The particle production in photon-photon ($\gamma \gamma$) interactions present in electron-ion collisions is investigated. We present calculations for the total cross sections and event rates related to the production of light mesons $[\eta ,{\eta }^{\prime },f_0$ and $f_2]$, charmonium $[{\eta }_c$ and ${\chi }_c]$, and charmoniumlike $\left[X\right(3915),X(3940),X(4140)$, and $X\left(6900\right)]$ states, considering the Electron - Ion Collider, Electron - ion collider in China, Large Hadron electron Collider, and Future Circular Collider - electron hadron energies. Our predictions demonstrate that experimental studies of these processes are feasible and useful to constrain the properties of light mesons and quarkonium states and shed some light on the configuration of the considered charmoniumlike states.

We present an extensive study of quadrupole-hexadecapole correlation effects in even-even Sm and Gd isotopes with neutron number $N=88\text{–}106$. The calculations are performed in the framework of the Gogny energy density functional (EDF) with the D1S parametrization and the $sdg$ interacting boson model (IBM). The quadrupole-hexadecapole constrained self-consistent mean-field potential energy surface is mapped onto the expectation value of the $sdg$-boson Hamiltonian. This procedure determines the parameters of the $sdg$-IBM Hamiltonian microscopically. Calculated excitation energies and transition strengths are compared to the ones obtained with a simpler $sd$-IBM, as well as with the experimental data. The Gogny-EDF mapped $sdg$-IBM reproduces spectroscopic properties of the studied nuclei as reasonably as in the case of the previous $sdg$-boson mapping calculations that were based on the relativistic EDF, indicating that the axial quadrupole-hexadecapole method is sound regardless of whether relativistic or nonrelativistic EDF is employed. The mapped $sdg$-IBM improves some of the results in lighter Sm and Gd isotopes compared to the mapped $sd$-IBM, implying the existence of significant hexadecapole correlations in those nuclei. For those nuclei with $N\ge 94$, hexadecapole effects are minor, and the only significant difference between the two boson models can be found in the description of $E0$ monopole transitions.

The critical spin $I_c$ or critical frequency $\hslash {\omega }_c$ denotes the minimum spin or rotational frequency required for the onset of aplanar chiral rotation in nuclear systems. The triaxial deformation dependence of the pairing effect on $I_c$ and $\hslash {\omega }_c$ are investigated for the particle-hole configuration $\pi \left(1h_{11/2}\right)\otimes \nu {\left(1h_{11/2}\right)}^{-1}$, with triaxial deformation parameters $\gamma ={15}^{\circ }$, ${20}^{\circ }$, ${25}^{\circ }$, ${35}^{\circ }$, ${40}^{\circ }$, and ${45}^{\circ }$, within the particle rotor model. The analyses of the energy-splitting $\mathrm{\Delta }E\left(I\right)$ between the doublet bands, the probability density profiles $\mathcal{P}(\theta ,\phi )$ of the total angular momentum orientation $(\theta ,\phi )$, the maximal and average orientation angles ${\phi }_m$ and $\overline{\phi }$, as well as the derived rotational frequency $\hslash \omega \left(I\right)$ reveal that with increasing neutron pairing interaction, the critical spin $I_c$ is delayed for $\gamma <{30}^{\circ }$, whereas it is advanced for $\gamma >{30}^{\circ }$. In contrast, the critical frequency $\hslash {\omega }_c$ decreases monotonically across all values of $\gamma$. These behaviors are further elucidated by analyzing the angular momentum components of both protons and neutrons along the three principal axes.

High spin states in ${}^{142}\mathrm{Eu}$ have been investigated using the fusion-evaporation reaction ${}^{116}\mathrm{Cd}$ (${}^{31}\mathrm{P}$, $5n$) at the beam energy of 148 MeV using the Indian National Gamma Array. The spin-parity of the observed dipole bands (DB1 and DB2) and the quadrupole bands (QB1 and QB2) have been firmly established from the spectroscopic measurements. Level lifetimes of several levels in the dipole and quadrupole bands have been measured using the Doppler shift attenuation method. Variation of the $B\left(M1\right)$ values with spin has been reproduced well in the framework of the semiclassical shears mechanism with the principal axis cranking model calculations, exhibiting the fact that DB1 and DB2 have been generated from the symmetric shears mechanism with significant collective contribution from the nuclear core. The experimental excitation energy and spin values throughout QB1 and QB2 are compared with the pairing-independent cranked Nilsson-Strutinsky model, which reveals that they are generated due to the rotation of a triaxially deformed core.

