Estimation of cross section of a Δ resonance in <Emphasis Type="Italic">pA</Emphasis> collisions for internal target of nuclotron

The two-stage process of a Δ-resonance production in a target nucleus is considered. The analysis of the reaction uses a kinematic calculation of a cross section in pion production in pA inelastic collisions, and the subsequent interaction of produced pions with the target nucleus is analyzed. In particular, a simple technique to estimate the cross section of Δ-resonance production is offered for the carbon target. This technique allows one to estimate the total cross section of Δ-resonance production at rest, depending on the energy value of incident particles. Calculations of the efficiency of a resonance registration for the internal target of the Nuclotron at the SCAN setup are presented.     

Zur Produktion von π-Mesonen bei peripheren Kern-Kern-Stößen

The production of pions by nucleus nucleus scattering under particular kinematical conditions is discussed. Taking into account only electromagnetic interactions between the nuclei a simple formalism is presented which allows to express the differential cross section and the angular distribution of the produced pions in terms of so called generalized form factors. Using the concept of impulse approximation these form factors can be explicitely calculated.     

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.     

Bose-einstein correlations in the target fragmentation region in 200A GeV16O+nucleus collisions

Correlations between positive pions are investigated in the target fragmentation region of 200A GeV¹⁶O+nucleus collisions. The pions are measured with the Plastic Ball detector in the WA80 experiment at the CERN SPS. The target mass dependence of the radii and the correlation strength extracted by interferometry is studied. A new approach to the fit of the correlation function is introduced. The correlation strength and both invariant and transverse radii increase with decreasing target mass. The transverse radius for¹⁶O+C reactions appears to be much larger than the geometrical radius of the nuclei involved. For the Au target only a small fraction of the measured pions contributes to the apparent correlation. Hints for a much larger second component in¹⁶O+Au reactions are observed. Rescattering phenomena may provide a clue to understand these phenomena.     

K^＋ Scattering with the Nuclear Pion from Chiral Effective Lagrangian

The K^ scattering cross section with the in-medium virtual pion is evaluated in the lowest-order chiral perturbation theory with the density-dependent pion decay constant and mass.The contribution of nuclear pions to the total K^ -nucleus cross section is found to be about 5% and 12% when the excess pion numbers per nucleon nπ=0.057 and 0.13 are used.The inclusion of the off-mass-shell behavior of the K^ π amplitude produced a significant improvement in the K^ -nucleus cross section.     

Transverse momentum spectra of the produced hadrons at SPS energy and a random walk model

The transverse momentum spectra of the produced hadrons have been compared to a model, which is based on the assumption that a nucleus–nucleus collision is a superposition of isotropically decaying thermal sources at a given freeze-out temperature. The freeze-out temperature in nucleus–nucleus collisions is fixed from the inverse slope of the transverse momentum spectra of hadrons in nucleon–nucleon collision. The successive collisions in thessss nuclear reactions leads to gain in transverse momentum, as the nucleons propagate in the nucleus following a random walk pattern. The average transverse rapidity shift per collision is determined from the nucleon–nucleus collision data. Using this information, we obtain parameter-free result for the transverse momentum distribution of produced hadrons in nucleus–nucleus collisions. It is observed that such a model is able to explain the transverse mass spectra of the produced pions at SPS energies. However, it fails to satisfactorily explain the transverse mass spectra of kaons and protons. This indicates the presence of collective effect which cannot be accounted for, by the initial state collision broadening of transverse momentum of produced hadrons, the basis of random walk model.     

Pion production in mass- symmetric heavy ion collisions at 0.8–1.8 AGeV

Double differential cross sections of positively charged pions and protons have been measured in nuclear collisions of mass-symmetric systems (Ne+NaF, Ni+Ni, Au+Au, Bi+Pb) at incident energies between 0.8 and 1.8 AGeV as a function of the centrality of the reaction. Using a magnetic spectrometer pions and protons were detected with laboratory angles between 40 and 48 degrees, and with momenta up to about 1400 MeV/c. This setting allows for the study of pions and protons emitted close to midrapidity. The center-of-mass pion spectra deviate from a Boltzmann distribution. The inverse slope parameters of the high-energetic pions are smaller than those of the proton spectra and they exhibit a weaker centrality dependence. A scenario is presented where the shape of the pion spectra reflects the decay kinematics of nucleonic resonances embedded in the thermal and the collective motion of the nucleons in the reaction zone. The number of emitted pions per participating nucleon is higher for light than for heavy mass systems. For a given mass system, the total pion multiplicity increases linearly with the number of participating nucleons, whereas the multiplicity of high-energy pions increases more than linearly. This result is consistent with a scenario where the high-energy pions are produced in multiple energetic baryon-baryon collisions occurring in the high-density phase of the collision.     

