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.     

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.     

Pion production in central high energy nuclear collisions

A model for pion production is studied in the context of a cascade calculation. The pions are produced through Δ-resonances which are allowed to decay. The emitted pions are assumed to interact with the other nucleons by forming new Δ -resonances. The time evolution of the pion and Δ-population is studied; it is found that Δs are always more numerous than pions during the sequence of baryon-baryon collisions. The spectrum of the pions is in considerably better agreement with experiment than the one obtained with frozen Δ-isobars. The presence of Δ-resonances appears to be important for the cooling of the pion system. The pion multiplicity is found to deviate from a Poisson distribution. The pion yield is overestimated by at least 25%; this result is discussed within the framework of conventional dynamics.     

Pion production in dAu collisions at RHIC energy

We present our results on neutral pion (π⁰) production in pp and dAu collisions at RHIC energy. Pion spectra are calculated in a next-to-leading order (NLO) perturbative QCD-based model. The model includes the transverse component of the initial parton distribution (“intrinsic k_T”). We compare our results to the available experimental data from RHIC, and fit the data with high precision. The calculation tuned this way is repeated for the dAu collision, and used to investigate the interplay of shadowing and multiple scattering at RHIC. The centrality dependence of the nuclear modification factor shows a measurable difference between different shadowing parameterizations.     

Pion production mechanism in high energy heavy ion collisions

We study the mechanism of pion production in high energy heavy ion collisions with the nuclear cascade model developed in a previous paper. We describe time-dependently the reaction rates of the processes N+N→N+Δ, N+Δ→N+N, Δ→N+π, and N+π→Δ, to discuss the achievement of the chemical equilibrium. The importance of final Δ−N and π−N interactions is pointed out. The present cascade model reproduces well the observed pion yields in Ar+KCl high multiplicity events, without introducing the nuclear compression effect. The reason of the agreement is discussed by decomposing a variety of conditions employed in this model, and by reproducing the situations of Cugnon's cascade model and others.     

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     

Pion production in two-body pd collisions

The reactions d(p, π⁺)t and d(p, π^o)³ He are analyzed in terms of the pp → π⁺d sub-process. The exclusion principle induces an additional exchange contribution involving the πd scattering amplitude. Acceptable deuteron and tritron (³H) wave functions which reproduce the charge form factors yield results consistent with experimental angular distributions at 470, 590, 754 and 1192 proton lab. energies. The forward peak is not sensitive to details of the wave functions. Both the direct and exchange form factors develop a zero which is imposed by the dip in the ³He charge form factor. The associated interference pattern governs the backward cross section.     

Muon pair production in very high energy nucleus-nucleus collisions

Characteristics of muon pair production in very high energy nucleus-nucleus collisions are discussed. Particular attention is given to comparing the rate of muon pairs produced from thermalized quark-gluon matter to that of pairs produced via the usual Drell-Yan mechanism. The thermal rate is at least of the same order of magnitude as the direct Drell-Yan rate and will certainly dominate whenx _F for the pair approaches 1. Beyondx _F =1 the thermal rate is also substantial. This region is particularly easily accessible in fixed target experiments.     

Higher order QED corrections toW pair production at LEP II

The problem of evaluating initial state QED corrections to theW pair production cross section, taking into account the effects of finiteW width, is investigated. The contributions of higher order corrections compared with the ones, hard bremsstrahlung and experimental photon energy cuts are computed within the structure functions formalism both in the threshold and peak region. The numerical impact of the corrections on theW mass measurement is studied.