A negative search using 1/12 of the eventual MACRO detector has yielded nuclearite flux limits of 1.1×${10}^{\mathrm{-}14}$ ${\mathrm{cm}}^{\mathrm{-}2}$ ${\mathrm{sr}}^{\mathrm{-}1}$ ${\mathrm{s}}^{\mathrm{-}1}$ for ${10}^{\mathrm{-}10}$<m<0.1 g, and 5.5×${10}^{\mathrm{-}15}$ ${\mathrm{cm}}^{\mathrm{-}2}$ ${\mathrm{sr}}^{\mathrm{-}1}$ ${\mathrm{s}}^{\mathrm{-}1}$ for m>0.1 g. We have modified the formula of De Rújula and Glashow for the light yield of nuclearites to include the uv light absorbed and reemitted in the visible region, and proved that the MACRO sensitivity extends almost to the escape velocity of the Earth. Our flux limit, therefore, can be used to address nuclearites that are possibly trapped in the solar system.

We compare the implications for ${}_{}{}^7{}_{}{}^{}\mathrm{Be}$ and pp neutrinos of the two Mikheyev-Smirnov-Wolfenstein fits to the new GALLEX solar neutrino measurements. Small-mixing-angle solutions tend to suppress the former as electron neutrinos, but not the latter, and large-angle solutions tend to reduce both by about a factor of 2. The consequences for BOREXINO and similar solar neutrino-electron scattering experiments are discussed.

We find the conditions for the existence of a new class of defects. These defects arise from the nontrivial homotopy associated with the spontaneous breaking of subgroups of the full symmetry group of the field theory and may be viewed as topological defects embedded in a larger theory. Examples include monopole configurations in an O(4) model and a new vortex solution in the Weinberg-Salam model.

The quark and lepton mass matrices possess approximate flavor symmetries. Several results follow if the interactions of new scalars possess these approximate symmetries. Present experimental bounds allow these exotic scalars to have a weak-scale mass. The Glashow-Weinberg criterion is rendered unnecessary. Finally, rare leptonic B meson decays provide powerful probes of these scalars, expecially if they are leptoquarks.

The chiral soliton of the Nambu–Jona-Lasinio model is investigated including vector and axial-vector mesons. It is found that the baryon number is carried by the polarized Dirac sea of the quarks in contrast to the case without the (axial-) vector mesons where the baryon number resides on (positive-energy) valence quarks. Thus the model strongly supports the picture of the baryon as a topological soliton of the meson fields.

We discuss a low-energy theorem of threshold pion electroproduction which allows one to determine the axial root-mean-square radius ${\mathit{r}}_{\mathit{A}}$ of the nucleon. We show that at the same order where the radius appears, pion loops induce a small correction to the momentum dependence of the electric dipole amplitude ${\mathit{E}}_{0+}^{\left(\mathrm{-}\right)}$. This correction amounts to a decrease of the axial mean-square radius by 5% from the electroproduction data and allows one to explain the small discrepancy to the values determined from (anti)neutrino-proton scattering data.

Results from three independent measurements of the p(γ→,${\mathrm{\pi }}^0$) reaction are presented for incident photon energies between 243 and 314 MeV, and for c.m. angles of 105°, 122°, and 150°. The ratio of cross sections measured with orthogonal states of linear polarization is sensitive to the E2 excitation of the Δ resonace. Comparisons are made to the predictions of various models, all of which fail to reproduce these data.

Nuclear disks and bubbles formed in central heavy-ion collisions, as simulated by means of Boltzmann-like kinetic equations, break up into several fragments due to a new kind of Rayleigh-Taylor-like surface instability. We demonstrate that a sheet of liquid, stable in the limit of noninteracting surfaces, becomes unstable due to surface-surface interactions. The onset of this instability is determined analytically. The relevance of this instability to nuclear multifragmentation is discussed.

Nuclear transport theory predicts the formation of unstable bubbles and rings in the central collision between equal-mass heavy ions. This phenomenon occurs around a beam energy of E/A≊50 MeV/nucleon and is due to the compression of the nuclei in the early stage of the collision. As observable consequences, we predict a higher number of intermediate-mass fragments than expected from multifragmentation of a homogeneously filled sphere, and a different pattern of fragment Coulomb accelerations.

The nuclear-breakup and Coulomb-breakup contributions to the sequential breakup of 156-MeV ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ by ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ via the $3^{+}$ resonance state of ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ at 2.186 MeV are investigated by three-body calculations based on the continuum-discretized coupled-channels method. The nuclear-breakup contribution is as important as the Coulomb-breakup one up to forward angles where the Coulomb breakup was believed to be dominant. The effects of multistep processes via various nonresonant continuum states due to the three-body dynamics are also found to be important.

The excitation function of the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(α,γ${)}^{16}$O reaction and the angular distribution of its γ rays were measured at nine energies ranging from ${\mathit{E}}_{\mathrm{c}.\mathrm{m}.\mathrm{}}$=1.37 to 2.98 MeV. These measurements allowed the separation of the E1 and E2 contributions and their extrapolation to the region of astrophysical interest. The analysis of the E1 cross sections with a three-level R-matrix method yields a prediction of $1_{\mathrm{-}1}^{+6\mathrm{}}$ keV b for the S factor at ${\mathit{E}}_{\mathrm{c}.\mathrm{m}.\mathrm{}}$=0.3 MeV. The contribution of E2 transitions at the same energy is 40±7 keV b from a cluster-model analysis of the present data.

