Resonance Production Through The Two Neutral-pion Channel

Double-pion photoproduction cross sections (for the γp → pπ+π− , nπ+π0, and pπ0π0 reaction channels) can be used to extend the study of nucleon resonances beyond the previous work done with pion-nucleon scattering and with single-pion photoproduction. The γ p → Nππ channels not only can be used to extract information on the electromagnetic excitation and decay mechanisms of resonances, but should be particularly useful for disentangling the broad and overlapping resonances at higher excitation energies and for exciting those resonances that do not couple strongly to the pion-nucleon channel. Two-π0 production, in particular, is not contaminated with background events from the Born terms. Moreover, intermediate ρ 0(770) processes are forbidden since the ρ0 cannot decay into two neutral pions. We have obtained data for the γp → pπ0π0 reaction with the CLAS at Jefferson Lab for photon energies 0.4 < Eγ < 1.7 GeV. These data extend previous total-cross-section measurements of the γp → pπ 0π0 channel beyond Eγ = 800 MeV, and show a strong enhancement of resonance production at Eγ ≃ 1.1 GeV (Wcms ≃ 1.7 GeV). The proton and forward-going π0 were detected in the CLAS, while the backward π0 has been reconstructed from missing mass. An intermediate Δ state can be reconstructed from the proton and the backward π0, which provides information on the sequential decay of the γp → N* → Δ +π0 → pπ0π 0 process. The Δ+π0 intermediate state is a significant part of the cross section at center-of-mass energies of W ≃ 1700 MeV and W ≃ 1930 MeV. The enhancement at W ≃ 1.7 GeV is broad, and coincides with the structure seen in the total cross-section measurement. This is most likely the signature of a nucleon resonance having a mass of 1650–1750 MeV, with strong Δπ and γp couplings. The enhancement at W ≃ 1.9 GeV, however, is narrow and therefore is unlikely to arise from resonance production or the interference of a resonance with non-resonant background. The proximity of the enhancement to the pη′ threshold (W = 1896 MeV), as well as evidence from other analyses for the presence of a resonance at this mass, suggests a cusp phenomenon. There is also evidence for the γp → (N* → D13(1520)π 0) → (pπ0)π0 two-step process, that is enhanced at W ≃ 1.83 GeV. No resonances have been previously observed at this energy. Finally, for events that did not proceed through the two decay chains mentioned above, there is an enhancement at W ≃ 1.7 GeV . This could possibly be the signature of a resonance decaying via the γp → pσ → p(π0π0) process. If this enhancement arises from the same resonance in the Δπ channel, then its branching fraction would be roughly 1/2 that of the Δπ channel