Final state interaction effects in production near threshold

1. Final state interaction effects in production near threshold V.F. Dmitriev and A.I. Milstein Budker Institute of Nuclear Physics

2. Ratio of electric and magnetic formfactors In the threshold region. p (Ν) q R.Bijker, F.Jachello, Phys.Rev. C69 (2004) Γ _ -p (N)

5. General features of the process _ p P P formation scale q -p At distances 0.1 < r < 1.4 fm there is a strong interaction between slowly moving P and Pbar. This scale corresponds to the energies starting from few ten’s MeV. Strong final state interaction in this energy scale is very likely responsible for the energy dependence of the formfactors near threshold. QUANTUM NUMBERS of P Pbar state P Pbar Isospin not fixed I=0,1 PHOTON J=1, P=-1, C=-1 L - even S = 1

6. Elementary amplitude of N Nbar production • polarization of virtual • photon M – proton mass, E – nucleon energy, • Dirac formfactors responsible for short distance • behaviour are smooth functions, nearly constants in threshold region

7. Accounting for N Nbar interaction - N Nbar scattering state with asymptotic behaviour is the spin polarization vector for S=1 state of N Nbar pair

8. Paris N Nbar potential, Phys.Rev. C 50 (1994) 2710 Real part - meson exchange for r > 0.84 - 1 fm and fit for r < 0.84 - 1 fm. In both I=0,1 channels p n Coupled channels Imaginary part

9. Production amplitude becomes D-wave S-wave Electromagnetic formfactors

10. Results for

13. Summary • We developed an approach to account for final state interaction (FSI) in P Pbar production near threshold. • The effects of FSI are considerable; they lead to strong energy dependence near threshold. • The approach is not a parameter free one. More data are necessary to check its validity.