abstract

Gluon and graviton radiation in strong field shockwave scattering are described by effective Lipatov vertices, with the graviton Lipatov vertex proportional to the bilinear of its QCD counterpart. We show here that the \(n\)-particle gluon radiation spectrum can be described as a generalized Susskind–Glogower (gSG) squeezed coherent state and discuss the properties of such squeezed states. The double-copy structure of the radiative frameworks suggests that multi-graviton radiation can be similarly described as a gSG state. We examine the physical parameter space and show that very large squeezing parameters \(\sim \ln ({\bar n})\) (where \({\bar n}\) is the mean graviton occupancy) are feasible for nearly minimal uncertainty configurations of the gSG state. Quantum noise in the corresponding gravitational wave spectrum is enhanced above the sensitivity of current and future gravitational wave detectors. Our results point to the importance of a comprehensive study of the strong field Lipatov regime of gravitational radiation.

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Different bases for the spin-1 density matrix are discussed to clarify the connection between its components and observables measured in heavy-ion collisions. The theoretical advantage of using the adjoint representation for spin matrices is emphasized. Next, the equilibrium spin density matrix and the corresponding Wigner function are introduced. With appropriate definitions of the energy-momentum and spin tensors, this framework allows for the formulation of perfect spin hydrodynamics in the same way as previously done for spin-\(\nicefrac {1}{2}\) particles. Together, these results provide a unified description of spin-\(\nicefrac {1}{2}\) and spin-1 particles.

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Hadron production in relativistic nuclear collisions is well described in the framework of the statistical hadronization model, over a broad range of collision energies. We outline this for hadrons composed of light (\(u, d, s\)) and heavy (charm and beauty) quarks, discuss recent findings relevant for understanding the phase structure of QCD, and formulate some open issues.

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Exploring the QCD phase diagram through relativistic heavy-ion collisions is a primary goal of modern nuclear physics. This contribution focuses on fluctuations and correlations of conserved charges — specifically net-baryon and net-charge cumulants — as sensitive probes of the phase structure and the QCD critical point. We discuss recent theoretical and experimental advancements, highlighting constraints from lattice QCD and new results from the RHIC Beam Energy Scan program. Key challenges in theory-to-experiment comparisons at high baryon density are discussed, alongside the systematic requirements for meaningful physical interpretation. Finally, we identify open issues and outline the discovery potential of future low-energy experiments, such as CBM, in resolving the high-density regime of the phase diagram.

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QCD at finite isospin density is considered for a large number of colors \(N_c\). A linear sigma model is used to model the meson content of the theory at low density. At isospin chemical potential \(\mu _I \ll {\mit \Lambda }_{\mathrm {QCD}}\), this matter forms a Bose condensate. For \(\mu _I \gg \sqrt {N_c} {\mit \Lambda }_{\mathrm {QCD}}\), unlike QCD, remains confined, but the degrees of freedom of the system are mesons and Cooper pairs bound on size scales small compared to the QCD size scale determined by the superfluid gap. For most purposes, this matter may be analyzed using weak coupling methods. For \( {\mit \Lambda }_{\mathrm {QCD}} \le \mu _I \le \sqrt {N_c} {\mit \Lambda }_{\mathrm {QCD}}\), we argue that meson matter is quarkyonic, with quarks bound into mesons on a size scale of order \({\mit \Lambda }_{\mathrm {QCD}}\) corresponding to a filled Fermi sea of quarks, with possible Bose condensation at the Fermi surface and/or Cooper pairs with a finite width of the surface of order \({\mit \Lambda }_{\mathrm {QCD}}\).

abstract

The wounded nucleon model was invented to account for the multiplicity of particles produced in nucleus–nucleus collisions. As such, it emphasizes the features of particle production along the collision axis. It contains, however, another important aspect, which is the main focus of this note, that the early stages of high-energy nuclear collisions are determined by the locations of the wounded nucleons in the transverse plane. Thanks to the high-precision data of the LHC, this may, in return, be used to infer fine nuclear structure aspects of the colliding nuclei.

abstract

Femtoscopy, historically associated with Hanbury-Brown–Twiss (HBT) interferometry, has evolved into a precision tool for investigating the space-time structure of particle-emitting sources created in high-energy collisions. While the term HBT is often used, it captures only a subset of the broader class of femtoscopic correlation techniques based on quantum-statistical correlations. This review provides a systematic overview of femtoscopic measurements across a broad range of collision energies and system sizes. We explore the “sizes” of homogeneity regions through detailed energy scans, spanning from the low-energy regime of HADES to the highest energies of the RHIC Beam Energy Scan and the LHC. We examine the system-size dependence of these regions, including the intermediate regime of \(p\)–Pb collisions at the LHC, which serves as a bridge between elementary \(p\)–\(p\) and dense \(A\)–\(A\) systems, while highlighting the breakdown of universal multiplicity scaling due to initial-state geometric effects. Furthermore, we discuss non-identical particle correlations, such as \(\pi \)–\(K\), as a unique probe of relative space-time emission asymmetries, providing independent evidence for collective transverse expansion. The review also addresses “interaction femtoscopy” exploiting the sensitivity of correlation functions to final-state interactions in order to extract scattering parameters for (anti)protons, \({\mit \Lambda }\) hyperons, and light nuclei. These measurements impose important constraints on hyperon–nucleon and three-body interactions, supplying essential input for chiral effective field theory and the equation of state of dense nuclear matter, with significant implications for the internal structure of neutron stars.

