(Benmouna, Berman)

A current topic of great interest is the modification of the mass of the r^o meson in the nuclear medium predicted by theories of chiral-symmetry restoration [Rap99]. The measurements of the invariant mass of the r^o to date [Dre96, Ste97], which report evidence for a r^o mass renormalization, have so far been indirect and inconclusive. Photoproduction of the r^o on a simple nuclear system such as ³He or ⁴He, in which the interpretation of the results is less complex, allows for a more direct measurement.

The r^o meson can be easily identified experimentally since it has a short lifetime and decays into two charged pions. We have therefore been analyzing the p⁺p^--photoproduction channel on ³He and ⁴He from the g3a data, in order to isolate the events which arise from the decay of the r^o. The r^o production threshold is 900 MeV for ³He and 850 MeV for ⁴He. In ³He we do not expect to see a modification of the mass of the r^o since it will usually decay outside of the nucleus. In the subthreshold region, however, the production of r^o mesons is possible due to the Fermi motion of the nucleons inside the nucleus. The conditions to observe a modification of the mass of the r^o meson are then ideal, since these mesons are produced with small kinetic energy and will therefore decay inside the nucleus. There has been an investigation of the r^o mass modification in the subthreshold region for ³He [Hub98], which showed evidence for a mass shift of 290± 40 MeV/c², which is surprisingly large. Clearly, this claim needs to be checked.

The reaction of interest for this analysis is: g+³He ® p⁺+p^-+X. We require the detection of a p⁺ and a p^- and no other charged particle. The p⁺p^- missing-mass spectra are shown in Fig. 12, both above and below the r^o threshold. We see a distinct peak at the three-nucleon mass, and we select the events that fall inside the cuts on the missing mass shown.

Figure 12. p⁺p^- missing-mass spectra above threshold (top), and below threshold (bottom). The blue vertical lines denote the mass cut used in our analysis.

We then select events for which the detected pions have a large opening angle because, the pions which arise from r^o decay at rest or with small kinetic energy have a much larger opening angle than uncorrelated pions. We then plot the p⁺p^- invariant mass both above and below the threshold production energy in Fig. 13. Above threshold, we obtain a distribution which is peaked at 760 MeV, 10 MeV below the free r^o mass. This small shift is under investigation, and we will be able to determine its origin. Below threshold, the invariant-mass spectrum from the data is much broader, which we attribute to the presence of other processes that produce two charged pions in the final state. Our simulations below threshold will be extended to include the dominant processes below threshold to provide us with a good comparison with the data.

Figure 13. p⁺p^- invariant-mass spectra above threshold (top) and below threshold (bottom). The solid line is placed at the value for the free r ⁰ mass; the dashed line indicates the centroid of the Gaussian fit to the data.

In order to further distinguish the r^o mesons, we look at the angular distribution of the decay pions with respect to the direction of the r^o in its rest frame. The two decay pions are in a relative l = 1 state, which translates into an angular distribution of the form cos²(q_c^p_m,), where q_c^p_m is the pion-production angle described above. This angular distribution is plotted for the detected p⁺ both above and below threshold in Fig. 14. Above threshold, we see a clear p-wave distribution (except for very forward and backward angles, which are strongly influenced by the CLAS acceptance), which confirms that the event sample is dominated by r^o mesons. Below

threshold, the r^o mesons are far from being dominant, but our simulations will allow us to fit this angular distribution and obtain an accurate determination of the fraction of the events which come from the decay of r^o mesons.

Figure 14. p⁺ production angle in the r ^o rest frame above threshold (top) and below threshold (bottom). The solid curve is a fit to the data for –0.70 < cos(q_c^p_m,) < 0.55, and clearly shows the characteristic p-wave decay of the r^o.

The analysis will soon be extended to ⁴He, which will provide more data to investigate the possible medium modification of the r^o meson at higher nuclear density. This work was presented in a paper at the Spring 2003 APS meeting in Philadelphia [Ben03]. It also has been presented in a poster at the 17th International Conference on Few-Body Problems in Physics.

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Last modified: 03/31/05