(Philips, Heimberg, Berman)

Double-pion photoproduction cross sections (for the gp ® pp⁺p^-, np⁺p⁰, and pp⁰p⁰ reaction channels) extend the study of nucleon resonances beyond the previous work done with pion-nucleon scattering and with single-pion production. The gp ® Npp channels not only can be used to extract information on the electromagnetic excitation and decay mechanisms of resonances at higher excitation energies, but will also be essential for disentangling the broad, overlapping resonances that do not couple strongly to the p-N channel. Two-p⁰ production, in particular, is not contaminated with background events from the Born terms. Moreover, intermediate r^o(770) processes are forbidden since the r^o cannot decay into two neutral pions.

We have obtained data for the reaction gp ® pp⁰p⁰ using CLAS at JLab for photon energies 400 < E_g < 1700 MeV. The g1b data were analyzed using two different analysis methods, which provided different ways of extracting the physics information from the data. In the first method, a proton and a single coincident p^o decay photon are detected. The two-p⁰ channel is identified by looking at the missing-mass spectrum of the proton and applying cuts to eliminate background from other two-body channels. For background channels that could not be eliminated on the basis of this method of particle identification (e.g., gp ® pp⁰p⁰p⁰), a second analysis method was used to quantify the background. Our second analysis technique requires a coincidence of a proton and 2gs from one of the decay p⁰s. The second p⁰ is identified by plotting the missing-mass spectrum gp ® pp⁰X. The kinematics of all three final-state particles is obtained and various differential cross sections can be extracted. This method allows an estimation of background from 3p channels.

The total cross section as a function of photon energy obtained using the first method is shown in the left panel of Fig. 3, compared with data from previous measurements [Br95][Au01][Wo00]. This analysis extends the total-cross-section measurements beyond E_g = 800 MeV. A prominent peak is seen at E_g = 1.1 GeV, corresponding to a center-of-mass energy W @ 1.7 GeV, and another is seen at E_g = 1.4 GeV (W @ 1.9 GeV). The cross section up to 800 MeV agrees very well with the earlier work done at Mainz; the peak at 1.1 GeV has been recently reported as well by the GRAAL group [Ass03]; the peak at 1.4 GeV is beyond the range of the GRAAL facility.

Figure 3. Left: the gp ® pp⁰p⁰ total cross section as a function of photon energy using CLAS compared with data from previous measurements using TAPS and DAPHNE [Bra95][Aud01][Wo00]. Right: the normalized yield as a function of W from our second analysis technique. Events that proceed through the D⁺p⁰ intermediate state show enhancements at W = 1.7 GeV and at W = 1.92 GeV. The W-distribution of the remaining events is shown below for comparison.

The structure at W = 1.7 GeV seen using the first method is also seen for the 2p⁰ events that were identified with the second method. To see if this is indicative of a resonance that subsequently decays via a two-step process, the pp⁰ invariant-mass systems were reconstructed and the presence of an intermediate D⁺ state was clearly evident in the pp⁰ spectrum. Events that proceed through the D⁺ intermediate process were then selected and the resulting W-distribution for these events is shown in Fig. 3 (right panel). (The W-distribution of the remaining events is also shown on the same plot for comparison.) It is seen that the structure in the region of W = 1.7 GeV is further enhanced through the D⁺p^o channel, providing evidence for a resonance at this energy that decays ultimately into the pp⁰p⁰ final state. An additional sharp structure is seen at W = 1.92 GeV, which also decays through the D⁺p^o channel.

This work was the dissertation project for S.A. Philips under the supervision of B.L. Berman. These results were presented at an APS Meeting in 2000 [Phi00] and at the International Nuclear Physics Conference in 2001 [Phi01]. An analysis note is currently under review by the CLAS collaboration in preparation for their publication.

This site is best viewed in Internet Explorer 6.0 or higher. Send mail to Web Master with questions or comments about this web site.
Copyright © 2004 GW Experimental Nuclear Physics Research Group
Last modified: 03/31/05