title: Constraints on the [Formula: see text] form factor from analyticity and unitarity.
abstract: Motivated by the discrepancies noted recently between the theoretical calculations of the electromagnetic [Formula: see text] form factor and certain experimental data, we investigate this form factor using analyticity and unitarity in a framework known as the method of unitarity bounds. We use a QCD correlator computed on the spacelike axis by operator product expansion and perturbative QCD as input, and exploit unitarity and the positivity of its spectral function, including the two-pion contribution that can be reliably calculated using high-precision data on the pion form factor. From this information, we derive upper and lower bounds on the modulus of the [Formula: see text] form factor in the elastic region. The results provide a significant check on those obtained with standard dispersion relations, confirming the existence of a disagreement with experimental data in the region around [Formula: see text].
authors.affiliation: Centre for High Energy Physics, Indian Institute of Science, Bangalore, 560 012 India.
authors.forename: B
authors.initials: B
authors.lastname: Ananthanarayan
authors.affiliation: Horia Hulubei National Institute for Physics and Nuclear Engineering, P.O.B. MG-6, 077125 Magurele, Romania.
authors.forename: I
authors.initials: I
authors.lastname: Caprini
authors.affiliation: Helmholtz-Institut für Strahlen-und Kernphysik (Theorie) and Bethe Center for Theoretical Physics, Universität Bonn, 53115 Bonn, Germany.
authors.forename: B
authors.initials: B
authors.lastname: Kubis