We propose a new approach to probe the initial fireball geometry in relativistic heavy-ion collisions using spin polarization. Specifically, we introduce polarization harmonics of open heavy hadrons as a novel observable sensitive to geometric anisotropies. Heavy quarks are produced in early hard scatterings and can acquire spin polarization from the strong, transient electromagnetic fields present at early times. As they propagate through the anisotropic quark-gluon plasma, medium-induced interactions lead to path-length dependent depolarization, imprinting an azimuthally anisotropic polarization pattern. Within the framework of rotational Brownian motion, we show that the resulting polarization harmonics are directly related to the initial spatial eccentricities, thereby establishing heavy-flavor polarization anisotropies as a sensitive and complementary probe of the early-time collision geometry. We present quantitative estimates of the second (elliptic) polarization harmonic associated with the recently observed <math altimg='si1.svg'><msup><mi>D</mi><mrow><mo>*</mo><mo>+</mo></mrow></msup></math> spin alignment reported by the ALICE Collaboration.

India

Amaresh Jaiswal

Heavy quark polarization anisotropy as a novel probe of fireball geometry

We investigate the role of <italic>&#964;</italic>-lepton polarization in ultraperipheral heavy-ion collisions (UPCs) as a novel application of the intense electromagnetic fields generated in such processes. In particular, we analyze the decay distributions of polarized <italic>&#964;</italic>-leptons produced via photon-photon fusion, focusing on both leptonic and semi-leptonic channels. We show that the external magnetic field present in UPCs induces a preferred spin quantization axis, which modifies the angular and energy distributions of <italic>&#964;</italic> decay products relative to the standard helicity frame. By formulating the spin polarization along the magnetic field direction, we derive modified polarization-sensitive observables and demonstrate how kinematic selections can retain nonvanishing polarization signals even after ensemble averaging. Furthermore, we propose that the relative polarization of <math altimg='si1.svg'><msup><mi>&#964;</mi><mo>&#8722;</mo></msup></math> and <math altimg='si2.svg'><msup><mi>&#964;</mi><mo>+</mo></msup></math>, accessible through complementary angular ranges of their decay products, serves as a sensitive observable for potential <italic>CP</italic>-violating effects. This framework provides a pathway for future experimental studies at the LHC and future colliders to exploit polarized <italic>&#964;</italic> decays in UPCs as an application of the strong electromagnetic fields to probe new sources of <italic>CP</italic> violation.

Polarized tau decay and <italic>CP</italic> violation in ultraperipheral heavy-ion collisions

We consider the rotational Brownian motion of heavy quark in QCD medium and provide analytical results for polarization of open heavy-flavor hadrons. We calculate expressions for vector and tensor polarization, corresponding to baryon spin polarization and vector meson spin alignment, respectively. Assuming that heavy quarks are initially fully spin polarized along the direction of magnetic field, we compare our results with recent experimental data from ALICE collaboration for <math altimg='si1.svg'><msup><mi>D</mi><mrow><mo>*</mo><mo>+</mo></mrow></msup></math> meson and provide predictions for spin polarization of open charm baryons. We propose that the transverse momentum dependence of heavy quark polarization may serve as a distinctive signature of the intense initial magnetic field generated in off-central relativistic heavy-ion collisions.

Sourav Dey

Rotational Brownian motion and heavy quark polarization in QCD medium

We employ a hybrid approach to describe the light nuclei production mechanism where the nucleons are assumed to be thermally produced, and are allowed to form light nuclei using a coalescence prescription. In this approach, we first fit transverse momentum ( $$p_{T}$$ ) distribution of nucleons using hydro-inspired boost-invariant Blast-wave model. The extracted parameters are then used to describe the deuteron $$p_{T}$$ spectra, along with two additional parameters that characterize the coalescence prescription employed in this study. We refer this combined approach as “thermo-coalescence model” and it is designed to study the deuteron production and describe the experimental measurements. In this work, we analyze the measured $$p_{T}$$ distribution of protons and deuterons from Pb–Pb collisions at the ALICE Collaboration at the LHC. We also evaluate the $$p_{T}$$ -integrated deuteron yields using this approach and compare with experimental measurements. A Bayesian inference framework is employed to determine the best-fit parameters of the thermo-coalescence model. Finally, we estimate the traditionally used experimental coalescence parameter ( $$B_{A}$$ ) within our framework in order to establish a connection between our model and the conventional coalescence approach commonly used to relate experimental data with theoretical descriptions of light nuclei production.

Arun Yadav

Nachiketa Sarkar

Russia

Sudhir Rode

Partha Bhaduri

Abhijit Bhattacharyya

Thermo-coalescence model for light nuclei production in relativistic heavy-ion collisions

In this work, we address the problem of longitudinal spin polarization of the <math xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="normal">Λ</mi></math> hyperons produced in relativistic heavy-ion collisions. We combine a relativistic kinetic-theory framework that includes spin degrees of freedom treated in a classical way with the freeze-out parametrization used in previous investigations. The use of the kinetic theory allows us to incorporate dissipative corrections (due to the thermal shear and gradients of thermal vorticity) into the Pauli-Lubanski vector that determines spin polarization and can be directly compared with the experimental data. As in earlier similar studies, it turns out that a successful description of data can only be achieved with additional assumptions—in our case, they involve the use of projected thermal vorticity and a suitably adjusted time for spin relaxation (<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>τ</mi><mi>s</mi></msub></math>). From our analysis, we find that <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msub><mi>τ</mi><mi>s</mi></msub><mo>∼</mo><mn>5</mn></mrow><mspace width="4pt"></mspace><mrow><mi>fm</mi><mo>/</mo><mi>c</mi></mrow></math>, which is comparable with other estimates.

