Separated response functions in exclusive, forward pi(+/-) electroproduction on deuterium

G.M. Huber, H.P. Blok, C. Butuceanu, D. Gaskell, T. Horn, D.J. Mack, D. Abbott, K. Aniol, H. Anklin, C. Armstrong, J. Arrington, K. Assamagan, S. Avery, O.K. Baker, B. Barrett, E.J. Beise, C. Bochna, W. Boeglin, E.J. Brash, H. BreuerC.C. Chang, N. Chant, M.E. Christy, J. Dunne, T. Eden, R. Ent, H. Fenker, E.F. Gibson, R. Gilman, K. Gustafsson, W. IIinton, R.J. IIolt, I.I. Jackson, S. Jin, M.K. Jones, C.E. Keppel, P.H. Kim, W. Kim, P.M. King, A. Klein, D. Koltenuk, V. Kovaltchouk, M. Liang, J. Liu, G.J. Lolos, A. Lung, D.J. Margaziotis, P. Markowitz, A. Matsumura, D. Mckee, D. Meekins, J. Mitchell, T. Miyoshi, H. Mkrtchyan, B. Mueller, G. Niculescu, I. Niculescu, Y. Okayasu, L. Pentchev, C. Perdrisat, D. Pitz, D. Potterveld, V. Punjabi, L.M. Qin, P.E. Reimer, J. Reinhold, J. Roche, P.G. Roos, A. Sarty, I.K. Shin, G.R. Smith, S. Stepanyan, L.G. Tang, V. Tadevosyan, V. Tvaskis, R.L.J. Van der Meer, K. Vansyoc, D. Van Westrum, S. Vidakovic, J. Volmer, W. Vulcan, G. Warren, S.A. Wood, C. Xu, C. Yan, W.-. Zhao, X. Zheng, B. Zihlmann

Research output: Contribution to JournalArticleAcademicpeer-review


Background: Measurements of forward exclusive meson production at different squared four-momenta of the exchanged virtual photon, Q2, 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. Purpose: We perform the first complete separation of the four unpolarized electromagnetic structure functions L/T/LT/TT in forward, exclusive π± electroproduction on deuterium above the dominant resonances. Method: Data were acquired with 2.6-5.2-GeV electron beams and the HMS+SOS spectrometers in Jefferson Lab Hall C at central Q2 values of 0.6, 1.0, and 1.6 GeV2 at W=1.95 GeV, and Q2=2.45 GeV2 at W=2.22 GeV. There was significant coverage in φ and ε, which allowed separation of σL,T,LT,TT. Results: σL shows a clear signature of the pion pole, with a sharp rise at small -t. In contrast, σT is much flatter versus t. The longitudinal/transverse ratios evolve with Q2 and t and at the highest Q2=2.45 GeV2 show a slight enhancement for π- production compared to π+. The π-/π+ ratio for transverse photons exhibits only a small Q2 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 σTT/σT ratio also drops rapidly with Q2, 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. Conclusions: The separated cross sections are compared to a variety of theoretical models, which generally describe σL but have varying success with σT. Further theoretical input is required to provide a more profound insight into the relevant reaction mechanisms for longitudinal and transverse photons, such as whether the observed transverse ratio is indeed attributable to a transition from pion to quark knockout mechanisms and provide useful information regarding the twist-3 transversity generalized parton distribution, HT.
Original languageEnglish
Pages (from-to)015202-015202
JournalPhysical Review C
Issue number1
Publication statusPublished - 2015

Bibliographical note

PT: J; NR: 42; TC: 0; J9: PHYS REV C; PG: 23; GA: AZ9YA; UT: WOS:000348569100003


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