Source parameters deduced from Bose-Einstein correlations of two and three soft pions in symmetric heavy-ion interactions at 650 A MeV

The analysis of Bose-Einstein correlations for systems of two and three low-momentum pions in interactions of Au+Au and Nb+Nb at 650 A MeV kinetic energy are presented. The data were taken with the Plastic Ball spectrometer. The pion-emitting source sizes obtained from specific three-pion correlations are significantly smaller than those extracted from two-pion correlations. The dependence of the source parameters on the multiplicity of produced pions was examined. The low-p _t pions are emitted from a larger region than the high-p _t ones.     

Evidence of non-statistical structures in the elastic and inelastic scattering of58Ni+58Ni and58Ni+62Ni and intermediate dinuclear states

Excitation functions and angular distributions of⁵⁸Ni+⁵⁸Ni and⁵⁸Ni+⁶²Ni scattering at energies just above the Coulomb barrier have been measured aroundθ _cm=90° in energy stepsΔE _cm=0.25 MeV fromE _cm ? 110 MeV toE _cm ? 120 MeV for⁵⁸Ni+⁵⁸Ni and fromE _cm ? 110 MeV toE _cm ? 118 MeV for⁵⁸Ni+⁶²Ni. Evidence for structure of non-statistical character has been found in the angle-summed excitation functions; this evidence is corroborated by the analysis of the angular distributions. This is the first time that non-statistical structure in elastic and inelastic scattering is reported with high confidence level for this mass and excitation energy ranges. Attempts are presented to understand the nature of this structure, including the presence of intermediate dinuclear states and virtual states in a potential well.     

Positive pion photoproduction from hydrogen at photon energies between 500 MeV and 1,400 MeV in forward direction

The differential cross section of the reactionγ+p→π ⁺ was measured at pion CM-angles of 20° and 30° for photon energies between 500 MeV and 1,400 MeV. The pions were detected in a magnetic spectrometer. By measuring each pion trajectory and by offline calculation of the initial pion parameters an energy resolution of about 2.5% FWHM was achieved. The results complete a set of data which were measured in recent years at the Bonn 2.5 GeV synchrotron. In comparison to photoproduction analyses two effects were revealed: The η cusp appears in the energy dependence of the cross section as a sharp drop atK _γ=710 MeV. In the region of the third resonance the data show a greater enhancement than predicted by most of the analyses.     

π− Production in16O +27Al at 38, 65 and 93 MeV/u

Negative pion production cross-sections are measured at 38, 65 and 93 MeV per nucleon in O + Al reaction. Rough estimations of the total cross sections are given and compared to previous neutral pion data. Double differential cross sections at 93 and 65 MeV/u show enhancement for pions with velocity close to the beam. At variance no effect is observed at 38 MeV, where π^? and π⁰ distributions are identical. Fewπ ⁺ detected at 38 MeV support this result. This indicates that at the lowest incident energy nearly the whole projectile participates in the production reaction.     

Induced nuclear fission accompanied by pion emission

Basic features of the nuclear-fission process induced by protons of incident energy in the range 150<E _p<600 MeV and accompanied by pion emission are predicted on the basis of the cascade-evaporation-fission model. Specific calculations are performed for the total cross section; and the angular and double-differential distributions of pions; excitation-energy,mass number, and charge-number distributions of compound nuclei; and the mass-energy distributions of fission fragments. Various lines of possible experimental investigations into this fission channel are discussed, including searches for the pionic channel of nuclear decay induced by protons of energy close to the meson-production threshold, advancements to the energy region E _p<100 MeV in order to study of new mechanisms of pion production in nuclear fission, and an extension of investigations to the case of nuclear fission accompanied by kaon emission.     

Thermal hadron production in Si-Au collisions

The most abundantly produced hadron species in Si - Au collisions at the BNL-AGS (nucleons, pions, kaons, antikaons and hyperons) are shown to be in accord with emission from a thermal resonance gas source. Within the uncertainties of the present data, two freeze-out points are possible. The best agreement is obtained for a temperature T ? 110 MeV and a baryochemical potential μ_B ? 540 MeV, corresponding to about 1/3 standard nuclear density. Another possible point lies at about twice nuclear density, with T ? 160 MeV and μ_B ? 620 MeV. Our analysis takes the isopin asymmetry of the initial state fully into account.     

