Antiproton production inp+A andd+A reactions at subthreshold energies

The production of antiprotons in proton-nucleus and deuteron-nucleus collisions is analyzed with respect to one-step nucleon-nucleon (NN → NNp¯ p) and secondary Δ-nucleon (ΔN → NNN¯ p) production channels within a phase-space model incorporating the selfenergies of the baryons. It is found that contrary to nucleus-nucleus collisions the Δ induced reaction channels are of minor importance except at very low bombarding energies. The data from KEK are approximately reproduced when accounting for the internal momentum distribution of the deuteron which provides a natural explanation for the large enhancement of the ¯p cross section ind+A as compared top+A.     

Properties ofΛ-hypernuclei from p+A reactions

We study the production ofΛ hyperons inp+A reactions on the basis of a BUU transport approach for³² S,⁶⁵ Cu,¹²⁰ Sn,¹⁹⁷ Au,²⁰⁸ Pb and²³⁸ U from 1.1 to 1.7 GeV beam energy and evaluate the properties of the hypernuclei produced with respect to excitation energy, transverse and longitudinal momentum, angular momentum and mass. Furthermore, the decay of the hypernuclei by particle emission in competition with fission channels is followed by a statistical model calculation. We find the systemp+¹⁹⁷ Au at about 1.5 GeV to be optimally suited for the production of heavy hypernuclei, where theΛ-decay is expected to be dominated by nonmesonic decay channels.     

\pi\pi correlations in \gamma + A reactions

Preliminary differential cross-sections of the reactions and with A = ¹ H, ¹² C, and ^nat Pb are presented. A significant nuclear-mass dependence of the invariant-mass distribution is found in the channel. The dependence is not observed in the channel. The in-medium observation in the channel is consistent with an in-medium modification of the interaction in the I = J = 0 channel, changing width and pole position of a resonant state.Received: 30 September 2002, Published online: 22 October 2003PACS: 13.60.Le Meson production - 21.65. + f Nuclear matterJ.G. Messchendorp: For the TAPS and A2 Collaborations     

Nuclear effects on open charm production in p+A reactions

We calculate and resum the nuclear enhanced power corrections from the final state partonic scattering in nuclear matter to open charm production and correlations. In p+A reactions, we find that single and double inclusive D mesons can be suppressed as much as the neutral pions from the dynamical high twist shadowing. Effects of initial state energy loss in p+A collisions are also investigated and may lead to significantly weaker transverse momentum dependence of the nuclear suppression.     

Meson production in p + d reactions

Total and differential cross sections for the reactions p + d → ³He + m ⁰ with m=π, η and p + d → ³H+π⁺ were measured with the GEM detector at COSY for beam momenta between threshold and the maximum of the corresponding baryon resonance. For both reactions a strong forward-backward asymmetry was found. The data were compared with model calculations. The aspect of isospin symmetry breaking is studied. Representing the GEM Collaboration     

Higgs-boson pairproduction ine + e − reactions

We calculate the production cross section of Higgs-boson pairs ine ⁺ e ^? annihilation. In this reaction the lowest order contribution comes from one loop graphs. We study the behaviour of the cross section as a function of the energy, the Higgs-boson mass and the scattering angle.     

A study of four- and six-body reactions in π+ andK + p interactions at 250 GeV/c

Data on the reactions (K ⁺/π⁺)p→(K ⁺/π⁺)pπ⁺π^- and (K ⁺/π⁺)p→(K ⁺/π⁺)p2π⁺2π^-, obtained with the European Hybrid Spectrometer, are presented and compared with data at lower energies. The contribution of beam and target diffractive dissociation and double Pomeron exchange, and porperties of these reactions are discussed.     

Effective interactions for 208Pb + p reactions

RPA hole-particle wave functions for the lowest 2⁺ and 3^? states in ²⁰⁸Pb were used to test the validity of effective nucleon-nucleon interactions for (p, p') excitations. Excellent agreement with experiment was obtained for the long-range part of the Hamada-Johnston S-state potential. Equally good agreement was found for the elastic scattering from ²⁰⁸Pb and the (p, n) transition to the analog state. It was found to be important to include an imaginary part in the interaction. The transitions to the two excited states of ²⁰⁸Pb were found to be almost pure isoscalar, but the transitions are dominated by the neutron excitations.     

Model for rebound reactions: M+RI

A simple model, consisting of ‘two-body’ reactant and product trajectories joined by an instantaneous switch, is proposed for the alkali atom-alkyl iodide rebound reactions. The electronic potential surfaces governing these trajectories are justified in terms of the electron jump model. Encouraging overall agreement is found with extensive molecular beam data for the K, Cs + CH₃I reactions.     

H + reactions from 1.96 to 6.20 MeV

The differential cross sections of ²H(d, d)²H, ²H(d, ³He)n, ²H(d, p)³H and ²H(d, t)p have been measured in a gas scattering chamber at ten bombarding energies between 2.0 and 6.2 MeV with accuracies ranging from ±1.7 % to ±3.4 %. The differential cross section of ²H(d, n)³He has been measured at forward angles to an accuracy of ±2.5 % at the same energies using a time-of-flight detection system. The neutron detection efficiency was calibrated from the ²H(d, ³He)n cross sections, so the neutron production cross sections reported here do not depend on any previous neutron measurements. The ²H(d, ³He)n cross sections have been converted into ²H(d, n) ³He cross sections and Legendre polynomial fits are given for the complete angular distributions of ²H(d, n)³He.     

