Consistent meson-field-theoretical description ofppbremsstrahlung

Abstract

A parameter-free and relativistic extension of the RuhrPot meson-baryon model is used to define the dominant isoscalar meson-exchange currents. We compute pp-bremsstrahlung observables below the π−production threshold using a relativistic hadronic current density that includes impulse, wave function re-orthonormalization, meson-recoil, NN creation and annihilation, ρπγ + ωπγ + ρηγ + ωηγ vector-meson decay and N∆γ(π, ρ) exchange currents. We obtain a good description of the available data. The N∆γ(π) current is shown to dominate the large two-body contributions and closed-form expressions for various non-relativistic approximations are analyzed. An experimental sensitivity to the admixture of pseudo-scalar and pseudo-vector admixture of the NNπ interaction is demonstrated. We examine the Lorentz invariance of the NN ⇀ ↽NN t-matrices and show a dominantly pseudo-vector NNπ coupling renders impulse approximation calculations without boost operators to be essentially exact. Conversely, a similar analysis of the ∆N ⇀ ↽NN transitions shows that boost operators and the two-body N ∆γ wave function re-orthonormalization meson-recoil currents are required in NN, ∆N and ∆∆ coupled channel t-matrix applications. The need for additional data is stressed.

Two-nucleon electromagnetic current in chiral effective field theory: One-pion exchange and short-range contributions

“…The obtained results in momentum space read: 28) where the scalar functions f i (k) are given by 29) and the loop function L(k) is defined in Eq. (B.4).…”

High-Precision Proton-Proton Bremsstrahlung Measurements below the Pion-Production Threshold

“…In short, the discrepancy of our data with the microscopic-model calculations indicates that an explicit treatment of additional higherorder effects is still needed, or that the problem resides in the approximations made in modeling the off-shell NN [3,32]. The one exception is a calculation by Eden and Gari [8,33] which produces systematically higher cross sections than other microscopic-model calculations. Their result is depicted by the dash-dotted curve in Fig.…”

“…This led to attempts to investigate the sensitivity of the observables to higher-order off-shell effects. The effects considered include negative-energy states [4], the virtual D isobar and the magnetic meson-exchange currents (MEC) [5][6][7][8][9], and the explicit off-shell dependence of the electromagnetic NNg vertex [10,11]. For the kinematics presented in this Letter, the inclusion of the virtual D isobar and MEC contribute up to 8% to the cross section [9].…”

“…The solid curve includes in addition MEC and the D isobar [9]. The dashed curve is a SPA calculation [29] and the dash-dotted curve is a microscopic calculation by Eden and Gari [8,33]. scattering angle is fixed at 145 ± , with a bin size of 10 ± (140 ± -150 ± ).…”

Field transformations and simple models illustrating the impossibility of measuring off-shell effects

“…For nucleon-nucleon bremsstrahlung there have been a large number of calculations, mostly in nonrelativistic potential models which have this aim [1][2][3][4][5]. The usual procedure is to calculate an off-shell nucleon-nucleon amplitude as a separate building block and attach photons to the external legs of this amplitude.…”