Pion production via isobar giant resonance formation and decay

A spin, isotopic-spin formalism for the production of pions due to decays of isobar giant resonances formed in peripheral heavy-ion collisions is presented. The projectile nucleus isobar giant resonance state is assumed to coherently form and then incoherently decay to produce the pions. Total spin and isotopic spin for the system are conserved through the concomitant excitation of the target nucleus to an isobaric analog giant resonance state. Comparisons of the predicted total pion cross sections, over a range of energies, are made with heavy-ion pion data.     

Coherent π− production in He3 + Li6 → π− + X at 303 MeV/N

The production of pions by the He³ nucleus is calculated in the Weizsäcker-Williams approximation. The pion cloud of the He³ nucleus acts like a beam of pions scattering incoherently on the Li⁶ nucleus. The size and energy dependence of the observed π-cross section at 0° degrees is in good agreement with this theoretical model.     

Low-energy antiproton annihilation in nuclei

The annihilation of 100 MeV antiprotons in atomic nuclei is studied in the frame of an intranuclear cascade model. It is found that the antiproton is annihilated about 1 fm inside the nucleus on the average. The energy released by the annihilation is carried by an average of 5 pions, which cascade throughout the nucleus. About 5% (10%) of the primordial pions are absorbed by the ⁴⁰Ca (¹⁰⁸Ag) target. The pions transfer around 5&#x0303;50 MeV (7&#x0303;00 MeV) to the nucleons and eject about one-fifth of the nucleons. The pion and proton cross sections are calculated. In particular, the relative transparency of the nucleus to high-energy pions (and not to pions in the Δ-resonance region) gives rise to a peculiar pion emission pattern. The time evolution of the baryon density and of the spectrum of the participant nucleons is investigated. It turns out that the cascade does not generate high energy density.     

A numerical formula for the total cross section of the lepton-pair production via the Bethe-Heitler process of a light pseudoscalar particle

We present in detail the calculations of the total cross section for coherent lepton-pair production of a light pseudoscalar particle off a nucleus via the Bethe-Heitler process.For future reference to experiments which are aimed to search for the pseudoscalar particle, we construct a numerical formula for the total cross section as a function of the incident energy of the pseudoscalar particle and the mass number of the target nucleus.     

Pion production in nuclear collisions

The production of pions in nuclear collisions is analyzed in terms of a microscopic reaction model, where the free energy of the entrance channel is transferred to the final channel through the coupling of the relative motion to the internal excitation of N^*-resonances. Such a model allows quite naturally for a consistent analysis of the exclusive and the inclusive production of pions. It turns out that the pionic fusion cross section is determined predominantly by the spectroscopic parentage between the initial target projectile combination and the final nucleus, whereas the inclusive part is dominated by the energy dependence of the phase space factor, i.e. by the number of the degrees of freedom which are relevant in a given kinematical situation. This model is applied to the analysis of available experimental data from threshold to the (1232)-region. A consistent analysis of both the inclusive and the exclusive part of the pion spectrum is presented.Dedicated to Prof. Dr. P. Kienle on the occasion of his 60th birthday     

Nuclear absorption of stopped antiprotons: Multipion-nucleus interactions

It is proposed to use the nuclear absorption of stopped antiprotons to study the interaction arising when several pions enter a nucleus simultaneously. A model is constructed for the incoherent multipion-nucleus interaction, when the annihilation pions interact with the nucleus independently of each other. Energy spectra and the multiplicities of secondary particles generated after the antiproton absorption are compared with the experimental data. Distributions of the number of pions interacting with the nucleus are calculated as well as the distributions of residual nuclei with excitation energy, momentum, angular momentum and isotope yield. It is shown that the effect of the excitation of high-spin isomers must be observed in the antiproton absorption. The example of the two-nucleon absorption of η- and ω-mesons is used to discuss the possibility of investigating interactions of pionic resonances with nuclei during the process of the annihilation of antiprotons.     

Transverse mass distributions in central heavy-ion collisions at high energies

The transverse mass spectra of protons, pions, kaons, Lambda and Antilambda produced in cen- tral nucleus-nucleus collisions at high energies are described by using one-temperature and two-temperature emission pictures. The calculated results are compared and found to be in good agreement with the experimen- tal data of the E895, E866 and E917 Collaborations measured in central Au-Au collisions at the Alternating Gradient Synchrotron (AGS) energies and the NA49 Collaboration measured in central Pb-Pb collisions at the Super Proton Synchrotron (SPS) energies. It is demonstrated that the transverse mass distributions of protons, kaons, Lambda and Antilambda, except for Lambda hyperons produced in central Pb-Pb collisions at 158 A GeV, can be described by using the one-temperature emission picture, and for pions, we need to use the two-temperature emission picture.     

