(Ilieva, Berman)

Short-range phenomena (Virtual Isobars, Short-Range Correlations, and Meson-Exchange Currents) in medium-energy nuclear physics have been extensively studied over the years. However, the above processes, as well as final-state interactions, all contribute to the measured observables, and the interpretation of the experimental results must therefore rely heavily on theoretical calculations and thus is strongly model-dependent. Only extensive experimental data for a broad range of processes sensitive to the high-momentum components of the nuclear wave function, combined with improved theoretical calculations, can lead to a better understanding of short-range nuclear dynamics. In this respect, the study of virtual nucleon excitations (and specifically isobar configurations) in the nuclear ground state is an important part of the more general effort.

Over the years, many theoretical calculations predicting the probability of finding one or more nucleons in an excited state have been performed. In some of the studies [Gre76, Ana79], the isobar excitations are explicitly included in the few-body problem as nuclear constituents supplementing the few-nucleon wave function with configurations involving one or several nucleon resonances.

Another way of approaching this problem is to use effective operators and keep only nucleons in the wave function. In this framework, the resonance contributions are exclusively taken into account by use of meson-exchange NN potentials [Web78, Pic91]. Several authors [Fri02, Ana79, Cen89, and Haj83] have found sensitivities of the predicted probabilities on the model used for the baryon-baryon interaction, mass cutoffs, coupling constants, and single-particle wave functions used in the calculations. Therefore, the theoretical predictions for P_NND in a three-nucleon ground state like ³He vary, and the values range from 2 to 8%.

Although the deuteron is the simplest bound-nucleon system, we study the ³He ground state since it can contain the energetically lowest configuration, NND. Our experimental method is to measure the D⁺⁺(1232) knocked out from the nucleus by a real photon: ³He(g,D⁺⁺)nn. The D⁺⁺ is the best choice among the D states since it couples strongly to the photon – the cross section for D⁺⁺ compared with D⁺ knockout is six times larger [Lip87] – and the ratio of the probabilities to find charged-D states in ³He is P(D⁺⁺nn):P(D⁺np) = 1/2:1/3. Most important, the D⁺⁺ cannot be produced by the photon on a single nucleon in a conventional one-step process, so that the main D⁺⁺ background contribution to our signal can come only from a two-step process in which a p⁺ is produced (nonresonantly or through D⁺ excitation) on a proton via charge exchange and rescatters on the second proton, forming a D⁺⁺.

Since the D isobar has a very short lifetime, we measure it by detecting its decay products p and p⁺, and reconstruct their missing mass and invariant mass. In order to select the D⁺⁺ signal, we restrict the invariant mass of a good event to be between 1.00 and 1.35 GeV/c². In order to separate the D⁺⁺ knockout events from those formed in a two-step process, we map out the event distribution for each bin (q^D_L⁺_S⁺ , p ^D_L⁺_S⁺ , E_g) looking for regions in the phase space where the two-step process is suppressed compared with the knockout. Such detailed mapping is possible since the experiment covers large energy and angular ranges. Theoretical calculations, together with a comparison to the g³He ® pppp^- phase space, will be used to extract the knockout signal. This channel does not contain knockout events, since there is no preformed D^o in ³He and the final state is dominated by D^o production (cutting out spectator protons in order to eliminate one-body events). In order to obtain clean spectra, we perform a background subtraction of the multipion events using the missing-mass spectrum. Background-subtracted momentum distributions for E_g between 0.65 and 0.70 GeV and several angular bins are shown in Fig. 18.

Figure 18. Preliminary cross sections ds/(dW^D++ dp^D++ dIM) for the reaction g³He ®   D⁺⁺nn for one 50-MeV wide photon-energy bin and four D⁺⁺ scattering-angle bins.

By using this analysis method, we probe virtual D s excited on a pp pair in ³He. Theoretical calculations will also be used to understand how to separate the contribution of knocked-out D s from MEC. We will be able to provide a clear upper limit for the probability of finding virtually excited D s in ³He. This analysis is in progress.

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Copyright © 2004 GW Experimental Nuclear Physics Research Group
Last modified: 03/31/05