Photon and neutral pion distributions in 60 and 200 A GeV16O+nucleus and proton+nucleus reactions

Transverse momentum (p_T) distributions of inclusive photons and neutral pions at midrapidity are measured with a lead glass calorimeter in $\text{60}\text{and}\text{200 A}\text{GeV}{}^{16}\text{O}\text{+}\text{nucleus and proton}\text{+}\text{nucleus reactions}$. The variation of the average transverse momentum is investigated as function of centrality, determined by measurements of the remaining energy of the projectile and the charged particle multiplicity. For small values of the entropy, deduced from the multiplicity density, an increase in average p_T is observed levelling off for larger values of entropy. The target-mass and energy dependence of π⁰ p_T distributions are presented.     

Neutral transverse momentum spectra in 60 and 200A·GeV16O+nucleus and proton+nucleus reactions

Transverse momentum (p _T) distributions fo inclusive photons and neutral pions at midrapidity are measured with a lead glass calorimeter in 60 and are measured with a lead glass calorimeter in 60 and 200A·Gev¹⁶O+nucleus and and proton+nucleus reactions. Inclusive photon distributions are compared for central and peripheral reactions. The degree of centrality is determined either from the charged particle multiplicity or from the remaining projectile energy in the forward direction. Deviations from a nucleus+nucleus interaction model based upon linear extrapolation from p+p reactions are observed in central¹⁶O+Au data. The variation of theaverage transverse momentum is investigated as function of centrality. The target-mass and energy dependence of π⁰ p _T distributions are presented. For¹⁶O+Au a change of slope in these distributions is observed atp _t ≈0.8 GeV/c compatible with hydrodynamic expansion models.     

Incomplete fusion reactions in 16O+165Ho

Excitation functions for evaporation residues of the system ¹⁶O + ¹⁶⁵Ho have been measured up to 100 MeV. Recoil range distribution of long lived reaction products were measured at ¹⁶O beam energy of 100 MeV. Detailed Monte Carlo simulation of recoil range distributions of products were performed with the help of PACE2 code, in order to extract the contributions of incomplete fusion in the individual channels. The results clearly show the incomplete fusion contributions in the tantalum and thulium products. This is confirmed by the predictions of breakup fusion model of the incomplete fusion.     

Dynamical approach to spectator fragmentation in Au+Au reactions at 35 MeV/A

The characteristics of fragment emission in peripheral ¹⁹⁷Au+¹⁹⁷Au collisions 35 MeV/A are studied using the two clusterization approaches within framework of quantum molecular dynamics model. Our model calculations using minimum spanning tree (MST) algorithm and advanced clusterization method namely simulated annealing clusterization algorithm (SACA) showed that fragment structure can be realized at an earlier time when spectators contribute significantly toward the fragment production even at such a low incident energy. Comparison of model predictions with experimental data reveals that SACA method can nicely reproduce the fragment charge yields and mean charge of the heaviest fragment. This reflects suitability of SACA method over conventional clusterization techniques to investigate spectator matter fragmentation in low energy domain.     

Proton transfer reactions of HCO+ at 298 K

Rate constants measured with the flowing afterglow technique at 298 ± 2 K are reported for the proton transfer reactions of HCO⁺ with CH₂O, CH₃CHO, (CH₃)₂CO, HCOOH, CH₃COOH, HCOOCH₃, CH₃OH, C₂H₅OH, (CH₃)₂O, and CH₂CO. Dissociative proton transfer channels were observed with C₂H₅OH and CH₃COOH. The rate constants are compared with the predictions of various theories for ion—molecule collisions. The protonation is discussed in terms of the energetics of possible sites of protonation and in terms of the energetics and mechanisms of various modes of dissociation.     

Coherent control of resonance-mediated reactions: F+HD

Cross sections resulting from scattering that proceeds via an intermediate resonance are shown to be exceptionally controllable using a coherent superposition of only two initial states. Full quantum computations on F+HD(v=0;j=0,1)-->H+DF, D+HF, which exhibits a resonance in one of the reactive channels, support the formal arguments, showing that control is indeed vast. In this case the ratio of reactive integral cross sections can be altered by a factor of 62 (compared to a noncoherent factor of only 3.3), while the ratio of reactive differential cross sections can be altered by a factor of over 6000 (compared to a noncoherent factor of less than 7). These results constitute the first prediction of extensive quantum control in a collisional process.     

Low-energy fusion cross sections of D + D and D + 3He reactions

The D(d, n)³He and D(d, p)T reactions have been investigated at E_c.m = 2.98 to 162.5 keV and the ³He(d, p)⁴He reaction at E_c.m. = 6.95 to 141.8 keV. The studies involved high-current accelerators with well-known beam characteristics and windowless gas target systems of the extended and quasi-point supersonic jet type. The measurement of absolute cross sections, angular distributions and excitation functions is reported. The data extend into the thermal energy region of future fusion reactors. The results for the D(d, n)³He and D(d, p)T reactions are in good agreement with previous work, while substantial differences are found for ³He(d, p)⁴He.