Medium Modi cations of Vector Mesons in the Subthreshold Region

We propose to investigate the subthreshold photoproduction of the three lowest mass vector mesons, V = , !, and . In the subthreshold energy region, de ned here as the energy region below the +N ! V +N reaction threshold on the free nucleon, VMD-driven vector meson production is suppressed and the in uence of hadronic matter on the masses of the vector mesons is emphasized. This method is the optimum vehicle for the study of in-medium modi cations and, speci cally for the 0 meson, experimental evidence already exists for a dramatic reduction in m o, which is dependent on the incident photon energy or the 0 N relative momentum. Di erent regions of phase space may be more sensitive to di erent reaction mechanisms and this assertion will be tested for the 0 ! + channel, and the study will be extended also to the unexplored domains of the !! + 0 and ! + 0 vector mesons. In addition, the hypothesis of vector mesons forming bound states in nuclei will be explored. An e ective nuclear target for this investigation is He. The minimum trigger requirement is the detection of two charged particles (with an open neutral trigger). We request 420 hours of nominal tagger photon beam ( 10 =s ) in Hall B, in the tagged photon region between 320 MeV and 1520 MeV, of which 150 hours can be collected in the already approved g3 period. Nature of the Update in this Proposal This proposal is an update (modi cation) to E99-001, which was deferred by PAC15. The changes, which address the issues raised by PAC15, have been incorporated into Sections 4 and 5. These two sections are critical in understanding the proposed analysis with CLAS, leading to the nal extraction of medium induced mass and width modi cations of the vector mesons. Other changes to the proposal consist mostly of trimming and rearranging. The conclusions, physics objectives, beam time request, and methodology remain the same as those in the original E99001, thus, in every aspect, this is an update not a new proposal. This is a Hall B Collaboration experiment. G.J. Lolos et al.: Medium Modi cations of Vector Mesons 1 1 Scienti c Motivation Recently, much attention has been directed to the coupling of vector mesons to nucleons, especially under conditions of high nuclear matter density where it is expected that this coupling will be modi ed in comparison to the situation on an unbound nucleon. Clearly, the 0 N coupling in nuclei, for example, must be known accurately as it is an important ingredient in the calculation of the N N interaction. Examples of this interest in vector mesons range from e A and A physics at Je erson Lab, to relativistic heavy ion collision experiments at CERN, GSI and in the future at RHIC (with an implication on the transition from normal hadronic matter to the conjectured but elusive quark-gluon plasma [1]). The latter has cosmological signi cance, since the universe was in this decon ned phase a few microseconds after the Big Bang. Other elds of interest include the determination of the equation of state for nuclear matter in connection to the underlying mechanism of supernovae explosions [2] and the properties of remnant neutron stars [3]. All the experiments above involve e ects in systems which transit from nucleonic to quark degrees of freedom. A transition of this nature occurs already in normal nuclear matter. Our aim is to photoproduce the light vector mesons in the interior of the nucleus, below the free production threshold, by exploiting the Fermi momentum of nucleons. We will demonstrate that this is the most direct method in the study of in-medium vector meson coupling. 1.1 Theoretical Overview The interior of a nucleus belongs to the long-range and non-perturbative transition region of QCD, which is largely unexplored. Here, QCD-driven interactions among nuclei and mesons a ect fundamental particle properties such as masses, lifetimes, and coupling constants. At high hadronic matter temperatures, T 150 MeV, lattice QCD calculations [4] support the expectation that the scalar quark (chiral) condensate < qq > vanishes, an e ect which signi es the transition of nuclear matter from a spontaneously broken symmetry phase to a chirally restored one. However, the condensate itself is not an experimental observable, and therefore other physical quantities associated with it need to be measured to test hadronic medium e ects. The masses of the light vector mesons are excellent candidates for this task. The theoretical basis for mass modi cation of hadrons in the nuclear medium is overwhelming. Various theoretical methods have been employed in this e ort: QCD sum rules (QSR), chiral pertubation theory ( PT ), quark-meson coupling (QMC) model, e ective Lagrangian theories, Nambu-Jona-Lasinio models, etc. Several review articles have appeared in print on this topic recently [5, 6, 7], and the reader is referred to the signi cant number of publications therein. Even though either high temperature T or high nuclear matter density nuc will result in vector meson G.J. Lolos et al.: Medium Modi cations of Vector Mesons 2 mass modi cation, there are signi cant di erences between these two conditions. For the mass modi cation to take place, the hadronic medium temperature must approach a critical temperature Tc 150 MeV [8, 9]. On the other hand, while chiral restoration is expected to take place at nuclear matter densities 5 nuc [7], substantial changes in the < qq > condensate (of 30%) can take place even at normal nuclear matter densities, nuc = 0.16 fm . This transition occurs when the scalar eld energy reaches a value of 250 MeV/fm, which corresponds to an energy density of = 1 GeV/fm [8]. Such a density can be reached in heavy ion collisions or inside a nucleon. Clearly, experiments probing such density e ects stand to yield valuable information. The e ect of mass modi cations on hadrons has been postulated [10]. This is the original scaling argument by Brown and Rho, who predict that the masses of other light vector mesons scale approximately like those of hadrons: m N =mN m =m m V =mV f =f (1) where the denotes the medium-modi ed quantities. Numerical predictions for the m V =mV ratio have been produced by a large spectrum of other models. It should be noted that most authors treat the 0 and the ! in the same manner, whereas they quote a di erent modi cation for the meson. The latter is of particular interest since it is modulated by the strangeness content of the nucleon (OZI-breaking parameter), y = 0:1 0:2, in QCD [11, 12]. The modi ed masses obey the following relations: m ;! =m ;! = 1 (A A)( = nuc) (2) m =m = 1 (B B)y( = nuc) (3)