: Pietro Rescigno (University of Milano-Bicocca, INFN Milano-Bicocca)

sewm24_rescigno_final.pdf

12:15 PM Lunch Break

Non-Perturbative Phenomena

Convener: Adrien Florio

45 The Cubic Casimir and photon --> Chi_c (in the background field of a proton) at the EIC

Exclusive C = +1 quarkonium production in high-energy electron-proton
scattering requires a C-odd t-channel exchange of a photon or three
gluons. The relative phase of the amplitudes is determined by the sign
of the light-front matrix element of the eikonal color current
operator $d^{abc} J^{+a} J^{+b} J^{+c}$. Model calculations predict
constructive interference, which is particularly strong for momentum
transfer $|t| \sim 1$ GeV$^2$ where the cross section for $\chi_{cJ}$ production
exceeds that for pure photon exchange by up to a factor of 4.

Speaker: Adrian Dumitru (Baruch College (CUNY))

chi_c-sewm.pdf

46 Are parton showers inside a quark-gluon plasma strongly coupled?

Jet quenching plays an important role as a hard probe to study properties of the quark–gluon plasma (QGP) in heavy-ion collisions at both the relativistic heavy-ion collider and the large hadron collider. During their travel in the QGP, high energy partons lose their energy mainly through splitting processes like bremsstrahlung and pair production, induced by elastic scatterings with the medium. When repeated, a shower of lower energy particles is produced. In the high energy limit, these splitting processes are coherent over large distances and the underlying elastic scatterings can no longer be treated as quantum mechanically independent, leading to a suppression of the splitting rate known as the Landau-Pomeranchuk-Migdal effect. We studied whether high-energy parton showers inside a QGP can be treated as a sequence of individual splitting processes, or whether there is significant quantum overlap between where one splitting ends and the next begins. Previously, the overlap of splitting rates has been calculated at leading-log order, and it was found that such corrections are large but can be absorbed into an effective value of the jet-quenching parameter $\hat{q}$ that characterizes the medium. Going beyond leading logarithms (in the simplest theoretical situation), we investigated a measure of overlap effects that cannot be absorbed into an effective value of $\hat{q}$. In this talk, I will summarize our recent results and comment on the implications of our study.

Speaker: Omar Elgedawy (South China Normal University)

OmarElgedawy_SEWM2024.pdf

47 Bottomonium suppression from the three-loop QCD potential

We present results for bottomonium suppression in the QGP based on pNRQCD and the open quantum system framework. We extend previous studies by including the three loop potential from pNRQCD, which provides a good prediction for the spin-averaged bottomonium spectrum. We solve the corresponding Lindblad equation for the quarkonium density matrix to obtain predictions for the nuclear modification factor. The new potential leads to results that are consistent with experimental measurements of the nuclear modification factor as well as lattice results for the in medium width and thermal mass shift of bottomonium.

Speaker: Tom Magorsch (TUM)

Tom_Magorsch_SEWM_24.pdf

3:15 PM Break

Non Equilibrium Dynamics

Convener: Jorge Noronha

48 A Unified Adiabatic Description of Hydrodynamization in Kinetic Theory

The far-from equilibrium dynamics of the pre-hydrodynamic quark-gluon plasma (QGP) formed in heavy ion collisions can be characterized by distinct stages, during each of which the system loses some memory of its initial condition, until only the hydrodynamic modes remain. This attractor behavior has been characterized previously in both strongly and weakly coupled descriptions. In particular, at weak coupling it has been found in kinetic theory descriptions in terms of self-similar scaling solutions for the particle distribution function, even at times well before hydrodynamization. However, even though it has been repeatedly observed, there has been an absence of an intuitive physical explanation of how and why attractor behavior occurs. The Adiabatic Hydrodynamization (AH) framework provides exactly such an explanation, showing that the attractor solution can be thought of as the ground state of an analog to quantum mechanical adiabatic evolution, provided we identify appropriate coordinate rescalings. Using the example of a simplified QCD kinetic theory in the small-angle scattering limit, we show how AH can explain both the early pre-hydrodynamic attractor and the later hydrodynamizing attractor in a longitudinally expanding gluon gas in a unified framework. By doing this, we provide a unified description of, and intuition for, all the stages of what in QCD would be bottom-up thermalization, starting from a pre-hydrodynamic attractor and ending with hydrodynamization.

