Bulletin of the American Physical Society
2008 Annual Meeting of the Division of Nuclear Physics
Volume 53, Number 12
Thursday–Sunday, October 23–26, 2008; Oakland, California
Session BA: Signatures for Chiral Symmetry Restoration in Nuclei |
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Chair: Volker Koch, Lawrence Berkeley National Laboratory Room: Simmons Ballroom 2-3 |
Friday, October 24, 2008 8:30AM - 9:06AM |
BA.00001: Hadrons in Medium Invited Speaker: That hadrons can change their properties, i.e.\ spectral functions and interactions, when they are embedded in the nuclear environment should come as no surprise since they interact with their surroundings. The interest in this phenomenon has been reawakened about 2 decades ago when possible connections of these in-medium changes with properties of QCD were pointed out. In particular, interest focussed on chiral symmetry restoration, expected to take place in the nuclear medium. In this talk I will assess the theoretical basis for these expectations and then discuss our present calculations of in-medium properties of vector mesons. I will then confront these expectations with experiments performed on cold matter where the interpretation of observables should be cleanest. [Preview Abstract] |
Friday, October 24, 2008 9:06AM - 9:42AM |
BA.00002: Search for Medium Modifications of the Light Vector Mesons at Jefferson Lab Invited Speaker: The E01-112 experiment at the Thomas Jefferson National Laboratory was an investigation of the properties of light vector mesons in dense nuclear matter, such as a shift in their masses and/or broadening of their widths. Theoretical calculations relate the modifications to partial restoration of chiral symmetry at high density or temperature. In the experiment, the $\rho$, $\omega$, and $\phi$ mesons were photo-produced of off $^{2}$H, C, Ti, Fe, and Pb targets and reconstructed with the CEBAF Large Acceptance Spectrometer (CLAS). The incident beam was tagged photons with energies up to 4~GeV. The mesons were detected via their rare leptonic decay to $e^{+}e^{-}$. This decay channel is preferred over hadronic modes in order to eliminate final state interactions in the nuclear matter. The $\rho$ meson mass spectrum was extracted after the subtraction of a combinatorial background and after the removal of the $\omega$ and $\phi$ signals in a nearly model-independent way. The $\rho$ mass spectra from the heavy targets (A $>$ 2) were compared with the mass spectrum extracted from the deuterium target. We obtain a mass-shift compatible with zero for the $\rho$ meson. For the $\rho$-mesons widths, our result is consistent with standard nuclear many-body effects, i.e. collisional broadening and Fermi motion. Even though the $\omega$ and $\phi$ mesons have a high probability of decaying outside the nucleus in their vacuum state, their in-medium widths can be accessed through their absorption inside the nucleus. The signature of absorption is a decrease of the nuclear transparencies of these mesons as a function of the number of target nucleons. Preliminary results indicate a substantial widening of the $\omega$ and $\phi$ mesons in the medium. [Preview Abstract] |
Friday, October 24, 2008 9:42AM - 10:18AM |
BA.00003: Photoproduction of mesons off nuclei and in-medium modifications of hadrons Invited Speaker: During the last few years, the TAPS, Crystal Barrel, and Crystal Ball collaborations have investigated in-medium effects on hadrons at the MAMI accelerator in Mainz and the ELSA accelerator in Bonn in photon induced meson production reactions. There are many predictions that vector mesons change mass and width in dense and hot nuclear matter, due to partial chiral symmetry restoration. The predicted size of the effects is related to nuclear density and temperature, so that many efforts have been directed towards heavy ion collisions. However, the baryon density varies dramatically with time due to the formation and expansion of the `fireball', which complicates the interpretation. Furthermore, FSI effects are large, so that only meson decays into leptons (Dalitz-decays of $\rho$ and $\omega$ mesons) could be used. In an alternative approach, photo-production of $\omega$ mesons from stable nuclei has been investigated at ELSA with the Crystal Barrel/TAPS setup. The $\omega$ mesons were identified via their $\pi^o\gamma$ decay. The advantages of this experiment are the much larger decay branching ratio (8.5\% for $\omega\rightarrow\gamma\pi^o$ compared to $7\times 10^{-7}$ for $\omega\rightarrow e^+e^-$), the almost complete suppression of background from the $\rho$ meson ($\rho\rightarrow\gamma\pi^o$ decay branching ratio: $8\times 10^{-4}$) and the better control over experimental parameters like nuclear density. The experiment has for the first time directly established a downward shift of the $\omega$-mass in nuclear matter via a comparison of the line shape of the $\omega$ invariant mass peak observed in photo-production off the free nucleon to the nuclear data. A detailed analysis of the scaling of the observed cross sections with nuclear mass number in the framework of different models has found an inelastic in-medium width of the $\omega$ meson in the range 130 - 150 MeV/$c^2$ at normal nuclear matter density for an average three-momentum of 1.1 GeV/$c$. Furthermore, a momentum dependent $\omega$N cross section in the range of 70 mb has been extracted. In the sector of scalar mesons, in a series of experiments, double pion photo-production off heavy nuclei has been studied in view of possible in-medium effects on the much discussed $\sigma$-meson. Results from a measurement of double $\pi^0$ and $\pi^0\pi^{\pm}$ photo-production off carbon and lead have shown for the heavier nucleus a shift of the strength to lower invariant masses for the $\pi^0\pi^0$ channel; but not for the mixed charge channel. This is a possible argument, that the effect does not arise from FSI, which is assumed to be similar for neutral and charged pions. However, more detailed comparisons to model calculations have shown, that at least part of the effect can be explained by FSI. [Preview Abstract] |
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