R. Thompson

The CLAS drift chamber system

Abstract Experimental Hall B at Jefferson Laboratory houses the CEBAF Large Acceptance Spectrometer, the magnetic field of which is produced by a superconducting toroid. The six coils of this toroid divide the detector azimuthally into six sectors, each of which contains three large multi-layer drift chambers for tracking charged particles produced from a fixed target on the toroidal axis. Within the 18 drift chambers are a total of 35,148 individually instrumented hexagonal drift cells. The novel geometry of these chambers provides for good tracking resolution and efficiency, along with large acceptance. The design and construction challenges posed by these large-scale detectors are described, and detailed results are presented from in-beam measurements.

THE REGION ONE DRIFT CHAMBER FOR THE CLAS SPECTROMETER

Abstract The Region One detector is the innermost of three nested drift chamber packages of the CLAS spectrometer at Jefferson Laboratory. This detector, with 7776 drift cells, was optimized for the CLAS toroidal magnet geometry, and consists of six wedge-shaped sectors integrated into a single unit. Each sector has thin and relatively weak aluminum endplates which support the large mechanical loads from the wire tensions and associated readout hardware. The unusual feature of the detector is its self-supporting design, wherein the wire tensions of neighboring sectors balance each other without the use of massive structural support. We discuss the design criteria, mechanical components, and assembly procedures.

The ep → e′pη reaction at and above the S11 (1535) baryon resonance

New cross sections for the reaction e p-->e p eta are reported for total center of mass energy W = 1.5--1.86 GeV and invariant momentum transfer Q2 = 0.25--1.5 (GeV/c)(2). This large kinematic range allows extraction of important new information about response functions, photocouplings, and eta N coupling strengths of baryon resonances. Newly observed structure at W approximately 1.65 GeV is shown to come from interference between S and P waves and can be interpreted with known resonances. Improved values are derived for the photon coupling amplitude for the S11(1535) resonance.

Analyzing powers in p→p→pnπ+ and determination of pion S and P wave amplitudes near threshold

Abstract Analyzing powers were measured and used to quantify the observation that s-wave processes dominate near threshold. Values of Ay(θπ,φπ=0°) are presented for η values of 0.22, 0.42 and 0.50. Maximum analyzing powers AN0 are equal to −0.13, −0.24, and −0.28, respectively. A partial wave analysis, made possible by the new analyzing powers and available cross sections, indicates that the s-wave contribution to the cross section constitutes about 91% of the total cross section at η=0.22, or 300 MeV. It decreases to about 75% for η=0.5.