Overview

Introduction

This proposal has been prepared for the review of the G⁰ experiment by the Jefferson Lab Program Advisory Committee. The G⁰ experiment was previously approved as experiment 91-017 with A priority in December 1993. This proposal has been prepared in accordance with the ``jeopardy'' rules of Jefferson Lab.

In this experiment, the parity-violating asymmetry in elastic electron scattering from the proton will be measured at both forward and backward angles and over a range of momentum transfers from about 0.1 - 1.0 GeV². A single measurement of the backward angle parity-violating quasi-elastic scattering from the deuteron will be measured. The primary purpose of the experiment is to separate the s quark contributions to the overall charge and magnetization densities of the nucleon using these measurements. No other proposed experiment will perform directly this separation. A special purpose, superconducting toroidal spectrometer with large azimuthally symmetric angular acceptance is being constructed for these measurements.

There has been a great deal of progress in development of the experiment as will be summarized in this report:

• funding of $2.25M from NSF, October 1995 for the magnet and target subsystems;
• magnet RFP, May 1996;
• magnet contract signed, May 1997;
• Cost and Schedule Review, February 1998;
• G⁰ Management Plan, November 1998;
• agreement on funding $0.952M from DOE, $2.194M from Jefferson Lab, $0.493M additional funding from NSF, $0.229M from ongoing grants, December 1998
• target Preliminary Design Review, December 1998; and
• expansion of collaboration from about 40 to about 80 members, including addition of large groups from Canada and France (Table 1)

Table 1. Participants in the G⁰ collaboration ( indicates contact person).

We request at this time that the PAC approve the original 46 days (1100 hours) for commissioning the experiment (see Section  at the end of this proposal). The commissioning plan is, at this time, basically unchanged from what was envisioned at the time of the original approval. The recent decision (October 1998) to locate the experiment downstream of the standard Hall C pivot may preclude breaking the commissioning run into three pieces; nevertheless the same jobs must be completed. At a future date we will request approval for the first physics running.

A summary of the experiment is provided in the following subsection. This is followed by a more detailed discussion of the physics, presentation of the management and schedule for the experiment, and more detailed descriptions and updates on the status of various subsystems. We conclude with the beam time request.

Experiment

In this experiment, parity-violating electron scattering asymmetries will be measured in the range GeV² at both forward and backward angles. These pairs of measurements will allow us to separate the form factors and (neutral weak current analogs of the ordinary and ). The asymmetries range from about -3 to ; we are planning to measure the asymmetries with statistical uncertainties of and systematic uncertainties related to helicity-correlated effects of . We note that the small systematic uncertainties achieved in the recent HAPPEX experiment at Jefferson Lab suggest that we can meet this goal. Initially, we will measure concurrently the forward angle asymmetries binned in seven values of momentum transfer in the range GeV². Assuming a beam polarization of 49%, the time required to reach this precision for the initial measurement will be about 30 days. There is good reason to expect that by the time of the experiment, higher beam polarizations will be available for parity- violation experiments, which would improve the statistical precision by a factor of about 1.5 in this running period. (Using the  GeV²) result from the SAMPLE experiment now completed at Bates, it would be possible to separate the charge and magnetic form factors in the lowest bin after this first measurement.) Each subsequent backward angle asymmetry, measured with comparable precision to the forward asymmetries, would require 30 days of running time.

To achieve the desired precision in a reasonable amount of time, this experiment must be run at high luminosity with a large-acceptance detector. The layout of the experiment is shown in Figure 1. First, for the forward angle asymmetries, we propose to measure elastically scattered protons ( GeV with , respectively; the electron beam energy will be 3.0 GeV and the beam current 40 A. The solid angle acceptance for the forward angle measurement is about 0.9 sr. Second, for the backward angle asymmetries, the spectrometer will be turned around to detect electrons at the complementary angle centered at about 110° with a solid angle acceptance of from 0.9 sr at GeV² to about 0.5 sr at GeV²; the beam energies will range from 0.34 to 0.93 GeV. There is also acceptance for inelastically scattered electrons in the backward angle measurement; this is the subject of approved proposal 97-104.

Figure 1. Schematic view of the G⁰ experiment showing the spectrometer in the forward angle configuration (protons detected). Two of the eight coils and one sector of detectors have been removed for clarity.

The polarized electron source requirements for this experiment are the responsibility of the injector group at Jefferson Lab. It must operate in a ``pulsed'' mode to allow for time- of-flight measurements (see below) wherein only one of every sixteen of the normal beam buckets (at 499 MHz) are filled. This mode will be effected with a special laser running at 499/16 MHz. The average current for which the experiment is designed, 40 A, is therefore generated from pulses with peak currents 16 times as large as ``normal'' (but about 3 times the peak current required for a 200 A beam). These high peak currents will require careful study and possible modification of the gun optics to account for the increased effects of space charge. We are, of course, interested in utilizing higher polarization cathodes for the experiment. Strained crystals pose special problems for parity-violation experiments because of larger helicity-correlated current and position differences (resulting at least in part from the effective analyzing power of the strained surface for residual linearly polarized components of the laser light). Mark Pitt of Virginia Tech will be the injector group liaison for the G⁰ Collaboration.