Speaker: Rachel Steinhorst (Massachusetts Institute of Technology)

sewm2024.pdf

49 Simulating stochastic diffusion in critical and near-critical fluids in (3+1) dimensions

We describe numerical simulations of stochastic fluid dynamics
with a conserved charge coupled to the momentum density of the
fluid. This theory is known as model H, and it is expected to
describe universal dynamics in the vicinity of a possible
critical endpoint in the QCD phase diagram.

Speaker: Thomas Schaefer (North Carolina State University)

talk_sewm_2024.pdf

Thu, August 29

Thermal Field Theory

Convener: Risto Paatelainen (University of Helsinki)

50 Tackling the four-loop pressure of dense and hot QCD

Due to its phenomenological relevance in heavy-ion collisions, cosmology and astrophysics, the determination of the QCD pressure - either at high temperature or large baryon density - has driven a number of important theoretical advances in perturbative thermal field theory applicable to equilibrium thermodynamics. In particular, the long-standing infrared problems that obstruct the perturbative series have been overcome by a systematic use of dimensionally reduced effective theories at high temperatures, and hard thermal loops at large baryon density. This allows mapping the problem of determining the full next-to-next-to-next-to-leading-order (N3LO) pressure to a challenging four-loop computation in thermal QCD. In this talk, we present advances in organizing this formidable calculation both in the hot Yang-Mills sector and for cold and dense quark matter. By classifying the distinct contributions, we filter out a large fraction of sub-diagrams that exhibit a factorized structure, and push ahead systematic simplifications taking into account linear relations that originate from the graphs' internal symmetries. This will enable us to gauge the grade of difficulty of a full determination of the pressure of QCD at N3LO in two phenomenologically interesting regimes, serving as the first steps toward the automation of large-scale perturbative computations in thermal field theory.

Speaker: Pablo Navarrete (University of Helsinki)

pablonavarrete_SEWM.pdf

51 Bose condensation in QCD

In this talk, I will present some recent result for Bose condensation in two and three-flavor QCD using chiral perturbation theory (chipt). In the pion-condensed phase, we calculate the pressure, energy density, isospin density, and the speed of sound to next-to leading order in chipt. The results are compared with recent high-precision lattice data. The phase diagram in the mu_I-mu_S plane is mapped out, including electromagnetic interactions.
An effective theory for the Goldstone boson is constructed using Son's recipe, valid for momenta p much smaller than mu_I.
Finally, it is shown that the system behaves as a dilute Bose gas close to the phase transition: classic results such as the energy density and the damping rate are derived.

Speaker: Jens Oluf Andersen

andersensewm24.pdf

52 Order of the chiral phase transition for N_f flavors

Based on the epsilon expansion of the renormalization group flows of the Ginzburg-Landau potential for the chiral phase transition of QCD, it has been widely accepted that it is of first order in the zero quark mass limit for N_f > 2, with the possibility of a second order transition for N_f = 2, given the axial anomaly remains strong enough at the critical point. Going beyond the perturbative approach, I reanalyze the renormalization group flows of the Ginzburg-Landau potential using the functional renormalization group technique. Instead of employing the epsilon expansion around d = 4, with the inclusion of all (perturbatively) relevant and marginal interactions directly in d = 3, the method predicts that there exist new fixed points in the whole flavor number range potentially belonging to conformal systems going through a finite-temperature chiral transition. The results indicate that the chiral transition can be of second order for N_f >= 5, while its nature depends on the strength of the U_A(1) anomaly for N_f = 2,3,4.

Speaker: Gergely Fejos (Eötvös University Budapest)

fejos.pdf

10:15 AM Break

Non-Perturbative Phenomena

Convener: Aleksas Mazeliauskas (Institute for Theoretical Physics, Heidelberg University)

53 Chiral Soliton Lattice turns into 3D crystal

Strongly interacting matter in large magnetic fields occurs in heavy-ion collisions and neutron stars and is of theoretical interest for the phase structure of QCD. It was previously found that a so-called Chiral Soliton Lattice is formed at sufficiently large magnetic fields and baryon chemical potentials due to the chiral anomaly. Using chiral perturbation theory and methods from ordinary type-II superconductivity I will discuss how the known instability of the Chiral Soliton Lattice results in a transition to a 3D crystalline structure. This structure is an inhomogeneous condensate of neutral and charged pions and thus a superconductor with spatially modulated magnetic field and, via the axial anomaly, spatially modulated baryon number. I will also speculate what this new phase implies for the QCD phase structure, in particular a possible liquid-solid transition in nuclear matter.

