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Two-Body Photodisintegration of 3He (Ilieva, Berman) The reaction g3He ® pd, together with its inverse (radiative capture), have been extensively studied over the years in the low- and intermediate-energy regions, where two-nucleon mechanisms dominate. However, higher photon energies give us access to higher momentum transfers where three-nucleon mechanisms are expected to play an important role. Therefore, an analysis of this reaction channel, together with the three-body photodisintegration (which also is part of our data set; see Section 1 above) at higher energies (up to 1.2 GeV), is expected to shed more light on the details of these reaction mechanisms. Laget has developed a model [Lag88] which uses meson double-scattering graphs, shown in the upper part of Fig. 4, in addition to one- and two-body mechanisms, in order to account for the large-momentum-transfer part of the differential cross sections. In the model, the forward differential cross sections are dominated by two-body mechanisms, whereas the three-body mechanisms become important at backward angles. A previous measurement of this reaction [Isb94], performed at photon energies up to 750 MeV, shows a structure in the differential cross section in this angular range which is indicative of a three-body absorption mechanism. However, a comparison of the model predictions with the data shows that double-meson scattering is not sufficient to account for the data and that other three-body graphs containing r and w exchange or a D in the intermediate state, as shown in the bottom part of Fig. 4, should be considered.
Figure 4. Double-scattering diagrams which are important in order to account for the data at large momentum transfer [Lag88]. The lower plots include D propagation or vector-meson exchange.Our data overlap in energy with those of [Isb94], and extend higher in energy, up to Eg ~ 1.2 GeV. In addition, the two-body breakup channel is analyzed in parallel with the three-body breakup, so that the whole data set can be expected to improve our present understanding of the relevant three-body mechanisms. The data for this reaction were obtained during the g3 running period in Hall B. The accessible range of photon energies is between 0.35 and 1.55 GeV (however, because the cross section decreases with the incident beam energy, we expect to cover just the range up to 1.2 GeV). Because the CLAS detector is very well suited for detecting charged particles, the selection of the reaction is very straightforward. We look for events containing identically two charged tracks. For each of the particles, we calculate the missing mass assuming a two-body final state. Then the p-d events are cleanly identified using the correlation spectrum of both missing masses, as shown in Fig. 5.
Figure 5. Missing-mass correlation distribution used for our reaction selection. One can see that almost no background subtraction is necessary. In order to reduce the background, we use the advantage of having detected both final-state particles: we constrain the difference between both momenta in the CM, the sum cos(qcmp ) + cos(qcmd), and the difference of the missing masses. After applying these additional cuts, the total background that remains is at the level of <3%. This allows us to extract the proton and deuteron yields as a function of the CM scattering angle without the necessity of any additional background subtraction. The yields obtained in this procedure are presented in Fig. 6 for 64M events (8% of our full data set). The proton and deuteron distributions should be completely symmetric around cos(qcm) = 0, which our distributions are, within the statistical uncertainties. Analysis of the full data set will improve the situation. In order to obtain differential cross sections from the yields, the detector acceptance and inefficiencies as well as the photon flux will be calculated. This final part of the analysis is in progress.
Figure 6. Yields for the g3He ® pd reaction. Full circles: deuteron distributions; empty squares: proton distributions. |
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