One of the superdeformed rotational bands in ${}_{}{}^{194}{}_{}{}^{}\mathrm{Hg}$ has transition energies (rotational frequencies) that are equal within about 0.1% to those of the (only known) band in ${}_{}{}^{192}{}_{}{}^{}\mathrm{Hg}$. This excited (two-quasiparticle) band in ${}_{}{}^{194}{}_{}{}^{}\mathrm{Hg}$ has an aligned spin of (1.00±0.04)ħ relative to the very similar band in ${}_{}{}^{192}{}_{}{}^{}\mathrm{Hg}$, suggesting an interpretation in terms of aligned pseudo (intrinsic) spin. The implied pseudospin symmetry may be a clue as to why the rotational frequencies are so similar.

Peripheral ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$${+}^{232}$Th collisions at a bombarding energy E/A=30 MeV were studied by measuring coincidences of fission fragments with projectile fragments detected near the grazing angle. It is found that the sequential fission of the highly excited target residue nuclei is severely hindered if their excitation energies, determined from the measured recoil momenta, exceed a value of abour 50–75 MeV. A transient time of fission of the order of ${10}^{\mathrm{-}20}$ s is deduced.

The production rate for soft dileptons in a hot quark-gluon plasma is computed to leading order in the QCD coupling constant through a systematic resummation of perturbation theory. The partial rate from the annihilation and decay of soft quarks and antiquarks exhibits unexpected structure—sharp peaks due to Van Hove singularities. These peaks are overwhelmed by the partial rate from processes involving hard quarks and gluons. The total rate for soft dileptons is larger than the naive one-loop prediction by orders of magnitude.

The Michigan State University 4π array has been used to select central-impact-parameter events from the reaction ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$${+}^{51}$V at incident energies from 35 to 85 MeV/nucleon. The event shape in momentum space is an observable which is shown to be sensitive to the dynamics of the fragmentation process. A comparison of the experimental event-shape distribution to sequential- and simultaneous-decay predictions suggests that a transition in the breakup process may have occurred. At 35 MeV/nucleon, a sequential-decay simulation reproduces the data. For the higher energies, the experimental distributions fall between the two contrasting predictions.

The ratio of transverse and longitudinal response functions is investigated for the quasielastic reaction ${\mathrm{Li}}^6$(e,e′p). The ratio itself and its dependence on the missing energy cannot be explained in the distorted-wave impulse approximation. It is proposed that a reaction mechanism other than quasielastic one-body knockout plays a significant role. © 1990 The American Physical Society

A model accounting for the time dependence of thermal shape fluctuations in hot nuclei and their effects on the giant dipole resonance (GDR) is presented. Experimental data for Er and Sn isotopes are compared with theoretical calculations, and evidence for the motional narrowing of the GDR is investigated.

A simple model for understanding the qualitative features of a fragmentation process is proposed. At a particular point of a tuning parameter of this model, the fragment-size distribution has a scale-invariant power-law behavior. At relatively low values of this parameter, evaporation modes dominate and at high values of the parameter, multifragmentation takes place. The model is exactly soluble in all regimes of the tuning parameter. A unifying connection between evaporation and multifragmentation is found.

Energy spectra and angular distributions of evaporated protons from the reaction ${}_{}{}^{52}{}_{}{}^{}\mathrm{Cr}$${(}^{34}$S, 2p2n${)}^{82}$Sr at 130 MeV were measured in coincidence with discrete γ transitions. Large shifts and changes in the shape of the proton spectra were observed when high-spin states in different rotational bands are populated. They are interpreted as due to near-yrast stretched proton emission, which preferentially populates the yrast band by subbarrier protons.

Recoil-γ coincidences have been measured in the reaction ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$${+}^{159}$Tb at beam energies of 300 and 400 MeV. In the γ-ray spectra corresponding to excitation energies above 140 MeV a split giant dipole resonance is observed with widths substantially smaller than found at lower excitation energy. The possible influence of excitation energy and angular momentum on this effect is discussed. The intensity ratio of the two dipole resonance components is found to change with excitation energy, indicating a transition of the nuclear shape from prolate towards oblate.

The neutrino-induced nuclear transition ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(${\nu }_{\mathit{e}}$,${\mathit{e}}^{\mathrm{-}}$${)}^{12}$N (g.s.) has been observed with a 15-ton fine-grained tracking detector. The exclusive final state is identified by both an electron track and the positron decay of the ${}_{}{}^{12}{}_{}{}^{}\mathrm{N}$. From the observation of 181±17 ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(${\nu }_{\mathit{e}}$,${\mathit{e}}^{\mathrm{-}}$${)}^{12}$N (g.s.) events with subsequent positron decays the flux-averaged cross section, equivalent to the cross section for 35.0-MeV neutrinos, is [1.05±0.10(stat)±0.10(syst)]×${10}^{\mathrm{-}41}$ ${\mathrm{cm}}^2$. This result is in good agreement with recent calculations.

We solve a set of Faddeev equations for some realistic two-nucleon potentials, taking into account the two-pion-exchange three-nucleon interaction, the Coulomb interaction, and the charge-independence- and charge-symmetry-breaking (CIB and CSB) interactions. After 32 case studies, we find a very good linear relationship between ${}_{}{}^3{}_{}{}^{}\mathrm{H}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ binding energies, from which we deduce a model-independent value for the Coulomb energy difference (648±4 keV for the finite-size protons) and the CIB and CSB nuclear effects. With other small effects, these effects reasonably account for the ${}_{}{}^3{}_{}{}^{}\mathrm{-}_{}^3{}_{}{}^{}$he binding-energy difference.

