Measurements of inclusive and direct photon production at midrapidity in $pp$ collisions at $\sqrt{s}=2.76$ and 8 TeV are presented by the ALICE experiment at the LHC. The results are reported in transverse momentum ranges of $0.4<p_{\mathrm{T}}<10 \text{GeV}/c$ and $0.3<p_{\mathrm{T}}<16 \text{GeV}/c$, respectively. Photons are detected with the electromagnetic calorimeter (EMCal) and via reconstruction of $e^{+}{e}^{-}$ pairs from conversions in the ALICE detector material using the central tracking system. For the final measurement of the inclusive photon spectra the results are combined in the overlapping $p_{\mathrm{T}}$ interval of both methods. Direct photon spectra, or their upper limits at 90% C.L. are extracted using the direct photon excess ratio $R_{\gamma }$, which quantifies the ratio of inclusive photons over decay photons generated with a decay-photon simulation. An additional hybrid method, combining photons reconstructed from conversions with those identified in the EMCal, is used for the combination of the direct photon excess ratio $R_{\gamma }$, as well as the extraction of direct photon spectra or their upper limits. While no significant signal of direct photons is seen over the full $p_{\mathrm{T}}$ range, $R_{\gamma }$ for $p_{\mathrm{T}}>7 \text{GeV}/c$ is at least one $\sigma$ above unity and consistent with expectations from next-to-leading order pQCD calculations.

We discuss recent experimental results concerning the cross section ratio of positron over electron elastic scattering on protons, and compare with the predictions of a pre-existent calculation. The deviation from unity of this ratio, i.e., a charge asymmetry different from zero, is the signature of contributions beyond the Born approximation. After reviewing the published results, we compare the elastic data to a calculation which includes the diagram corresponding to two-photon exchange. It turns out that all the data on the cross section ratio, in the limit of their precision, do not show evidence of enhanced two-photon contribution beyond the expected percent level. Our results confirm that experimental evidence for a large contribution of two-photon exchange is not yet found.

Two-particle, pair-number correlation distributions on two-dimensional transverse momentum ($p_{t1},p_{t2}$) constructed from the particle production in relativistic heavy-ion collisions allow access to dynamical processes in these systems beyond what can be studied with angular correlations alone. Only a few measurements of this type have been reported in the literature, and phenomenological models, which facilitate physical interpretation of the correlation structures, are nonexistent. Ongoing efforts at the Relativistic Heavy-Ion Collider (RHIC) will provide a significant volume of these correlation measurements in the future. In anticipation of these new data, two phenomenological models are developed which describe two-dimensional (2D) correlation distributions on transverse momentum. One model is based on a collision event-by-event fluctuating blast wave. The other is based on event-by-event fluctuations in fragmenting color-flux tubes and in jets. Both models are shown to be capable of accurately describing the measured single-particle $p_t$ distributions for minimum-bias Au+Au collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. Both models are then applied to preliminary, charged-particle correlation measurements on 2D transverse momentum. The capabilities of the two models for describing the overall structure of these correlations, the stability of the fitting results with respect to collision centrality, and the resulting trends of the dynamical fluctuations are evaluated. In general, both phenomenological models are capable of qualitatively describing the major correlation structures on transverse momentum and can be used to establish the required magnitudes and centrality trends of the fluctuations. Both models will be useful for interpreting the forthcoming correlation data from the RHIC.

A quantitative knowledge of the nuclear equation of state (EoS) requires an accurate estimation of the uncertainties on the EoS parameters and their mutual correlations. Such correlations are empirically observed in a large set of EoS models by different authors, but they are not always fully understood. We show that some of these correlations can be interpreted from basic physical constraints imposed on a simple Taylor expansion of the binding energy around saturation density. In particular, we investigate the correlations among the following empirical parameters: the symmetry energy $E_{\mathrm{sym}}$, the slope and curvature of the symmetry energy $L_{\mathrm{sym}}$ and $K_{\mathrm{sym}}$, and the curvature and skewness of the binding energy in symmetric matter $K_{\mathrm{sat}}$ and $Q_{\mathrm{sat}}$. The uncertainties on these correlations is estimated through analytical modeling as well as a metamodeling analysis of the EoS subject to physical constraints. We show that a huge dispersion of the correlations among low-order empirical parameters is induced by the unknown higher-order empirical parameters, such as $K_{\mathrm{sym}}$ (second order) and $Q_{\mathrm{sat}}$ and $Q_{\mathrm{sym}}$ (third order). We also propose an explanation of the reason why $Q_{\mathrm{sat}}$ is weakly constrained by present experimental data. We conclude that selective observables on high-order parameters, such as $K_{\mathrm{sym}}$ and $Q_{\mathrm{sat}}$, should be better determined before the present uncertainties on the EoS can be further reduced.

