A relativistic meson-baryon description ofpp-Bremsstrahlung

Abstract

Abstract. We describe a relativistic approach to the calculation of nucleon-nucleon Bremsstrahlung, where all meson-baryon and meson-baryon-photon interactions can be calculated consistently and microscopically. In this first relativistic approach to the problem, we present numerical results including both single-scatter and rescatter contributions with a relativistic current density, within a model where the explicit photon coupling to meson-exchange currents are small. The need for high precision (p, PY) measurements is stressed. PACS: 13.75.Cs; 25.10.+s; 27.30.+y The consistent microscopic prediction of (p, PT) observables requires the calculation of the photon coupling to an interacting meson-baryon system. For processes where the photon couples to a free nucleon before or after strong-interactions, one can express the (P, PT) invariant amplitude in the two-potential formalism of GellMann and Goldberger [1], consisting of a one-body nuclear current density and an off-shell NN T-matrix. The essential advantage of this approach is that the NN Tmatrix is defined to all orders in the strong-interaction coupling constant. All recent (p, p 7) calculations use this two-potential formalism, and obtain the NN T-matrices from the Lippmann-Schwinger [24] or BlankenbeclerSugar [5,6] equations for a non-relativistic boson-exchange potential such as Bonn [7] or Paris [8], or from inverse-scattering methods [9], and therefore include only the positive frequency components of the two-nucleon wavefunctions. Under this restriction, only the positive frequency processes that are shown in Fig. 1 are included.In all existing (p, P7) calculations, where the non-relativistic strong-interaction models neglect the negativefrequency components of the off-shell nulceons, it is necThis work is supported by COSY-KFA Jfilich (41140512) and Deutsche Forschungsgemeinschaft (Ga 153/11-4) essary to apply some kind of non-relativistic reduction to the current operator. This may take the form [2, 41 of a Foldy Wouthuysen transformation to obtain a nonrelativistic current with 'relativistic' spin corrections, which are retained to some approximate order, according to where the infinite p/m-expansion is truncated. In a more approximate scheme, it may take the form [6] of a direct Pauli reduction. These two approaches lead to differences of order 7% in the (p, pT) cross section at 280 MeV [10], and differ from the results that are obtained with a completely non-relativistic current density [3,5] by as much as 15% [-2, 6]. The corresponding differences in the spin-observables are generally even larger.This suggests the need for relativistic wave functions in (p, PT) calculations, so that the relativistic current density can be calculated without recourse to any (p/m)_ expansion, and the off-shell two-nucleon state can be properly described as including both its the positive and negative frequency contributions, as shown in Fig. 1. In the Feynman-Stfikelberg interpretation, the negative frequency contributions represent NN creation and annihilat...

Sensitivity of pp-bremsstrahlung to meson-exchange currents

“…In earlier work [8] we compared this data from TRIUMF with relativistic meson-baryon model calculations that include only the processes of Fig. 1(a-c), but we obtained results that share the discrepancy with experiment reported elsewhere [9][10][11][12][13][14][15].…”

“…Our procedure for calculating the impulse contributions of Figs 1(a-c) has been described in ref [8]. We add to this the perturbative ρπγ, ωπγ, ρηγ, ωηγ and N∆γ(π, ρ) exchange currents currents shown in Fig 1(d-e), all of which are easily calculated in the usual way [20][21][22][23] with,…”

Consistent meson-field-theoretical description ofppbremsstrahlung

“…In the present work we develop our earlier description [1,14,15] of the RuhrPot meson-baryon interactions in the pp-bremsstrahlung data below the π-production threshold. In refs [14] we included the relativistic single and rescattering impulse-current amplitudes, and in refs [1,15] we introduced the fully relativistic description of the radiative vector-meson decay currents and the non-relativistic description of the N∆γ π− and ρ−meson exchange currents without recourse to the soft-photon approximation. In the present work we investigate a number of important extensions.…”

“…A very long list of pp-bremsstrahlung calculations have been reported over the last 45 years. We will make no attempt to review them all since more recent works [8][9][10][11][12][13][14][15][16][17][18][19][20] already contain appropriate citation and serve to remove a number of questionable approximations. A notable exception to this trend is found in the very detailed r-space calculations reported some 25 years ago by Brown [21,22], where the rescattering amplitudes of Fig.…”

“…(1.1) was used to constrain the longitudinal meson-exchange currents. Noteworthy calculations since that time have generally been less complete, but find their merit in the application of more reliable NNinteractions and the exploration of coulomb corrections [17] and relativistic corrections to the impulse current [8,11,14,16].…”