Within the framework of an isospin-dependent quantum molecular dynamics model in which the initial neutron and proton densities are sampled according to the densities calculated from the Skyrme-Hartree-Fock method and the initial Fermi momenta of neutrons and protons are calculated from the Fermi gas model, we study systematically the transverse collective flow of different fragment types at an energy of 55 MeV/nucleon and the balance energy in the reactions ${}^{58}\mathrm{Fe}{+}^{58}\mathrm{Fe}$ and ${}^{58}\mathrm{Ni}{+}^{58}\mathrm{Ni}.$ The results from the present calculations indicate that the neutron-rich system ${(}^{58}\mathrm{Fe}{+}^{58}\mathrm{Fe})$ displays stronger negative deflection and has a higher balance energy, which are qualitatively in agreement with the experimental data. Furthermore, the effects of the isospin-dependent symmetry energy and nucleon-nucleon cross sections on collective flow are studied.

The energy dependence of forward angle pion double charge exchange is calculated in the energy range of 0–250 MeV. The most striking feature is a peak around 40 MeV which is in excellent agreement with the data when distorted waves obtained from a realistic optical model are used. Two possible short-range corrections to the reaction mechanism are addressed.

On the basis of a nine-parameter expanding source model that includes special relativity, quantum statistics, resonance decays, and freeze-out on a realistic hypersurface in spacetime, we analyze in detail invariant ${\pi }^{+},$ ${\pi }^{-},$ $K^{+},$ and $K^{-}$ one-particle multiplicity distributions and ${\pi }^{+}$ and $K^{+}$ two-particle correlations in nearly central collisions of $\mathrm{S}\mathrm{i}+\mathrm{A}\mathrm{u}$ at $p_{\mathrm{lab}}/A=14.6\mathrm{}\mathrm{GeV}/c.\mathrm{}$ By considering separately the one-particle data and the correlation data, we find that the central baryon density, nuclear temperature, transverse collective velocity, longitudinal collective velocity, and source velocity are determined primarily by one-particle multiplicity distributions and that the transverse radius, longitudinal proper time, width in proper time, and pion incoherence fraction are determined primarily by two-particle correlations. By considering separately the pion data and the kaon data, we find that although the pion freeze-out occurs somewhat later than the kaon freeze-out, the 99% confidence-level error bars associated with the two freeze-outs overlap. By constraining the transverse freeze-out to the same source time for all points with the same longitudinal position and by allowing a more flexible freeze-out in the longitudinal direction, we find that the precise shape of the freeze-out hypersurface is relatively unimportant. By regarding the pion and kaon one-particle data to be unnormalized, we find that the nuclear temperature increases slightly, but that its uncertainty increases substantially. By including proton one-particle data (which are contaminated by spectator protons), we find that the nuclear temperature increases slightly. These detailed studies confirm our earlier conclusion based on the simultaneous consideration of the pion and kaon one-particle and correlation data that the freeze-out temperature is less than 100 MeV and that both the longitudinal and transverse collective velocities—which are anticorrelated with the temperature—are substantial. We also discuss the flaws in several previous analyses that yielded a much higher freeze-out temperature of approximately 140 MeV for both this reaction and other reactions involving heavier projectiles and/or higher bombarding energies.

The multiparticle Bose-Einstein correlations for quark-gluon plasma (QGP) formation in high energy heavy ion collisions are analyzed. The average correlation intensities of multipions as a function of pion multiplicity are investigated. It is shown that the average correlation intensities of multipions have obvious enhancements in the high multiplicity region. This signature can be used to detect the existence of QGP in rare high pion multiplicity events in high energy heavy ion collisions.

Bose-Einstein correlations of pions are calculated in the presence of both statistical and coherent pion production. An isotopic spin conservation is taken into account for the coherent component. Additional contributions due to the isospin effect in pion correlation functions are found. Moreover, the experimental data of two-particle Bose-Einstein correlations for neutral pions are analyzed in this scheme.