We propose a dynamical mechanism which provides an interaction between the spins of hyperons and antihyperons and the vorticity of the baryon current in noncentral high energy nuclear collisions. The interaction is mediated by massive vector and scalar bosons, which is well known to describe the nuclear spin-orbit force. It follows from the Foldy-Wouthuysen transformation and leads to a strong-interaction Zeeman effect. The interaction may explain the difference in polarizations of $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ hyperons as measured by the STAR Collaboration at the BNL Relativistic Heavy Ion Collider. The signs and magnitudes of the meson-baryon couplings are closely connected to the binding energies of hypernuclei and to the abundance of hyperons in neutron stars.

Within the framework of chiral effective field theory, perturbative calculation for $NN$ scattering is carried out in partial waves with orbital angular momentum $L⩾1$. The primary goal is to identify the lowest angular momenta at which perturbative treatment of chiral forces can apply. Results up to the order where the subleading two-pion exchange appears are shown. It is concluded that perturbation theory applies to all partial waves but ${}^{1}S_0, {}^{3}S_1–{}^{3}D_1$, and ${}^{3}P_0$. Where it is applicable, perturbation theory with the delta-less chiral forces produces good agreement with the empirical phase shifts up to $k_{\text{c.m.}}\simeq 300$ MeV.

Asymptotic normalization coefficients (ANCs) are fundamental nuclear constants that play an important role in nuclear reactions, nuclear structure, and nuclear astrophysics. In this paper a connection between ANCs and resonance widths of the mirror states is established. Using the Pinkston-Satchler equation the ratio for resonance widths and ANCs of mirror nuclei is obtained in terms of the Wronskians from the radial overlap functions and regular solutions of the two-body Schrödinger equation with the short-range interaction excluded. This ratio allows one to use microscopic overlap functions for mirror nuclei in the internal region, where they are the most accurate, to correctly predict the ratio of the resonance widths and ANCs for mirror nuclei, which determine the amplitudes of the tails of the overlap functions. If the microscopic overlap functions are not available one can express the Wronskians for the resonances and mirror bound states in terms of the corresponding mirror two-body potential-model wave functions. A further simplification of the Wronskian ratio leads to the equation for the ratio of the resonance widths and mirror ANCs, which is expressed in terms of the ratio of the two-body Coulomb scattering wave functions at the resonance energy and at the binding energy [N. K. Timofeyuk, R. C. Johnson, and A. M. Mukhamedzhanov, Phys. Rev. Lett. 91, 232501 (2003)]. Calculations of the ratios of resonance widths and mirror ANCs for different nuclei are presented. From this ratio one can determine the resonance width if the mirror ANC is known and vice versa. Comparisons with available experimental ratios are done.

We propose a new variational method for describing nuclear matter from nucleon-nucleon interaction. We use the unitary correlation operator method (UCOM) for central correlation to treat the short-range repulsion and further include the two-particle two-hole (2p2h) excitations of nucleon pair involving a large relative momentum, which is called high-momentum pair (HM). We describe nuclear matter in finite size with finite particle number on periodic boundary condition and increase the 2p2h configurations until we get the convergence of the total energy per particle. We demonstrate the validity of this UCOM+HM framework by applying it to the symmetric nuclear and neutron matters with the Argonne $\mathrm{V}{4}^{\prime }$ potential having short-range repulsion. The nuclear equations of state obtained in UCOM+HM are fairly consistent to those of other calculations such as Brueckner-Hartree-Fock and auxiliary field diffusion Monte Carlo in the overall density region.