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We discuss first the discovery of the odderon by the TOTEM and D0 collaborations. We then describe the gap between jet measurements sensitive to the high gluon density regime and the possible observation of the saturation phenomenon in PbPb interactions. We also mention the sensitivity to beyond the Standard Model physics and to the production of axion-like particles via photon–photon interactions.

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We describe the physics program at the Relativistic Heavy Ion Collider (RHIC) with forward protons measured in the Roman Pot system. The program started as a standalone PP2PP experiment with a goal of measuring of proton–proton elastic scattering. The PP2PP experiment took data at RHIC as a dedicated experiment at the beginning of RHIC operations. To expand the physics program to include non-elastic channels with forward protons, such as Central Exclusive Production (CEP), Central Production (CP), and Single Diffraction Dissociation (SD), the experiment with its equipment was merged with the STAR experiment at RHIC. This allowed for a more comprehensive diffractive physics program with measured forward protons. The expanded program, which included both elastic and inelastic proton–proton scattering, became part of the physics program and operations of the STAR experiment. The results obtained by both programs are described here. Given that proton beams at RHIC were polarized, the results include spin dependence in proton–proton elastic scattering.

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We examine the Regge theoretical properties for the scaling observed in \(pp\) elastic scattering differential cross sections at the LHC. A positive signature amplitude (i.e. the Pomeron) with scaling properties has been derived. It is found to describe the dip–bump region of momentum transfer at LHC energies in agreement with data. We derive the analytic continuation in the whole plane of the \(t\)-channel partial waves of index \(l_t\) specific to the Regge formalism. The analytic form of the partial-wave amplitude exhibits a specific scaling property without singularities, except for a series of poles in the \(l_t\) real axis at fractional values.

abstract

Potential and corresponding electric and magnetic fields of a moving point-like charge are computed in several ways. The subtle limit of the charge velocity equal to the speed of light is taken either in the final formulas or in the equation of motion. The results are discussed.

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This contribution reviews recent progress in the low-lying scalar mesons and glueballs. We propose a new classification for the scalar nonet that includes \(f_0(980)\) and \(a_0(980)\) as the lowest states, while we identify \(f_0(1500)\) as a primary glueball candidate. We demonstrate that the production yields of these states in heavy-ion collisions are mutually consistent across statistical, coalescence, and S-matrix frameworks. To investigate their internal structure, we move beyond standard phenomenology by describing glueballs as topological solitons. This approach yields an energy spectrum in excellent agreement with lattice QCD and experimental data, while interpreting \(f_0(2470)\) as a tightly bound glueballonium to explain its anomalously long lifetime. This non-perturbative framework provides a predictive basis for the future experimental verification of exotic scalar states.

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This year marks the \(350^{\mathrm {th}}\) anniversary of the discovery of the first animalcula (little animals) by van Leeuwenhoek in 1676. Flavour physics makes it possible to search for new animalcula at distance scales far shorter than those resolved by van Leeuwenhoek in 1676, and even shorter than those directly accessible at the Large Hadron Collider and the planned colliders in this century. I summarize various strategies for achieving this goal. While precise measurements of a wide variety of observables and their precise theoretical calculations, both within the Standard Model (SM) and beyond it, are indispensable in this context, in my view, it is crucial to develop strategies for the search for New Physics (NP) that go beyond the global fits that are very popular today. While effective field theories such as WET and SMEFT are formulated in terms of Wilson coefficients of the relevant operators, with correlations characteristic of the SM and of specific NP scenarios, the most direct tests of the SM and its extensions are, in my opinion, correlations among different observables, like branching ratios of numerous decays, that are characteristic of particular new animalcula at work.

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In a recently proposed approach to testing models of inflation by Cosmic Microwave Background (CMB) radiation, the reheating temperature is directly expressed in terms of the CMB observables. Its model-independent bounds translate in a given model into narrow ranges of those observables. In that approach, we analyse the polynomial class of the \(\alpha \)-attractor inflaton potential models (P-models), in a broad range of polynomials and with the inflaton decays and fragmentation in the reheating period taken into account. The predictions for the CMB observables, the scalar spectral index \(n_{\mathrm {s}}\) and tensor-to-scalar ratio \(r\), are compared with the Planck and Planck combined with ACT data. Both can be accommodated by that class of the \(\alpha \)-attractor models. The sensitivity of the results of that comparison to the reheating temperature and to the upper bound on the ratio \(r\) is clearly demonstrated.

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Teleportation, introduced in science fiction literature, is an instantaneous change of the position of a macroscopic object. Two teleportation-like phenomena have been predicted by quantum mechanics: quantum teleportation and, more recently, quantum particle teleportation. Here, we introduce the third teleportation-like phenomenon — apparent teleportation. It seems to be a natural consequence of the Standard Model’s indistinguishable elementary particles and antiparticles. We illustrate the idea within a 1+1D toy model of particle–antiparticle creation and space-time evolution obeying transport locality. Furthermore, we propose a novel method to observe apparent teleportation driven by strong interactions through measurements of correlations between the momenta of charm and anticharm hadrons in nuclear collisions. Observing the apparent teleportation would uncover the basic transport properties of indistinguishable particles.