[Clascomment] OPT-IN: Double-pion electroproduction off protons in deuterium: quasi-free cross sections and final state interactions

[Igor Strakovsky]

Dear Iuliia, Gleb, Ralf, Phil, Marco, and Ian,

This paper presented a good analysis for double charged pion electroproduction with CLAS6. The Final-State Interactions (FSI) have a significant effect on the results, leading to an increase in events, in particular, with low-momentum and large polar angles for all final-state hadrons. This is discussed in Section VII ("Final-State Interactions"). Previously published classical works indicate that it is not possible, in pion photoproduction, to measure the effects of FSI experimentally, it requires model-dependent nuclear corrections called FSI [A.B. Migdal, JETP 1, 2 (1955); K.M. Watson, Phys Rev 95, 228 (1954)]. Our GW-ITEP group studied the effect of FSI in the case of the bound neutron in detail to get gn-->pi-p and gn-->pi0n cross sections from single-pion photoproduction measurements using the deuteron target. We summarize our work in Ref [W.J. Briscoe, A.E. Kudryavtsev, I.I. Strakovsky, V.E. Tarasov, and R.L. Workman, Eur. Phys. J. A 58, 23 (2022)]. In particular, the CLAS Col
 laboration published a paper [P. Mattione et al. [CLAS Collaboration], Phys. Rev. C 96, 035204 (2017)] showed the importance of the FSI corrections to get cross sections for the reaction gn-->pi-p. Unfortunately, there are no theoretical/phenomenological works addressed to the double pion photo- and electroproduction. Obviously, in this three-body case, one has to take into account NN interaction and piN interactions for each pion with each nucleon. Additionally, one needs to take into account 2-loop diagrams. My suggestion is that the paper has to spell it out accordingly. 

Kind Regards, Igor