- Published on 02 February 2011
A key to our understanding of Quantum Chromodynamics (QCD) in the strong regime is our ability to reproduce the hadronic excitation spectrum. Up to now, and due to their limited predictive power, quark models forecast of this spectrum at high excitation energies is unsatisfactory and is dubbed ``the missing resonances problem”. To explore the high excitation energies in the hadron spectrum production or scattering of heavier mesons from a nucleon target is essential.
In a recent tour-de-force experiment [Eur. Phys. J. A, Volume 47, n. 1, January 2011] I. Jaegle et al. report on an impressive first measurement of ?’ photoproduction off a deuteron target at beam energies between 1.47 - 2.45 GeV at the tagged photon beam of the ELSA electron accelerator. Differential cross sections with a wide angular coverage were derived for quasi-free production both on protons and neutrons validating the quasi-free picture. And the first estimate of the coherent ?d ? d?’ contribution is found consistent with an impulse approximation, pointing to a viable isospin composition model amplitudes and weak final state interactions.
Legendre polynomials coefficients from angular distributions fits of this experiment and world data are reported in Fig. 1 where proton and neutron cross sections for photon energies above 2 GeV, in a region where contributions from t-channel exchange are important, display a similar behavior. At lower photon energies from where the proton cross section peaks, the behavior is different and would require polarization observables for future investigation.
From left to right of panels in Fig. 1, we have the inclusive reaction data first, then the world free and, this experiment, quasi-free proton data and last quasi-free neutron data from coincidence and an extraction of proton from deuteron inclusive data. Solid lines: solution (I) NH model, dashed lines: ??-MAID for neutron and dotted lines: CLAS proton data.