The formation of the compound nucleus in heavy-ion collisions is considered in two steps. The first step is related to the penetration of the ions through the capture barrier, the formation of the sticking nuclei in the capture well, and the transfer of kinetic energy of relative motion into the intrinsic energy of the sticking nuclei. The second step is associated with the compound-nucleus formation from the sticking nuclei in competition to the quasielastic scattering. The compound nucleus is preferably formed when the height of the compound nucleus formation barrier is substantially smaller than the height of the quasielastic barrier. This leads to the values of the capture and compound nucleus formation cross sections being close. In contrast, the formation of the compound nuclei is suppressed when the height of the compound-nucleus formation barrier is close to or larger than that of the quasielastic barrier. In this case, the capture cross sections are greater than the compound-nucleus formation cross sections. The suppression of the compound-nucleus formation is increased with the mass and charge of ions colliding with lead because the quasielastic barrier height obtained relative to the compound nucleus ground state is decreased with the mass and charge of ions colliding with lead and the role of the compound-nucleus formation barrier rises. The dependence of the compound-nucleus formation suppression on the shell-correction energy of the compound nucleus is shown. The available experimental data for 14 heavy-ion reactions induced by lead are well described in the model.

Various azimuthal anisotropies ($v_1,v_2,v_3,v_4$), at high transverse momentum (high $p_T$), are shown to arise from the asymmetric scattering of transverse polarized quarks and gluons, arising from unpolarized nucleons (the Boer-Mulders effect) and resulting in unpolarized hadrons (the Collins effect). Combined with the asymmetric scattering of partons from polarization independent but transverse momentum dependent (TMD) distributions, we obtain a possible mechanism to understand the azimuthal anisotropy of hadrons at large transverse momentum observed in $p\text{−}p$ collisions. Constraining the ratio of polarization dependent TMD distributions to polarization independent distributions by comparing with the data from $p\text{−}p$ collisions, we find that scaling the acquired transverse momentum of the initial state partons from the proton, due to prescattering before the hard interaction, with the length of the nucleus ($k_{\perp }^2\propto A^{1/3}$), straightforwardly yields the azimuthal anisotropy at high $p_T$ in $p\text{−}A$ collisions.

In noncentral heavy-ion collisions, the particle-emitting source can be tilted away from the beam direction, an effect that becomes particularly significant at collision energies of a few GeV and lower. This phenomenon, manifest itself in many observables such as directed flow, polarization, and vorticity, is therefore important to investigate. In this paper, we study the consistency between the tilt extracted directly from the freeze-out distribution of pions and the tilt parameter obtained using the azimuthally sensitive femtoscopy (asHBT) method. Using the Ultrarelativistic Quantum Molecular Dynamics model, we demonstrate a strong dependence of the tilt parameter extracted with asHBT on the momentum of the particle pair. Considering the experimental challenges in accessing low particle momenta—where the tilt parameter extracted with asHBT closely matches the tilt of the freeze-out distribution of pions—we propose an exponential extrapolation method to obtain the tilt of the entire freeze-out distribution. This approach aims to enhance the accuracy of experimental measurements of tilt in noncentral heavy-ion collisions.

Fluctuations between charged and neutral kaons measured by the ALICE Collaboration in Pb-Pb collisions at the CERN Large Hadron Collider (LHC) exceed conventional explanations. Previously it was shown that if the scalar condensate is accompanied by an electrically neutral isospin-1 field then the combination can produce large equilibrium fluctuations where $\langle \overline{u}u\rangle \ne \langle \overline{d}d\rangle$. Hadronizing strange and antistrange quarks might then strongly fluctuate between charged ($u\overline{s}$ or $s\overline{u}$) and neutral ($d\overline{s}$ or $s\overline{d}$) kaons. Here, we estimate the times for the condensates to achieve their equilibrium probability distributions within causal volumes in high energy heavy ion collisions. This is achieved by modeling the temperature dependence of the condensates, mesonic collective excitations, decay rates of the associated fields, and employing the Langevin and Fokker-Planck equations. Within this model, we find that the equilibration times are short compared with the expansion time, making disoriented isospin condensates a viable explanation for the anomalous fluctuations observed at the LHC.

