(Ilieva, Berman)

Coherent pion photoproduction on deuterium is characterized by very large momentum transfer in certain parts of phase space, which makes it very suitable for studying large-momentum-transfer phenomena and the underlying reaction mechanisms.

One of the interesting aspects of the investigation of this reaction is connected with a structure in the energy dependence of the cross section at pion backward scattering angles and photon energies between 600 and 800 MeV, which was observed in a previous measurement [Ima85]. A calculation [Miy87] describing the reaction by single- and double-pion rescattering diagrams reproduces the data fairly well up to 700 MeV but underestimates the cross sections at higher energies and does not account for the observed enhancement. A possible mechanism suggested by Kondratyuk and Lev [Kon76], originally to account for similar structure in the excitation function for backward p -d elastic scattering, invokes near-threshold production of intermediate on-shell h-mesons in the double-scattering diagram. Since the elementary amplitude for the h intermediate state is peaked at backward pion-scattering angles, in contrast to the one for a p^o intermediate state, an enhancement of the differential cross sections at backward pion angles around 700 MeV beam energy, which is the threshold for h production on a nucleon, would be a clear signature for this mechanism [Str02a]. In view of this possibility, the reaction gd ® dp^o can be used for studying the properties of the hN interaction.

Another aspect of the study of the gd ® dp^o reaction is connected with the question of the limits of applicability of traditional meson-exchange theories and pQCD to exclusive nuclear processes. The transition between meson-nucleon and quark-gluon descriptions of the reactions is still not well understood. pQCD provides models such as Constituent Counting Rules (CCR) and Reduced Amplitude Formalism (RAF) [Bro01] that should be applicable in the few-GeV energy region. The CCR predict that the asymptotic behavior of the invariant cross section is given by the relation

The RAF is based on the assumption that since the binding energy of the deuteron is small (2.2 MeV), it can be neglected. Because the coherent process can proceed only if each nucleon absorbs an equal fraction of the overall momentum transfer and must scatter while remaining nearly on its mass shell, in the zero-binding limit the photoproduction amplitude factorizes as

where M_{gN1® N1p}o(u/4,t/4) is the nucleon photoproduction amplitude at half the overall momentum transfer, F_N2(t/4) is the nucleon form factor at half the overall momentum transfer, and f_d(t) is the reduced deuteron form factor. The large-q² behavior of the reduced amplitude is related by dimensional counting rules to the number of the elementary constituents in the initial and final states:

where p_T is the transverse momentum and t and u are the Mandelstam invariants.

We expect that our analysis of this reaction over a wide energy and CM angular range will allow us to determine the limits of the validity of pQCD predictions for this reaction and might enable us to determine the energy (for a given pion angle) of the transition between traditional models and pQCD.

The data were obtained during the g2 running period in Hall B using a tagged-photon beam. The deuterons were detected by the CLAS detector, which provides information about the three-momentum vectors and masses of the particles. The latter is used to identify the deuterons and to separate them from the other charged particles. The dp^o channel is identified by means of the missing-mass technique applied to the selected deuterons. Since our goal is to determine differential cross sections ds/dcos(q_cm^p) and ds/dt, we divide the accessible photon energy range into 50-MeV wide bins and the cos(q_cm^p) interval into 10 (or 20, depending on the beam energy) bins. For each individual bin we extract the yield by fitting a polynomial to the background on both sides of the missing-mass peak and subtracting it from the original distribution. Then the extracted yields are normalized to the photon flux and corrected for detector acceptance.

Preliminary excitation functions for values of cos(q_cm^p) from 0.0 to - 0.85 are shown in Fig. 9. The statistical uncertainties range from 6 to 16%. The systematic uncertainties due to background subtraction vary between 0.5 and 3%. Those associated with the acceptance determination are still to be studied. Detailed systematics studies are also in progress. Our data are in good agreement with the previous measurements and also show the existence of a structure near 700 MeV (the h threshold) at q_cm^p = 130° and at more backward scattering angles, where the effect is even more pronounced.

Figure 9. CLAS preliminary cross sections for several pion CM scattering angles, compared with previous data [Ima85, Mee99]. The h- and w-photoproduction thresholds are indicated by the arrows. The large-angle data show a prominent structure near the h threshold at about 700 MeV.

Figure 10 shows the scaled invariant cross section for dcos(q_cm^p) = -0.75. Although the authors of [Mee99] conclude that the data at this scattering angle are consistent with CCR predictions, our measurement shows that scaling is not observed, at least up to Eg  = 1.2 GeV. Scaling properties of the scattering amplitude are being studied further. Preliminary results of this analysis were presented at the International Conference on Few-Body Problems in Physics [Ili03].

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Last modified: 03/31/05