Separated response functions in exclusive, forward π [superscript ±] electroproduction on deuterium
Author(s)
Huber, G. M.; Blok, H. P.; Butuceanu, C.; Gaskell, D.; Horn, T.; Mack, D. J.; Abbott, D.; Aniol, K.; Anklin, H.; Armstrong, C.; Arrington, J.; Assamagan, K.; Avery, S.; Baker, O. K.; Barrett, B.; Beise, E. J.; Bochna, C.; Boeglin, W.; Brash, E. J.; Breuer, H.; Chang, C. C.; Chant, N.; Christy, M. E.; Dunne, J.; Eden, T.; Ent, R.; Fenker, H.; Gibson, E. F.; Gilman, R.; Gustafsson, K.; Hinton, W.; Holt, R. J.; Jackson, H.; Jin, S.; Jones, M. K.; Keppel, C. E.; Kim, P. H.; Kim, W.; King, P. M.; Klein, A.; Koltenuk, D.; Kovaltchouk, V.; Liang, M.; Liu, J.; Lolos, G. J.; Lung, A.; Margaziotis, D. J.; Markowitz, P.; Matsumura, A.; McKee, D.; Meekins, D.; Mitchell, J.; Miyoshi, T.; Mkrtchyan, H.; Mueller, B.; Niculescu, G.; Niculescu, I.; Okayasu, Y.; Pentchev, L.; Perdrisat, C.; Pitz, D.; Potterveld, D.; Punjabi, V.; Qin, L. M.; Reimer, P. E.; Reinhold, J.; Roche, J.; Roos, P. G.; Sarty, A.; Shin, I. K.; Smith, G. R.; Stepanyan, S.; Tang, L. G.; Tadevosyan, V.; Tvaskis, V.; van der Meer, R. L. J.; Vansyoc, K.; Van Westrum, D.; Vidakovic, S.; Volmer, J.; Vulcan, W.; Warren, G.; Wood, S. A.; Xu, C.; Yan, C.; Zhao, W.-X.; Zheng, X.; Zihlmann, B.; ... Show more Show less![Thumbnail](/bitstream/handle/1721.1/93141/PhysRevC.91.015202.pdf.jpg?sequence=6&isAllowed=y)
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Background: Measurements of forward exclusive meson production at different squared four-momenta of the exchanged virtual photon, Q[superscript 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 [superscript −] and π [superscript +] 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 [superscript ±] 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 Q [superscript 2] values of 0.6, 1.0, and 1.6 GeV [superscript 2] at W=1.95 GeV, and Q [superscript 2] = 2.45 GeV [superscript 2] at W = 2.22 GeV. There was significant coverage in ϕ and ε, which allowed separation of σ [subscript L,T,LT,TT].
Results: σ [subscript L] shows a clear signature of the pion pole, with a sharp rise at small −t. In contrast, σ [subscript T] is much flatter versus t. The longitudinal/transverse ratios evolve with Q [superscript 2] and t and at the highest Q [superscript 2] = 2.45 GeV [superscript 2] show a slight enhancement for π [superscript −] production compared to π [superscript +]. The π [[superscript -] over π [superscript +]] ratio for transverse photons exhibits only a small Q [superscript 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 [[subscript TT] over σ[subscript T]] ratio also drops rapidly with Q [superscript 2], qualitatively consistent with s-channel helicity conservation. The [π [superscript −] over π [superscript +]] 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 σ [subscript L] but have varying success with σ [subscript T]. 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, H [subscript T].
Date issued
2015-01Department
Massachusetts Institute of Technology. Department of PhysicsJournal
Physical Review C
Publisher
American Physical Society
Citation
Huber, G. M. et al. “Separated Response Functions in Exclusive, Forward π [superscript ±] Electroproduction on Deuterium.” Physical Review C 91.1 (2015): 015202-1-015202-23. © 2015 American Physical Society
Version: Final published version
ISSN
0556-2813
1089-490X