We have measured missing-energy spectra for the reaction ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(e,e’p) in the quasielastic region in parallel kinematics at momentum transfers q of 585, 775, and 827 MeV/c. We observed ${}_{}{}^1{}_{}{}^{}$p and ${}_{}{}^1{}_{}{}^{}$s single-proton knockout peaks and large broad strength attributed to multinucleon knockout. We saw no increase in strength at pion threshold. The ratio of single-particle knockout to a distorted-wave impulse-approximation calculation is approximately constant with q. Multinucleon knockout is 25%–40% of the total cross section; it increases with q. This quasielastic non-single-nucleon knockout is much stronger than previously observed.

Excited superdeformed bands have been observed in the N=86 isotones ${}_{}{}^{150}{}_{}{}^{}\mathrm{Gd}$ and ${}_{}{}^{151}{}_{}{}^{}\mathrm{Tb}$. The properties of these bands were found to closely resemble those of the yrast superdeformed bands in their Z+1 N=86 isotones ${(}^{151}$Tb and ${}_{}{}^{52}{}_{}{}^{}\mathrm{Dy}$) such that, on average, the transition energies are identical to an accuracy of 1:1000. It is proposed that these excited superdeformed bands are based on single-proton excitations from the [301]1/2 orbital into the [651]3/2 intruder orbital and thus they are the first obsrevation of proton excitations in superdeformed nuclei.

The structure of recently discovered identical superdeformed bands in ${}_{}{}^{151}{}_{}{}^{}\mathrm{Tb}$ and ${}_{}{}^{152}{}_{}{}^{}\mathrm{Dy}$, and in ${}_{}{}^{150}{}_{}{}^{}\mathrm{Gd}$ and ${}_{}{}^{151}{}_{}{}^{}\mathrm{Tb}$ are discussed in terms of the strong-coupling approach. Based on the experimental evidence that the superdeformed core of ${}_{}{}^{152}{}_{}{}^{}\mathrm{Dy}$ is extremely insensitive to the polarization effects induced by the odd particle, the bands are shown to exhibit the presence of the pseudo SU(3) symmetry at extreme conditions of large elongations and high spins.

We calculate the pion spectrum and its electromagnetic annihilation vertex in nuclear matter taking into account the strong πNΔ interaction. For nuclear densities the pion kinetic energy is approximately compensated by the interaction, but also the free-pion annihilation matrix element is almost canceled by the contribution from the interaction with the medium. Consequently, the enhancement of the dilepton production rate due to the softening of the pion spectrum is strongly reduced.

The observed shadowing and antishadowing phenomena of quark structure functions in nuclei at small x are interpreted as a consequence of an antiquark-nucleus multiscattering process.

Collisions of ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$+Al, Cu, Pb at ${\mathit{E}}_{\mathrm{lab}}$/A=14.6 GeV were studied in a calorimetry-based experiment at the BNL Alternating Gradient Synchrotron. Transverse-energy production was measured for pseudorapidities -0.5<η<0.8. Correlations with the spectra and multiplicity of neutrons and protons emitted into a forward 0.8° cone demonstrate quantitatively the large amount of nuclear stopping observed in these reactions. Calculations in hadronic-fireball or nucleon-nucleon based models underpredict the measured transverse-energy production for Si+Pb and indicate the need to include rescattering of secondaries and /or contributions from target fragmentation.

The dissociation of neutron-rich nuclei from secondary beams incident on several targets can be explained within two distinct models: (a) The weakly bound neutrons form clusters near the nuclear surface, and (b) all protons can vibrate against all neutrons in a soft mode. We show that the momentum widths of the projectile fragments, as well as the total cross sections for the dissociation, are consistent with both hypotheses. Consequently, the interpretation of almost all recent experimental studies with secondary radioactive beams is ambiguous.

We show that the QCD van der Waals interaction due to multiple gluon exchange provides a new kind of attractive nuclear force capable of binding heavy quarkonia to nuclei. The parameters of the potential are estimated by identifying multigluon exchange with the Pomeron contributions to elastic meson-nucleon scattering. The gluonic potential is then used to study the properties of cc¯ nuclear-bound states. In particular, we predict bound states of the ${\eta }_c$ with ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and heavier nuclei. Production modes and rates are also discussed.

Semi-inclusive spectra of ${\pi }^{\pm }$, $K^{\pm }$, and p have been measured near midrapidity for central collisions of 14.6A-GeV/c ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ with ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ nuclei. The invariant cross sections are all well fit by exponential distributions in transverse mass, within the measured ranges. The π and K cross sections can also be fit by Boltzmann distributions. From integrated yields, at midrapidity, the kaon/pion yield ratios are (19.2±3)% for $K^{+}$/${\pi }^{+}$ and (3.6±0.8)% for $K^{\mathrm{-}}$/${\pi }^{\mathrm{-}}$; the ${\pi }^{+}$/${\pi }^{\mathrm{-}}$ ratio is 1.00±0.04.