Non-statistical structures in12C(15N,4He)23Na reaction

The data on the? _lab=7° excitation functions of¹²C(¹⁵N,⁴He)²³Na reaction betweenE _cm=9.42 and 17.33 MeV for 28 states upto an excitation energy of 8.940 MeV in²³Na have been subjected to statistical analysis. In addition to statistical fluctuations, the results of the analysis indicate the existence of non-statistical structures atE _cm=10.66, 10.93, 11.38, 12.62, 13.16, 15.32 and 16.18 MeV.     

The production of charged particles with high transverse momentum in proton-proton collisions at the CERN ISR

Results are given on the inclusive production of charged pions, kaons and nucleons in high-energy proton-proton collisions in the centre-of-mass rapidity range |y_cm| < 1 and for transverse momenta up to 4.8 GeV/c. The dependence of the composition of produced particles on y_cm, the total energy $\text{s}$, and transverse momentum p_T is discussed.     

A search for πNN bound systems

The d(π⁺, π⁻)T⁺⁺⁺ and the d(π⁻, π⁺) T⁻ reactions were studied at an incident pion energy of 260 MeV. Outgoing negative pions were detected at 35° and 55° in the first reaction, and the positive pions at 45° in the second. Indications for structures corresponding to a (π⁺pp) system with a mass of 2003.0±1.0 MeV and a (π⁻nn) system with a mass of 200.1±1.5 MeV were observed with marginal statistical significance. Upper limits for the cross sections are obtained.     

Near threshold production of neutral pions by complex particles

Cross sections for the inclusive neutral pion production cross section have been measured for near threshold energy ³He particles on complex nuclei. The results of these measurements are of order 10^?36 cm²/sr · MeV. The neutral pion cross section for 710 MeV ⁴He on ¹²C has been found to be comparable to the production cross section for nucleons, 10^?30 cm²/sr · MeV. Comparisons are made with a naive model relating complex particle production to nucleon production of pions.     

Positive pion production at threshold on A 10B target

The angular distribution of low-energy positive pions (E_π ≈ 11 MeV) produced by 154 MeV protons on a ¹⁰B target has been measured between 16.2° and 54.8° c.m. angles. Only pions corresponding to the ground state of ¹¹B were observed. The data have been compared with theoretical angular distributions calculated in the framework of the one-nucleon mechanism using the DWBA formalism.     

Vector analyzing power of π+7Li scattering in the region of the Δ33 resonance

Within Glauber diffraction theory, the vector analyzing power iT ₁₁ is calculated at three energies of positively charged pions, 134, 164, and 194 MeV, incident to ⁷Li nuclei. These energy values lie in the region of the Δ₃₃ resonance in π^± N interaction, the resonance maximum being at 180 MeV. The calculation of iT ₁₁ was performed with several model ⁷Li wave functions, including the αt-cluster and shell-model ones. The properties of π⁺⁷Li scattering are found to be sensitive to the structural features of the target nucleus. A comparison of the results of the calculations with experimental data shows that the wave functions in question and the potentials used to calculate them are quite appropriate.     

Thermalization in high energy nuclear collisions

complete events of the symmetric collisions Ca + Ca at E/A = 400 MeV and 1050 MeV and Nb + Nb at E/A = 400 MeV and 650 MeV have been measured with the Plastic Ball/Wall 4π spectrometer. The degree of isotropy expressed by the ratio of the mean transverse to the mean longitudinal momentum in each event is determined as a function of the charged particle multiplicity. Also discussed are proton energy distributions at θ_cm = 90° and their multiplicity dependences.     

Low-energy pion scattering from nuclei and the α-particle model

The elastic scattering of pions on carbon and oxygen for T_π = 50–80 MeV is calculated in the Glauber theory using a non-interacting α-particle model wave function for nuclei. The results are in agreement with experimental data.     

