Neutral pion photoproduction from the proton near threshold

Fuchs, Markus; Ahrens, J.; Anton, G.; Averbeck, R.; Beck, R.; Bernstein, A. M.; Gabler, A.R.; Härter, F.; Harty, P.D.; Hlaváč, S.; Krusche, B.; McGregor, Ijd; Metag, V.; Novotny, R.; Owens, R.O.; Peise, J.; Röbig-Landau, M.; Schubert, András; Simon, R. S.; Ströher, H.; Tries, V.

doi:10.1016/0370-2693(95)01488-8

Cited by 107 publications

(51 citation statements)

References 19 publications

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“…In this framework and calculating the S-waves to order q 4 , we have already reexamined neutral pion photoproduction [9,10]. Novel P-wave low-energy theorems (LETs) have been found together with a good description of the new TAPS and SAL data for γp → π 0 p [11] [12]. In particular, chiral loops are necessary to understand the small value of the electric dipole amplitude E 0+ at the π 0 p and π + n thresholds.…”

Section: Introductionmentioning

confidence: 99%

Threshold neutral pion electroproduction in heavy baryon chiral perturbation theory

1996

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We investigate neutral pion electroproduction off protons in the framework of heavy baryon chiral perturbation theory. The chiral expansion of the S-wave multipoles E 0+ and L 0+ is carried out to three orders. There appear several undetermined lowenergy constants. Three are taken from a recent study of the new TAPS threshold π 0 photoproduction data, one is fixed from the proton Dirac form factor and the novel S-wave constants appearing at orders q 4 and q 5 are determined from a best fit (constrained by resonance exchange) to the recent NIKHEF and MAMI data at photon momentum transfer squared k 2 = −0.1 GeV 2 . The inclusion of a particular set of dimension five operators is forced upon the fit by a soft-pion theorem which severely constrains the momentum dependence of the longitudinal S-wave multipole L 0+ at order q 4 . We give predictions for lower photon virtualities for the various multipoles and differential cross sections. Further improvements are briefly touched upon.1

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Section: Introductionmentioning

confidence: 99%

Threshold neutral pion electroproduction in heavy baryon chiral perturbation theory

1996

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“…Analysis of measurements from Mainz [1] and Saskatoon [2] of the photoproduction reaction p(γ, π 0 )p at the pion production threshold have determined that the s-wave multipole (E 0+ ) is well reproduced by ChPT (after adding loop terms [3]). On the other hand, cross section measurements of electroproduction at Mainz [4] and the Netherland National Institute for Nuclear and High Energy Physics (NIKHEF) [5] where ChPT LEC's were fitted at four momentum transfers Q 2 = 0.1[GeV /c] 2 , disagreed significantly with ChPT predictions at Q 2 = 0.05[GeV /c] 2 [6,7].…”

Section: Introductionmentioning

confidence: 99%

“…The solid line is the prediction of ChPT; the data are from ref. [8]. This figure is taken from ref.…”

Section: Introductionmentioning

confidence: 99%

Precision Measurement of Electroproduction of pi<sup>0</sup> near Threshold

Chirapatpimol

2012

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0near Threshold ABSTRACT 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, I would also like to thank the other people who were involved in the experiment. But, let me start by apologizing if I forget to mention anyone:

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“…The photoproduction of pions near threshold has been a topic of considerable experimental and theoretical activities over the past years, ever since the results of the experiments, performed in Saclay [30], Mainz ( [31,32]) and Saskatoon [33], were at variance with the prediction of a low energy theorem (LET), which was derived in the early 70s [34,35]. Being based on fundamental principles, this LET predicted the value of the S-wave threshold amplitude E 0+ in a power series in µ = m π /m N , the ratio of the masses of the pion and nucleon.…”

Section: Pion Polarizabilitymentioning

confidence: 99%

“…The experimental value for P 3 is higher than the value of ChPT, which can be explained by the smaller total and differential cross sections of ref. [32], used by ChPT to determine the dominant low-energy constant b P for this multipole [45]. A new fourth-order calculation in heavy-baryon ChPT by Bernard et al, introduced in [46] and compared ChPT [8] E thr n + =151.4 MeV to the new MAMI data presented in this letter, shows, that the potentially large ∆-isobar contributions are cancelled by the fourth-order loop corrections to the P -wave low-energy theorems.…”

Section: Pion Polarizabilitymentioning

confidence: 99%

Experiments with photons at MAMI

Beck¹

Many Body Structure of Strongly Interacting Systems

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Abstract. A very successful experimental program with real photons has been achieved in 20 years of operation at the Mainz Microtron (MAMI) facility. The different detector setups, like DAPHNE, TAPS and the Crystal Ball are centered around the tagged photon facility the so-called Glasgow Tagger. From the rich spectrum of results only a few highlights will be discussed here, the proton polarizabilities, the pion polarizabilities, pion photoproduction close to the pion threshold and in the ∆(1232)-resonance region.

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Neutral pion photoproduction from the proton near threshold

Cited by 107 publications

References 19 publications

Threshold neutral pion electroproduction in heavy baryon chiral perturbation theory

Threshold neutral pion electroproduction in heavy baryon chiral perturbation theory

Precision Measurement of Electroproduction of pi<sup>0</sup> near Threshold

Experiments with photons at MAMI

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