[Paper Review] Polarization observables in double neutral pion photoproduction

TAPS Collaboration, J. Hartmann+74 more|arXiv (Cornell University)|Jul 5, 2022

Quantum Chromodynamics and Particle Interactions4 citations

TL;DR

This study presents new polarization and double-polarization observables in double neutral pion photoproduction ($\gamma p \to p\pi^0\pi^0$) using data from the CBELSA/TAPS experiment at ELSA. The results, incorporated into the BnGa partial-wave analysis, confirm that resonances with two-oscillator quark configurations exhibit enhanced branching ratios into intermediate states with non-zero orbital angular momentum, supporting quark-model interpretations of nucleon excitation spectra.

ABSTRACT

Measurements of target asymmetries and double-polarization observables for the reaction $γp o pπ^0π^0$ are reported. The data were taken with the CBELSA/TAPS experiment at the ELSA facility (Bonn University) using the Bonn frozen-spin butanol (C$_4$H$_9$OH) target, which provided transversely polarized protons. Linearly polarized photons were produced via bremsstrahlung off a diamond crystal. The data cover the photon energy range from $E_γ$=650 MeV to $E_γ$=2600 MeV and nearly the complete angular range. The results have been included in the BnGa partial wave analysis. Experimental results and the fit agree very well. Observed systematic differences in the branching ratios for decays of $N^*$ and $Δ^*$ resonances are attributed to the internal structure of these excited nucleon states. Resonances which can be assigned to SU(6)$ imes$O(3) two-oscillator configurations show larger branching ratios to intermediate states with non-zero intrinsic orbital angular momenta than resonances assigned to one-oscillator configurations.

Motivation & Objective

• To measure target asymmetries and double-polarization observables in double neutral pion photoproduction to probe the internal structure of nucleon resonances.
• To test quark-model predictions regarding the role of oscillator configurations in resonance decays.
• To investigate whether resonances with two-oscillator excitations preferentially decay into intermediate states with non-zero orbital angular momentum.
• To resolve discrepancies in branching ratios by including data from reactions with two charged pions to disentangle interference effects between same-spin-parity, different-isospin resonances.

Proposed method

• Data were collected using the CBELSA/TAPS detector at the ELSA facility with a frozen-spin butanol target providing transversely polarized protons.
• Linearly polarized photons were generated via bremsstrahlung off a diamond crystal, enabling precise measurement of polarization observables.
• The experiment covered photon energies from 650 MeV to 2600 MeV and nearly full angular coverage, ensuring high-statistics and kinematically complete data.
• The results were incorporated into the BnGa partial-wave analysis (PWA), which includes a comprehensive database of pion- and photo-induced reactions off protons and neutrons.
• Resonance contributions were extracted by fitting the data, with special attention to interference between same-spin-parity, different-isospin resonances.
• Branching ratios were calculated for resonances assigned to one-oscillator versus two-oscillator configurations, comparing decay into ground states versus orbitally excited intermediate states.

Experimental results

Research questions

• RQ1 Do resonances with two-oscillator quark configurations exhibit enhanced branching ratios into intermediate states with non-zero orbital angular momentum?
• RQ2 How do the branching ratios of $N^*$ and $\Delta^*$ resonances with similar masses and quantum numbers differ when classified by their oscillator configuration?
• RQ3 To what extent do interference effects between same-spin-parity, different-isospin resonances distort previous branching ratio estimates in $\gamma p \to p\pi^0\pi^0$?
• RQ4 Can the inclusion of two-charged-pion data improve the separation of overlapping resonance contributions in partial-wave analyses?

Key findings

• Resonances assigned to SU(6) × O(3) two-oscillator configurations show significantly larger branching ratios into intermediate states with non-zero intrinsic orbital angular momentum than those with one-oscillator configurations.
• The $\Delta^*$ resonances with two-oscillator excitations have an average branching ratio of (44 ± 7)% to $N^*\pi^0/\Delta^*\pi^0$ final states, compared to only (5 ± 2)% to orbitally excited states.
• The $N^*$ resonances with mixed-oscillator excitations exhibit a branching ratio of (34 ± 6)% to $N^*\pi^0/\Delta^*\pi^0$ and (21 ± 5)% to orbitally excited states, indicating a preference for sequential decay into excited intermediate states.
• The new branching ratios are mostly compatible with prior results from Sokhoyan et al. (2015), but in some cases are significantly smaller due to improved separation of interfering resonances with same spin-parity but different isospin.
• The observed differences in decay patterns are attributed to the internal quark-diquark structure and oscillator configuration, supporting the idea that such configurations influence decay dynamics.
• The consistency of the data with the BnGa PWA confirms the reliability of the partial-wave analysis framework and highlights the importance of including multi-pion channels for resonance spectroscopy.