Exclusive production of heavy mesons in photon-photon collisions: The double scattering mechanism

We present two mechanisms which can compete with constituent interchange in the exclusive production at wide angle of heavy mesons in photon-photon collisions. We show that they have the same energy behaviour as constituent interchange, and that their amplitudes are free from large logarithms. We exploit this to calculate the double-scattering diagrams numerically. We find that while constituent interchange dominates at 90°, double scattering becomes dominant for angles less than about 45°.     

Multipion production in photon-photon collisions

The magnitudes of the matrix elements for γγ→2π⁺2π^?, γγ→π⁺π^?2π⁰, γγ→2π⁺2π^?π⁰, γγ→2π⁺2π^?2π⁰, γγ→3π⁺3π^?3π⁺ and γγ→3π⁺3π^?π⁰are deduced from experimental data as a function of the invariant massW of the system (from 1.0 to 3.0 GeV). It is shown that the data are consistent with a universal energy dependence of the reduced channel amplitudes. A statistical model is shown to describe the peaks in the energy dependence of multipion production in γγ collisions quite well without introducing explicit intermediate resonances.     

Two-pion production in photon-photon collisions

We calculate within chiral perturbation theory the cross section for γγ→π⁰π⁰ and γγ→π⁺π⁻ to next-to-leading order. The first process only depends on the one-loop contributions and forms a test of chiral perturbation theory. We also calculate the pion electromagnetic form factor and compare it to recent data.     

Pion pair production in photon-photon collisions

The process γγ→π⁺π^? was measured using the detector MD-1 at VEPP-4. The two-photon reactionse ⁺ e ^?, μ⁺ μ^? and π⁺ π^? pair production were separated using scintillation counters, Cherenkov counters and shower-range chambers. A radiation widthГ _γγ(f ₂(1270))=3.1±0.35±0.35 keV was obtained.     

K + production inp-nucleus collisions

We present a non-perturbative dynamical study ofK ⁺ meson production in proton-nucleus collisions from 1.2 to 2.5 GeV bombarding energy. The dynamical evolution of the proton-nucleus collision is described by a transport equation of the Boltzmann-Uehling-Uhlenbeck type evolving phase-space distribution functions for nucleons, δ’s,N(1440)’s,N(1535)’s, poins and η’s with their isospin degrees of freedom. We incorporate all known sources forK ⁺ production and study their momentum and angular distributions, and the excitation function. We show that at lower energies (E _b<1.5 GeV)NΔ andNN* channels dominate the kaon yield for light systems, and the πN channel for heavy systems. At higher bombarding energies the directNN channel accounts for almost the whole cross-section.     

Pion pair production in photon-photon collisions at DCI

This paper presents experimental results on π⁺π⁻ production threshold from the collision of quasi-real photons. The data, obtained at the e⁺e⁻ collider DCI, are a combination of the results from the DM1 and DM2 experiments. Using the e⁺e⁻ and π⁺π⁻ production for normalization and cross-checks, we observe a pion pair yield at low invariant mass (W < 500 MeV/c²) which is approximately twice the one expected from Born terms.     

Pion pair production in photon-photon collisions at spear

We report a measurement of the cross section for the process γγ→π⁺π^? at invariant masses 500 MeV/c² < m_ππ<2000 MeV/c². A value for the radiative width of the f(1270) tensor meson Γ_f→γγ = 3.6 ± 0.3 ± 0.5 KeV (helicity λ = 2) has been obtained from a fit to the observed ππ mass spectrum.     

Subthreshold K+ production in proton-nucleus collisions

Double differential K⁺ cross sections have been measured in p+C collisions at 1.2, 1.5 and 2.5 GeV beam energy and in p+Pb collisions at 1.2 and 1.5 GeV. The K⁺ spectrum taken at 2.5 GeV can be reproduced quantitatively by a model calculation which takes into account first chance proton-nucleon collisions and internal momentum with energy distribution of nucleons according to the spectral function. At 1.2 and 1.5 GeV beam energy the K⁺ data excess significantly the model predictions for first chance collisions. When taking secondary processes into account the results of the calculations are in much better agreement with the data.     

Subthreshold K+ production in proton-nucleus collisions

Double differential K⁺cross sections have been measured in p+C collisions at 1.2, 1.5 and 2.5 GeV beam energy and in p+Pb collisions at 1.2 and 1.5 GeV. The K⁺ spectrum taken at 2.5 GeV can be reproduced quantitatively by a model calculation which takes into account first chance proton-nucleon collisions and internal momentum with energy distribution of nucleons according to the spectral function. At 1.2 and 1.5 GeV beam energy the K⁺ data excess significantly the model predictions for first chance collisions. When taking secondary processes into account the results of the calculations are in much better agreement with the data.