Speaker: Andreas Schmitt

schmitt.pdf

54 Chiral crossover vs chiral density wave in dense nuclear matter

We employ a model based on nucleonic and mesonic degrees of freedom to describe cold and dense matter, in order to discuss the competition between isotropic and anisotropic phases, in the region of the chiral phase transition. Anisotropic phases have been extensively studied in quark models of dense matter, but equivalent progress was lacking in models using nucleonic degrees of freedom. When such models were employed, the nucleonic Dirac sea contribution to the pressure was neglected. We show that including it produces a significant effect both for the isotropic and anisotropic phases. Allowing for an anisotropic phase in the form of a chiral density wave, we observe a smooth crossover being disrupted. We identify the regions in the parameter space of the model where a chiral density wave is energetically preferred. A high-density re-appearance of the chiral density wave with unphysical behavior, which is present in previous studies, is avoided by choosing a suitable renormalization scheme.

Speaker: Savvas Pitsinigkos (University of Southampton)

Pitsinigkos_SEWM_2024_split.pdf

SEWM Frankfurt 2024.pptx

55 Rotating strongly interacting matter in a cylindrical geometry

We study the effect of rotation on the confining and chiral properties of QCD using the linear sigma model coupled to the Polyakov loop in an attempt to resolve discrepancies between the first-principle numerical and model-based analytical results. The attractive features of this model are renormalizability and the presence of the coupling between chiral and confining degrees of freedom.
We carefully enforce the causality constraint by implementing the spectral boundary conditions at the boundary of a cylinder. Working in a homogeneous approximation, we obtain the phase diagram at finite temperature, baryon density, and angular frequency. We demonstrate that in this model, the critical temperatures of both transitions diminish in response to the increasing rotation, being in contradiction with the first-principle lattice results. Surprisingly, despite the complicated structure of the spectrum caused by the presence of the boundaries, the thermodynamics of the model follows closely the simple Tolman-Ehrenfest law.

Speaker: pracheta singha (West University of Timisoara)

Pracheta_Singha_SEWM2024.pdf

12:00 PM Lunch Break

56 Excursion Information

Some information concerning the different excursions and lists of excursion participants

Speaker: Owe Philipsen

2:00 PM Excursions

7:00 PM Social Dinner

Fri, August 30

Thermal Field Theory

Convener: Jacopo Ghiglieri (Subatech, CNRS)

57 BSM Physics from Sp(2N) Gauge Theories: Meson Spectroscopy and Mixing Angles

Field theories with symplectic gauge group are compelling candidates for composite physics extending the Standard Model. They are of interest for both composite Dark Matter and composite Higgs models and could give rise to first-order phase transitions in the early universe. With the aim of providing a robust non-perturbative characterisation of a model of potential phenomenological relevance, we study on the lattice the Sp(4) gauge theory with fundamental fermions as well as fermions in the two-index antisymmetric representation. We report our results on their spectrum for both flavor singlet and non-singlet mesons, and the mixing between singlet mesons.

Speaker: Fabian Zierler (Swansea University)

zierler_sewm2024.pdf

58 Surface tension in confinement-deconfinement phase transition in SU(N) gauge theory at large N

SU($N$) pure gauge theories have a first order confinement-deconfinement phase transition when $N \ge 3$, and are prototype models for studying strongly coupled phase transitions. We use lattice simulations to determine the surface tension in SU($N$) gauge theory up to $N=16$. We observe that the surface tension $\sigma$ scales as $N^2$, and in the continuum limit can be described with $\sigma/T_c^3 = -0.16(4) + 0.0173(11) N^2$ ($N \ge 4$).

Speaker: Kari Rummukainen (University of Helsinki)

