We show that hadronization via quark coalescence enhances hadron elliptic flow at large $p_{\perp }$ relative to that of partons at the same transverse momentum. Therefore, compared to earlier results based on covariant parton transport theory, more moderate initial parton densities $dN/d\eta (b=0)\sim 1500–3000$ can explain the differential elliptic flow $v_2(p_{\perp })$ data for $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ reactions at $\sqrt{s}=130$ and $200A\mathrm{G}\mathrm{e}\mathrm{V}$ from BNL RHIC. In addition, $v_2(p_{\perp })$ could saturate at about 50% higher values for baryons than for mesons. If strange quarks have weaker flow than light quarks, hadron $v_2$ at high $p_{\perp }$ decreases with relative strangeness content.

Using $\beta$ decays of a clean source of ${}^{12}\mathrm{N}$ produced at the IGISOL facility, we have measured the breakup of the ${}^{12}\mathrm{C}$ (12.71 MeV) state into three $\alpha$ particles with a segmented particle detector setup. The high quality of the data permits solving the question of the breakup mechanism of the 12.71 MeV state, a longstanding problem in few-body nuclear physics. Among existing models, a modified sequential model fits the data best, but systematic deviations indicate that a three-body description is needed.

The half-life, 3.8755(12) s, and superallowed branching ratio, 0.5315(12), for ${}^{22}\mathrm{M}\mathrm{g}$ $\beta$ decay have been measured with high precision. The latter depended on $\gamma$-ray intensities being measured with an HPGe detector calibrated for relative efficiencies to an unprecedented $0.15\%$. Previous precise measurements of $0^{+}\to 0^{+}$ transitions have been restricted to the nine that populate stable daughter nuclei. No more such cases exist, and any improvement in a critical Cabibbo-Kobayashi-Maskawa unitarity test must depend on precise measurements of more exotic nuclei. With this branching-ratio measurement, we show those to be possible for $T_z=-1$ parents. We obtain a corrected $\mathcal{F}t$ value of 3071(9) s, in good agreement with expectations.

We investigate the suppression factor and the azimuthal correlation function for high $p_T$ hadrons in central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{s_{NN}}=200\mathrm{G}\mathrm{e}\mathrm{V}$ by using a dynamical model in which hydrodynamics is combined with explicitly traveling jets. We study the effects of parton energy loss in a hot medium, intrinsic $k_T$ of partons in a nucleus, and $p_{\perp }$ broadening of jets on the back-to-back correlations of high $p_T$ hadrons. Parton energy loss is found to be a dominant effect on the reduction of the awayside peaks in the correlation function.

We have measured transverse momentum distributions of charged hadrons produced in $d+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{s_{NN}}=200\mathrm{G}\mathrm{e}\mathrm{V}$. The spectra were obtained for transverse momenta $0.25<p_T<6.0\mathrm{G}\mathrm{e}\mathrm{V}/c$, in a pseudorapidity range of $0.2<\eta <1.4$ in the deuteron direction. The evolution of the spectra with collision centrality is presented in comparison to $p+\overline{p}$ collisions at the same collision energy. With increasing centrality, the yield at high transverse momenta increases more rapidly than the overall particle density, leading to a strong modification of the spectral shape. This change in spectral shape is qualitatively different from observations in $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at the same energy. The results provide important information for discriminating between different models for the suppression of high-$p_T$ hadrons observed in $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions.

Transverse momentum spectra of charged hadrons with $p_T<8\mathrm{G}\mathrm{e}\mathrm{V}/c$ and neutral pions with $p_T<10\mathrm{G}\mathrm{e}\mathrm{V}/c$ have been measured at midrapidity by the PHENIX experiment at BNL RHIC in $d+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{s_{NN}}=200\mathrm{G}\mathrm{e}\mathrm{V}$. The measured yields are compared to those in $p+p$ collisions at the same $\sqrt{s_{NN}}$ scaled up by the number of underlying nucleon-nucleon collisions in $d+\mathrm{A}\mathrm{u}$. The yield ratio does not show the suppression observed in central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at RHIC. Instead, there is a small enhancement in the yield of high momentum particles.

We report measurements of single-particle inclusive spectra and two-particle azimuthal distributions of charged hadrons at high transverse momentum (high $p_T$) in minimum bias and central $d+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{s_{NN}}=200\mathrm{G}\mathrm{e}\mathrm{V}$. The inclusive yield is enhanced in $d+\mathrm{A}\mathrm{u}$ collisions relative to binary-scaled $p+p$ collisions, while the two-particle azimuthal distributions are very similar to those observed in $p+p$ collisions. These results demonstrate that the strong suppression of the inclusive yield and back-to-back correlations at high $p_T$ previously observed in central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions are due to final-state interactions with the dense medium generated in such collisions.

