Attenuation of ϕ mesons in γA reactions

Abstract

We present a theoretical analysis of inclusive photoproduction of φ mesons in nuclei. In particular the dependence of the total φ meson yield on the target mass number is investigated. The calculations are done using the semi-classical BUU transport approach that combines the initial state interaction of the incoming photon with the coupled-channel dynamics of the final state particles. The conditions of the calculations are chosen such as to match the set-up of a recent experiment performed at SPring8/Osaka. Whereas the observables prove to be rather sensitive to the φ self energy in the medium, the attribution of deviations from the standard scenario to a particular in-medium effect seems to be impossible.  2005 Elsevier B.V. All rights reserved. PACS: 25.20.Lj; 13.25.-k; 14.40.Cs Keywords: Photonuclear reactions; φ decay in nuclei; Medium properties of mesonsThe question of how the properties of the light vector mesons change once they are put into a strongly interacting environment provides a field of very active research. In particular the theoretical and experimental determination of the vector and isoscalar spectral densities in systems with either finite temperature or baryon density has attracted much attention [1]. In particular the φ meson provides a well suited probe as it is a very sharp resonance in vacuum. One could thus expect to observe even smallest deviations of its spectral distribution in the nuclear medium from its properties in vacuum. Numerous authors have studied the medium modifications of the φ properties in different approaches as effective Lagrangians and QCD sum rules [2-6]. As a general picture from these studies emerges a quite small shift of the φ meson pole mass, but a sizeable renormalization of its width as temperature and/or density increase. Particular numbers

φ-meson production and the in-medium φ-width in proton-nucleus collisions

“…A momentum-dependent increase of the φ in-medium broadening, as suggested by Wood et al [11], and shown for the collisional part of the broadening by Mühlich et al [9], might partly explain the different results, but this has certainly not been unambiguously proven so far. The high statistics of 7000-10000 φ per target provided by the existing ANKE data allows us to search for such a possible variation in the momentum range of 0.6 GeV/c < p φ < 1.6 GeV/c, and an investigation of this is in progress.…”

“…From this they deduced an increase in the width by a factor 3.6 at normal nuclear density [7]. A much larger in-medium broadening was reported by SPring-8 [8] (compare also [9,10]) and the very recent JLab [11] measurements. Both these experiments studied the variation of the φ production cross section with the atomic number A.…”

“…The production depends on attenuation of the φ flux in a nuclear target which, in turn, is governed by the imaginary part of the φ in-medium self-energy or width. In the low-density approximation [12], this width can be related to an effective φ N total cross section, though this is less obvious at higher densities where two-nucleon mechanisms are important (compare [9,13,14,15,16,17]). The big advantage of this method is that one can exploit the large K + K − branching ratio (≈ 50%) in order to identify the φ meson.…”

Hadrons in Nuclear Matter

“…With this method we have first analysed both results on the experimental determination of the nuclear transparency ratio for φ mesons [7], measured by a Japanese group at SPRING8 that lead to an unexpectedly large in-elastic cross section for φN interactions. We have found that indeed cross sections about a factor 3 larger than those theoretically expected are needed to explain the mentioned data, in line with a simple Glauber analysis by the SPRING8 group.…”

In-Medium Properties of the ϕ Meson with ϕ N Resonant Contributions

“…The ϕ production in nuclei has been intensively studied both experimentally [1][2][3][4], as well as in theoretical analyses [4][5][6][7][8]. The ϕ nuclear transparency ratio, measured in proton-induced and photoproduction experiments, points to a large absorption in nuclei, consistent with widths about one order of magnitude larger than the one from its dominant decay toKK in vacuum.…”