{'fraction_non_alphanumeric': 0.026203807390817468, 'fraction_numerical': 0.0035834266517357225, 'mean_word_length': 4.970588235294118, 'pattern_counts': {'":': 0, '<': 2, '<?xml version=': 0, '>': 1, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
( Proposal to Jefferson Lab PAC 39 ) Measurement of the Ratio R = σ L / σ T in Exclusive and Semi-Inclusive π ◦ Production

The neutral pion electroproduction reaction is an important tool in our study of hadron structure. The quantum numbers in the π production process introduce sensitivity to axial vector configurations. The pionexchange contributions in the t-channel are expected to be suppressed. Despite these positive aspects, this reaction has been relatively unexploited to date because of the lack of the necessary experimental facilities. As a result, there are practically no L/T separated data for semi-inclusive (elasticity z < 1) or exclusive (z → 1) π production from the proton. With the unique capabilities of the 12 GeV upgrade, we have the opportunity to dramatically improve upon this situation and measure observables, in particular the ratio R = σL/σT for neutral-pion electroproduction, hitherto only known as model-dependent guesses. This proposal has the following primary goals: • The L/T separated cross sections in the exclusive limit z → 1 Separated p(e, eπ)p cross sections allow investigations of the transition from hadronic to partonic degrees of freedom in exclusive processes. Recent separated π data from JLab suggest that the power-law behavior expected from the hard-scattering mechanism is reasonably consistent with the dependence on Q, the fourmomentum transfer squared, of longitudinal cross section data. However, the transverse cross section does not show a corresponding behavior, indicating that transversely polarized photons may play an important role in pion electroproduction even at relatively high values of Q. The Q-dependence of the pion form factor is likewise consistent with the Q-scaling expectation already at values of Q > 1 GeV, while the observed magnitude is larger than the hard QCD prediction. Charged pion processes feature a meson exchange contribution in the t-channel (pole term), whose impact on factorization has to be understood. Hence, it is of great interest to study a similar system in which the pole term is suppressed. A direct comparison of the π longitudinal and transverse cross sections and their ratio for Q up to 5.1 GeV would thus be essential for our understanding of the transition from the hadronic to the partonic regime. We thus propose a systematic measurement of the Q-dependence of the L/T separated cross sections at fixed xB=0.5 in neutral pion electroproduction. In the hard scattering regime, one may expect that the longitudinal and transverse cross section scale as Q and Q, respectively. A large value of their ratio R would be indicative of the realization of the scaling expectation of the factorization theorem for helicity non-flip GPDs, while a large transverse cross section could suggest contributions from transversity (helicity flip) GPDs. The coupling to a virtual photon is suppressed for the neutral pion, and so the pion exchange contribution in the t-channel (pion pole) to the longitudinal cross section is suppressed as well. This is different from the charged pion case, where the pion pole is dominant. Our proposed measurement of the t and x dependence of the π longitudinal cross section, using L/T separated p(e, eπ)p cross section data, will thus also allow to constrain the size of the non-pole contributions in charged-pion electroproduction. One study predicts that the perturbative QCD longitudinal cross section is comparable in size for both the π and π [1]. This is different from the simpler interpretation that σL ≈ 0 for neutral-pion electroproduction, since the pion exchange diagram does not contribute. Thus, a significant longitudinal response in forward π electroproduction may be indicative of non-pole contributions in forward charged-pion electroproduction. • The L/T separated cross sections in the regime z < 1 Whereas inclusive scattering cannot distinguish between the quark flavor, there is great promise in flavor decompositions of regular or transverse-momentum dependent parton distributions through semi-inclusive deep inelastic scattering, and of generalized parton distributions through deep exclusive scattering. For the latter, the ratio R = σL/σT asymptotically scales like Q 2 at fixed Bjorken x. For the former, the ratio R is usually assumed by analyzers of semi-inclusive deep inelastic scattering (SIDIS) data to be either zero or equal to the values determined from inclusive deep inelastic scattering.