Incoherentπ0photoproduction from complex nuclei

Abstract

Incoherent π 0 photoproduction from nuclei is evaluated via a multicollisional intranuclear cascade framework. In-medium modifications are taken into account, including a realistic dynamical treatment of multiple πN and N scattering processes throughout the cascade. This time-dependent analysis yields structures in the 12 C π 0 differential cross section both in the region and in the photon energy range from 5 to 6 GeV, with the former in very nice agreement with recent results from Mainz Microton. For heavy nuclei, however, such structures disappear because of a more effective Fermi motion and a relatively higher final state interaction of the produced pions as they exit the nucleus. The calculation of the incoherent part of the total π 0 photoproduction propitiates a clean and powerful kinematical separation from competitive (electromagnetic/nuclear) production processes, which currently is a theoretical challenge for the PrimEx experiment at the Jefferson Lab. PACS number(s): 25.20. Lj, 21.65.+f, 25.80.Ls Various experiments have been recently proposed in an effort to investigate in-medium modifications in meson photoproduction from complex nuclei. Single π 0 , double π 0 , and π 0 π +,− photoproduction channels from 200 to 800 MeV in nuclei were explored by the Mainz Microton (MAMI) collaboration [1]. In these experiments, they were able to separate the incoherent and coherent parts of the single π 0 differential cross section near the (1232), where the incoherent contribution exhibited remarkable structures for the 12 C. Such an intriguing result propitiates an accurate verification of the available models in describing nuclear matter effects. Another interesting result showed up in the total photoabsorption cross section, where the peak of the second resonance, the so-called N(1520), is highly suppressed for both light and heavy nuclei.Another example of the in-medium influence in the higher photon energy regime, is the incoherent π 0 photoproduction background at forward polar angles when extracting the π 0 lifetime via the Primakoff effect [2,3], as proposed by the PrimEx collaboration at the Jefferson Lab [4]. In fact, one of the major theoretical challenges in this experiment is the kinematical separation of the electromagnetic production, which comes from the coupling of the incident photon with the Coulomb field of the nucleus, from competitive (coherent and incoherent) nuclear mechanisms. For more details see Ref.[5] and references therein.In this Rapid Communication we report for the first time a full dynamical model, based on a time-dependent multicollisional intranuclear cascade framework, to calculate the incoherent π 0 photoproduction from the pion threshold up to 6 GeV incident photon energies. The model is an extension of the original MCMC (Monte Carlo Multi-Collisional) model [6-9] with a novel approach focused on the quasideuteron energy region [10]. This newest version has incorporated substantial theoretical improvements with respect to similar cascade calculations [11] and has complete...

The nuclear matter effects in pi0 photoproduction at high energies

“…The elementary photoproduction cross section dσ N dΩ can be written in terms of the helicity amplitudes, which are calculated in the context of the Regge theory under the assumption that the production process is largely dominated by vector meson (ρ and ω) exchange for low t. Such elementary operator was calculated in ref. [10], reproducing quite succesfully the proton data at few GeV [15,16]. For details on the calculations of the helicity amplitudes see ref.…”

“…The MCMC model was first developed to study high energy photonuclear reactions [4][5][6][7], being more recently successfully extended to intermediate energies [8,9]. A special and very sophisticated version of the MCMC code were also proposed in a recent paper [10] in order to study the incoherent π 0 photoproduction both at the ∆(1232) resonance energy region, as well as in the 4.0 − 6.0 GeV range and forward angles [10]. Such work showed the accuracy of the cascade model in predicting recent and intriguing results of the π 0 differential cross section for 12 C in the Delta region [11].…”

Photon-induced multiple particle emissions ofZr90andnatZr from 10 to 140 MeV

“…The MCMC cascade has also been useful to calculate angular distributions of mesons produced via incoherent nuclear excitation processes [7]. In this work, we have reproduced quite successfully recent data from the Mainz Microton collaboration (MAMI) for the differential cross sections for incoherent π 0 photoproduction at the (1232) resonance energy region, for both 12 C and 208 Pb [8].…”

Nuclear incoherent photoproduction ofπ0andηfrom 4 to 12 GeV

“…The photoproduction mechanism is described via a two-step process: (i) meson photoproduction from a single nucleon (IA) and (ii) meson-nucleus FSI via a cascade approach. 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.…”