High precision measurements of the differential cross sections for π0 photoproduction at forward angles for two nuclei, 12C and 208Pb, have been performed for incident photon energies of 4.9-5.5 GeV to extract the π0→γγ decay width. The experiment was done at Jefferson Lab using the Hall B photon tagger and a high-resolution multichannel calorimeter. The π0→γγ decay width was extracted by fitting the measured cross sections using recently updated theoretical models for the process. The resulting value for the decay width is Γ(π0→γγ)=7.82±0.14(stat)±0.17(syst)  eV. With the 2.8% total uncertainty, this result is a factor of 2.5 more precise than the current Particle Data Group average of this fundamental quantity, and it is consistent with current theoretical predictions.

The two-step process that characterizes the intermediate-and high-energy photonuclear reactions (between 40 MeV and 4 GeV) has been successfully described by Monte Carlo calculations. Recently, a new class of codes capable to perform those calculations according to a more realistic method has been developed, improving the possibilities for simulating the reactions in more details. In this work we present the CRISP package (standing for Rio-São Paulo Collaboration), which is a coupling of the multi collisional Monte Carlo (MCMC) and the Monte Carlo for evaporation-fission (MCEF) codes. The first one describes the intranuclear cascade process, while the second one is dedicated to the evaporation/fission competition step. Both codes have already shown to be useful for calculating several features of different nuclear reactions. The CRISP code, coupling these two software, represents a good tool to describe the complex characteristics of the nuclear reactions, and opens the opportunity for applications in quite different fields, such as studies of hadron physics inside the nucleus, specific nuclear reactions, spallation and/or fission processes initiated by different probes and many others.

The mechanism of incoherent π 0 and η photoproduction from complex nuclei is investigated from 4 to 12 GeV with an extended version of the multicollisional Monte Carlo (MCMC) intranuclear cascade model. The calculations take into account the elementary photoproduction amplitudes via a Regge model and the nuclear effects of photon shadowing, Pauli blocking, and meson-nucleus final-state interactions. The results for π 0 photoproduction reproduced for the first time the magnitude and energy dependence of the measured rations σ γ A /σ γ N for several nuclei (Be, C, Al, Cu, Ag, and Pb) from a Cornell experiment. The results for η photoproduction fitted the inelastic background in Cornell's yields remarkably well, which is clearly not isotropic as previously considered in Cornell's analysis. With this constraint for the background, the η → γ γ decay width was extracted using the Primakoff method, combining Be and Cu data [ η→γ γ = 0.476 (62)

Incoherentphotoproduction in nuclei is evaluated at forward angles within 4 to 9 GeV using a multiple scattering Monte Carlo cascade calculation with full -nucleus final-state interactions. The Primakoff, nuclear coherent and nuclear incoherent components of the cross sections fit remarkably well previous measurements for Be and Cu from Cornell, suggesting a destructive interference between the Coulomb and nuclear coherent amplitudes for Cu. The inelastic background of the data is consistently attributed to the nuclear incoherent part, which is clearly not isotropic as previously considered in Cornell's analysis. A renewed interest in the Primakoff method appeared with the advent of the PrimEx Collaboration at the Jefferson Laboratory, which will provide a more precise measurement of ÿ 0 ! [9]. Furthermore, high precision and 0 photoproduction experiments are strongly encouraged by the forthcoming 12 GeV upgrade of the electron beam, demanding reliable methods for the accurate delineation of the nuclear background.The approaches developed so far for incoherent photoproduction from nuclei are restricted to 1 GeV, where the contribution from the S 11 1535 resonance largely dominates. The final-state interactions (FSI) of the mesons are taken into account either using optical potentials [10], the quantum molecular dynamics (QMD) model of Ref. [11], or the Boltzmann-Uehling-Uhlenbeck (BUU) transport model [12]. Obviously, these important theoretical developments are not suitable to describe incoherent production at higher energies.In this Letter, we present for the first time a consistent solution for the puzzling scenario of ÿ ! 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 forward angle photoproduction cross section is assumed to be in the form [16 -18] where T P , T NC , and T NI are the Primakoff (P), nuclear coherent (NC), and nuclear incoherent (NI) amplitudes, respectively, with ' representing the P-NC phase shift. The Coulomb amplitude is the sum of the amplitudes from the protons [6], such thatwhere 1=137, Z is the atomic number, k the photon energy, Q the four momentum transfer,F C k; the Coulomb form factor (FF) including -nucleus FSI; with ÿ ! , , , and representing the decay width, velocity, mass, and production angle of the meson, respectively. PRL 101, 012301 (2008)

The Monte Carlo multicollisional (MCMC) intranuclear cascade model is used to study photonuclear reactions at intermediate energies ͑20ഛ E ␥ ഛ 140 MeV͒. This version of the code differs from previous versions in the following aspects: (i) the quasideuteron model of photoabsorption is consistently included by taking into account relative momentum correlations of the neutron-proton pair in a relativistic kinematics; (ii) a realistic treatment of the Pauli-blocking mechanism at the initial photoabsorption and at each binary nucleon-nucleon scattering during the cascade process is incorporated throughout the calculations; (iii) a criterion based on energy considerations is required by the end of the cascade. Differently from other transport models used so far, which are based on a randomly generated nuclear ground state with a stochastic treatment of the Pauli blocking, the present model incorporates a shell constrained momentum space of the nucleons which is preserved as the cascade evolves along time. The transition between the pre-equilibrium and evaporation phases is energetically determined, allowing the description of the cascade process without any free parameter, such as some ambiguous stopping time parameters adopted in similar time-structured cascade models. The occupation number distribution after the cascade corresponds to a typical Fermi distribution at a finite nuclear temperature, and the long-standing spurious depletion of the Fermi sphere, usually present in other cascade models, no longer appears. The Pauli-blocking factors are calculated and compared with previous approaches based on Fermi gas level density calculations. The evaporation-fission process of the compound nucleus is described in the framework of a Monte Carlo algorithm. Experimental data of the total photoabsorption cross section and the neutron multiplicities for Sn, Ce, Ta, and Pb in the 20-140-MeV range are described fairly well by the present calculations.

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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...