To investigate the source size and time dependence of multifragmentation reactions, small- and large-angle relative velocity correlations between coincident complex fragments have been measured for the 1.8–4.8 GeV ${}^3\mathrm{He}{+}^{\mathrm{nat}}\mathrm{Ag},{\hspace{0.5em}}^{197}\mathrm{Au}$ systems. The results support an evolutionary scenario for the fragment emission process in which lighter IMFs $(Z\lesssim 6)$ are emitted from a hot, more dense source prior to breakup of an expanded residue. For the most highly excited residues, for which there is a significant yield of fragments with very soft energy spectra $(E/A<~3\hspace{0.5em}\mathrm{MeV}),$ comparisons with an N-body simulation suggest a breakup time of $\tau \sim 50\hspace{0.5em}\mathrm{fm}/c$ for the expanded residue. Comparison of these data with both the evolutionary expanding emitting source model and the Copenhagen statistical multifragmentation model shows good agreement for heavier IMF’s formed in the final breakup stage, but only the evolutionary model is successful in accounting for the lighter IMFs.

A semiclassical description of multiple giant resonance excitation that incorporates incoherent fluctuation contributions of the Brink-Axel type is developed. Numerical calculations show that the incoherent contributions are important at low to intermediate bombarding energies.

The general microscopic formalism is presented for the description of the quasielastic knock-out of $\alpha$ clusters from p-shell nuclei by ultrarelativistic electrons. Manifestations of nuclear structure in differential cross sections and angular distributions are studied. The typical ${}^{12}\mathrm{C}{(e,e}^{\prime }\alpha {)}^8\mathrm{Be}$ reaction is considered in the PWIA and DWIA approximations; particular attention is paid to the effects of virtually excited cluster states inside the initial nucleus. Suggestions for the observation of nuclear restructuring effects (interplay between diagonal and off-diagonal transitions with respect to the intrinsic state of the cluster) are proposed.

A simple rescattering model is used to predict three-pion correlation functions in high-energy $\mathrm{S}+\mathrm{Pb}$ and $\mathrm{Pb}+\mathrm{Pb}$ collisions in anticipation of new experimental results which are expected soon for both of these systems. Gaussian pion source parameters are extracted from these calculations as well as the three-pion phase factor $\cos \phi .$ The three-pion phase factor is found to be close to unity for both systems and for all regions of rapidity and transverse momentum investigated. This is in agreement with the expectations of a previous theoretical study.

A modified version of a multiple scattering model is applied to describe nuclear inclusive reactions of the type $p\overrightarrow{A}\mathrm{pX}$ and investigate the behavior of the inelasticity in nuclear processes. The modifications are such that some recent developments in the Pomeron physics are incorporated into the new theoretical scheme. The particular attention paid to the diffractive region of the spectrum results in a very good description of the diffractive cross section in terms of the atomic mass. Another important outcome resulting from this analysis is the average total inelasticity whose atomic mass and energy dependences are shown to be in agreement with the available data. Moreover, the behavior of partial inelasticities in intranuclear collisions is also discussed.

Exclusive vector meson production reactions such as Au+Au$\to \mathrm{Au}+\mathrm{Au}+\mathrm{V},$ where $V=\rho ,\hspace{0.5em}\omega ,\hspace{0.5em}\phi$, or $J/\psi ,$ can proceed through photon-Pomeron and photon-meson interactions. Photons from the electromagnetic field of one nucleus interact coherently with the other nucleus. Photonuclear cross sections are scaled from $\gamma p$ data and convoluted with the photon spectrum to find the exclusive rates. The cross sections at the RHIC and LHC heavy ion colliders are huge, 10% of the total hadronic cross section at RHIC and 50% at LHC. These accelerators may be useful as vector meson factories. With iodine beams at RHIC, 640 $\rho$ are produced each second ${(10\mathrm{}}^{10}/\mathrm{yr});$ with calcium at the LHC the rate is 240 kHz. The $\phi$ rates are 39 Hz at RHIC and 15 kHz at LHC, while the $J/\psi$ rate is 0.3 Hz at RHIC and 780 Hz at the LHC. Because of the coherent couplings, the reactions kinematics are similar to coherent two-photon interactions; we discuss the interplay between the two reactions.

We compute the pion inclusive momentum distribution in heavy-ion collisions at AGS energies, assuming thermal equilibrium and accounting for density and expansion effects at the time of decoupling. We compare to data on midrapidity charged pions produced in central Au+Au collisions and find very good agreement. The shape of the distribution at low $m_t-m$ is explained in part as an effect arising from the high mean pion density achieved in these reactions. The difference between the positive and negative pion distributions in the same region is attributed in part to the different average yields of each kind of charged pions.

