Precision Measurement of Electroproduction of π⁰ near Threshold

Electromagnetic production of neutral pions near threshold is the most basic, lowest energy reaction in which a new hadron is created. The electromagnetic interaction is well understood so measurements of this reaction can yield direct insight into the hadronic production mechanism. During the past three decades there have been many developments in both the measurement and theory of threshold pion production, starting with measurements of photoproduction, p(γ,π 0 )p, at Saclay in 1986 and at Mainz in 1990. These measurements indicated a surprising discrepancy with so-called Low Energy Theorems (LETs) which are based on quite fundamental symmetries and considerations. Chiral Perturbation Theory (ChPT) is an effective field theoretic description of the nuclear force which contains the underlying symmetries of the force but deals with nucleons and pions rather than quarks and gluons. It has the advantage of being applicable at low energies but requires tuning some parameters to experimental data. Once these parameters have been determined ChPT predicts how the reaction should behave as a function of the kinematic vaiables. When applied to the reaction, p(γ,π 0 )p, near threshold it explained the discrepancy with the LETs and made predictions for electroproduction, p(e,e ′ p)π 0 . Electroproduction measurements at Mainz in the 1990's showed a clear discrepancy with these predictions of ChPT; with parameters determined from one set of kinematics the data for a second set lay far from the predicted value. However, recently completed measurements at Mainz disagreed with their previous measurements. In the experiment presented here, measurements of neutral pion electroproduction, iii p(e,e ′ p)π 0 , were made in fine bins of momentum transfer, Q 2 , between Q 2 = 0.05 [GeV/c] 2 and Q 2 = 0.15 [GeV/c] 2 and of center-of-mass energy, W, between 0 ≤ W ≤ 30 MeV (above threshold). The experiment was performed in Hall A at the Jefferson Laboratory. Scattered electrons were detected in one of the two High Resolution Spectrometers while the recoiling protons were detected in the BigBite spectrometer. The results of the present experiment agree within uncertainties with the recent Mainz measurements and extend in W significantly beyond those measurements. The present data show significant disagreement with some terms in the cross section above Q 2 ≈ ( ) 0.10 [GeV/c] 2 : the total cross section, , and the transverse-transverse cross section, A T+L 0 ( ) A TT 0 . For W ≥ 5 MeV the disagreement is very strong and consistent. Whether fitting new ChPT parameters to the recent data or adding more terms to the calculations would improve the agreement remains to be seen.

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