Since large-$p_T$ particles in high-energy hadronic or nuclear collisions come from jet fragmentation, jet quenching due to parton energy loss in dense matter will cause the suppression of large-$p_T$ hadron spectra in high-energy heavy-ion collisions. Assuming an effective energy loss $dE/dx$ for the high-$E_T$ partons, effective jet fragmentation functions are constructed in which leading hadrons will be suppressed. Using such effective fragmentation functions, high-$p_T$ hadron spectra and particle suppression factors relative to $\mathrm{pp}$ collisions are estimated in central high-energy nuclear collisions with a given range of the assumed $dE/dx.\mathrm{}$ It is found that the suppression factors are very sensitive to the value of the effective energy loss. Systematic nuclear and flavor dependence of the hadron spectra are also studied.

Using the influence functional formalism, classical equations of motion for the $O\left(N\right)\mathrm{}$ model are derived in the presence of a heat bath, in both the symmetric phase as well as the phase of spontaneously broken symmetry. The heat bath leads to dissipation and fluctuation terms in the classical equations of motion, which are explicitly computed to lowest order in perturbation theory. In the broken phase these terms are found to be large for the $\sigma$ field, even at zero temperature, due to the decay process $\overrightarrow{\sigma }\pi \pi ,$ while they are small for the $\pi$ fields at temperatures below $T_c\simeq 160\hspace{0.5em}\mathrm{MeV}.$ It is shown that in large volumes the presence of dissipation and fluctuations suppresses the formation of disoriented chiral condensates (DCC’s). In small volumes, however, fluctuations become sufficiently large to induce the formation of DCC’s even if chiral symmetry has not been restored in the initial stage of the system’s evolution.

We examine the effects that the new equilibration scenario of the increasingly strongly interacting parton plasma and the nonequilibrium environment have on the production of open charms, photons, and dileptons at Large Hadron Collider (LHC) and at Relativistic Heavy Ion Collider (RHIC) energies. We show that an out-of-equilibrium effect, not shown before, changes significantly the relative yield of the two main partonic contributions to photon production in the higher $p_T$ range, and higher orders for electromagnetic emissions have increased significance in the new scenario especially at RHIC energies. We argue that the effects of the new scenario are not restricted just to the parton phase but will continue through to the later hadron gas phase with potentially positive implications for the detection of the quark-gluon plasma.

We have measured the K-series x rays from kaonic hydrogen atoms and have succeeded for the first time in observing a distinct $K\alpha$ peak. The L x-ray peak has also been observed. The strong-interaction shift and width of the kaonic hydrogen atom $1s$ state were determined from the transition energy and the line width of the $K\alpha$ x rays. The sign of the strong-interaction shift was confirmed to be repulsive, with the results $\Delta E_{1s}=-323\pm 63\hspace{0.5em}\left(\mathrm{statistical}\right)\pm 11\hspace{0.5em}\left(\mathrm{systematic}\right)\hspace{0.5em}\mathrm{eV}$ and ${\Gamma }_{1s}=407\mathrm{}\pm 208\pm 100\hspace{0.5em}\mathrm{eV}.\mathrm{}$ The measured shift and width of the $1s$ state are consistent with those expected from other low energy $\overline{K}N$ data, and thus the long-standing kaonic hydrogen puzzle is resolved. The $K\alpha$ x-ray yield per stopped kaon was estimated to be $1.5\pm 0.5\%$ at a hydrogen target density of $10{\rho }_{\mathrm{STP}}.$