In the present study, we employ three distinct, physically motivated speed of sound bounds to construct hybrid models, where the high-density phase is described by the maximally stiff equation of state. In particular, we consider the bounds related to special relativity, relativistic kinetic theory, and conformality. The low-density hadronic phase is described by a state-of-the-art microscopic relativistic Brueckner-Hartree-Fock theory. This work aims to access the effect of the different speed of sound constraints on the relevant parameter space of the key parameters of first-order phase transitions by utilizing recent astronomical data. This involves a systematic analysis that also includes two distinct schemes for the construction of hybrid models (abrupt and smooth). Finally, a relevant discussion is conducted on the possible occurrence of a thermodynamic inconsistency that is related to the stability of the high-density phase over hadronic matter at large densities.

In the SMASH-CLVisc-hybrid framework, including SMASH for the initial conditions and the hadronic rescattering stage, and CLVisc for the quark gluon plasma evolution, we investigate net baryon number fluctuations via considering the equation of state (EoS) with and without a critical end point (CEP) in the quantum chromodynamic (QCD) phase transition. Specifically, two distinct QCD EoSs are utilized: one with smooth crossover derived from NEOS and another with a critical end point sourced from the rPNJL model. Our results show that nonmonotonic behavior of $\kappa {\sigma }^2$ is not observed above the collision energy $7.7\mathrm{GeV}$, nor are there explicit differences between the EoS characterized by crossover and that by CEP. This could be attributed to the significant deviation of the freeze-out line from the location of CEP. It is also found that the pure SMASH result of $\kappa {\sigma }^2$ is positive and close to zero at 3 GeV, which is different from the negative value observed from STAR.

We report on the masses and hyperfine structure of ground and isomeric states in ${}^{114,116,118,120}\mathrm{Ag}$ isotopes, measured with the phase-imaging ion-cyclotron-resonance technique (PI-ICR) with the JYFLTRAP mass spectrometer and the collinear laser spectroscopy beamline at the Ion Guide Isotope Separator On-Line facility, Jyväskylä, Finland. We measured the masses and excitation energies, electromagnetic moments, and charge radii, and firmly established the nuclear spins of the long-lived states. A new isomer was discovered in ${}^{118}\mathrm{Ag}$ and the half-lives of ${}^{118}\mathrm{Ag}$ long-lived states were reevaluated. We unambiguously pinned down the level ordering of all long-lived states, placing the inversion of the $I=0^{-}$ and $I=4^{+}$ states at $A=118$ $(N=71)$. Last, we compared the electromagnetic moments of each state to empirical single-particle moments to identify the dominant configuration where possible.

