Elliptic flow and Hanbury-Brown–Twiss correlations (HBT) are studied for noncentral relativistic nuclear collisions. Azimuthal asymmetries show up in both elliptic flow and HBT radii and are calculated in both collisionless and hydrodynamic limits relevant for peripheral and central collisions, respectively. Determining the reaction plane and measuring the HBT radii as a function of the angle between the reaction plane and the particle momenta can determine the physical quantities as source sizes, deformations, emission times, duration of emission, and opacities. Comparison to SPS data and predictions for RHIC and LHC energies are given. The centrality dependence with and without a phase transition to a quark-gluon plasma is discussed—in particular, how the physical quantities are expected to display a qualitative different behavior in the case of a phase transition.

A hadron and string cascade model, JPCIAE, for simulating ultrarelativistic nucleus-nucleus collisions based on the LUND model and PYTHIA event generator especially, is used to investigate $J/\psi$ suppression due to nuclear absorption in minimum bias $\mathrm{pA}$ and $\mathrm{AB}$ collisions at $200A\mathrm{GeV}/c$. With different sets of reasonable formation time for a $J/\psi$ and a meson the results of the $J/\psi$ suppression factor from both the usual scenario simulation (i.e., standard cascade simulation) and the Glauber-like simulation are comparable with all the NA38 $\mathrm{pA}$ and $\mathrm{AB}$ data, except the NA50 data of Pb+Pb collisions. However, the difference between the usual scenario simulation and the Glauber-like simulation, with the same parameter set of formation time, is noticeable. Meanwhile, the sensitive effect of formation time and the effect of the possible uncertainty in the absorption cross sections on $J/\psi$ suppression are studied in detail.

We consider the orthopositronium and parapositronium production in the process $A\overrightarrow{A}AA+$positronium where A is a nucleus with the charge number Z. The inclusive cross section and the energy distribution of the relativistic positronium are calculated which are of primary interest from the experimental point of view. The accuracy of the corresponding cross sections is given by omitting terms $\sim (Z\alpha {)}^2{/L}^2$ for the parapositronium and $\sim (Z\alpha {)}^2/L$ for the orthopositronium production where $L=\ln {\gamma }^2\approx 9$ and 16 for the RHIC and the LHC. Within this accuracy the multiphoton (Coulomb) corrections are taken into account. We show that the RHIC and the LHC will be positronium factories with a production rate of about ${10}^5–{10}^8$ relativistic positronium per day. The fraction of the orthopositronium is expected to be of the same order as that of the parapositronium for Au-Au and Pb-Pb collisions.

The extent to which geometrical effects contribute to the production and suppression of the $J/\psi$ and $q\overline{q}$ minijet pairs in general is investigated for high-energy heavy-ion collisions at SPS, RHIC, and LHC energies. For the energy range under investigation, the geometrical effects referred to are shadowing and antishadowing, respectively. Due to those effects, the parton distributions in nuclei deviate from the naive extrapolation from the free nucleon result; $f_A\ne {\mathrm{Af}}_N.$ The strength of the shadowing/antishadowing effect increases with the mass number. Therefore it is interesting to see the difference between cross sections for, e.g., $S+U$ vs Pb+Pb at SPS. The recent NA50 results for the survival probability of produced $J/\psi$’s has attracted great attention and are often interpreted as a signature of a quark gluon plasma. This paper will present a fresh look on hard QCD effects for the charmonium production level. It is shown that the apparent suppression of $J/\psi$’s must also be linked to the production process. Due to the uncertainty in the shadowing of gluons the suppression of charmonium states might not give reliable information on a created plasma phase at the collider energies now available. The consequences of shadowing effects for the $x_F$ distribution of $J/\psi$’s at $\sqrt{s}=20\mathrm{}\mathrm{GeV},$ $\sqrt{s}=200\mathrm{}\mathrm{GeV},$ and $\sqrt{s}=6\mathrm{}\mathrm{TeV}$ are calculated for some relevant combinations of nuclei, as well as the $p_T$ distribution of minijets at midrapidity for $N_f=4\mathrm{}$ in the final state.

