Light vector mesons in the nuclear medium

Abstract

The light vector mesons (ρ, ω, and φ) were produced in deuterium, carbon, titanium, and iron targets in a search for possible in-medium modifications to the properties of the ρ meson at normal nuclear densities and zero temperature. The vector mesons were detected with the CEBAF Large Acceptance Spectrometer (CLAS) via their decays to e + e − . The rare leptonic decay was chosen to reduce final-state interactions. A combinatorial background was subtracted from the invariant mass spectra using a well-established event-mixing technique. The ρ-meson mass spectrum was extracted after the ω and φ signals were removed in a nearly model-independent way. Comparisons were made between the ρ mass spectra from the heavy targets (A > 2) with the mass spectrum extracted from the deuterium target. With respect to the ρ-meson mass, we obtain a small shift compatible with zero. Also, we measure widths consistent with standard nuclear many-body effects such as collisional broadening and Fermi motion.

K̄*mesons in dense matter

“…This spectacular increase is much larger than the width of the ρ meson in matter, evaluated theoretically in Refs. [6][7][8][9] or measured recently [12,14,17,18].…”

“…Similarly, a null shift in the γ -induced dilepton production at CLAS was also found in Refs. [17] and [18]. In contrast, the KEK team had earlier reported an attractive mass shift of the ρ in [19] and [20].…”

“…As explained in detail in Ref. [17], the different conclusions can be traced back to the way the background is subtracted.…”

Medium modifications of theρmeson in nuclear photoproduction

“…2 we compare our earlier calculations for the 56 Fe target with our improved calculation and with the CLAS data at JLab [15,16]. It also includes the comparison for the 12 C target.…”

“…Using a 12-GeV proton beam at KEK, the E325 Collaboration reported a significant reduction in the ρ mass [14]. However, with a 1-3.5-GeV incident-energy photon beam at JLab, the CLAS Collaboration [15,16], using an absolutely normalized background subtraction procedure, found not a significant mass shift but rather a moderate broadening of the dilepton excess spectrum associated with ρ decays (cf. also Ref.…”