The nuclear matter effects in pi0 photoproduction at high energies

Abstract

The in-medium influence on π 0 photoproduction from spin zero nuclei is carefully studied in the GeV range using a straightforward Monte Carlo analysis. The calculation takes into account the relativistic nuclear recoil for coherent mechanisms (electromagnetic and nuclear amplitudes) plus a time dependent multi-collisional intranuclear cascade approach (MCMC) to describe the transport properties of mesons produced in the surroundings of the nucleon. A detailed analysis of the meson energy spectra for the photoproduction on 12 C at 5.5 GeV indicates that both the Coulomb and nuclear coherent events are associated with a small energy transfer to the nucleus ( 5 MeV), while the contribution of the nuclear incoherent mechanism is vanishing small within this kinematical range. The angular distributions are dominated by the Primakoff peak at extreme forward angles, with the nuclear incoherent process being the most important contribution above θ π 0 2 0 . Such consistent Monte Carlo approach provides a suitable method to clean up nuclear backgrounds in some recent high precision experiments, such as the PrimEx experiment at the Jefferson Laboratory Facility.

Nuclear incoherent photoproduction ofπ0andηfrom 4 to 12 GeV

“…The results obtained with Eq. (28) are in nice agreement with the measured values (σ Total γp = 123.6 µb for p 4.5 GeV and σ Total γp = 117.9 µb for p 7.5 GeV). Another important step for the evaluation of the shadowing effect for photonucleus interactions is the concept of formation time, which is the time interval that the physical photon is momentarily in a vector-meson state.…”

“…An earlier version of the MCMC model [27] has been successfully applied for the interpretation of recent data of incoherent π 0 photoproduction from MAMI [5]. Another version [28] also investigated coherent π 0 photoproduction (electromagnetic/nuclear) including the nuclear recoil.…”

η→γγDecay Width via the Primakoff Cross Section

“…from Cornell using the multicollisional intranuclear cascade model MCMC [13][14][15] to describe the nuclear background. The Monte Carlo (MC) method takes into account incoherent photoproduction from nuclei at forward angles within 4 to 9 GeV, including -nucleus FSI via a multiple scattering framework.…”

The Contribution of Regge Cuts on π[sup 0] Photoproduction from Complex Nuclei within 4 to 12 GeV

“…The elementary π 0 photoproduction mechanism (γ + N −−> π 0 + N) for low momentum transfer and photon energies typically above 4 GeV can be described in terms of vector meson exchange (VMD) using t-channel helicity amplitudes [1][2][3]: where p * and k * are the meson and photon momentum, respectively, in the center of mass of the schannel with m N and μ representing the nucleon and meson masses. The helicity amplitudes F 1 and F 3 are calculated assuming ρ and ω exchange plus the Reggeon cuts (F 1 →F 1…”

“…The model consists of a time-dependent multi-collisional algorithm [1][2][3] that incorporates the following features: i) relativistic kinematics, ii) the inclusion of the meson photoproduction mechanism within 4.0 to 12.0 GeV via two different Regge models (Model 1 and 2), iii) the incorporation of accurate momentum distributions for light nuclei taken from nucleon knock-out reactions, iv) a rigorous non-stochastic Pauli-blocking both for the photoproduction and binary mesonnucleon scatterings, v) photon shadowing effects using a VMD model with vector meson formation time constraint, vi) a consistent analysis of the meson-nucleus FSI via a multiple scattering framework, and vii) the inclusion of realistic (diffractive) angular distributions for the meson-nucleon elastic scattering. A more comprehensive and detailed description of the cascade model both for π 0 and η photoproduction from complex nuclei will be provided shortly [7].…”