The renormalized form factors for pseudoscalar meson-baryon coupling are computed in the chiral color dielectric model. This has been done by rearranging the Lippmann-Schwinger series for the meson-baryon scattering matrix so that it can be expressed as a baryon pole term with renormalized form factors and baryon masses and the rest of the terms which arise from the crossed diagrams. Thus we are able to obtain an integral equation for the renormalized meson-baryon form factors in terms of the bare form factors as well as an expression for the meson self-energy. This integral equation is solved, and renormalized meson baryon form factors and renormalized baryon masses are computed. The parameters of the model are adjusted to obtain a best fit to the physical baryon masses. The calculations show that the renormalized form factors are energy dependent and differ from the bare form factors primarily at momentum transfers smaller than 1 GeV. At nucleon mass, the change in the form factors is about 10% at zero momentum transfer. The computed form factors are soft with the equivalent monopole cutoff mass of about 500 MeV. The renormalized coupling constants are obtained by comparing the chiral color dielectric model interaction Hamiltonian with the standard form of the meson-nucleon interaction Hamiltonian. The ratio of $\Delta N\pi$ and $\mathrm{NN}\pi$ coupling constants is found to be about 2.15. This value is very close to the experimental value.

A quark model approach to the photoproduction of vector mesons off nucleons is proposed. Its starting point is an effective Lagrangian of the interaction between the vector meson and the quarks inside the baryon, which generates the nondiffractive s- and u-channel resonance contributions. Additional t-channel ${\pi }^0$ and σ exchanges are included for the ω and ${\rho }^0$ production, respectively, to account for the large diffractive behavior in the small $t$ region as suggested by Friman and Soyeur. The numerical results are presented for the ω and ρ productions in four isospin channels with the same set of parameters, and they are in good agreement with the available data not only in ω and ${\rho }^0$ productions, but also in the charged ρ productions where the additional t-channel σ exchange does not contribute so that it provides an important test to this approach. The investigation is also extended to the φ photoproduction, and the initial results show that the nondiffractive behavior of the φ productions in the large $t$ region can be described by the s- and u-channel contributions with significantly smaller coupling constants, which is consistent with the findings in the similar studies in the QHD framework. The numerical investigation has also shown that polarization observables are essential for identifying so-called “missing resonances.”

Under certain circumstances, a three-body decay width can be approximated by an integral involving a product of two off-shell two-body decay widths. This “angle-average” approximation is used to calculate the $\pi \mathrm{NN}$ decay width of the $d^{*}{(J}^{\pi }{=3\mathrm{}}^{+},T=0)$ dibaryon in a simple ${\Delta }^2$ model for the most important Feynman diagrams describing pion emissions with baryon-baryon recoil and meson retardation. The decay width is found to be about 0.006 (0.07, 0.5) MeV at the $d^{*}$ mass of 2065 (2100, 2150) MeV for input dynamics derived from the full Bonn potential. The smallness of this width is qualitatively understood as the result of the three-body decay being “third forbidden.” The concept of l forbiddenness and the threshold behavior of a three-body decay are further studied in connection with the $\pi \mathrm{NN}$ decay of the dibaryon $d^{\prime }{(J}^{\pi }{=0\mathrm{}}^{-},T=0\mathrm{}or2)$ where the idea of “unfavoredness” has to be introduced. The implications of these results are briefly discussed.

The polarization observables in $\phi$ meson photoproduction are studied to probe the strangeness content of the nucleon. In addition to the dominant diffractive production and the one-pion-exchange process, we take into account the direct knockout mechanism that arises from the possible hidden strangeness content of the nucleon. We find that some double polarization observables are very sensitive to the strangeness content of the proton because of the different spin structures of the amplitudes associated with different mechanisms. This suggests that such measurements could be very useful in probing the strangeness content in the proton. The orbitally excited quark-cluster configurations in the proton are included in the calculation and found to have little effect.

A light-front treatment for the scalar and vector meson momentum distribution functions is developed using a model in which the nucleus is treated a static source of radius $R.\mathrm{}$ The limit $\overrightarrow{R}\infty$ corresponds to infinite nuclear matter. In this limit, the scalar mesons are shown to carry a vanishing momentum fraction of the nuclear plus momentum $k^{+},$ but the vector mesons carry a significant fraction, all occuring at the experimentally inaccessible value of $k^{+}=0.\mathrm{}$ This confirms earlier work. A study of the R dependence for sizes of real nuclei shows that a fraction of the scalar and vector mesons could be observable.

