Perturbative QCD and factorization of coherent pion photoproduction on the deuteron

Abstract

We analyze the predictions of perturbative QCD for pion photoproduction on the deuteron γD → π 0 D at large momentum transfer using the reduced amplitude formalism. The cluster decomposition of the deuteron wave function at small binding only allows the nuclear coherent process to proceed if each nucleon absorbs an equal fraction of the overall momentum transfer. Furthermore, each nucleon must scatter while remaining close to its mass shell. Thus the nuclear photoproduction amplitude M γD→π 0 D (u, t) factorizes as a product of three factors: (1) the nucleon photoproduction amplitude M γN 1 →π 0 N 1 (u/4, t/4) at half of the overall momentum transfer, (2) a nucleon form factor F N 2 (t/4) at half the overall momentum transfer, and (3) the reduced deuteron form factor f d (t), which according to perturbative QCD, has * Work supported by the Department of Energy, contracts DE-AC03-76SF00515, DE-FG02-98ER41087, 1 the same monopole falloff as a meson form factor. A comparison with the recent JLAB data for γD → π 0 D of Meekins et al. [Phys. Rev. C 60, 052201 (1999)] and the available γp → π 0 p shows good agreement between the perturbative QCD prediction and experiment over a large range of momentum transfers and center-of-mass angles. The reduced amplitude prediction is consistent with the constituent counting rule p 11 T M γD→π 0 D → F (θ c.m. ) at large momentum transfer. This is found to be consistent with measurements for photon lab energies E lab > 3 GeV at θ c.m. = 90 • and E lab >

Cross section measurements of charged pion photoproduction in hydrogen and deuterium from 1.1 to 5.5 GeV

“…The scaled amplitude P 5 T |M| is plotted here for the γ N → π ± N process, whereas P 11 T |M| was plotted in Ref. [64] P T can be estimated by dimensional counting. Because the arrows in different colors (indicating the possible onset of scaling for different angles) are closer in terms of transverse momentum than in terms of the center-of-mass energy, the transverse momentum may be a better choice to describe the scaling onset than the center-of-mass energy, which was also stated previously [9,29,65].…”

“…15 and 16 against center-of-mass energy √ s and transverse momentum P T , similar to what was performed for neutral pion photoproduction on the deuteron [64]. The invariant amplitude M was calculated from the differential cross section by using the following:…”

Probing Nuclear Dynamics in Real-Photon Experiments

“…[3][4][5] Comparison between our CLAS data for the reaction γd → π 0 d and the meson-nucleon model calculations 3,4 is shown in Fig. 8 In order to study the transition region between short and long distance dynamics in nuclear reactions, extensions of pQCD, such as the Reduced Nuclear Amplitudes (RNA) approach, 9 have been developed. The differential cross sections are generally well described up to around 0.9 GeV by the most sophisticated up-to-date model calculation, 3 whereas the calculation of 4 reproduces well the data only up to around 0.7 GeV.…”

Photodisintegration of light nuclei

“…As neither QCD nor chiral effective field theories are able to predict the available data at these intermediate energies, the interpretation of the observed dimensional scaling relies on phenomenological models. The main QCD based models for deuteron photodisintegration are the reduced nuclear amplitudes (RNA) model [16,17], the hard-rescattering mechanism (HRM) [18,19,20], and the quark-gluon string model (QGSM) [21,22]. The HRM studies the absorption of the photon by a quark of one nucleon followed by a high-momentum transfer interaction with a quark in the other nuclean.…”