Transcript Introduction The Thomas Jefferson National Accelerator Facility (JLab) in Newport News, Virginia, is used to understand the fundamental properties of matter in terms of.

Introduction
The Thomas Jefferson National Accelerator Facility
(JLab) in Newport News, Virginia, is used to
understand the fundamental properties of matter in
terms of quarks. We describe here how data is
collected at Jefferson Lab and how to improve the
measurement of the electron momenta.
Momentum Corrections for E5 Data Set
R. Burrell, G.P. Gilfoyle
University of Richmond, Physics Department
CEBAF
Physics Motivation
The Continuous Electron Beam Accelerating Facility(CEBAF)
is the central particle accelerator at JLab. CEBAF is capable of
producing electron beams of 2-6 GeV. The accelerator is about
7/8 of a mile around and is 25 feet underground. The electron
beam is accelerated through the straight sections and magnets
are used to make the beam travel around the bends(See Fig. 1).
An electron beam can travel around the accelerator up to five
times at the speed of light. The beam is sent to one of three
halls where the beam collides with a target and causes
particles to scatter into the detectors.
The CLAS detector is a large (10 m diameter, 45 ton) spectrometer designed to measure and identify the
debris from a nuclear collision. A toroidal magnet field bends the trajectories of charged particles through
CLAS and enables us to measure their momenta. Improving the accuracy and precision of those
measurements is the goal of this project. There are many small deviations within CLAS. These include things
like wire misalignments, imperfect knowledge of the magnetic field, and wire sag, among others. Despite
these imperfections, we are able to measure the angle at which particles are scattered quite precisely. These
angles provide a more accurate depiction of the momentum under the appropriate conditions.
Hall B
Fig. 1 JLab Accelerator and Halls A, B, and C
CLAS
The CEBAF Large Acceptance Spectrometer(CLAS), located in
Hall B, is used to detect electrons, protons, pions and other
subatomic particles. CLAS is able to detect most particles
created in a nuclear reaction, because it covers a large range of
angles. The particles go through each region of CLAS leaving
behind information that is collected and stored on tape. The
event rate is high (about 3000 Hz), so the initial data analysis is
done at JLab, and we analyze more deeply those results at the
University of Richmond. There are six different layers of CLAS,
visible in Figure 2, that produce electrical signals that provide
information on velocity, mass, and energy, and allows us to
identify and separate different subatomic particles. The drift
chambers make up the first three layers, and determine the
path of different particles. Also in CLAS is a toroidal magnet
that causes charged particles to bend as they pass through the
drift chambers. This bending is then used to determine
momentum, which leads to a calculation of mass that is used
to identify the particle[1].
Fig. 2 CLAS Event
Display(CED), displays
signals received from each
layer of CLAS.
Procedure (1)
In an e + p  e’ + π+ + X reaction, we detect the scattered
electron and pion, and use conservation laws to
determine missing mass (MM2) of X. We then compare
this value with the known mass of a neutron. The
momentum corrections for this set of data should move
the peak in this missing mass plot closer to the accepted,
known value. It should also make the peak narrower
(more precise), and should even out sector by sector
variations. We use this method of detecting two particles
and calculating the third because it is much easier than
detecting three particles at once.
Procedure (2)
First we calculate curvature, qB/pm, event by event, where
q is particle charge, B is the ratio of the torus current to
3860A, and pm is the reconstructed particle momentum.
We then calculate the curvature again using pc, which is
derived only from the polar angle of the track, θ. We then
separate events into 16 Θe bins and 24 φe bins. Next, we
make a 2-dimensional plot of the difference between
these two curvatures and qB/pc for each Θe- φe bin (See
Fig. 3).
Below we show results for MM2 and W2 for 2.56 GeV,
reversed field and 4.23 GeV, normal field running
conditions before and after the correction.
Fig. 5. Corrected missing
mass 2.56 GeV
Fig. 6. Corrected W2 vs φe
for 4.23 GeV
Table 1. Centroid and width of uncorrected and
corrected missing mass for 4.23 GeV
Table 2. Centroid and width of uncorrected and
corrected W2 vs φe for 4.23 GeV
Fig.4. Profile Histogram of ΔqB/p vs. qB/pc
with fitted line.
Procedure (3)
From these total 384 histograms, a profile
histogram is constructed for each bin. A profile
histogram makes slices along the x-axis and
displays the mean y-value and standard
deviation within that slice. These profile
histograms are then fitted with a first order
polynomial (see Fig. 4).
Fig. 7. Corrected missing
mass for 2.56 GeV
Fig. 8. Corrected W2 vs φe
for 2.56 GeV
Procedure (4)
The parameters from this fit, α (slope) and β (yintercept) are then inserted into the equation:
ΔqB/p = α + β(qB/pc)
And we solve for pc, which becomes the corrected
momentum. We then use this to calculate the
corrected missing mass.
Procedure (5)
Fig. 3. 2D Histogram of ΔqB/p vs. qB/pc .
Results
As an addition test of the accuracy and precision
of this procedure we also compared W2 (the
square of the recoiling mass for e + p  e’ + X)
before and after applying the momentum
corrections for the electron.
Table 3. Centroid and width of uncorrected and
corrected missing mass for 2.56 GeV
Table 4. Centroid and width of uncorrected and
corrected W2 vs φe by sector for 2.56 GeV
Conclusion
We find the uncorrected data was already quite accurate
and this procedure did little to improve the peak position
or width.