Search for the population of the low-energy continuum of a tetraneutron system was performed for reactions of a ${}^8\mathrm{He}$ beam on a deuterium target. These studies are based on the data [I. A. Muzalevskii et al., Phys. Rev. C 103, 044313 (2021)] previously used for the studies of ${}^7\mathrm{H}$ and ${}^6\mathrm{H}$ in the ${}^2\mathrm{H}{({}^8\mathrm{He},{}^3\mathrm{He})}^7\mathrm{H}$ and ${}^2\mathrm{H}({}^8\mathrm{He},{}^4\mathrm{He}){}^6\mathrm{H}$ reactions. Evidence for a hump in the ${}^4\mathrm{n}$ continuum at $3.5\pm 0.7$ and $3.2\pm 0.8\mathrm{MeV}$ was observed in the ${}^2\mathrm{H}({}^8\mathrm{He},{}^6\mathrm{Li}){}^4\mathrm{n}$ and ${}^2\mathrm{H}({}^8\mathrm{He},{}^3\mathrm{He}){}^7\mathrm{H}\to {}^3\mathrm{H}+{}^4\mathrm{n}$ reactions, respectively. The observed statistics is quite low (6 events and up to 40 events) corresponding to very low cross sections of few microbarns or tens of microbarns. The background conditions for the ${}^2\mathrm{H}({}^8\mathrm{He},{}^6\mathrm{Li}){}^4\mathrm{n}$ reaction are shown to be good, favoring the physical nature of the observed events. The ${}^2\mathrm{H}({}^8\mathrm{He},{}^3\mathrm{He}){}^7\mathrm{H}\to {}^3\mathrm{H}+{}^4\mathrm{n}$ process transforms to the ${}^2\mathrm{H}({}^8\mathrm{He},{}^6\mathrm{Li}^{*}){}^4\mathrm{n}$ reaction in the limit of the highest ${}^7\mathrm{H}$ decay energies. The population of the low-energy region in the ${}^4\mathrm{n}$ spectrum is found to be perfectly correlated with the population of the lowest ${}^6\mathrm{Li}$ state in the ${}^3\mathrm{He}+{}^3\mathrm{H}$ continuum with $E^{*}=18\mathrm{MeV}$. Theoretical calculations of ${}^8\mathrm{He}$ in five-body $\alpha +4n$ and of ${}^4\mathrm{n}$ in four-body hyperspherical models are presented. The ${}^8\mathrm{He}$ wave function is shown to contain strong specific correlations, which may give rise to very low-energy structures in the tetraneutron continuum in extreme-peripheral reaction scenarios.

This paper investigates the dynamics of the net-baryon multiplicity fluctuations near the QCD critical point, within the inhomogeneous temperature and baryon chemical potential profiles of quark-gluon plasma from the hydrodynamics. The Langevin dynamics of conserved net-baryon density are solved numerically within the temperature and chemical potential profiles borrowing from hydrodynamic simulation. It is found that the local systems at different rapidities reach the critical point at different proper times, owing to the inhomogeneous temperature and chemical potential. As a result, the pronounced enhancement of the magnitude for the net-baryon multiplicity fluctuations with large rapidity acceptance at the freeze-out surface is observed, which is the consequence of the combined effect of critical slowing down and an inhomogeneous profile.

We discuss the emission of $\varphi$ mesons with longitudinal and transverse polarization in ultrarelativistic heavy ion collisions. In the hadronic phase and in leading order in the kaon diluteness, $\varphi$ emission is isotropic and driven by the flavor singlet and octet vector spectral functions. At next-to-leading order, the emissivities receive a nonisotropic correction from the flavor octet spectral function, and the nonequilibrium contributions originating from the shear and bulk viscosities. Implications for the $\varphi$ meson alignment in heavy-ion collisions are discussed.

Single-particle and collective excitations in ${}^{116}\mathrm{Sb}$ are studied using the reaction ${}^{115}\mathrm{In}(\alpha ,3n){}^{116}\mathrm{Sb}$ at a beam energy of 40 MeV and investigated using $\gamma$-ray spectroscopic techniques. The existing level scheme is extended with the observation of several new transitions. The previously known band structures are extended to higher spins. Measurements of the directional correlation from oriented states (DCO) ratio and polarization asymmetry of the observed $\gamma$ rays are carried out to assign the spin-parities of the excited levels, populated in the present work. New band structures are interpreted as being due to the coupling of the valence particles with the 2p-2h intruder states of the Sn core. Further, the collective band structures are discussed on the basis of their rotational properties, total Routhian surface calculations, and the triaxial projected shell model approach.

In this work, we present new data on the ${}^{197}\mathrm{Au}$ photoneutron reactions in and above the giant dipole resonance region, obtained by using 8 to 39 MeV quasimonochromatic $\gamma$-ray beams produced at the NewSUBARU facility in Japan and a high-and-flat efficiency neutron detection system. We report absolute cross sections and mean photoneutron energies for the ${}^{197}\mathrm{Au}(\gamma ,inX)$ reactions with $i=1$ to 4. The photoabsorption cross section was obtained as the sum of the $(\gamma ,inX)$ reaction cross sections. The giant dipole resonance parameter values were obtained by fitting the experimental photoabsorption cross sections. The present photoabsorption cross sections are in good agreement with the Saclay results of Veyssiere et al., Nucl. Phys. A 159, 561 (1970). Thus, our study does not support the recommendation of Berman et al., Phys. Rev. C 36, 1286 (1987), of lowering the Saclay photoabsorption cross sections by $8\%$. We observed a nonstatistical high-energy neutron emission in the $(\gamma ,n)$ reaction in the low-energy region between $S_n$ and 10 MeV. The present results are compared with data from the literature and statistical model calculations performed with the TALYS and EMPIRE codes.