The spectrometer being constructed for this experiment provides the unique capability of measuring both the forward and backward angle asymmetries. It consists of a toroidal array of eight superconducting coils with a field integral of approximately 1.6 T m. The spectrometer is designed to focus particles of the same momentum and scattering angle from the length of the extended target to a single point (zero magnification in the dispersion direction) in each of the eight identical sectors of the spectrometer. The bend angle of about 35° at the highest momentum is sufficient to allow complete shielding of the detectors. Careful collimation reduces the contamination of inelastic protons (electrons) in the acceptance of elastically scattered protons (electrons). The spectrometer has a number of advantages for this parity-violation experiment. We are able to access relatively high momentum transfers using a magnet that has a maximum momentum of less than 1 GeV. It has very large solid angle and momentum acceptance. The solid angle acceptance is axially symmetric and thus susceptibility to systematic uncertainties is reduced. The shape of the field is determined by the current conductors, there is no polarized iron in the system, and the magnetic field at the target is zero.

The target for the experiment is a thin-walled, 5 cm diameter, 20 cm long vessel of liquid hydrogen; cooling required for the experiment is about 250 W. The design is a combination of those used for the successful SAMPLE (500 W) and Jefferson Lab targets. It consists of the hydrogen cell, a helium cell to maintain consistent curvatures at both ends of the primary cell, together with a cooling loop containing a heat exchanger, pump and heaters. This loop is situated inside the bore of the spectrometer magnet; tests have been performed to ensure the operation of the motor in the magnetic field.

In the G⁰ experiment, we will count individual particles rather than to integrate the signal in the detectors. Particle counting affords the possibility of using standard time-of-flight and coincidence techniques to supplement the resolution of the spectrometer and suppress backgrounds. For both the forward and backward measurements, there will be 16 scintillators in each sector of the focal surface shaped to accept a narrow range of particle momenta. In the case of the forward measurement each of the scintillators will be paired with a second identically shaped partner to reduce background from neutral particles (this set of detectors together are the Focal Plane Detectors - FPDs). Time-of-flight (using a beam with only one of sixteen of the normal 499 MHz beam buckets filled) will be used to separate prompt particles, including pions, photons and electrons, from protons in the forward measurement. In the case of the backward measurement, a set of smaller scintillators (Cryostat Exit Detectors – CEDs) located near the magnet exit window will be used in conjunction with the 16 focal surface detectors to effectively determine the momentum and scattering angle of the electrons. These detectors are therefore used to separate the elastic and inelastic electrons.

The electronics used for the experiment will involve a mixture of custom and commercial components. Two different types of readout electronics systems will be utilized. In each case, time-of-flight will be decoded for each event and effectively histogrammed in scalers. Four sectors (``North American'') will be instrumented with shift-register-based time encoding; four sectors (``French'') will be instrumented with either integrated shift-register-based encoders or flash-TDC/Digital-Signal-Processor encoders. Some time-of-flight capability will be retained for the backward angle measurements; in addition the combinations of CEDs and FPDs necessary to record both elastic and inelastic events will be accommodated.

Various types of backgrounds have been investigated for both forward- and backward-angle measurements. Prior to the original proposal, the inelastic proton background in the forward measurement was measured at SLAC under essentially the same kinematic conditions. It was found to be approximately consistent with the predictions of Lightbody and O'Connell. Backgrounds from pions, neutrons and positrons have been simulated and found to be small in the time region of interest. We note that the asymmetry of the combined backgrounds (inelastic protons and electrons, neutrons, pions, photons, etc.) is measured simultaneously in time bins which do not contain the elastic protons or electrons. In the case of the backward angle measurements where the electrons are detected, s are kinematically forbidden from the elastic acceptance for momentum transfers extending above 1 GeV²; by means of the CEDs, elastic and inelastic electrons are adequately separated. A more complete version of the GEANT MC used for many of these studies is currently being constructed to include more precisely the actual geometry of the spectrometer.

Precise monitoring and control of the beam will be required for this experiment. For each measurement interval the beam characteristics - position, angle, energy and charge must be measured. Based on the present design of the experiment, position measurements with precision on the order of 25  m will be required for each measurement interval (the most stringent requirements are for the position measurements used to determine the beam energy centroid). A system to allow the entire experiment, including the key beam monitoring devices, to be ``rolled'' in and out of the beamline is being presently being developed at Jefferson Lab. It will allow the experiment to be reinstalled with minimum cryogenic, mechanical and electrical work.