Separated Response Functions in Exclusive, Forward $π^{\pm}$ Electroproduction on Deuterium
Creators: name: Huber, G. M.
name: Blok, H. P.
name: Butuceanu, C.
name: Gaskell, D.
name: Horn, T.
name: Mack, D. J.
name: Abbott, D.
name: Aniol, K.
name: Anklin, H.
name: Armstrong, C.
name: Arrington, J.
name: Assamagan, K.
name: Avery, S.
name: Baker, O. K.
name: Barrett, B.
name: Beise, E. J.
name: Bochna, C.
name: Boeglin, W.
name: Brash, E. J.
name: Breuer, H.
name: Chang, C. C.
name: Chant, N.
name: Christy, M. E.
name: Dunne, J.
name: Eden, T.
name: Ent, R.
name: Fenker, H.
name: Gibson, E. F.
name: Gilman, R.
name: Gustafsson, K.
name: Hinton, W.
name: Holt, R. J.
name: Jackson, H.
name: Jin, S.
name: Jones, M. K.
name: Keppel, C. E.
name: Kim, P. H.
name: Kim, W.
name: King, P. M.
name: Klein, A.
name: Koltenuk, D.
name: Kovaltchouk, V.
name: Liang, M.
name: Liu, J.
name: Lolos, G. J.
name: Lung, A.
name: Margaziotis, D. J.
name: Markowitz, P.
name: Matsumura, A.
name: McKee, D.
name: Meekins, D.
name: Mitchell, J.
name: Miyoshi, T.
name: Mkrtchyan, H.
name: Mueller, B.
name: Niculescu, G.
name: Niculescu, I.
name: Okayasu, Y.
name: Pentchev, L.
name: Perdrisat, C.
name: Pitz, D.
name: Potterveld, D.
name: Punjabi, V.
name: Qin, L. M.
name: Reimer, P. E.
name: Reinhold, J.
name: Roche, J.
name: Roos, P. G.
name: Sarty, A.
name: Shin, I. K.
name: Smith, G. R.
name: Stepanyan, S.
name: Tang, L. G.
name: Tadevosyan, V.
name: Tvaskis, V.
name: van der Meer, R. L. J.
name: Vansyoc, K.
name: Van Westrum, D.
name: Vidakovic, S.
name: Volmer, J.
name: Vulcan, W.
name: Warren, G.
name: Wood, S. A.
name: Xu, C.
name: Yan, C.
name: Zhao, W. -X.
name: Zheng, X.
name: Zihlmann, B.
name: Collaboration, The Jefferson Lab Fpi
Publisher: name: arXiv
Year: 2014
Subjects: subject: Nuclear Experiment (nucl-ex)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: Background: Measurements of forward exclusive meson production at different squared four-momenta of the exchanged virtual photon, $Q^2$, and at different four-momentum transfer, t, can be used to probe QCD's transition from meson-nucleon degrees of freedom at long distances to quark-gluon degrees of freedom at short scales. Ratios of separated response functions in $π^-$ and $π^+$ electroproduction are particularly informative. The ratio for transverse photons may allow this transition to be more easily observed, while the ratio for longitudinal photons provides a crucial verification of the assumed pole dominance, needed for reliable extraction of the pion form factor from electroproduction data. Method: Data were acquired with 2.6-5.2 GeV electron beams and the HMS+SOS spectrometers in Jefferson Lab Hall C, at central $Q^2$ values of 0.6, 1.0, 1.6 GeV$^2$ at W=1.95 GeV, and $Q^2$=2.45 GeV$^2$ at W=2.22 GeV. There was significant coverage in $ϕ$ and $ε$, which allowed separation of $σ_{L,T,LT,TT}$. Results: $σ_L$ shows a clear signature of the pion pole, with a sharp rise at small -t. In contrast, $σ_T$ is much flatter versus t. The longitudinal/transverse ratios evolve with $Q^2$ and t, and at the highest $Q^2$=2.45 GeV$^2$ show a slight enhancement for $π^-$ production compared to $π^+$. The $π^-/π^+$ ratio for transverse photons exhibits only a small $Q^2$-dependence, following a nearly universal curve with t, with a steep transition to a value of about 0.25, consistent with s-channel quark knockout. The $σ_{TT}/σ_T$ ratio also drops rapidly with $Q^2$, qualitatively consistent with s-channel helicity conservation. The $π^-/π^+$ ratio for longitudinal photons indicates a small isoscalar contamination at W=1.95 GeV, consistent with what was observed in our earlier determination of the pion form factor at these kinematics.
description: 25 pages, 16 figures, 6 tables