The structure of the nucleon is not well understood from the fundamental point of view of QCD, i.e. in terms of the quark and gluon degrees of freedom that appear in the QCD Lagrangian. The G0 experiment will measure two ground state proton matrix elements which are precisely defined in the context of QCD. They are sensitive to (point-like) s quarks and hence to the q-qBAR ocean in the proton. The matrix elements of interest are the elastic flavor singlet charge and magnetic form factors, GE0 and GM0, respectively, which can be extracted from a set of electron-proton parity-violation measurements. If a relationship between proton and neutron structure is assumed (for example that the proton and neutron differ only by the interchange of u and d quarks), the s quark contribution to the form factors of the nucleon can be determined using these flavor singlet form factors. This information is relevant to the discussions of the Ellis-Jaffe sum rule and of the PI-N-SIGMA term; there is evidence in both cases that the s quark contribution is larger than expected. The present measurements will allow the determination of the s quark contributions to proton observables in a much more straightforward manner than in either of the cases noted above. Both the charge and magnetic s quark form factors also have intrinsic interest as fundamental quantities, as they would contribute the first direct measurements of the ocean in low energy observables.
In this experiment, parity-violating electron scattering asymmetries will be measured in the region 0.1 <= Q2 <= 1.0 (GeV/c)2 at both forward and backward angles. These pairs of measurements will allow us to separate GE0 and GM0. The asymmetries range from -3E-6 to -35E-6; we are planning to measure the asymmetries with statistical uncertainties of DELTA-A/A of a few percent and systematic uncertainties of DELTA-A <= 2.5E-7. Initially we will measure concurrently the forward angle asymmetries at eight values of momentum transfer in the range 0.1 <= Q2 <= 1.0 (GeV/c)2. Assuming a beam polarization of 70%, the time required to reach this precision for the initial measurement will be about 1 month. We will also be able to make backward angle asymmetry measurements for momentum transfers of Q2 <= 3 (GeV/c)2; in addition, backward angle measurements of quasielastic scattering from a deuterium target will allow us to determine the axial form factor GA and its radiative correction.
The G0 spectrometer will provide the capability of measuring both the forward and backward angle asymmetries. It will consist of a toroidal array of eight super-conducting coils with a field integral of approximately 1.1 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. The bend angle of about 35 degrees at the highest momentum is sufficient to allow complete shielding of the detectors. The detector package will consist of 16 scintillator elements per segment, each element covering approximately 6.6% of the full momentum range.
The spectrometerbeing 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 targetfor 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.
For a summary of G0 in one page, clickHERE
The Superconducting Magnet System (SMS) is the heart of the G0 experiment. It consists of the superconducting coils which generate the magnetic field that focuses scattered particles onto the detectors. Also included in the scope of the SMS is any material that is part of the cold mass as well as cryogenic and vacuum support systems. In large part, the SMS will be fabricated by BWX Technology, which is responsible for the cryostat, coils, cryo reservoir and cooling system, and control system. Additional material and systems are provided either by JLab (power supply, integrated superconductor, vacuum system) or by UIUC (collimators, carriage, exit windows, lead collar) and integrated into the SMS at appropriate points in the assembly. A full test of the SMS will be carried out at UIUC before its shipment JLab and final installation in Hall C.
The instrumentation of the detectors with high speed electronics will be split between North American and French collaborating institutions. CMU will handle the fabrication of FPD custom electronics and and procurement of commercial electronics while Louisiana Tech will produce custom electronics the CEDs for the North American octants. will be carried out by. Electronics for the French octants will be provided by one of the French participating institutions (IPN Orsay or ISN Grenoble).
The G0 detection system will consist of 8 "octant" detector modules. Each module includes individual detector elements (both focal plane (FPD) and exit window (CED) detectors), PMTs, and mechanical support. Four of the modules will be constructed by a North American collaboration consisting of Norfolk State (overall management), William and Mary and U. Maryland (scintillator fabrication), CMU (light guide fabrication), and U. Manitoba (PMTs, and bases) and JLab (octant support design). The remaining 4 modules will be fabricated by the French participating institutions (IPN Orsay or ISN Grenoble). CEDs for both North American and French octants will be fabricated by the Louisiana Tech. Assembly and final testing will be carried out at JLab. A timing and gain monitoring system for all 8 octants will be provided by New Mexico State University.
