Vector Meson Production from Nuclei
Prepared by M. H. Wood, C. Djalali, R. Nasseripour, and D. Weygand, JLab Hall B collaboration, for the DNP webpage
Quantum chromodynamics (QCD), the theory of the strong interaction, has been very successful in describing high-energy and short-distance-scale experiments, and less successful in explaining low-energy and large-distance scales. However, the symmetries of QCD (such as chiral symmetry) provide guiding principles in treating the non-perturbative regime.
Hadrons are composite particles, but unlike molecules and atoms, most of their mass is generated dynamically. These masses are impacted by the spontaneous breaking of chiral symmetry. For example, the proton has a mass of approximately 1 GeV, that is much larger than the summed mass of its constituent quarks, which is a few MeV. In the early 1990's, various models [1, 2] related the hadron properties with chiral symmetry restoration. In particular, the hadron properties can be altered in cold nuclear matter by a partial restoration of chiral symmetry.
The E01-112 experiment at Jefferson Lab (JLab) has recently published results in the search for medium modifications to the ρ meson in a cold nuclear environment . The ρ mesons were produced in nuclei of deuterium, carbon, titanium, and iron. The mass spectra from the carbon and iron/titanium targets were compared with that from deuterium in hopes of seeing changes to the mass and width. One possible explanation of mass shifts and/or width broadening is a partial restoration of chiral symmetry.
The JLab experiment produced the ρ, ω, and φ mesons with an intense photon beam with energies up to 4 GeV. The ρ meson is a wide resonance (Γρ = 150 MeV) and will decay primarily inside the nucleus, making it ideal for observing medium effects. The ω and φ mesons have natural widths of 8 and 4 MeV, respectively, and most of them decay outside the nucleus. The narrow resonances provide an experimental check since all three mesons are present in the mass spectrum. This experiment is unique, in that it utilizes electromagnetic probes in both the production and decay channels. The photon beam interacts with nucleons throughout the entire nuclear volume. The vector mesons are reconstructed through their decays into e+e- pairs, which eliminates any strong final state interactions. The CEBAF Large Acceptance Spectrometer (CLAS)  in Hall B at JLab has been designed for multi-particle states and is the ideal for electron and positron identification with a 10-7 rejection of π+π- over lepton pairs. The figure shows a typical e+e- event.
The target assembly has been constructed such that each material is simultaneously in the beam, as shown below. In this view, the beam moves from the upper left to the lower right of the figure.
The resulting mass spectra for the deuterium, carbon, and iron/titanium data exhibit wide ρ and narrow ω- and φ-meson peaks. The data from the iron and titanium targets have been combined since effective densities are very close. The data from carbon are shown below as an example.
These spectra have been fit with the realistic functional forms based on the Giessen Boltzmann-Uehling-Uhlenbeck (GiBUU) simulation [5, 6]. This model is a transport calculation that treats many-body effects such as shadowing, Fermi motion, Pauli blocking, Coulomb interaction, and collisional broadening. The simulations also incorporate the CLAS acceptance and have been scaled to fit the mass spectra. The ω- and φ-meson contributions can be removed as well as a combinatorial background. What remains are background-free ρ-meson mass distributions. As an example the ρ-meson mass distribution from carbon is shown below.
Fits have been performed on the mass distributions and constrained by fits to the ratio of the carbon and iron/titanium spectra to the deuterium spectrum. The results for the iron/titanium data are consistent with no mass shift and an increase in the width by 67.7 +/- 14.5 MeV, which is consistent with many-body effects. This result is in disagreement with the one from the KEK-PS collaboration [7, 8, 9]. They reported a decrease in the ρ-meson mass by 9% in reactions of 12-GeV protons incident on carbon and copper targets. Other searches with the ρ meson have been performed with relativistic heavy-ion (RHI) collisions. The CERES  collaboration at CERN reported an excess in the e+e- mass spectrum in range between 300 and 700 MeV in Pb-Au collisions. The NA60  collaboration reported a doubling of the ρ-meson width from their di-muon measurement from In-In collisions with no change in the ρ mass. The temperature and density in RHI reactions are not constant but are evolving to an equilibrium state. Thus, it is difficult to relate with elementary reactions such as the JLab experiment.
Is there any way to reconcile the experimental results with the generally accepted prediction that chiral symmetry restoration leads to a shift in the ρ mass? A second experiment in Hall B has been approved to investigate the momentum dependence of the meson properties. It is possible that the predicted mass shift applies to ρ mesons at rest in the nucleus, but not the ρ mesons moving through the nucleus in the conditions of this experiment. This new measurement will provide improved statistics allowing the momentum dependence to be studied, and giving valuable information about the many-body effects in heavy nuclei. These effects need to be under control in order to have clear evidence of chiral symmetry restoration.
We acknowledge the support of Jefferson Lab, the U.S. Department of Energy, and the U. S. National Science Foundation.
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