suN.pdf

59 Finite-temperature effects in hot axion production

The QCD axion has long been a subject of both theoretical and experimental interest due to being an elegant solution to the strong CP problem. It was also realized that axions can contribute significantly to the energy density of the Universe as dark radiation, that is a population of ultra-relativistic (or "hot") axions thermally produced in the early Universe by interacting with the Standard Model (SM) plasma.
Today, one of the ways to constrain the axion-SM interactions is to compare the theoretically predicted axion contribution to the effective number of neutrinos with measurements obtained by experiments like Planck. The perspective of the next generation of experiments like the future CMB-S4 motivates newer, more precise determinations of thermal axion production.
In this talk I will present recently obtained results on that matter, in the Kim-Shifman-Vainshtein-Zakharov (KSVZ) model where the axion couples only to gluons. The most important step in computations of the thermal production rate, as well as the main source of theory uncertainty, is the implementation of collective effects due to the medium. Those are essential to cure would-be divergences caused by the exchange of "soft" gluons in the t channel.
I present two new computation schemes for determining the production rate and compare them to ones in the literature. Those new schemes solve or improve upon issues of production rate negativity and gauge dependence that can arise in the existing computation schemes. I also discuss how all considered computation schemes behave differently in the case of soft axion momentum and how this contributes to the uncertainty on the theoretical computation. Finally, I show results for quantities of phenomenological interest, like the aforementioned contribution to the effective number of neutrinos. I find that the theoretical contribution is in the ballpark of 0.032, with the error due to the computation scheme choice being around 0.002 at most.
As an outlook, I will also touch on the automated techniques that were used in the research that was presented, and offer some perspectives as to how they could be extended to automate the entire production rate computation.

The results presented in this talk were obtained in 2404.06113, written by myself and Jacopo Ghiglieri.

Speaker: Killian Bouzoud (SUBATECH)

sewm_2024_bouzoud.pdf

10:15 AM Break

Cosmology

Convener: Geraldine Servant

60 Smooth reheating via non-Abelian dark sector

We consider a model, where a single inflaton interacts weakly as an axion with Yang-Mills gauge bosons. As these rapidly thermalize, the friction felt by the inflaton field is increased, leading to a self-amplifying process.
If the gauge bosons of the thermal bath represent a dark sector, the reheating of the Standard Model is then realised through portal interactions.
The dark relic abundance in this scenario depends mostly on the departure from equilibrium after the universe cools down below the critical temperature, when the dark vectors confine into composite states. This constraints the confinement scale of the dark sector. Moreover, indirect detection and Big-Bang nucleosynthesis set bounds on the portal interactions, revealing a predictive parameter space.

Speaker: Simona Procacci (U. Geneva)

SProcacci_SEWM24_w.pdf

61 Corrections to the thermal neutrino interaction rate from QED

The cosmic neutrino background, for which there is ample indirect evidence, is a remnant from when the universe was just one second old. I will discuss the rate at which neutrinos interact with a QED plasma, around MeV temperatures (just prior to their decoupling). Computing the NLO interaction rate as a function of the neutrino momentum and flavour, we recently found relative corrections on the few percent level [1]. One of the main ingredients is the electron-positron spectral function, of which both physical polarization states are needed (transverse and longitudinal with respect to spatial momentum). In addition to the spectral function, the interaction rate also depends on an effective neutrino-antineutrino-photon vertex. The latter gives rise to scatterings with Bose-enhanced $t$-channel photons and hence necessitates the resummation of hard-thermal-loops.

[1] G. Jackson, M. Laine, JHEP 05 (2024), 089 [arXiv:2312.07015]

Speaker: Greg Jackson (SUBATECH, Nantes)

GJ_August2024.pdf

62 Upper bound on thermal gravitational waves from hidden sectors

Hot viscous plasmas unavoidably emit a stochastic gravitational wave background similar to electromagnetic black body radiation. Presenting work published in 2312.13855, we study the hidden particle contribution to the background emitted by the primordial plasma in the early universe. While this contribution can easily dominate over that from Standard Model particles, we find that both are capped by a generic upper bound that makes them difficult to detect with interferometers in the foreseeable future. We finally illustrate our results by considering axion-like particles and heavy neutral leptons.

Speaker: Philipp Klose (Bielefeld University)

klose_SEWM.pdf

12:00 PM Lunch Break

Invited

Convener: Laura Sagunski (Goethe University Frankfurt)

63 Probing dark matter’s particle physics parameters with the small scale structures

In this talk, I highlight the strong constraining power of small scale astrophysical observations. The small scales not only give us a window into the still not precisely measured small scale behaviour of dark matter, but it can also tell us about the particle physics properties of this component. We focus on the ultra-light dark matter models, specifically the fuzzy dark matter (FDM) model, since these models present a rich and distinct phenomenology on small scales. I will show how we can use the different predictions of this model and different astrophysical systems to put the strongest bounds to date on the mass of this ultra-light axion. We also show how these measurements can even give us hints on the spin, self-interaction, and fraction of these bosonic particles.

Speaker: Elisa Ferreira

ElisaFerreira_SEWM_2024_2_compressed.pdf

64 Exploring the cosmic history with gravitational waves

Speaker: Geraldine Servant

Servant-SEWM-Frankfurt2024.pdf

3:15 PM Prize Ceremony and Closing