Physics opportunities with meson beams

Abstract

Over the past two decades, meson photo-and electroproduction data of unprecedented quality and quantity have been measured at electromagnetic facilities worldwide. By contrast, the meson-beam data for the same hadronic final states are mostly outdated and largely of poor quality, or even nonexistent, and thus provide inadequate input to help interpret, analyze, and exploit the full potential of the new electromagnetic data. To reap the full benefit of the high-precision electromagnetic data, new high-statistics data from measurements with meson beams, with good angle and energy coverage for a wide range of reactions, are critically needed to advance our knowledge in baryon and meson spectroscopy and other related areas of hadron physics. To address this situation, a stateof-the-art meson-beam facility needs to be constructed. The present paper summarizes unresolved issues in hadron physics and outlines the vast opportunities and advances that only become possible with such a facility.

Baryons as relativistic three-quark bound states

“…Dispersion theory makes it clear that the timelike and spacelike properties are intrinsically related, and in this respect it is curious that many quark models capable of providing a good spacelike description do not have such mechanisms implemented at all (with exceptions discussed further below). In any case, the experimental situation may change in the future with new precision data on timelike nucleon form factors expected from Novosibirsk, BESIII/Beijing and PANDA/FAIR [449], and timelike reactions are also promising tools to extract the properties of nucleon resonances [485][486][487]. …”

γ*N→N*(1520) form factors in the timelike regime

“…Although different, both experiments, with strong and electromagnetic probes, complement each other [1].…”

“…Measurements of dielectron production in elementary pp collisions [1][2][3][4][5][6][7] expand to the timelike region (q 2 > 0, where q is the momentum transfer) the extraordinarily precise results obtained with the electron scattering CLAS N * program at Jefferson Lab [1,8,9], describing the electromagnetic structure of baryonic transitions in spacelike or q 2 < 0 region (program to be extended down to −12 GeV 2 , as well as up to −0.05 GeV 2 ). Although different, both experiments, with strong and electromagnetic probes, complement each other [1].…”

Eta photoproduction in a combined analysis of pion- and photon-induced reactions

“…Physics opportunities with a pion beam are discussed in Ref. [33]. In any case, the current database does not contain a complete set of observables, neither for photon-nor pion-induced eta production.…”