First Measurement of the Helicity Asymmetry forγp→pπ0in the Resonance Region

Abstract

The first measurement of the helicity dependence of the photoproduction cross section of single neutral pions off protons is reported for photon energies from 600 to 2300 MeV, covering nearly the full solid angle. The data are compared to predictions from the SAID, MAID, and BnGa partial wave analyses. Strikingly large differences between data and predictions are observed which are traced to differences in the helicity amplitudes of well known and established resonances. Precise values for the helicity amplitudes of several resonances are reported. PACS numbers:The spin structure of the proton has been of topical interest since the discovery that the quark spins constitute an unexpectedly small fraction of the proton spin [1]. Deep inelastic scattering experiments have revealed gluonic contributions to the proton spin to be consistent with zero, at least within the admittedly large errors [2]. At low energies, a different aspect of the proton spin structure is tested by a comparison of the helicity dependence of the total γp cross section integrated over all photon energies, ∞ 0 dE γ (σ 3/2 − σ 1/2 )/E γ , with the proton magnetic moment [3], a relation which is known as Gerasimov-Drell-Hearn (GDH) sum rule [4,5]. The subscripts denote the total helicity, h = 1/2 for photon and proton spin anti-aligned, h = 3/2 for both spins aligned. The GDH integral sums over all energies and all final states. A breakdown of the GDH integral into exclusive final states can provide a link between inclusive properties of the proton like its magnetic moment and the contributions of specific nucleon resonances to the GDH integral.In this letter, we report the first measurement of the double polarization observable(1) for the exclusive reaction γp → pπ 0 in the energy range from 600 to 2300 MeV, and compare the results with predictions of well established partial wave analyses (PWA) like SAID [6,7], MAID [8], and BnGa [9]. The double polarization observable G [10] -governing the correlation between linearly polarized photons and longitudinally polarized target protons -revealed remarkable differences in the predictions of the three partial wave analysis groups even in the well-studied 2 nd resonance region around E γ =750 MeV [11]. Here, we extend the covered energy regime to 2.3 GeV for the double polarization observable E. So far, data on E were published up to 780 MeV for a limited angular range [12,13]; further data from CLAS exist and were reported at a conference [14].The experiment was carried out using the tagged photon beam of the ELectron Stretcher Accelerator ELSA at Bonn [15]. Photons with circular polarization [16],were produced by scattering a 2.4 GeV beam of longitudinally polarized electrons with polarization P e − off a bremsstrahlung target. The electrons were deflected by a magnet into a tagging hodoscope which defines the energy of the bremsstrahlung photons. The electron polarization (P e − ≈ 0.60) was monitored in a Møller polarimeter [17]. The photon beam impinged on a butanol (C 4 H 9 OH) target, which was p...

Baryon spectroscopy - Recent results from the CBELSA/TAPS experiment

“…In this paper, the measurement of single and double polarization observables accessible with linearly polarized beam and a transversely polarized target are reported. They complement our published results with a longitudinally polarized target and linearly [5] and circularly [6] polarized beam.…”

“…Our data up to E γ = 930 MeV were used as a basis for an energy-independent PWA [20], allowing for the determination of the N(1520) 3/2 − helicity amplitudes with minimal model dependence. All the data were included in the BnGa multi-channel PWA, together with our recently published data on G [5] and E [6], and further data on other channels. 1 Starting from the previous solutions BnGa2011-01 and BnGa2011-02 [16] all parameters were reoptimized.…”

Light Baryon Spectroscopy

“…One of the well-measured double polarization observable is the observable E, which is closely related to the helicity asymmetry σ 3/2 − σ 1/2 , see for example [190,134]. This asymmetry has already been published by the GDH experiment at MAMI in 2002 and 2004 [191,124,192], but a normalization, which leads to the observable E had been missing.…”

“…This asymmetry has already been published by the GDH experiment at MAMI in 2002 and 2004 [191,124,192], but a normalization, which leads to the observable E had been missing. This was first done by the CBELSA/TAPS collaboration for the pπ 0 final state [190,134]. In the last years, the data base for this observable has been extend for all other final states by measurements from the CLAS collaboration for nπ + [188] and pπ − [193] Figure 24: The beam asymmetry Σ from the BGOegg experiment (closed circles) [144] and from CLAS (open squares) [181] for the reaction γp → pπ 0 .…”

A GEM-TPC for the CBELSA/TAPS experiment

“…It is optimized for the study of baryonic resonances with neutral final states [2], for example γ p → pπ 0 , γ p → pη, γ p → pη . The nearly totally absorbing calorimeter covers the full azimuthal angle and the polar angle from 1 • to 156 • , making this experiment an ideal candidate to measure a large set of polarization variables in reactions created by an linearly or cirularly polarized photon beam hitting a longitudinal or transversely polarized target [3,4]. However, the existing setup is optimised, but also restricted, to study reactions with neutral final states.…”