The chemical equilibration of a highly unsaturated quark-gluon plasma has been studied at finite baryon density. It is found that in the presence of a small amount of baryon density, the chemical equilibration for gluon becomes slower and the temperature decreases less steeply as compared to the baryon free plasma. As a result, the space time integrated yield of dilepton is enhanced if the initial temperature of the plasma is held fixed. Even at a fixed initial energy density, the suppression of the dilepton yields at higher baryochemical potential is compensated, to a large extent, by the slow cooling of the plasma.

The proton energy spectrum and the angular distribution of the probability of the ${\Lambda }_c^{+}\to {p+K}^{-}+{\pi }^{+}$ decay for the polarized ${\Lambda }_c^{+}$ and the unpolarized proton are calculated in the quark model with chiral $\mathrm{U}\mathrm{}\left(3\right)\mathrm{}\times \mathrm{U}\mathrm{}\left(3\right)\mathrm{}$ symmetry incorporating heavy quark effective theory and chiral perturbation theory at the quark level. The application of the obtained result to the analysis of the polarization of the ${\Lambda }_c^{+}$ produced in the processes of photo and hadroproduction is discussed. We draw the similarity between the measurements of the polarization of the ${\Lambda }_c^{+}$ in the ${\Lambda }_c^{+}\to {p+K}^{-}+{\pi }^{+}$ decay and the ${\mu }^{-}$ meson in the ${\mu }^{-}\to e^{-}+{\overline{\nu }}_e+{\nu }_{\mu }$ decay.

The new physics sensitivity of a variety of low-energy parity-violating (PV) observables is analyzed. A comparison is made between atomic PV for a single isotope, atomic PV using isotope ratios, and PV electron-hadron and electron-electron scattering. The complementarity among these observables, as well as with high-energy processes, is emphasized. Theoretical uncertainties entering the interpretation of low-energy measurements are discussed.

We report new measurements of the total cross section for the ${}^3\mathrm{H}{\mathrm{}(p,n)\mathrm{}}^3\mathrm{He}$ reaction from threshold ${(E}_p=1.02\mathrm{}$ MeV) to $E_p=4.5\mathrm{}$ MeV. The experiment utilized specially prepared $\mathrm{Ti}{-}^3\mathrm{H}$ targets, and neutrons were detected using a $4\pi$ detector. A weak resonant structure due to an excited state in ${}^4\mathrm{He}$ is observed which was not seen in previous cross section measurements. A new expression for the ${}^3\mathrm{He}{\mathrm{}(n,p)\mathrm{}}^3\mathrm{H}$ thermonuclear reaction rate for temperatures below 10 GK is presented which will allow for more accurate calculations of the yields of light elements produced by big-bang nucleosynthesis.

The effects of internal quark structure of baryons on the composition and structure of neutron star matter with hyperons are investigated in the quark-meson coupling (QMC) model. The QMC model is based on a mean-field description of nonoverlapping spherical bags bound by self-consistent exchange of scalar and vector mesons. The predictions of this model are compared with a quantum hadrodynamic (QHD) model calibrated to reproduce identical nuclear matter saturation properties. By employing a density-dependent bag constant through direct coupling to the scalar field, the QMC model is found to exhibit identical properties as QHD near saturation density. Furthermore, this modified QMC model provides well-behaved and continuous solutions at high densities relevant to the core of neutron stars. Two additional strange mesons are introduced which couple only to the strange quark in the QMC model and to the hyperons in the QHD model. The constitution and structure of stars with hyperons in the QMC and QHD models reveal interesting differences. This suggests the importance of quark structure effects in the baryons at high densities.

The T matrix of the $p\overrightarrow{n}p\Lambda$ reaction, which is a strangeness changing weak process, is derived. The explicit formulas of the spin observables are given for s-wave $p\Lambda$ final states which kinematically corresponds to inverse reaction of the weak nonmesonic decay of $\Lambda$ hypernuclei. One can study interferences between amplitudes of the parity-conserving and -violating, spin-singlet and -triplet, and isospin-singlet and -triplet. Most of them are not available in the study of the nonmesonic decay and will be measured in the coming experiment. They clarify the structure of the reaction and constrain strongly theoretical models for the weak hyperon nucleon interaction.

Using a pseudoparticle technique we simulate large-amplitude isoscalar giant octupole excitations in a finite nuclear system. Dependent on the initial conditions we observe either clear octupole modes or overdamped octupole modes which decay immediately into quadrupole ones. This shows clearly a behavior beyond linear response. We propose that octupole modes might be observed in central collisions of heavy ions.