The two-center Dirac equation for an electron in the external electromagnetic field of two colliding heavy ions in the limit in which the ions are moving at the speed of light is exactly solved off the light fronts, and nonperturbative amplitudes are obtained for free electron-positron pair production. We find the condition for the applicability of this solution for large but finite collision energy, and use it to explain recent experimental results. The observed scaling of positron yields as the square of the projectile and target charges is a result of a cancellation of a nonperturbative charge dependence and holds as well for large coupling. Other observables would be sensitive to nonperturbative phases.

The $\rho$ meson mass shift, width broadening, and spectral density at finite temperature and nucleon density are estimated using a general formula which relates the self-energy to the real and imaginary parts of the forward scattering amplitude on the constituents of the medium. We saturate the scattering amplitude at low energies with resonances, while at high energies a Regge approach is taken in combination with vector meson dominance; experimental data is used wherever possible. The main result is that the $\rho$ meson becomes increasingly broad with increasing nucleon density. The spectral density is suppressed in the resonance region $600<M<\mathrm{}900\mathrm{}\mathrm{MeV}$ and enhanced in the subresonance region $M<\mathrm{}600\mathrm{}\mathrm{MeV}$.

We study the HBT interferometry of ultrarelativistic nuclear collisions using a freeze-out model in which free pions emerge in the course of the last binary collisions in the hadron gas. We show that the HBT correlators of both identical and nonidentical pions change with respect to the case of independent pion production. Practical consequences for the design of the event generator with the built in Bose-Einstein correlations are discussed. We argue that the scheme of inclusive measurement of the HBT correlation function does not require the symmetrization of the multipion transition amplitudes (wave functions).

Changes in the properties of the vector mesons in hot and dense hadronic matter, as produced in heavy-ion collisions, lead to the intriguing possibility of the opening of the decay channel $\overrightarrow{\omega }\rho \pi ,$ for the $\omega$ meson, which is impossible in free space. This along with the channel $\omega \overrightarrow{\pi }\pi \pi$ would result in a decrease in its effective lifetime enabling it to decay within the hot zone and act as a chronometer in contradiction to the commonly held opinion and would have implications vis a vis determination of the size of the region through pion interferometry. A new peak and a radically altered shape of the low invariant mass dilepton spectra appears due to different shifts in the masses of $\rho$ and $\omega$ mesons. The Walecka model is used for the underlying calculation for the sake of illustration.

The mass spectra and binding radii of heavy quark bound states are studied on the basis of the reduced Bethe-Salpeter equation. The critical values of screening masses for $c\overline{c}$ and $b\overline{b}$ bound states at a finite temperature are obtained and compared with the previous results given by nonrelativistic models.

The preliminary data on hadron multiplicities measured in central Pb+Pb collisions at $158A$ $\mathrm{GeV}/c$ are analyzed. The ideal hadron gas model fails to give a reasonable explanation to the Pb+Pb data sets. We study the possible effects of pion enhancement due to different hard-core repulsion for pions and other hadrons and strangeness suppression because of incomplete chemical equilibrium. Each of these two modifications improves the results. The combined effect of these two mechanisms leads to an extremely good agreement with the data. An interpretation of the obtained results in terms of the possible quark-gluon plasma formation at the early stage of the collision is also discussed.

A thorough theoretical analysis of fast and slow hadrons emitted in the forward $(\theta <90\mathrm{}°)$ and backward $(\theta >~90°)$ hemispheres in the interactions of ${}^6\mathrm{Li}$ $\left(3.7A\mathrm{GeV}\right)$ with emulsion nuclei is made with a modified FRITIOF model (which allowance is made for secondary interactions) and a typical cascade model. It is found that, while the cascade model applies well in the region of limited cascading (backward hemispheres), it becomes less applicable in the forward hemisphere (where cascading becomes more branched). The modified FRITIOF model, on the other hand, could reproduce the experimental observations in both hemispheres.