Chiral effective field theory ($\chi \mathrm{EFT}$) has proved to be a powerful microscopic framework for predicting the properties of neutron-rich nuclear matter with quantified theoretical uncertainties up to about twice the nuclear saturation density. Tests of $\chi \mathrm{EFT}$ predictions are typically performed at low densities using nuclear experiments, with neutron star (NS) constraints only being considered at high densities. In this work, we discuss how asteroseismic quasinormal modes within NSs could be used to constrain specific matter properties at particular densities not just the integrated quantities to which bulk NS observables are sensitive. We focus on the crust-core interface mode, showing that measuring this mode's frequency would provide a meaningful test of $\chi \mathrm{EFT}$ at densities around half the saturation density. Conversely, we use nuclear matter properties predicted by $\chi \mathrm{EFT}$ to estimate that this mode's frequency is around $185\pm 50\text{Hz}$. Asteroseismic observables such as resonant phase shifts in gravitational-wave signals and multimessenger resonant shattering flare timings, therefore, have the potential to provide useful tests of $\chi \mathrm{EFT}$.

The implications of perturbative QCD (pQCD) calculations on neutron stars are carefully examined. While pQCD calculations above baryon chemical potentials ${\mu }_B\approx 2.4$ GeV demonstrate the potential of ruling out a wide range of neutron star equations of state (EOSs), such constraints only affect the most massive neutron stars in the vicinity of the Tolman-Oppenheimer-Volkoff limit, resulting in constraints that are orthogonal to current or expected astrophysical bounds. In the most constraining scenario, pQCD considerations favor low values of the squared speed sound $C_s$ at high ${\mu }_B$ relevant for the most massive neutron stars, but leave predictions of the radii and tidal deformabilities almost unchanged. Such considerations become irrelevant if the maximum speed of sound squared inside neutron stars does not exceed about $C_{s,\max }\approx 0.5$, or if pQCD breaks down below ${\mu }_B\approx 2.9$ GeV. Furthermore, the large pQCD uncertainties preclude any meaningful bounds on the neutron star EOS at the moment. Interestingly, if pQCD predictions for the pressure at around ${\mu }_B\approx 2.5$ GeV are refined and found to be low ($\lesssim 1.5$ GeV/${\mathrm{fm}}^3$), evidence for a soft neutron star inner core EOS in combination with the existence of two-solar-mass pulsars would indicate the presence of color superconductivity beyond neutron star densities. I point out that two-solar-mass pulsars place robust upper bounds on this nonperturbative effect and require the pairing gap to be less than ${\mathrm{\Delta }}_{\mathrm{CFL}}\le 500\text{MeV}$ at ${\mu }_B\approx 2.5\text{GeV}$.

The initial conditions of one-dimensional expanding viscous fluids in relativistic heavy-ion collisions are scrutinized in terms of nonlinear causality of the relativistic hydrodynamic equations. Conventionally, it is believed that the matter generated in relativistic heavy-ion collisions starts to behave as a fluid all at once at some initial time. However, it is by no means trivial how soon after the first contact of two high-energy nuclei the fluid picture can be applied. It is demonstrated that one-dimensional expanding viscous fluids violate the necessary and the sufficient conditions of nonlinear causality at large departures from local equilibrium. We therefore quantify the inverse Reynolds number to justify the validity of the hydrodynamic description. The initial conditions are strictly constrained not to violate the causality conditions during the time evolution. With the help of the transverse energies per rapidity measured at the BNL Relativistic Heavy Ion Collider (RHIC) and the CERN Large Hadron Collider (LHC), we obtain the minimum initial proper time and the maximum energy density allowed by nonlinear causality. This analysis strongly suggests that the initial stage of relativistic heavy-ion collisions needs to be described by a nonequilibrium framework other than relativistic dissipative hydrodynamics.