We present calculations of nucleon-deuteron scattering as well as ground and low-lying excited states of light nuclei in the mass range $A=3–16$ up through next-to-next-to-leading order in chiral effective field theory using semilocal coordinate-space regularized two- and three-nucleon forces. It is shown that both of the low-energy constants entering the three-nucleon force at this order can be determined from the triton binding energy and the differential cross section minimum in elastic nucleon-deuteron scattering. From all considered nucleon-deuteron scattering observables, the strongest constraint on these low-energy constants emerges from the precisely measured cross section minimum at $E_N=70$ MeV. The inclusion of the three-nucleon force is found to improve the agreement with the data for most of the considered observables.

The newly developed relativistic-mean-field and complex-momentum-representation (RMF-CMR) method is used to explore the exotic structure for the neutron-rich Ca, Ni, Sn, and Pb isotopes. The results are compared with the relativistic Hartree-Bogoliubov (RHB) calculations as well as the available data. The calculated root mean square (rms) radius of the neutron and proton suggests a halo or giant halo structure in the Ca isotopes with neutron number more than $N=40$, and a thick neutron skin in the extremely neutron-rich Ni, Sn, and Pb isotopes. The extracted two-neutron separation energies agree the experimental data as well as the RHB calculations. The conclusions are supported by the available single particle levels, the valence nucleon occupations, the contributions of resonant and weakly bound levels to the neutron rms radius, the density distributions of the occupation levels, and the total neutron, proton, and matter densities. Especially, it is found that the unusually increasing radius comes mainly from the contributions of the weakly bound levels with lower orbital angular momentum, and the diffuse density distributions, like the halo, originate mostly from the resonant levels with lower orbital angular momentum in the vicinity of the continuum threshold. The predication on the structure of halo and skin in the neutron-rich Ca, Ni, Sn, and Pb isotopes are of referential value for experiment.

We present an experimental investigation of thermal phase transition in atomic nuclei by measuring the $\gamma$ rays from the decay of the giant dipole resonance in ${}^{169}\mathrm{Tm}$ populated by using the reaction ${}^4\mathrm{He}+{}^{165}\mathrm{Ho}$. The systematic measurement confirms the prolate shape, similar to the ground-state value, till temperature $T=1.23$ MeV. Moreover, the present data, together with the previous experimental studies, point towards the persistence of prolate shape with deformation similar to that of the ground state till $T=1.5$ MeV. In addition, the emergence and evolution of thermal fluctuations observed directly in the experiment suggest that the sharp phase transition from prolate to near spherical at $T\approx 1.7$ MeV will not be evident experimentally due to statistical fluctuations owing to the finite size of the nucleus.

The experimental liquid-gas phase transition signals are examined for the quasiprojectile (QP) reconstructed from the reactions of ${}^{40}\mathrm{Ar}+{}^{27}\mathrm{Al}$, ${}^{48}\mathrm{Ti}$, ${}^{58}\mathrm{Ni}$ at 47 MeV/nucleon, using measures of caloric curve, multiplicity derivative, moment parameters, and fluctuation of maximum fragment charge number (NVZ). The QP source is reconstructed, using moving source parametrizations on an event-by-event basis. For the determination of the temperature, a quadrupole fluctuation thermometer is used. Deuterons are chosen for the thermometer to minimize the Coulomb and secondary sequential decay effects. A new event-by-event method is proposed for the thermometer to determine the temperature. All measures show a characteristic signature around the temperature $T=9.0\pm 0.4$ MeV, which may suggest that the QP system goes into the liquid-gas phase transition at $T_c=8.3\pm 0.4$ MeV after the Coulomb correction.

The equation of state and phase diagram of isospin-symmetric chemically equilibrated mixture of $\alpha$ particles and nucleons $N$ are studied in the mean-field approximation. The model takes into account the effects of Fermi and Bose statistics for $N$ and $\alpha$, respectively. We use Skyrme-like parametrization of the mean-field potentials as functions of partial densities $n_{\alpha }$ and $n_N$, which contain both attractive and repulsive terms. Parameters of these potentials are chosen by fitting known properties of pure $N$ and pure $\alpha$ matter at zero temperature. The sensitivity of results to the choice of the $\alpha N$ attraction strength is investigated. The phase diagram of the $\alpha -N$ mixture is studied with special attention paid to the liquid-gas phase transitions and the Bose-Einstein condensation of $\alpha$ particles. We have found two first-order phase transitions, stable and metastable, which differ significantly by the fractions of $\alpha$ particles. It is shown that states with $\alpha$ condensate are metastable.

