Determination of the Pion Charge Form Factor atQ2=1.60and2.45  (GeV/c)<

Abstract

The 1 H e; e 0 n cross section was measured at four-momentum transfers of Q 2 1:60 and 2:45 GeV 2 at an invariant mass of the photon nucleon system of W 2:22 GeV. The charged pion form factor (F ) was extracted from the data by comparing the separated longitudinal pion electroproduction cross section to a Regge model prediction in which F is a free parameter. The results indicate that the pion form factor deviates from the charge-radius constrained monopole form at these values of Q 2 by one sigma, but is still far from its perturbative quantum chromodynamics prediction. DOI: 10.1103/PhysRevLett.97.192001 PACS numbers: 14.40.Aq, 11.55.Jy, 13.40.Gp, 25.30.Rw A fundamental challenge in nuclear physics is the description of hadrons in terms of the constituents of the underlying theory of strong interactions, quarks, and gluons. Properties such as the total charge and magnetic moments are well described in a constituent quark framework, which effectively takes into account quark-gluon interactions. However, charge and current distributions, which are more sensitive to the underlying dynamic processes, are not well described.Hadronic form factors provide important information about hadronic structure. The coupling of a virtual photon to structureless particles is completely determined by their charge and magnetic moments. However, for composite particles one must account for the internal structure, which is accomplished by momentum transfer dependent functions. Examples of these functions are the electromagnetic form factors, which describe the distribution of charge and current.One of the simplest hadronic systems available for study is the pion, whose valence structure is a bound state of a quark and an antiquark. The electromagnetic structure of a spinless particle such as the pion is parametrized by a single form factor. Asymptotically, the pion charge form factor, F , is given in perturbative quantum chromodynamics (pQCD) [1]:

Dispersion relation for hadronic light-by-light scattering: two-pion contributions

“…Experimentally, the available constraints derive from e + e − → π + π − data, which determine the time-like form factor [74][75][76][77][78][79], and space-like measurements by scattering pions off an electron target [80,81]. We have also checked that our representation is consistent with extractions of the space-like form factor from e − p → e − π + n data [82][83][84][85], although due to the remaining model dependence of extrapolating to the pion pole we do not use these data in our fits. To obtain a representation that allows us to simultaneously fit space--36 -…”

“…We find the relation 82) where the ratio of η f factors compensates the sign (−1) λ 3 +λ 4 from (2.73). The HLbL tensor is written in terms of the redundant BTT Lorentz decomposition as…”

The mass and width differences of the neutral and charged ρ(770), ρ(1450) and ρ(1700) mesons from e+e−→ π+π− and τ−→ ντπ−π0 processes

“…An advantage of this approach is that it also utilizes the existing data on the pion EM FF in the spacelike region. [20][21][22] As a result, the errors of the determined masses and widths are thereby remarkably reduced. In Sec.…”

Nucleon electromagnetic form factors

“…Thus, information on the pion probes into the deepest part of QCD. New data [222] and the reanalysis of old data [223] are confronting theory, e.g. [224,225].…”