G0 will employ a 20 cm long liquid hydrogen target operating at a pressure of 2 atm connected to a 450 W cryogenic cooling loop. The target design incorporates a cell filled with helium gas to maintain equal radii of curvature of the entrance and exit windows. The construction of the cryo-target is the responsibility of and will be fabricated at Caltech. Caltech is also responsible for the design and construction of gas handling system which will supply gaseous hydrogen to the target cell and helium to the helium cell. The control system hardware and software will provided by U. Maryland. Caltech is responsible for construction of the transverse motionn mechanism to move the target in and out of the beam path. The responsibility for design and construction of the servicing and removal system has not yet been assigned.
The efforts of the G0 collaboration that relate to computers and software contribute both to the task of Installation and Construction and to the performance of the Experiment and the extraction of Physics results. Computation-related activities are divided among the following four sub-groups.
The slow controls sub-group is responsible for integrating slow controls hardware and software provided by other G0 groups into a unified slow controls system. This involves communication with stand-alone systems associated with the spectrometer magnet/cooling control system the target and the accelerator. Mechanisms for dealing with these slow controls front end systems programed with Lab-view and EPICS must be provided. The work of this sub-group will involve maintaining specialized software for communication with beam diagnostic systems (such as current, beam position, and helicity monitoring) as well as electronics which requires external parameter down-loads. User-friendly displays must be provided to indicate status and alarm conditions.
This area of the G0 software effort will intimately interface with both electronics and Hall C Infrastructure groups and the Slow Controls and Analysis Computation sub-groups. Its responsibilities involve first insuring that the computer hardware required by G0 will be present and working. This includes items such as terminals, monitors for slow controls output, processors for on-line analysis and data acquisition, disk space, tape backup (including the tape), network hardware, front-end (VME) computers etc.. It does not include DAQ electronics (ADC's, scalers, associated interconnecting cables etc,) which is the responsibility of the electronics group. Nor does it include crates (VME, Camac, fastbus) or HV hardware which is the responsibility of the Hall C infrastructure group. This sub-group must, certify that the computer hardware is capable of absorbing the data stream produced by the electronics the slow controls hardware and software, i.e. that the rate and volume of data produced by G0 can be accommodated by the on-line analysis processing power and the storage media.
The CODA software necessary for reading out the electronics to produce a data stream, for storing that data stream, for backing up that storage to tape and for providing hooks for on-line analysis of the data stream are also the responsibility of this sub-group. Additionally an interface to the slow controls software must be maintained by this sub-group to allow slow controls parameters to be inserted into and stored along with the data stream.
There are currently three directions, and in the future there is possible a fourth direction, for our simulation efforts:
The simulation sub-group is responsible for maintaining standard versions of the codes. This will include documenting them to a level which would allow the average grad student to use them with only minimal consulting from the sub-group. The sub-group is also be responsible for running the codes to answer a list of questions/issues provided by the experiment coordinator.
The analyis sub-group would provide analysis software for the experiment to indicate, in real time (or as close as possible to real time), the status and results of the measurement. Examples of the output of this analysis include: histograms of sampled ADC and TDC info, accumulated scaler values, plots of counts versus kinematic parameters, and run summaries. Special procedures such as determination of calibration constants would also need to be addressed. This sub-group would have to closely interface with the DAQ sub-group in order to clearly define the content and format of data that is "piped" from DAQ to on-line analysis and the mechanism by which this "piping" takes place. We will rely on experience from earlier parity experiments (SAMPLE for example) for a first approximation to the sort of diagnostics, output, and procedures needed.
As the experiment matures, it is expected that the on-line analysis code will evolve to become our production off-line code. The analysis sub-group will also be responsible for generating the final off-line results of the experiment. Probably the bulk of the effort will be carried out by thesis students. It will be important to organize their work to provide a small number of analysis tools, to resolve differences in parallel analysis tracks, to produce a consistent answer, and to document the analysis for eventual publications.
The provision of infrastructure for the experiment is generally the responsibility of JLab. Infrastructure equipment includes beamline monitors, the beam raster system, cryogenic systems to provide cryogens (LHe and LN2) to the target and SMS, detector shielding, portions of the cabling, high voltage supplies, modular electronics crates, data acquisition support hardware (computers, air conditioning, etc.), and the Hall C Moller Polarimeter facility.
A polarized source capable of producing a stable, high intensity electron beam which is free of systematic changes during polarization reversal is necessary for the G0 experiment. Development of the polarized electron source at JLab is an ongoing project undertaken by Charles Sinclair and his group as well as members of the G0 collaboration lead by Mark Pitt.
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