Elliptic flow ($v_2$) fluctuations in central heavy-ion collisions are direct probes of the fluctuating geometry of the quark-gluon plasma and, as such, are strongly sensitive to any deviation from spherical symmetry in the shape of the colliding nuclei. I investigate the consequences of nuclear deformation for $v_2$ fluctuations, and I assess whether current models of medium geometry are able to predict and capture such effects. Assuming linear hydrodynamic response between $v_2$ and the eccentricity of the medium, ${\varepsilon }_2$, I perform accurate comparisons between model calculations of ${\varepsilon }_2$ fluctuations and STAR data on cumulants of elliptic flow in central $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{U}+\mathrm{U}$ collisions. From these comparisons, I evince that the most distinct signatures of nuclear deformation appear in the non-Gaussianities of $v_2$ fluctuation, and I show, in particular, that the non-Gaussian $v_2$ fluctuations currently observed in central $\mathrm{Au}+\mathrm{Au}$ collisions are incompatible with model calculations that implement a quadrupole coefficient of order 12% in the ${}^{197}\mathrm{Au}$ nuclei. Finally, I make robust predictions for the behavior of higher order cumulants of $v_2$ in collisions of nonspherical nuclei.

The Bayesian approach for feedforward neural networks has been applied to the extraction of the nucleon axial form factor from the neutrino-deuteron-scattering data measured by the Argonne National Laboratory bubble-chamber experiment. This framework allows to perform a model-independent determination of the axial form factor from data. When the low $0.05<Q^2<0.10-{\mathrm{GeV}}^2$ data are included in the analysis, the resulting axial radius disagrees with available determinations. Furthermore, a large sensitivity to the corrections from the deuteron structure is obtained. In turn, when the low-$Q^2$ region is not taken into account with or without deuteron corrections, no significant deviations from previous determinations have been observed. A more accurate determination of the nucleon axial form factor requires new precise measurements of neutrino-induced quasielastic scattering on hydrogen and deuterium.

The ${}^{12}\mathrm{C}(\alpha ,\gamma ){}^{16}\mathrm{O}$ reaction plays a key role in the evolution of stars with masses of $M>0.55M_{\odot }$. The cross-section of the ${}^{12}\mathrm{C}(\alpha ,\gamma ){}^{16}\mathrm{O}$ reaction within the Gamow window ($E_{\text{c.m.}}=300\mathrm{keV}, T_9=0.2$) is extremely small (about ${10}^{-17}$ barn), which makes the direct measurement in a ground-based laboratory with existing techniques unfeasible. Up until now, the cross-sections at lower energies can only be extrapolated from the data at higher energies. However, two subthreshold resonances, located at $E_x=7.117\mathrm{MeV}$ and $E_x=6.917\mathrm{MeV}$, make this extrapolation more complicated. In this work, the 6.917 MeV subthreshold resonance in the ${}^{12}\mathrm{C}(\alpha ,\gamma ){}^{16}\mathrm{O}$ reaction was investigated via the ${}^{12}\mathrm{C}({}^{11}\mathrm{B},{}^7\mathrm{Li}){}^{16}\mathrm{O}$ reaction. The experiment was performed using the Q3D magnetic spectrograph at the HI-13 tandem accelerator. We measured the angular distribution of the ${}^{12}\mathrm{C}({}^{11}\mathrm{B},{}^7\mathrm{Li}){}^{16}\mathrm{O}$ transfer reaction leading to the 6.917 MeV state. Based on the finite-range distorted wave Born approximation (FRDWBA) and coupled-reaction-channel (CRC) analysis, we derived the asymptotic normalization coefficient (ANC) of the 6.917 MeV level in ${}^{16}\mathrm{O}$ to be ($1.10\pm 0.29$) $\times {10}^{10}{\mathrm{fm}}^{-1}$, with which the reduced $\alpha$ width was computed to be $18.0\pm 4.7\mathrm{keV}$ at the channel radius of 6.5 fm. Finally, we calculated the astrophysical $S_{E2}\left(300\right)$ factor of the ground-state transitions to be 46.2 $\pm$ 7.7 keV b. The result for the astrophysical $S_{E2}\left(300\right)$ factor confirms the values obtained in various direct and indirect measurements and presents an independent examination of the most important data in nuclear astrophysics.

