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Gluex Detector and Beamline Is Mesons, That Exotic Hybrid Mesons Exist Mapping the Spectrum of Light Quark Mesons and Gluonic Excitations with Linearly Polarized Photons Presentation to PAC30 – The GlueX Collaboration (Dated: July 6, 2006) The goal of the GlueX experiment is to provide critical data needed to address one of the out- standing and fundamental challenges in physics – the quantitative understanding of the confinement of quarks and gluons in quantum chromodynamics (QCD). Confinement is a unique property of QCD and understanding confinement requires an understanding of the soft gluonic field responsible for binding quarks in hadrons. Hybrid mesons, and in particular exotic hybrid mesons, provide the ideal laboratory for testing QCD in the confinement regime since these mesons explicitly manifest the gluonic degrees of freedom. Photoproduction is expected to be particularly effective in producing exotic hybrids but there is little data on the photoproduction of light mesons. GlueX will use the coherent bremsstrahlung technique to produce a linearly polarized photon beam. A solenoid-based hermetic detector will be used to collect data on meson production and decays with statistics after the first year of running that will exceed the current photoproduction data in hand by several orders of magnitude. These data will also be used to study the spectrum of conventional mesons, including the poorly understood excited vector mesons. In order to reach the ideal photon energy of 9 GeV for this mapping of the exotic spectrum, 12 GeV electrons are required. This document describes the physics goals, the beam and apparatus, and plans for the first two years of commissioning and data-taking. I. OVERVIEW brid mesons, one supported by lattice QCD [1], is one in which a gluonic flux tube forms between the quark and A. QCD and Light Meson Spectroscopy antiquark and the excitations of this flux tube lead to so-called hybrid mesons. Actually the idea of flux tubes, The observation, nearly four decades ago, that mesons or strings connecting the quarks, originated in the early are grouped in nonets, each characterized by unique val- 1970’s [2] to explain the observed linear dependence of ues of J PC – spin (J), parity (P ) and charge conjugation the mass-squared of hadrons on spin (Regge trajectories). (C) quantum numbers – led to the development of the Conventional qq¯ mesons arise when the flux tube is in its quark model. Within this picture, mesons are bound ground state. Hybrid mesons arise when the flux tube is excited and some hybrid mesons can have a unique sig- states of a quark (q) and antiquark (¯q). The three light- PC quark flavors (up, down and strange) suffice to explain nature, exotic J , and the spectroscopy of these exotic the spectroscopy of most – but not all – of the lighter- hybrid mesons is simplified because they do not mix with mass mesons (below 3 GeV/c2) that do not explicitly conventional qq¯ states. carry heavy flavors (charm or beauty). Early observa- The level splitting between the ground state flux tube tions yielded only those J PC quantum numbers consis- and the first excited transverse modes is π/r, where r is tent with a fermion-antifermion bound state. The J PC the separation between the quarks, so the hybrid spec- quantum numbers of a qq¯ system with total quark spin, trum should lie about 1 GeV/c2 above the ground state S~, and relative angular momentum, L~ , are determined spectrum. While the flux-tube model [3] has all hybrid as follows: J~ = L~ + S~, P = ( 1)L+1 and C = ( 1)L+S. nonets degenerate in mass, from lattice gauge calcula- PC + PC − −− +− −−+ tions [4], one expects the lightest J =1− exotic hy- Thus J quantum numbers such as 0 ,0 ,1 and 2 2+− are not allowed and are called exotic in this context. brid to have a mass of about 1.9 GeV/c . In this discus- Our understanding of how quarks form mesons has sion the motion of the quarks was ignored, but we know evolved within quantum chromodynamics (QCD) and we from general principles [3] that an approximation that now expect a richer spectrum of mesons that takes into ignores the impact of the flux tube excitation and quark account not only the quark degrees of freedom but also motion on each other seems to work quite well. It should the gluonic degrees of freedom. Gluonic mesons with no be noted, also, the in the large-Nc limit of qcd, exotic quarks (glueballs) are expected. These are bound states hybrids are expected to have narrow widths, comparable of pure glue and since the quantum numbers of low-lying to qq¯ states [5]. glueballs (below 4 GeV/c2) are not exotic, they should In the coming years there will be significant compu- manifest themselves as extraneous states that cannot be tational resources [6] dedicated to understanding non- accommodated within qq¯ nonets. But their unambigu- perturbative QCD including confinement using lattice ous identification is complicated by the fact that they techniques. The prediction of the hybrid spectrum, in- can mix with qq¯. Excitations of the gluonic field binding cluding decays, will be a key part of this program but the quarks can also give rise to so-called hybrid mesons experimental data will be needed to verify these calcu- that can be viewed as bound states of a quark, antiquark lations. The spectroscopy of exotic mesons provides a and valence gluon (qqg¯ ). An alternative picture of hy- clean and attractive starting point for the study of glu- 2 onic excitations. GlueX experiment will require an amplitude analysis The GlueX experiment is designed to collect data of based on measuring the energy and momentum of their unprecedented statistics and quality surpassing existing decay products. Linear polarization of the incident pho- data on the photoproduction of light mesons by several ton is required for a precision amplitude analysis to iden- orders of magnitude after the first year of data-taking. As tify exotic quantum numbers, to understand details of part of the program of identifying exotic hybrid mesons, the production mechanism of exotic and conventional these data will also be used to understand the conven- mesons and to remove backgrounds due to conventional tional meson spectrum including the poorly understood processes. Linear polarization will be achieved using the excited vector mesons. coherent bremsstrahlung technique. Details of how po- The physics of GlueX has been presented to an ex- larization information is used and how polarization is ternal review committee, several town meetings of the achieved will be given below. nuclear physics community leading up to the Nuclear We will show below, that for a solenoid-based detector Science Advisory Committee (NSAC) Long Range Plan, system, and given the required mass reach required for to three jlab Program Advisory Committees and most mapping the spectrum of exotic hybrid mesons, a photon recently to the Department of Energy (DOE) Office of energy of 9 GeV is ideal. To achieve the requisite de- Nuclear Physics Science Review of the Proposed 12 GeV gree of linear≈ polarization for 9 GeV photons using coher- Upgrade in April 2005. ent bremsstrahlung requires a minimum electron energy of 12 GeV. B. Using Linearly Polarized Photons C. Detector & Beam Overview As will be discussed below, there are tantalizing sug- gestions, mainly from experiments using beams of π A schematic of the GlueX detector and beamline is mesons, that exotic hybrid mesons exist. The evidence shown in Figure 1. Note that the drawing is not to scale. is by no means clear cut, owing in part, to the appar- To carry out the amplitude analysis that will identify ently small production rates for these states in the decay exotic hybrid mesons, the beam must have sufficient en- channels examined. It is safe to conclude that the ex- ergy and be linearly polarized and the detector must be tensive data collected to date with π probes have not hermetic and have excellent resolution. We now discuss uncovered the hybrid meson spectrum. Models, like the these features of the GlueX beam and detector. flux-tube model, however, indicate the photon is a probe that should be particularly effective in producing exotic hybrids, but data on photoproduction of light mesons are 1. Photon Beam sparse indeed. The first excited transverse modes of the flux tube are The photon beam is produced by having a low- degenerate and correspond to clockwise or counterclock- emittance 12 GeV electron beam incident on a thin wise rotations of the flux tube about the axis joining the ( 20 µm) diamond wafer. After passing through the quark and antiquark fixed in space with J = 1 [3]. Linear wafer,≈ the electron beam is bent by a dipole magnet (the combinations of these two modes are eigenstates of parity tagger magnet) into the beam dump. A small fraction, PC +− PC −+ and lead to J =1 and J =1 for the excited about 0.01% of the electrons, emit a photon via inco- flux tube. When these quantum numbers are combined herent bremsstrahlung or coherent bremsstrahlung, the with those of the qq¯ with L~ = 0 and S~ = 1 (quark spins latter leading to an enhancement over the incoherent aligned) three of the six possible J PC have exotic combi- spectrum at a photon energy determined by the angle + + + nations: 0 −,1− and 2 −. A photon probe is a virtual between the incident electron direction and the wafer. qq¯ with quark spins aligned. In contrast when the qq¯ By exploiting the tight energy–angle correlation for the have L~ = 0 and S~ = 0 (spins anti-aligned), the resulting coherent photons, collimation of the photon beam can quantum numbers of the hybrid meson are not exotic.
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