Shadowing,dipions and the EMC effect

The EMC effect is thought to be a compound phenomenon that appears when nucleons are bound in a nucleus. The shadowing effect shows up in a very low-x range. It is possible that in low- and medium-x ranges there exist weak-binding regions caused by the nucleon distortion in the nucleus as well as by exchange pions among the nucleons and other pions in weak-binding regions. Both these types of pions will be in the dipion state because of the Coulomb field. The dipion effect contributes to the structure function of the nucleus. Nuclear binding energy and Fermi motion in medium- and higher-x ranges cause a shift of the ratio of the nucleus structure function to the deuteron one. Calculations show that better agreement with recent data is obtained if the above three effects are mixed.     

Atomic mass dependence of hadron production in semi-inclusive deep inelastic lepton-nucleus scattering

Hadron production in lepton-nucleus deep inelastic scattering is studied in a quark energy loss model. The leading-order computations for hadron multiplicity ratios are presented and compared with the selected HERMES pions production data with the quark hadronization occurring outside the nucleus by means of the hadron formation time. It is found that the obtained energy loss per unit length is 0.440 ± 0.013 GeV/fm for an outgoing quark by the global fit. It is confirmed that the atomic mass number dependence of hadron attenuation is theoretically and experimentally in good agreement with the A^2/3 power law for quark hadronization occurring outside the nucleus.     

Fission studies of gold induced by (1665 MeV) π- using a CR-39 detector

The fission cross section and fission probability of ¹⁹⁷Au, induced by (1665 MeV) π^-, have been studied using CR-39 track detectors. A 4π-geometry was used to count track statistics. A beam of negative pions of 1665 MeV was produced at AGS of Brookhaven National Laboratory, USA, and allowed to fall normally on the stack. Two detectors from the stack were scanned for fission fragment tracks after etching in 6N NaOH at 70℃. The statistics of fission fragment tracks in both detectors were obtained. It was found that there was a marked asymmetry of registered tracks with respect to the forward and backward hemispheres. This asymmetry could be partly accounted for on the basis of momentum transfer to the struck nucleus. On the basis of counting statistics fission cross section was measured, and fission probability was determined by dividing the fission cross section with the reaction cross section. The fission cross-section and fission probability were compared with the computed values using the cascade-exciton model code CEM95.     

Nucleus excitation and deexcitation following at rest

The evolution of a nucleus after at rest is studied. The process is divided into two successive phases: a rapid phase, described by an intranuclear cascade model and a slow phase, akin to an evaporation process. The distribution of the residue masses after the second phase is calculated and compared to the recent radiochemical measurements for ⁹⁸Mo. The evolution of the number of pions, and the distribution of the number of fast ejected nucleons and of participants is studied. The latter two are fitted by negative binomials. The meaning of this observation is discussed. The mass dependence of several quantities, like the maximum excitation energy and the average mass loss is also investigated. Finally, the possibility of a multifragmentation is discussed.     

Production of classical pion field and/or disoriented chiral condensate

We discuss the possibility of production of a classical pion field in high energy nuclei-nuclei collisions. We show that the occupation number for the produced pions can be large as required for the case of a classical field. By examination of the cooling we conclude that in the “quench” scenario it is unlikely to produce a large domain of DCC. The size of domain can be essentially larger due to the temperature dependence of the parameters of the effective Lagrangian. However even in this case the decay of the DCC leads to the production of no more than several tenths of pions which are not easy to distinguish from the “normal” pions.     

On the energy spectrum of cosmogenic neutrons

The processes of the generation of cosmogenic neutrons (cg-neutrons) underground are considered. The neutrons produced by cosmic-ray muons in their interactions with matter are called cosmogenic. Deep-inelastic πA-collisions of pions in muon-induced hadronic showers are mainly their source at energies above 30 MeV. The characteristics of the energy spectrum for the generation of cg-neutrons have been determined by invoking the additive quark model of deep-inelastic soft processes and the mechanism for the interactions of high-energy nucleons in a nucleus. The three-component shape of the spectrum is explained, and the energy of the “knee” in the spectrum has been found to depend on the mass number A. The peculiarities of deep-inelastic πA-scattering lead to the conclusion that the spectrum of cg-neutrons steepens sharply at energies above 1 GeV. The calculated quantitative characteristics of the spectrum are compared with those obtained in measurements.