We show how the linear “low-density theorem” of Drukarev and Levin can be extended to arbitrary positive integer power of the baryon density $\rho$. The $n\mathrm{th}$ coefficient in the McLaurin expansion of the fermion condensate’s $\rho$ dependence is the connected n-nucleon $\Sigma$ term matrix element. We calculate the $O\left({\rho }^2\right)$ coefficient in lowest-order perturbative approximation to the linear σ model and then show how this and other terms can be iterated to arbitrarily high order. Convergence radius of the result is discussed.

Variational Monte Carlo calculations for the ground- and excited-state binding energies of s-shell hypernuclei using a new form of dispersive spin-dependent noncentral $\Lambda \mathrm{NN}$ force have been made to study its effect on the overbinding problem of ${}_{\mathrm{\Lambda }}^5\mathrm{He}$ and on the spin dependence of $\Lambda N$ force. A detailed analysis shows that the strength of the dispersive $\Lambda \mathrm{NN}$ force can be adjusted to resolve the overbinding problem using two-body correlations alone. Consequently, the ambiguity in the strength of the dispersive $\Lambda \mathrm{NN}$ force masks the effect of 2π-exchange $\Lambda \mathrm{NN}$ force and $\Lambda \mathrm{NN}$ correlations on the data. The contribution of the dispersive force to the $0^{+}{-1\mathrm{}}^{+}$ spin-flip splitting of $A=4\mathrm{}$ hypernuclei is not uniquely determined. Further $B_{\Lambda }$ data favor a small spin dependence of the $\Lambda N$ potential, a situation characteristically similar to other versions of dispersive $\Lambda \mathrm{NN}$ potentials.

In this work we resolve a problem that arises in the calculation of the two-photon decay of the ${\pi }^0,$ η, and ${\eta }^{\prime }$ mesons, when use is made of the Nambu–Jona-Lasinio (NJL) model. It has been found that satisfactory results for the widths are obtained if a momentum cutoff is not introduced in the evaluation of the (convergent) integrals obtained from an analysis of the triangle diagram, while only about one-half of the experimental value is obtained, if the cutoff used for the (divergent) loop integrals of the NJL model is introduced. The resolution of this problem lies in the introduction of pseudoscalar–axial-vector mixing for both the ${\pi }^0$ and the $\eta -{\eta }^{\prime }$ system. (In addition, one has singlet-octet mixing for the η and ${\eta }^{\prime },$ requiring the study of T matrices of dimension four in that case.) Since we have used a relativistic quark model that includes a (covariant) confinement model, we are able to treat the ${\eta }^{\prime }$ and η mesons on the same basis. We found that the ’t Hooft interaction does not work well in our study of $\eta -{\eta }^{\prime }$ mixing, while the parametrization of nonperturbative effects due to coupling to gluonic modes, which affects the strength of the interaction for singlet states only, yields satisfactory results. In the case of a pion, our results may be understood by the observation that the vertex $\mathrm{iP}/{\gamma }_5/\sqrt{P^2}$ yields a much larger amplitude for two-photon decay of a meson of momentum P than the vertex $i{\gamma }_5.$ Therefore, quite small mixing angles can yield a large modification of the calculated width. We have found that our results are sensitive to the form of regulator used. For example, with a sharp cutoff, ${\Gamma }_{{\eta }^{\prime }\to \gamma \gamma }$ is overestimated by about a factor of 3, while the use of a Gaussian regulator leads to a satisfactory fit to the experimental value.

We show that the $D^{-}$ meson will form narrow bound states with ${}^{208}\mathrm{Pb}.$ Mean-field potentials for the $D^0,$ $D^0,$and $D^{-}$ in ${}^{208}\mathrm{Pb}$ are calculated self-consistently using the quark-meson coupling model in the local density approximation. The meson-${}^{208}\mathrm{Pb}$ bound-state energies are then calculated by solving the Klein-Gordon equation with these potentials. The experimental confirmation and comparison with the $D^0$ and $D^0$ will provide distinctive information on the nature of the interaction between the charmed meson and matter.