We present spectra of charged hadrons from $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ and $d+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{s_{NN}}=200\mathrm{G}\mathrm{e}\mathrm{V}$ measured with the BRAHMS experiment at RHIC. The spectra for different collision centralities are compared to spectra from $p+\overline{p}$ collisions at the same energy scaled by the number of binary collisions. The resulting ratios (nuclear modification factors) for central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at $\eta =0$ and $\eta =2.2$ evidence a strong suppression in the high $p_T$ region ($>2\mathrm{G}\mathrm{e}\mathrm{V}/c$). In contrast, the $d+\mathrm{A}\mathrm{u}$ nuclear modification factor (at $\eta =0$) exhibits an enhancement of the high $p_T$ yields. These measurements indicate a high energy loss of the high $p_T$ particles in the medium created in the central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions. The lack of suppression in $d+\mathrm{A}\mathrm{u}$ collisions makes it unlikely that initial state effects can explain the suppression in the central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions.

Transverse momentum spectra of neutral pions in the range $1<p_T<10\mathrm{G}\mathrm{e}\mathrm{V}/c$ have been measured at midrapidity by the PHENIX experiment at BNL RHIC in $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{s_{NN}}=200\mathrm{G}\mathrm{e}\mathrm{V}$. The ${\pi }^0$ multiplicity in central reactions is significantly below the yields measured at the same $\sqrt{s_{NN}}$ in peripheral $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ and $p+p$ reactions scaled by the number of nucleon-nucleon collisions. For the most central bin, the suppression factor is $\sim 2.5$ at $p_T=2\mathrm{G}\mathrm{e}\mathrm{V}/c$ and increases to $\sim 4–5$ at $p_T\approx 4\mathrm{G}\mathrm{e}\mathrm{V}/c$. At larger $p_T$, the suppression remains constant within errors. The deficit is already apparent in semiperipheral reactions and increases smoothly with centrality.

We describe our high-resolution measurements of the ${}^{133}\mathrm{C}\mathrm{s}$ $6p{}^{2}P_{3/2}$ state hyperfine structure. An optically narrowed diode laser excites perpendicularly a highly collimated atomic beam. The spectra are calibrated with a stable reference diode laser using a rf locking scheme allowing us to determine the splittings with an accuracy of $\le 2\mathrm{k}\mathrm{H}\mathrm{z}$, an order of magnitude better than previous results. The magnetic dipole $a$, electric quadrupole $b$, and magnetic octupole $c$ hyperfine coupling constants are determined. The values we obtained are $a=50.28827(23)\mathrm{M}\mathrm{H}\mathrm{z}$, $b=-0.4934(17)\mathrm{M}\mathrm{H}\mathrm{z}$, and $c=0.56(7)\mathrm{k}\mathrm{H}\mathrm{z}$. This work represents the first observation of the magnetic octupole moment of the cesium nucleus. We carry out atomic-structure calculations and determine the nuclear electric quadrupole moment $Q=-3.55(4)\mathrm{m}\mathrm{b}$ and nuclear magnetic octupole moment $\Omega =0.82(10)\mathrm{b}\times {\mu }_N$.

The issue of the Gamow-Teller sum rule in atomic nuclei is reexamined in the framework of a relativistic description of nuclei. It is shown that the total Gamow-Teller strength in the nucleon sector is reduced by about $12\%$ as compared to the prediction of a nonrelativistic description. This reduction is due to the antinucleon degrees of freedom.

We calculate the net-baryon rapidity distribution in $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at the Relativistic Heavy Ion Collider (RHIC) in the framework of the parton cascade model (PCM). Parton rescattering and fragmentation leads to a substantial increase in the net-baryon density at midrapidity over the density produced by initial primary parton-parton scatterings. The PCM is able to describe the measured net-baryon density at RHIC.

We present measurements of the pseudorapidity distribution of primary charged particles produced in $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at three energies, $\sqrt{s_{NN}}=19.6$, 130, and 200 GeV, for a range of collision centralities. The distribution narrows for more central collisions and excess particles are produced at high pseudorapidity in peripheral collisions. For a given centrality, however, the distributions are found to scale with energy according to the “limiting fragmentation” hypothesis. The universal fragmentation region described by this scaling grows in pseudorapidity with increasing collision energy, extending well away from the beam rapidity and covering more than half of the pseudorapidity range over which particles are produced. This approach to a universal limiting curve appears to be a dominant feature of the pseudorapidity distribution and therefore of the total particle production in these collisions.

We have measured the proton recoil polarization in the ${}^4\mathrm{H}\mathrm{e}(\overrightarrow{e},e^{\prime }\overrightarrow{p}){}^4\mathrm{H}$ reaction at $Q^2=0.5$, 1.0, 1.6, and $2.6(\mathrm{G}\mathrm{e}\mathrm{V}/c{)}^2$. The measured ratio of polarization transfer coefficients differs from a fully relativistic calculation, favoring the inclusion of a medium modification of the proton form factors predicted by a quark-meson coupling model. In addition, the measured induced polarizations agree reasonably well with the fully relativistic calculation indicating that the treatment of final-state interactions is under control.

