The role of Δ(1700) excitation and ρ production in double pion photoproduction

Abstract

Recent information on invariant mass distributions of the γp → π + π 0 n reaction, where previous theoretical models had shown deficiencies, have made more evident the need for new mechanisms, so far neglected or inaccurately included. We have updated a previous model to include new necessary mechanisms. We find that the production of the ρ meson, and the ∆(1700) excitation, through interference with the dominant terms, are important mechanisms that solve the puzzle of the γp → π + π 0 n reaction without spoiling the early agreement with the γp → π + π − p and γp → π 0 π 0 p reactions.

Helicity asymmetries in double pion photoproduction on the proton

“…This prediction was confirmed by the experiment of [21]. Therefore, the model of [15] has widely proven his efficiency in reproducing and predicting unpolarized observables in the energy range from threshold up to E γ ∼ 800 MeV. Nonetheless, a more demanding test to the model can be done by evaluating polarization observables, since it can be sensible to details of the model not visible when integrating over polarization degrees of freedom in the unpolarized observables.…”

“…(The ∆KR term is not present in the γp → π 0 π 0 p channel). Several ρ and ∆(1700) terms were included in the last version of the model [15] because of important interference effects. The consideration of the ρ terms was of crucial importance in the analysis of Ref.…”

“…[15], using a non relativistic approximation exact up to order p/M p , that is, removing terms of order (p/M p ) 2 and higher.…”

“…The photoproduction of two pions on nucleons at low and intermediate energies (up to E γ ∼ 1 GeV) has been the subject of intense experimental [1][2][3][4][5][6] and theoretical [7][8][9][10][11][12][13][14][15][16][17][18][19] study. The works have been mainly motivated to understand the role of the many baryonic resonances involved in the process.…”

“…Nonetheless, a more demanding test to the model can be done by evaluating polarization observables, since it can be sensible to details of the model not visible when integrating over polarization degrees of freedom in the unpolarized observables. In this line, a test of the model of [15] was done in [22] when evaluating the spin 1/2 and 3/2 amplitudes and the contribution to the GDH sum rule of the double pion channel, in fair agreement with Mainz results [23,24], and the evaluation of beam asymmetries under experimental study at GRAAL [5]. These observables are based on differences of total differential cross sections dependent on polarization, which provide a valuable information on the internal dynamics of the reaction.…”