Three-Body Photodisintegration of ³He

(Niccolai, Strauch, Berman)

One of the main challenges of contemporary nuclear physics is to explain the nature of strong many-body interactions between the nucleons in nuclei. In particular, the nature of three-body forces (3BF) in the A ³ 3 nuclei is still an open problem. The main evidence for the need of such interactions lies in the failure of theoretical models based on two-body forces in reproducing the correct binding energy of light nuclei. The optimal starting point to study strong many-body interactions is the ³He nucleus, since kinematically complete experiments are possible due to the small number of nucleons involved, and exact Faddeev ground-state wave functions and exact wave functions for the continuum three-body final state are available. We have measured the three-body photodisintegration of ³He with the tagged photon beam and the CLAS in Hall B, in the photon energy range between 0.35 and 1.55 GeV. This measurement constitutes a wide-ranging survey of two- and three-body processes in this nucleus, thanks to the high statistics and large kinematic coverage obtained with the CLAS.

The 4p-integrated cross section, shown in Fig. 1, is in excellent agreement with previous experimental results from DAPHNE up to 800 MeV [Aud97]; no previous results have been obtained above this energy. Total and partially integrated differential cross sections, as well as momentum and angular distributions, for the full data set and for selected kinematics were extracted and are compared with theoretical predictions of Laget [Lag85, Lag87, Lag88]. (For details, see [Nic03a].) At low photon energies, the calculations are in fair agreement with the data (Fig. 2, top plot).

Figure 1. Total cross section for the g³He ® ppn reaction, plotted as a function of photon energy. Our data (red circles) are compared with the results from DAPHNE [Aud97] (black squares), and TAGX [Mar95] (blue triangles). The acceptance has been evaluated with a phase-space Monte-Carlo simulation, for all three experiments.

The comparison shows evidence of strong contributions of three-body absorption mechanisms, especially in the star kinematics (Fig. 2, middle plot), a symmetric configuration of the three final-state nucleons. Mostly the effects of two-body absorption mechanisms are seen, as expected, in the pp kinematics (Fig. 2, bottom plot), where the neutron is a spectator.

Figure 2. The g³He ® ppn cross section, integrated over the CLAS acceptance, plotted as a function of photon energy, on a logarithmic scale, for the full energy range. Our CLAS data are compared with Laget’s full model (black solid curve), and to Laget’s calculation suppressing the three-body mechanisms (blue dashed curve). Top: events for all measured kinematics (full Dalitz plot); middle: star-configuration events; bottom: neutron-spectator events.

The ratio of cross sections for the star configuration and for the two-body kinematics shows a maximum for three-body effects at a photon energy of about 0.55 GeV, corresponding to a reduced photon wavelength of 0.4 fm, as can be seen in Fig. 3. This apparent peaking of three-body strength for distances of the order of half the radius of the nucleon may have important implications for our understanding of the hierarchy of nuclear many-body forces [Fri96, Wei97].

The results of this analysis have been presented at the XVIII European Conference on Few-Body Problems in Physics [Nic03b], the DNP2002 Fall Meeting, and the XVII International IUPAP Conference on Few-Body Problems in Physics. They will also be presented at the Sixth Workshop on Electromagnetically Induced Two-Hadron Emission (Pavia, Italy, September 2003). The present analysis is undergoing the internal CLAS review process in preparation for publication.

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Last modified: 03/31/05