The baryon octet and decuplet masses and Bethe-Salpeter vertex and wave functions are calculated in the ladder approximation to the quark exchange between a scalar or axialvector diquark and a constituent quark. These functions reflecting full Lorentz covariance are given in terms of an expansion in Gegenbauer polynomials. In the rest frame of the baryon, a complete partial wave decomposition of the Bethe-Salpeter wave function is performed. The confinement of quarks and diquarks is implemented via a parametrization of the corresponding propagators. We also discuss some aspects of the momentum routing in the ladder approximation to the Bethe-Salpeter equation. Numerical results for the octet and decuplet masses with broken flavor SU(3) in the conserved isospin limit are presented.

Until now, calculations of the $E2\mathrm{}$ amplitude in the $\overrightarrow{N}\Delta$ transition using the spatial current operator have differed considerably from those using the one-body charge operator and Siegert’s theorem. We show that this difference is almost entirely explained by spatial two-body exchange currents consistent with the chosen quark-quark potentials. These were not explicitly included in previous comparisons. Furthermore, using Siegert’s theorem we find that the main contribution to the $E2\mathrm{}$ amplitude comes from a two-quark spin-flip mechanism originating from pion and gluon degrees of freedom in the charge operator. Our results provide insight into the microscopic origin of the intrinsic deformation of the nucleon.

A search for ${\nu }_{\mu }\to {\nu }_e$ oscillations has been conducted at the Los Alamos Meson Physics Facility using ${\nu }_{\mu }$ from ${\pi }^{+}$ decay in flight. An excess in the number of beam-related events from the ${\nu }_e\overrightarrow{C}{e}^{-}X$ inclusive reaction is observed. The excess is too large to be explained by normal ${\nu }_e$ contamination in the beam at a confidence level greater than 99%. If interpreted as an oscillation signal, the observed oscillation probability of $(2.6\mathrm{}\pm 1.0\pm 0.5)\times {10}^{-3}$ is consistent with the previously reported ${\overline{\nu }}_{\mu }\to {\overline{\nu }}_e$ oscillation evidence from LSND.

Neutrino-induced recoil spectra of ${}^{37}\mathrm{Cl}$ ions produced in the electron capture (EC) decay of ${}^{37}\mathrm{Ar}$ were measured and searched for the presence of massive neutrinos admixed to the dominant electron neutrino. Fractions of a monolayer of ${}^{37}\mathrm{Ar}$ were physisorbed on Au and on several underlayers of ${}^{40}\mathrm{Ar}$ adsorbed on both Au and graphite substrates cooled to $<~20\hspace{0.5em}\mathrm{K}$ under ultrahigh vacuum conditions. Time-of-flight spectra of the recoiling ions were recorded in coincidence with x rays and Auger electrons emitted following the EC decay. By searching these spectra for peaks with energies between 7.6 eV and 3.6 eV upper limits were placed on the mixing probability of the electron neutrino with heavy neutrinos in the 370–640 keV mass range. These limits vary from 1 to 4 %, at the 90% confidence level.

We use the quasiparticle random-phase approximation to include the effects of low-lying Gamow-Teller and first forbidden strength in neutrino capture by very neutron-rich nuclei with $N=50,$ 82, or 126. For electron neutrinos in what is currently considered the most likely r-process site the capture cross sections are two or more times previous estimates. We briefly discuss the reliability of our calculations and their implications for nucleosynthesis.

The cross section for thermal neutron capture by radioactive ${}^{44}\mathrm{Ti}$ was measured to be ${\sigma }_{\gamma }=1.1\mathrm{}\pm 0.2$ b. This result was shown to be in agreement with a direct capture calculation. Possible implications of the ${}^{44}\mathrm{Ti}(n,\gamma )$ reaction in nucleosynthesis are discussed.