Separated response functions in exclusive , forward [ superscript ± ] electroproduction on deuterium
نویسندگان
چکیده
Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. 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. Purpose: We perform the first complete separation of the four unpolarized electromagnetic structure functions L/T /LT /T T 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 Q 2 values of 0.6, 1.0, and 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 , which allowed separation of σ L,T ,LT ,T T. 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 T T /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. Conclusions: The …
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