Measurement of polarization observables T, P, and H in π0 and η photoproduction off quasi-free nucleons

Abstract

The target asymmetry T, recoil asymmetry P, and beam-target double polarization observable H were determined in exclusive ${\pi }^0$ and $\eta$ photoproduction off quasi-free protons and, for the first time, off quasi-free neutrons. The experiment was performed at the electron stretcher accelerator ELSA in Bonn, Germany, with the Crystal Barrel/TAPS detector setup, using a linearly polarized photon beam and a transversely polarized deuterated butanol target. Effects from the Fermi motion of the nucleons within deuterium were removed by a full kinematic reconstruction of the final state invariant mass. A comparison of the data obtained on the proton and on the neutron provides new insight into the isospin structure of the electromagnetic excitation of the nucleon. Earlier measurements of polarization observables in the $\gamma p\to {\pi }^{0}p$ and $\gamma p\to \eta p$ reactions are confirmed. The data obtained on the neutron are of particular relevance for clarifying the origin of the narrow structure in the $\eta n$ system at $W=1.68\text{GeV}$. A comparison with recent partial wave analyses favors the interpretation of this structure as arising from interference of the $S_{11}\left(1535\right)$ and $S_{11}\left(1650\right)$ resonances within the $S_{11}$-partial wave.

Fig. 1

Coincidence times from the reaction $\gamma n\to {\pi }^{0}n$. Blue histograms: time difference. Cyan dotted lines: cut positions. Within the canvas, the first text line shows the involved detectors (CB: Crystal Barrel, MT: MiniTAPS, Tagger: Tagging system) and the second one indicates the involved particle ($\gamma$: photon, m: meson, N: nucleon, e: Tagger electron)

Fig. 2

Kinematic background suppression quantities from the reaction $\gamma n\to {\pi }^{0}n$. Shown are from top left to bottom right: invariant mass $m_{\gamma \gamma }$, coplanarity $\Delta \varphi$, missing mass $\Delta m$, polar angle difference $\Delta \theta$, total Fermi momentum $p_F$, and x-component of the Fermi momentum $p_F^x$ for incident photon energies of 650–3100 MeV. All cuts except for the one on the shown quantity are applied. See text for more details. Histograms: deuterated butanol data (blue), carbon data scaled to the photon flux and target densities (green), deuterium data, i.e., difference between deuterated butanol and carbon data (red). Dotted cyan lines: static cut positions for Fermi momenta at $p_F=160\text{MeV}$ and $|{\overrightarrow{p}}_F^x|\approx 92.4\text{MeV}$. See Table 3 for typical cut values

Fig. 3

Background contamination determination in the invariant mass $m_{\gamma \gamma }$ spectra for all investigated reactions. Shown are the total angle- and energy-integrated data. Blue histograms: deuterium data. Curves: total fit (solid red) given by Eq. 5 + linear background function for $\gamma d\to {\pi }^{0}N(N)$ and $\gamma d\to \eta N(N)$ with $\eta \to 2\gamma$, whereas the $\eta \to 3{\pi }^0$ decay channel uses a quadratic background function, fit signal (dashed orange), linear (quadratic) background function (dashed yellow). Dotted cyan lines: $\pm 2.5\sigma$. Note the logarithmic y-scale

Fig. 4

Target asymmetry T, recoil asymmetry P, and polarization observable H as a function of the polar center-of-mass (c.m.) angle $\theta$ of the ${\pi }^0$ meson for bins at the given centroid c.m. energies W. Left (a): $\gamma p\to {\pi }^{0}p$. Right (b): $\gamma n\to {\pi }^{0}n$. Blue circles: this work. Magenta triangles: CBELSA/TAPS data [52]. Gray shaded areas: systematic uncertainties. Curves: model predictions from BnGa 2022-02 (solid black) [10], BnGa 2022-03 (solid green), SAID MA19 (dashed-dotted orange) [13]. BnGa 2022-03 is identical to BnGa 2022-02 but includes the results presented here in the fits

Fig. 5

Target asymmetry T, recoil asymmetry P, and polarization observable H as a function of the polar center-of-mass (c.m.) angle $\theta$ of the $\eta$ meson for bins at the given centroid c.m. energies W. Left (a): $\gamma p\to \eta p$. Right (b): $\gamma n\to \eta n$. Blue circles: this work. Orange open diamonds: A2 data [56]. Magenta triangles: CBELSA/TAPS data [55]. Gray shaded areas: systematic uncertainties. Curves: PWAs from BnGa 2022-02 (solid black) [10], BnGa 2022-02b (dashed black), BnGa 2022-03 (solid green), BnGa 2022-03b (dashed green curve), EtaMAID 2018 (dashed purple) [16]. The PWAs explain the narrow structure in $\eta n$ around $W=1.68\text{GeV}$ (yellow bins) as interference of $S_{11}(1535)1/2^{-}$ and $S_{11}(1650)1/2^{-}$ resonances within the $S_{11}$-partial wave (BnGa 2022-02/BnGa 2022-03), $P_{11}(1680)1/2^{+}$ resonance (BnGa 2022-02b/BnGa 2022-03b), or $S_{11}(1535)1/2^{-}-P_{11}(1710)1/2^{+}$ interference (EtaMAID 2018). BnGa 2022-03 is identical to BnGa 2022-02 but includes the results presented here in the fits