Elliptic flow from nuclear collisions is a hadronic observable sensitive to the early stages of system evolution. We report first results on elliptic flow of charged particles at midrapidity in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{\mathrm{NN}}}=130\mathrm{GeV}$ using the STAR Time Projection Chamber at the Relativistic Heavy Ion Collider. The elliptic flow signal, $v_2$, averaged over transverse momentum, reaches values of about $6\%$ for relatively peripheral collisions and decreases for the more central collisions. This can be interpreted as the observation of a higher degree of thermalization than at lower collision energies. Pseudorapidity and transverse momentum dependence of elliptic flow are also presented.

A change is made in a statistical framework by introducing a set of variables called ancestral or stochastic. This leads to an underlying dynamics based on branching laws, lines of descent in an hierarchical topology, period doublings, cascades, and clans. Above a certain branching probability, a percolative feature suddenly appears. Power laws emerge and cascade points arise and end at golden mean $(\sqrt{5}-1\mathrm{})/2\mathrm{}$.

The production of pions and kaons has been measured in ${}^{197}\mathrm{Au}{+}^{197}\mathrm{Au}$ collisions at beam energies from 0.6 to $1.5A\mathrm{GeV}$ with the kaon spectrometer at SIS/GSI. The $K^{+}$ meson multiplicity per nucleon is enhanced in $\mathrm{Au}+\mathrm{Au}$ collisions by factors up to 6 relative to $\mathrm{C}+\mathrm{C}$ reactions, whereas the corresponding pion ratio is reduced. The ratio of the $K^{+}$ meson excitation functions for $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{C}+\mathrm{C}$ collisions increases with decreasing beam energy. This behavior is expected for a soft nuclear equation-of-state.

Radioactive ion beams of ${}^{17}\mathrm{F}$ were used to study several resonance states in ${}^{18}\mathrm{Ne}$. Clear evidence for simultaneous two-proton emission from the 6.15 MeV state $\left(J^{\pi }{=1\mathrm{}}^{-}\right)$ in ${}^{18}\mathrm{Ne}$ has been observed with the reaction ${}^{17}\mathrm{F}{+}^1\mathrm{H}$. Because of limited angular coverage, the data did not differentiate between the two possible mechanisms of simultaneous decay, diproton ${(}^2\mathrm{He})$ emission or direct three-body decay. The two-proton partial width was found to be $21\pm 3\mathrm{eV}$ assuming ${}^2\mathrm{He}$ emission and $57\pm 6\mathrm{eV}$ assuming three-body decay. The total width of the $1^{-}$ state was measured to be $50\pm 5\mathrm{keV}$. Several additional resonances that decay by single proton emission were also studied.

We point out that the problem of kaon condensation in dense hadronic matter can be addressed in perturbative QCD. Indeed, perturbative calculations suggest that negative kaons are condensed in high-density quark matter if the presence of electrons is taken into account. This observation sheds new light on the proposal that the low-density hyperon and high-density quark matter phases of QCD are continuously connected.

A systematic expansion in opacity, $L/\lambda$, is used to clarify the nonlinear behavior of induced gluon radiation in quark-gluon plasmas. The inclusive differential gluon distribution is calculated up to second order in opacity and compared to the zeroth order (factorization) limit. The opacity expansion makes it possible to take finite kinematic constraints into account that suppress jet quenching in nuclear collisions below RHIC ( $\sqrt{s}=200\mathrm{AGeV}$) energies.

We report on an investigation of the $({\pi }^{-},{\pi }^0{\pi }^0)$ reaction by means of measurements of the ${\pi }^0{\pi }^0$ invariant mass distributions from ${\pi }^{-}$ interactions on H, D, C, Al, and Cu targets at $p_{{\pi }^{-}}=0.408\mathrm{}\mathrm{GeV}/c$. The sharp, strong peak in the ${\pi }^{+}{\pi }^{-}$ invariant mass near ${2m}_{\pi }$ reported by the CHAOS Collaboration is not seen in our ${\pi }^0{\pi }^0$ data. However, we do observe a change in the shape of the ${\pi }^0{\pi }^0$ invariant mass spectrum for the different targets, indicating that the ${\pi }^0{\pi }^0$ interaction diminishes in the nuclear medium as represented by nuclei D, C, Al, and Cu, compared to hydrogen.

A Comment on the Letter by H. Rauch et al., Phys. Rev. Lett. 83, 4955 (1999). The authors of the Letter offer a Reply.

A Reply to the Comment by J. Felber, R. Gahler, and R. Golub.

The root-mean-square radius for neutrons in nuclei is investigated in the Skyrme Hartree-Fock model. The main source of theoretical variation comes from the exchange part of the density-dependent interaction which can be related to a basic property of the neutron equation of state. A precise measurement of the neutron radius in ${}^{208}\mathrm{Pb}$ would place an important new constraint on the equation of state for neutron matter. The Friedman-Pandharipande neutron equation of state would lead to a very precise value of $0.16\pm 0.02\mathrm{fm}$ for the difference between the neutron and the proton root-mean-square radius in ${}^{208}\mathrm{Pb}$.