The exchange charge-density operators that correspond to the Fermi-invariant decomposition of quark-quark interactions have been constructed. Their effect on the electromagnetic charge radii of the nucleons, in combination with that of the relativistic corrections to the single-quark operator, has been studied with a constituent quark model with a spin and flavor dependent hyperfine and a linear confining interaction, which gives a quantitative description of the spectra for the light and strange baryons. The model gives proton and neutron charge radii in approximate agreement with the empirical results assuming reasonable values for the radii of the constituent quarks.

Parity violation has been studied recently for neutron resonances in ${}^{117}\mathrm{Sn}.$ The neutron resonance spectroscopy is essential for the analysis of the parity violation data. We have measured neutron resonances in ${}^{117}\mathrm{Sn}$ for neutron energies from 1 to 1500 eV using the time-of-flight method and the $(n,\gamma )$ reaction. The sample was enriched to 87.6% ${}^{117}\mathrm{Sn}.$ Neutron scattering and radiative widths were determined, and orbital angular momentum assignments were made with a Bayesian analysis. The s-wave and p-wave strength functions and average level spacings were determined.

We analyze the parity-violating (PV) components of the analyzing power $\mathcal{A}$ in elastic electron-proton scattering and discuss their sensitivity to the strange quark contributions to the proton weak form factors. We point out that the component of $\mathcal{A}$ along the momentum transfer is independent of the electric weak form factor and thus compares favorably with the PV beam asymmetry for a determination of the strangeness magnetic moment. We also show that the transverse component could be used for constraining the strangeness radius. Finally, we argue that a measurement of both components could give experimental information on the strangeness axial charge.

The photon spectra from radiative muon capture (RMC) on O, Al, Si, Ti, Zr, and Ag have been measured for photon energies greater than 57 MeV using a cylindrical pair spectrometer at the TRIUMF cyclotron. Values of $R_{\gamma },$ the ratio of the radiative rate for photon energies above 57 MeV, to the nonradiative rate, are $1.67\pm 0.18,$ $1.40\pm 0.11,$ $2.09\pm 0.20,$ $1.30\pm 0.12,$ $1.31\pm 0.15$, and $1.12\pm 0.13$, respectively, in units of ${10}^{-5}$. The Al/Si rate difference confirms an earlier result. The Ti/Ca rate difference and the rate suppression in Zr and Ag are new results which confirm that the RMC rate is a much smoother function of neutron excess than of atomic number. This suggests that Pauli blocking is relatively more important for radiative capture than for nonradiative capture. The value of the ratio of the induced weak pseudoscalar to axial coupling constants, $g_p{/g}_a,$ for O was found to be $4.9\pm 0.6,$ $6.3\pm 1.1$, or ${8.1}_{-2.1}^{+1.8\mathrm{}}$, depending on the theory used to extract it. These values are in good or fair agreement with the partially conserved axial-vector current (PCAC) hypothesis. For the other nuclei studied, large model dependences or a lack of detailed RMC calculations made tests of the PCAC hypothesis difficult.

The transfer reaction ${}^7\mathrm{Be}{\mathrm{}(d,n)\mathrm{}}^8\mathrm{B}$ is modeled for the recent low experimental energies using the distorted-wave Born approximation (DWBA). Tests of the validity of the asymptotic normalization constant method for the extraction of the $S_{17}$ astrophysical factor are performed. We show that a peripheral assumption is no longer accurate for center-of-mass energies greater than approximately 15 MeV. In all cases we found that core deformation and/or excitation effects to be small as long as the optical potential for the entrance channel is not strongly surface peaked. Multistep corrections to the DWBA were found to be small. We also emphasize that the lack of knowledge of the optical potentials for the entrance and outgoing channels induces severe uncertainties in the extracted $S_{17}$ astrophysical factor.