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Final-state interactions in two-nucleon knockout reactions

Dec 24, 2015
12 pages
Published in:
  • Phys.Rev.C 93 (2016) 3, 034608
  • Published: Mar 7, 2016
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Abstract: (APS)
Background: Exclusive two-nucleon knockout after electroexcitation of nuclei [A(e,e′NN) in brief] is considered to be a primary source of information about short-range correlations (SRCs) in nuclei. For a proper interpretation of the data, final-state interactions (FSIs) need to be theoretically controlled. Purpose: Our goal is to quantify the role of FSI effects in exclusive A(e,e′pN) reactions for four target nuclei representative of the whole mass region. Our focus is on processes that are SRC driven. We investigate the role of FSIs for two characteristic detector setups corresponding to “small” and “large” coverage of the available phase space. Method: Use is made of a factorized expression for the A(e,e′pN) cross section that is proportional to the two-body center-of-mass (c.m.) momentum distribution of close-proximity pairs. The A(e,e′pp) and A(e,e′pn) reactions for the target nuclei C12,Al27,Fe56, and Pb208 are investigated. The elastic attenuation mechanisms in the FSIs are included using the relativistic multiple-scattering Glauber approximation (RMSGA). Single-charge exchange (SCX) reactions are also included. We introduce the nuclear transparency TApN, defined as the ratio of exclusive (e,e′pN) cross sections on nuclei to those on “free” nucleon pairs, as a measure for the aggregated effect of FSIs in pN knockout reactions from nucleus A. A toy model is introduced in order to gain a better understanding of the A dependence of TApN. Results: The transparency TApN drops from 0.2–0.3 for C12 to 0.04–0.07 for Pb208. For all considered kinematics, the mass dependence of TApN can be captured by the power law TApN∝A−λ with 0.4≲λ≲0.5. Apart from an overall reduction factor, we find that FSIs only modestly affect the distinct features of SRC-driven A(e,e′pN) which are dictated by the c.m. distribution of close-proximity pairs. Conclusion: The SCX mechanisms represent a relatively small (order of a few percent) contribution of SRC-driven A(e,e′pN) processes. The mass dependence of FSI effects in exclusive A(e,e′pN) can be captured in a robust power law and is in agreement with the predictions obtained in a toy model.
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