The first measurement of dielectron (${\mathrm{e}}^{+}{\mathrm{e}}^{-}$) production in central $(0–10\%)$ Pb–Pb collisions at $\sqrt{{\mathit{s}}_{\mathrm{NN}}}=2.76\mathrm{TeV}$ at the LHC is presented. The dielectron invariant-mass spectrum is compared to the expected contributions from hadron decays in the invariant-mass range $0<m_{\mathrm{ee}}<3.5\mathrm{GeV}/{\mathit{c}}^2$. The ratio of data and the cocktail of hadronic contributions without vacuum ${\rho }^0$ is measured in the invariant-mass range $0.15<m_{\mathrm{ee}}<0.7\mathrm{GeV}/{\mathit{c}}^2$, where an excess of dielectrons is observed in other experiments, and its value is $1.40\pm 0.28(\mathrm{stat}.)\pm 0.08(\mathrm{syst}.)\pm 0.27\left(\mathrm{cocktail}\right)$. The dielectron spectrum measured in the invariant mass range $0<m_{\mathrm{ee}}<1\mathrm{GeV}/{\mathit{c}}^2$ is consistent with the predictions from two theoretical model calculations that include thermal dielectron production from both partonic and hadronic phases with in-medium broadened ${\rho }^0$ meson. The fraction of direct virtual photons over inclusive virtual photons is extracted for dielectron pairs with invariant mass $0.1<m_{\mathrm{ee}}<0.3\mathrm{GeV}/{\mathit{c}}^2$ and in the transverse-momentum intervals $1<p_{\mathrm{T},\mathrm{ee}}<2\mathrm{GeV}/\mathit{c}$ and $2<p_{\mathrm{T},\mathrm{ee}}<4\mathrm{GeV}/\mathit{c}$. The measured fraction of virtual direct photons is consistent with the measurement of real direct photons by ALICE and with the expectations from previous dielectron measurements at RHIC within the experimental uncertainties.

We present a new framework to treat the dissipation and fluctuation dynamics associated with nucleon-nucleon scattering in heavy-ion collisions. The two-body collision processes are effectively described in terms of the diffusion of nucleons in viscous nuclear media, governed by a set of Langevin equations in momentum space. The new framework combined with the usual mean-field dynamics can be used to simulate heavy-ion collisions at intermediate energies. As a proof of principle, we simulate Au + Au reactions and obtain results consistent with other existing codes under the same constrained conditions. We also study the formation of fragments in Sn + Sn reactions at 50 MeV/nucleon, and results are discussed and compared to two other models commonly employed for collisions.

Background: Previous studies of $(d,p)$ reactions in three-body (proton, neutron, nuclear core) systems revealed a nontrivial effect of the core excitation: the transfer cross section cannot be factorized into the spectroscopic factor and the single-particle cross section obtained neglecting the core excitation. This observable, up to a kinematic factor, is the angular distribution of the core nucleus in the $(p,d)$ reaction.

Purpose: The study of the core excitation effect for the most closely related observable in the $(p,pn)$ three-body breakup, i.e., the core angular distribution, is the aim in the present work.

Methods: Breakup of the one-neutron halo nucleus in the collision with the proton is described using three-body Faddeev-type equations extended to include the excitation of the nuclear core. The integral equations for transition operators are solved in the momentum-space partial-wave representation.

Results: Breakup of ${}^{11}\mathrm{Be}$ nucleus as well as of model $A=11p$-wave nuclei is studied at beam energies of 30, 60, and 200 MeV per nucleon. Angular and momentum distributions for the ${}^{10}\mathrm{Be}$ core in ground and excited states is calculated. In sharp contrast to $(p,d)$ reactions, the differential cross section in most cases factorizes quite well into the spectroscopic factor and the single-particle cross section.

Conclusions: Due to different reaction mechanisms the core excitation effect in the breakup is very different from transfer reactions. A commonly accepted approach to evaluate the cross section, i.e., the rescaling of single-particle model results by the corresponding spectroscopic factor, appears to be reliable for breakup though it fails in general for transfer reactions.