Soham Banerjee

Poland

Samapan Bhadury

Wojciech Florkowski

Radoslaw Ryblewski

Longitudinal spin polarization in a thermal model with dissipative corrections

We extend the derivation of second-order relativistic viscous hydrodynamics to incorporate the effects of baryon current, a nonvanishing chemical potential, and a realistic equation of state. Starting from a microscopic quantum theory, we employ a quasiparticle approximation to describe the evolution of hydrodynamic degrees of freedom and establish its connection to the Wigner formalism. Using methods from relativistic kinetic theory, we perform a second-order expansion to derive a closed set of equations for the components of the stress-energy tensor and the baryon current. The resulting transport coefficients, which depend on the equation of state, are obtained through a unified prescription that ensures thermodynamic consistency.

Asaad Daher

Leonardo Tinti

Quasiparticle second-order dissipative hydrodynamics at finite chemical potential

We present analytical solution of relativistic hydrodynamics for a system having cylindrical symmetry with boost-invariant longitudinal expansion and Hubble-like transverse expansion. We also consider analytical solution for Hubble-like spherically expanding system. For these two cases, we calculate analytical expression for transverse momentum spectra of hadrons, at constant temperature freeze-out hypersurface using Cooper–Frye prescription. We compare our results for transverse momentum spectra with experimental results from Large Hadron Collider and CERN SPS where one expects cylindrical and spherical geometry of the fireball, respectively. In the case of low-energy collisions with spherical geometry, we calculate rapidity spectra and compare with the results from CERN SPS.

Mahammad Ali

Deeptak Biswas

Italy

Sushant Singh

Hadron momentum spectra from analytical solutions of relativistic hydrodynamics

In this study, we analyze the transport properties of the quark-gluon plasma (QGP), focusing on bulk (<math><mi>ζ</mi></math>) and shear (<math><mi>η</mi></math>) viscosities at vanishing chemical potential. To describe the QGP, we employ a quasiparticle model for quarks along with Gribov’s prescription for gluons, which effectively captures nonperturbative dynamics. The Gribov parameter <math><msub><mi>γ</mi><mi>G</mi></msub></math> and the dynamical mass <math><msub><mi>m</mi><mi>g</mi></msub></math> are obtained by solving the one-loop gap equation in the <math><mover><mi>MS</mi><mo>¯</mo></mover></math> renormalization scheme and further using lattice QCD data for the equation of state (EOS) of pure gluonic matter. The interaction between quarks and gluons is reflected in the quark quasimass <math><msub><mi>m</mi><mi>q</mi></msub></math>, again obtained using lattice EOS data for (<math><mrow><mn>2</mn><mo>+</mo><mn>1</mn></mrow></math>)-flavor QCD. Our primary goal is to investigate the influence of quasiquarks on the transport coefficients of QGP. Interestingly, we find a substantial decrease in the scaled transport coefficients with rising temperatures within the range (<math><mn>1</mn><mo>≤</mo><mi>T</mi><mo>/</mo><msub><mi>T</mi><mi>c</mi></msub><mo>≤</mo><mn>3.5</mn></math>).

Sadaf Madni

Arghya Mukherjee

Najmul Haque

Shear and bulk viscosity of the quark-gluon plasma with Gribov gluons and quasiparticle quarks

The strongly interacting matter created in relativistic heavy-ion collisions possesses several conserved quantum numbers, such as baryon number, strangeness, and electric charge. The diffusion process of these charges can be characterized by a diffusion matrix that describes the mutual influence of the diffusion of various charges. We derive the Kubo relations for evaluating diffusion coefficients as elements of a diffusion matrix. We further demonstrate that in the weak coupling limit, the diffusion matrix elements obtained through Kubo relations reduce to those obtained from kinetic theory with an appropriate identification of the relaxation times. We illustrate this evaluation in a toy model of two interacting scalar fields with two conserved charges.

Hiranmaya Mishra

Diffusion coefficient matrix for multiple conserved charges: a Kubo approach

Equal-time commutators of different components of the energy-momentum tensor at spatially separated points are calculated for a relativistic quantum Fermi gas at finite temperature and density. Different definitions of such components, also known as different pseudogauges, are used and smeared with a Gaussian profile characterized by the width <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>σ</mi></math>. In this way, we introduce observables that may represent measurements of energy and momentum in a spatial region of size <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>σ</mi></math>. We find that the obtained commutators are sensitive to the pseudogauge chosen if the probed systems or the spatial separation are small. The pseudogauge dependence is expected as different quantum operators are analyzed in this case. On the other hand, we find that for sufficiently large probed systems or with large separation, the studied commutators are pseudogauge independent.