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GlueX Experiment Document 389-v2

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GlueX Collaboration Response to Proposed Changes in the GlueX Experiment and Electron Beam Energy

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Document type:
Hall D Note
Submitted by:
Alex R. Dzierba
Updated by:
Alex R. Dzierba
Document Created:
29 Nov 2004, 09:39
Contents Revised:
29 Nov 2004, 11:02
Metadata Revised:
29 Nov 2004, 11:52
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The GlueX collaboration has been asked to study the impact on the GlueX physics goals of moving the detector downstream of Hall~B and consequently lowering the electron beam energy from the planned 12~GeV to 11~ GeV . The goal of GlueX is to map out the spectrum of hybrid mesons starting with exotic hybrids -- this means measuring a spectrum of states not just discovering a single exotic state. Lowering the \emph{photon beam energy} by 1~GeV would effectively make inaccessible between one-third to one-half of the meson mass range (2 to 3~GeV/$c^2$) most relevant for discovery. Lowering the \emph{electron beam energy} by 1~GeV (or 2~GeV) will reduce the polarization figure of merit by a factor of about 3.8 (or 70). Linear polarization is required for a precision amplitude analysis to identify exotic quantum numbers, to understand details of the production mechanism of exotic and conventional mesons and to remove backgrounds due to conventional processes. Finally, sharing a beam line with Hall~B will introduce operational complications for both Hall~B and GlueX. All this would have a severe negative impact on the discovery potential of GlueX. Increasing the electron beam energy to 13~GeV -- which should be achievable as a future upgrade with the additional arc of magnets and the additional accelerator pass as in the current plan for locating GlueX in Hall~D -- will increase the figure of merit by a factor of two.
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