We report on first measurements of low-mass electron-positron pairs in Pb-Au collisions at the CERN SPS beam energy of $40A\mathrm{G}\mathrm{e}\mathrm{V}$. The observed pair yield integrated over the range of invariant masses $0.2<m\le 1\mathrm{G}\mathrm{e}\mathrm{V}/c^2$ is enhanced over the expectation from neutral meson decays by a factor of $5.9\pm 1.5(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 1.2(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{a})\pm 1.8(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{m}\mathrm{e}\mathrm{s}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{c}\mathrm{a}\mathrm{y}\mathrm{s})$, somewhat larger than previously observed at the higher energy of $158A\mathrm{G}\mathrm{e}\mathrm{V}$. The results are discussed with reference to model calculations based on ${\pi }^{+}{\pi }^{-}\to e^{+}{e}^{-}$ annihilation with a modified $\rho$ propagator. They may be linked to chiral symmetry restoration and support the notion that the in-medium modifications of the $\rho$ are more driven by baryon density than by temperature.

We investigate nuclear fragmentation in the central proton-nucleus and nucleus-nucleus collisions at the energies of CERN LHC. Within the semiclassical approximation we argue that because of the fast increase with energy of the cross sections of soft and hard interactions each nucleon is stripped in the average process off “soft” partons and fragments into a collection of leading quarks and gluons with large $p_t$. Valence quarks and gluons are streaming in the opposite directions when viewed in the c.m. of the produced system. The resulting pattern of the fragmentation of the colliding nuclei leads to an implosion of the quark and gluon constituents of the nuclei. The nonequilibrium state produced at the initial stage in the nucleus fragmentation region is estimated to have densities $\ge 50\mathrm{G}\mathrm{e}\mathrm{V}/{\mathrm{f}\mathrm{m}}^3$ at the LHC energies and probably $\ge 10\mathrm{G}\mathrm{e}\mathrm{V}/{\mathrm{f}\mathrm{m}}^3$ at BNL RHIC.

A large enhancement in the production of neutron-rich projectile residues is observed in the reactions of a $25\mathrm{M}\mathrm{e}\mathrm{V}/\mathrm{n}\mathrm{u}\mathrm{c}\mathrm{l}\mathrm{e}\mathrm{o}\mathrm{n}$ ${}^{86}\mathrm{K}\mathrm{r}$ beam with the neutron-rich ${}^{124}\mathrm{S}\mathrm{n}$ and ${}^{64}\mathrm{N}\mathrm{i}$ targets relative to the predictions of the EPAX parametrization of high-energy fragmentation, as well as relative to the reaction with the less neutron-rich ${}^{112}\mathrm{S}\mathrm{n}$ target. A hybrid model based on a deep-inelastic transfer (DIT) code followed by a statistical deexcitation code accounts for part of the observed large cross sections. The DIT simulation indicates that the production of neutron-rich nuclides in these reactions is associated with peripheral nucleon exchange in which the neutron skins of the neutron-rich ${}^{124}\mathrm{S}\mathrm{n}$ and ${}^{64}\mathrm{N}\mathrm{i}$ target nuclei may play an important role. From a practical viewpoint, such reactions offer a novel synthetic avenue to access extremely neutron-rich rare isotopes towards the neutron-drip line.

The reaction ${}^9\mathrm{B}\mathrm{e}({}^{28}\mathrm{M}\mathrm{g},{}^{26}\mathrm{N}\mathrm{e}+\gamma )X$ has been studied at $82\mathrm{M}\mathrm{e}\mathrm{V}/\mathrm{n}\mathrm{u}\mathrm{c}\mathrm{l}\mathrm{e}\mathrm{o}\mathrm{n}$ together with two similar cases, ${}^{30}\mathrm{M}\mathrm{g}$ and ${}^{34}\mathrm{S}\mathrm{i}$. Strong evidence that the reactions are direct is offered by the parallel-momentum distributions of the reaction residues and by the inclusive cross sections. The pattern of the partial cross sections for ${}^{28}\mathrm{M}\mathrm{g}$ suggests the presence of correlations. A preliminary theoretical discussion based on eikonal reaction theory and the many-body shell model is presented. The reaction holds great promise for the study of neutron-rich nuclei.

We investigate cross sections for neutrino-${}^{12}\mathrm{C}$ exclusive scattering and for muon capture on ${}^{12}\mathrm{C}$ using wave functions obtained in the ab initio no-core shell model. In our parameter-free calculations with basis spaces up to the $6\hslash \Omega$ we show that realistic nucleon-nucleon interactions, like, e.g., the CD-Bonn, underpredict the experimental cross sections by more than a factor of 2. By including a realistic three-body interaction, Tucson-Melbourne ${\mathrm{T}\mathrm{M}}^{\prime }(99)$, the cross sections are enhanced significantly and a much better agreement with experiment is achieved. At the same time, the ${\mathrm{T}\mathrm{M}}^{\prime }(99)$ interaction improves the calculated level ordering in ${}^{12}\mathrm{C}$. The comparison between the CD-Bonn and the three-body calculations provides strong confirmation for the need to include a realistic three-body interaction to account for the spin-orbit strength in $p$-shell nuclei.

I show that it does not seem possible to change modern nuclear Hamiltonians to bind a tetraneutron without destroying many other successful predictions of those Hamiltonians. This means that, should a recent experimental claim of a bound tetraneutron be confirmed, our understanding of nuclear forces will have to be significantly changed. I also point out some errors in previous theoretical studies of this problem.