Double arm spectrometer SAVEC has been used to identify spontaneous collinear tripartition of ${}^{252}\mathrm{Cf}$ nuclei: ${}^{M_0}{\overrightarrow{Z_0}}^{M_1}{Z}_1{+}^{M_3}{Z}_3{+}^{M_2}{Z}_2.$ No effect has been observed for $M_3>75\mathrm{}\hspace{0.5em}\mathrm{u}$ at the level of $7.5\times {10}^{-6}$ with respect to the probability of the binary fission process.

Excitation functions of the ${}^{58}\mathrm{Ni}{\mathrm{}(n,p)\mathrm{}}^{58}{\mathrm{Co}}^{m,g}$ reactions were measured in the energy range from 2 to 15 MeV. The energy dependence of the isomeric cross-section ratio $R={\sigma }_m/({\sigma }_m+{\sigma }_g)$ is deduced from the measured data. The shape and magnitude of the ${R(E}_n)$ function are described by model calculations using a consistent parameter set. Questions of the input level scheme were solved based on the accurate isomeric ratio measured at low energy region. The measured R-value prefers the mixing ratio value of $-2.3$ for the transition of the 52.8 keV state to the 24.9 keV state, over the $-0.33$ value. The transition has predominantly $E2\mathrm{}$ multipolarity.

The neutron capture cross section of ${}^{26}\mathrm{Mg}$ was measured relative to the known gold cross section at $kT=52\mathrm{}\hspace{0.5em}\mathrm{keV}$ using the fast cyclic activation technique. The experiment was performed at the Van de Graaff accelerator at the Universität Tübingen. From the new experimental result we deduce the resonance strength for the resonance at $E_n=68.7\mathrm{}\hspace{0.5em}\mathrm{keV}$ which is in good agreement with a direct measurement by Weigmann et al., whereas for the resonance at $E_n=220\mathrm{}\hspace{0.5em}\mathrm{keV}$ a discrepancy was found between the Weigmann data and our previous experiment. Finally, we calculate the Maxwellian-averaged capture cross section $〈\sigma 〉$ in the astrophysically relevant energy region from the available experimental data, and we discuss the discrepancies of $〈\sigma 〉$ between our work and a previous calculation by Weigmann et al.

We confirm the importance of standard medium effects (hadronic rescattering) in heavy-ion collisions by using a perturbative-QCD-motivated hadronic model to investigate the dilepton spectra from $\mathrm{Pb}+\mathrm{Au}$ collisions at 158 GeV/nucleon. These same effects, namely, prompt $\pi \overrightarrow{\rho }\pi e^{+}{e}^{-}$, have been studied in several CERN SPS systems [J. Murray, W. Bauer, and K. Haglin, Phys. Rev. C 57, 882 (1998)]. The results presented here are consistent with previous studies stating that this type of rescattering effect explains a portion of the “excess” lepton pairs seen by the CERES experiment, but not the entire effect.

In-medium pion-pion correlations were studied via their effect on experimental observables measured for the ${\pi }^{+}\to {\pi }^{+}{\pi }^{\pm }$ reaction in nuclei $(A:{\hspace{0.5em}}^{12}{\mathrm{C},\mathrm{\hspace{0.5em}}}^{40}\mathrm{Ca}$, and ${}^{208}\mathrm{Pb})$ and in deuterium, from which it is inferred the observables for the nucleon $(N:$ n and $p)$. The measurements were performed at TRIUMF using a positive-pion flux with a bombarding energy of 283 MeV. The $\pi \pi$ invariant mass distributions $M_{\pi \pi }$ were measured from the ${2m}_{\pi }$ threshold up to the kinematic maximum, and total cross sections ${\sigma }_T$ deduced. The $\pi \pi$ medium modifications were investigated by forming the composite ratio $C_{\pi \pi }^A{=(M}_{\pi \pi }^A/{\sigma }_T^A{)/(M}_{\pi \pi }^N/{\sigma }_T^N)$ which proved to be only weakly affected by the $(\pi ,2\mathrm{}\pi )$ reaction mechanism. In the $I=J=0\mathrm{}$ channel the $C_{\pi \pi }^A$ distributions are peaked at the ${2m}_{\pi }$ threshold and increases with A. For pion pairs in the $I=2,$ $J=0\mathrm{}$ channel the $C_{\pi \pi }^A$ distributions are nearly independent of A. The distinctive behavior of $C_{\pi \pi }^A$ may be related to the appearance of the σ meson.

Two recent experiments have demonstrated that the effective $\rho$-mass in nuclear medium, as extracted from the ${}^3\mathrm{He}(\gamma ,{\pi }^{+}{\pi }^{-})$ reaction, is substantially reduced. This has been advocated as an indication of partial restoration of chiral symmetry in nuclear matter. We show that even in the absence of chiral symmetry, effective mean field nuclear matter models can explain these findings quantitatively.