The states of ${}^{208}\mathrm{Pb}$ are calculated in a 24 orbit model space which includes all excitations up to two particles and two holes beyond the closed shell. All of the known low-lying states are reproduced by this model. The observed distributions of electromagnetic excitation strength from the ground state to low-lying excited states are well described by the model. The double-octupole excitation strength is calculated and found to be concentrated in $0^{+}$, $2^{+}$, and $4^{+}$ states around 5.2 MeV, but it is fragmented over many $6^{+}$ states, in agreement with recent experiments.

The first detailed measurements of the centrality dependence of strangeness production in $p-A$ collisions are presented. $\Lambda$ and $K_S$ $\mathrm{dn}/\mathrm{dy}$ distributions from 17.5 GeV/ $c$ $p-\mathrm{Au}$ collisions are shown as a function of “grey” track multiplicity and the estimated number of collisions, $\nu$, made by the proton. The $\nu$ dependence of the $\Lambda$ yield deviates from a scaling of $p-p$ data by the number of participants, increasing faster than this scaling for $\nu \le 5$ and saturating for larger $\nu$. A slower growth in $K_S$ multiplicity with $\nu$ is observed, consistent with a weaker $\nu$ dependence of $K\overline{K}$ production than $\mathrm{YK}$ production.

We apply the canonical quantum statistical model of nuclear multifragmentation generalized in the framework of recently proposed Tsallis nonextensive thermostatistics for the description of the nuclear multifragmentation process. The test calculation in the system with $A=197\mathrm{}$ nucleons shows strong modification of the “critical” behavior associated with the nuclear liquid-gas phase transition for small deviations from the conventional Boltzmann-Gibbs statistical mechanics.

We formulate and study a mechanical instability criterion for sudden hadronization of dense matter fireballs formed in $158A\mathrm{GeV}$ Pb-Pb collisions. Considering properties of quark-gluon matter and hadron gas we obtain the phase boundary between these two phases and demonstrate that the required deep quark-gluon-plasma supercooling prior to sudden hadronization has occurred.

Discrete Gamow-Teller (GT) transitions ${}^{176}\mathrm{Yb}{\to }^{176}\mathrm{Lu}$ at low excitation energies have been measured via the ( ${}^3\mathrm{He},t$) reaction at 450 MeV and at $0°$. For ${}^{176}\mathrm{Yb}$, two low-lying states are observed, setting low thresholds $Q\left(\nu \right)=301\mathrm{}$ and 445 keV for neutrino ( $\nu$) capture. Capture rates estimated from the measured GT strengths, the simple two-state excitation structure, and the low $Q\left(\nu \right)$ in Yb-Lu indicate that Yb-based $\nu$ detectors are well suited for a direct measurement of the sub-MeV solar electron-neutrino ( ${\nu }_e$) spectrum including $\mathrm{pp}$ neutrinos.

We report a $0°$ ${}^{176}\mathrm{Yb}(p,n{)}^{176}\mathrm{Lu}$ measurement at IUCF where we used 120 and 160 MeV protons and the energy dependence method to determine Gamow-Teller (GT) matrix elements relative to the model independent Fermi matrix element. The data show that there is an isolated concentration of GT strength in the low-lying $1^{+}$ states making the proposed Low Energy Neutrino Spectroscopy detector (based on neutrino captures on ${}^{176}\mathrm{Yb}$) sensitive to $\mathrm{pp}$ and ${}^7\mathrm{Be}$ neutrinos and a promising detector to resolve the solar neutrino problem.

The neutron-proton differential flow is shown to be a very useful probe of the isospin-dependence of the nuclear equation of state (EOS). This novel approach utilizes constructively both the isospin fractionation and the nuclear collective flow as well as their sensitivities to the isospin-dependence of the nuclear EOS. It also avoids effectively uncertainties associated with other dynamical ingredients of heavy-ion reactions at intermediate energies.

The data of $J/\psi$ suppression at large $x_F$ in $\mathrm{pA}$ collisions are used to infer the existence of gluon depletion as the projectile proton traverses the nucleus. The modification of the gluon distribution is studied by use of a convolution equation whose nonperturbative splitting function is determined phenomenologically. The depletion factor at $x_1=0.8\mathrm{}$ is found to be about $25\%$ at $A=100\mathrm{}$.

The NA50 Collaboration has recently observed that the $J/\psi$ production rate in Pb-Pb collisions decreases more rapidly as a function of the transverse energy for the most central collisions than for less central ones. We show that this phenomenon can be understood as an effect of transverse energy fluctuations in central collisions. A good fit of the data is obtained using a model which relates $J/\psi$ suppression to the local energy density. Our results suggest that the $J/\psi$ is completely suppressed at the highest densities achieved in Pb-Pb collisions.

A rotationally invariant random interaction ensemble was realized in a single- $j$ fermion model. A statistical approach reveals the random coupling of individual angular momenta as a source for the empirically known dominance of ground states with zero and maximum spin. The interpretation is supported by the structure of the ground state wave functions.