Hard photons and neutral pions as probes of hot and dense nuclear matter

The dynamics of heavy-ion collisions is studied in an energy domain in the vicinity of the Fermi energy. The early history of the collision is analyzed from the theoretical and experimental point of view in which the message conveyed by bremsstrahlung photons and neutral pions is exploited. The Boltzmann-Uehling-Uhlenbeck model and the Dubna Cascade Model, both based on similar principles but each adopting different computation technics, are briefly described and their respective predictions are discussed. In particular the emission pattern of bremsstrahlung photons is discussed. The photon production has been measured in the systems ⁸⁶Kr+⁵⁸Ni at 60A MeV, ¹⁸¹Ta+¹⁹⁷Au at 40A MeV and ²⁰⁸Pb+¹⁹⁷Au at 30A MeV and energy spectra, angular distributions and two-photon correlations have been analyzed. We find that bremsstrahlung photons are emitted from two distinct sources that can be correlated with nuclear-matter density oscillations. The properties of photon emission are discussed in terms of collective properties of nuclear matter. The high energy tail of the photon spectrum is interpreted by π⁰ and Δ decay but predominantly by radiative capture of pions. The π⁰ absorption in the nuclear medium is further analyzed by examining their emission pattern.     

Experimental investigation of the reaction12C(γ, π0)X in the photon energy range between threshold and 450 MeV

The photoproduction of neutral pions on¹²C has been investigated between threshold and 450 MeV photon energy using the tagged photon beam at the Bonn 500 MeV synchrotron and a large acceptance detector. Theπ ⁰ detector consists of cylindrical wire chambers covering a solid angle of nearly 4π. Using a scintillation counter as an active¹²C target an attempt was made to separate coherent and incoherentπ ⁰ photoproduction. The measured total and differential cross sections are compared to corresponding theoretical calculations.     

Excitation of isovector giant resonances by inelastic scattering of positive pions on90Zr at 226 MeV

We studied the region of giant resonances with positive pions of 226 MeV scattered inelastically on⁹⁰Zr. Two groups of resonances were seen: the first structure between 12 and 19 MeV excitation energy is explained as a sum of the isoscalar quadrupole resonance at 14 MeV, the isovector dipole resonance at 16.5 MeV and possibly some E₀ strength. The second group between 24 and 34 MeV excitation energy also corresponds to more than a simple multipolarity and may be described as a sum of a monopole and a quadrupole isovector resonance.     

Pion and proton emission in interactions induced by relativistic heavy nuclei withZ≧12

The emission of pions and protons in interactions between heavy nuclei from the cosmic radiation (12≦Z≦26) and photoemulsion nuclei has been studied. The angular and energy distributions of target protons are similar to corresponding distributions in proton-nucleus interactions for emission angles >30°. In heavy ion interactions a forward peak of high energy target protons (E>200 MeV)is observed. The energy spectrum of target protons for angles <30° is remarkably flat for heavy ion interactions. For increasing disintegration of the target nucleus the number of produced pions per emitted recoil target proton is almost constant in heavy ion interactions, while it decreases rapidly in proton-nucleus interactions. The logtgΘ distributions of pions have small standard deviations even in interactions with a large target disintegration.     

Neutral pion photoproduction on tensor-polarized deuterium on the VEPP-3 storage ring

An experiment to measure component T ₂₀ of the tensor analyzing power for the coherent photo-production of neutral pions on polarized deuterons is described. The measurements cover the photon energies in the ranges E _γ = 200–500 MeV and E _γ = 300–600 MeV, and the emission angles of neutral pions in the ranges = \(\theta _{\pi ^0 }^{c.m.} \) = 100°–140° and = \(\theta _{\pi ^0 }^{c.m.} \) = 60°–65°, respectively. The experiment uses a hyperfine internal gas polarized target. Detectors of neutral pions and deuterons register these particles in coincidence. The target’s degree of polarization and luminosity is measured by detecting elastic electron-deuteron scattering at low momentum transfer. A selection of the preliminary results is presented.     

A search for resonant two-step α-exchange in 6Li + 12C scattering

Excitation functions for ¹²C(⁶Li, ⁶Li)¹²C (gs, 4.43 MeV) have been measured at 10 angles (40° ? θ_cm ? 160°) over the energy range 20 MeV < E_lab < 36 MeV. A single anomaly of width Γ ≈ 800 keV is observed at E_lab = 22.8 MeV. The results casts doubts on the resonant two-step α-exchange mechanism suggested to occur in this system.