We have performed quasiparticle-random-phase-approximation (QRPA) calculations of the νν transition matrix for ${}_{}{}^{76}{}_{}{}^{}\mathrm{Ge}$, ${}_{}{}^{82}{}_{}{}^{}\mathrm{Se}$, and ${}_{}{}^{100}{}_{}{}^{}\mathrm{Mo}$, using effective interactions derived from the Paris and the Bonn NN potentials. Unlike earlier QRPA calculations where the self-energy corrections to the single-particle spectra were suppressed, we have retained these corrections as given by our interactions. In this way our calculations are able to avoid the commonly encountered difficulty of QRPA instability near ${\mathit{g}}_{\mathrm{pp}}$=1. The ${\mathit{M}}^{2\nu }$ matrix elements of ${}_{}{}^{76}{}_{}{}^{}\mathrm{Ge}$, ${}_{}{}^{82}{}_{}{}^{}\mathrm{Se}$, and ${}_{}{}^{100}{}_{}{}^{}\mathrm{Mo}$ given by our calculation, with no adjustable parameters, are in reasonably good agreement with recent direct counter experiments.

We report experimental results which imply that observed peaks in the short-pulse photoelectron spectrum of atoms in high fields result only from a resonant enhancement of the multiphonon ionization rate at the peak of the laser pulse, rather than a two-step process of real population transfer and subsequent single-photon ionization. These results consist of the first identification, in a high intensity (>${10}^{13}$ W/${\mathrm{cm}}^2$) short-pulse photoelectron spectrum, of low-lying states of argon with an ac Stark shift significantly different from the ponderomotive energy.

We present calculations of the $3^2$S and $3^2$P spin asymmetries and ${\mathit{L}}_{\mathrm{\perp }}$ for singlet and triplet scattering for projectile energies of 10 and 20 eV. Together these observables give a most stringent test of any electron-atom scattering theory. We find excellent agreement between the results of our coupled-channel optical method and experiment, which for the spin asymmetries can only be obtained by a good description of the couplings between the lower-lying target states and the target continuum.

Angular and momentum distributions have been measured for positrons from electron-positron pairs created in peripheral collisions of 6.4-TeV sulfur ions with fixed targets of Al, Pd, and Au. The data are compared with results of several theoretical treatments. Measured differential cross sections peak at low momentum (≤1 MeV/c), extend significantly to much higher momenta (>17 MeV/c), and concentrate sharply in the forward direction, along the ion-beam axis. Positron yields scale as the square of the target nuclear charge as predicted by theory.

The distribution of fragments resulting from collisions between 50–200-keV ${\mathrm{C}}_{60}^{+}$ ions and ${\mathrm{H}}_2$ and He is found to follow approximately a simple power law I(m)=${\mathit{cp}}^{\mathit{m}}$ where p is a constant depending on both energy and target gas, and m is the number of missing ‘‘pairs’’ of carbon atoms. Based on this observation, a new dynamical fragmentation model involving the ratio of two characteristic times is proposed. In collisions by 300-keV ${\mathrm{C}}_{60}^{++}$ ions, the singly charged products are distributed quite differently, which implies the first evidence of the presence of charge-separation reactions.

We observe and reproduce theoretically, Stueckelberg oscillations in the microwave excitation of helium (n=28) ${}_{}{}^3{}_{}{}^{}\mathrm{S}$ Rydberg atoms. They are produced by the coherent evolution along two isolated Floquet potential curves as the microwave electric field amplitude varies along a half-sine-wave pulse shape. The measured visibility and separation of the oscillations depends sensitively on the Floquet structure, on the atomic beam velocity, and particularly on additive noise, which smears and broadens the pattern.

We have compared the frequency of the hydrogen 1S-2S transition (resonance width Δν/ν=2.8×${10}^{\mathrm{-}11}$) with a reference frequency synthesized from an 88-THz infrared ${\mathrm{CH}}_4$-stabilized HeNe laser. In this first measurement of an optical frequency that overcomes the uncertainty limits of the visible ${\mathrm{I}}_2$-stabilized He-Ne laser standard, we find a 1S-2S frequency of 2 466 061 413.182(45) MHz with an 18-fold improved precision of 1.8 parts in ${10}^{11}$. A new value of the Rydberg constant, ${\mathit{R}}_{\mathrm{\infty }}$=109 737.315 684 1(42) ${\mathrm{cm}}^{\mathrm{-}1}$, is deduced.

We consider the quantum electrodynamics of an atom uniformly moving through a single spatially periodic field mode. The shape and periodicity of the field modulation can be designed by an appropriate choice of a defect in a periodic structure that possesses a forbidden spectral band (a ‘‘photonic band gap’’). This design of the periodic modulation can improve our control over the evolution and properties (e.g., photon statistics) of nonclassical ‘‘Schrödinger-cat’’ states of the field, generated by resonant interaction with the atom.

The concepts of absolute and convective instability are extended to nonlinear systems with broken Galilean invariance. As an illustrative model we describe the behavior of a flow, homogeneous in a semi-infinite domain, which undergoes a subcritical pitchfork bifurcation. The classical bifurcation phenomenology is shown to be nontrivially affected by the presence of a nonremovable advection term. In particular the existence of a hysteresis loop is shown to be restricted to the nonlinear absolute instability range. A qualitative description of the possible scenarios likely to arise in subcritically bifurcating open flows is outlined and a practical test is suggested to determine the nature of the bifurcation.