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)
Procedure (2)
In an e + p  e’ + π+ + X reaction, we detect the scattered
electron and pion, and use conservation laws to
determine missing mass of X. We then compare this value
with the known mass of a neutron. The momentum
corrections for this set of data will move the peak in this
missing mass plot closer to the accepted, known value. It
will also make the peak narrower (more precise), and will
even out sector by sector variations (See Fig. 3 & 6). We
use this method of detecting two particles and calculating
the third because it is much easier than detecting three
particles at once.
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 theta phi bin (See Fig. 4).
Procedure (4)
The parameters from this fit, α (slope) and β (y-intercept)
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. (see Fig. 6).
Fig.6. Corrected missing mass vs φe
Procedure (5)
Below are bin by bin plots of W2 vs. φe, before and after the
corrections. The central line is the neutron mass squared.
The centroid in sector 2 is noticeably better. The shape of
all of the sectors is the same or improved.
Fig. 7. Plot of W2 vs. φ e
Conclusions
Fig. 4. 2D Histogram of ΔqB/p vs. qB/pc .
Fig.3 Uncorrected missing mass vs. φe
We find the uncorrected data was already quite accurate
and this procedure did little to improve the peak position
(see columns 2 and 3 in Table 1. The expected value of
the centroid is 0.8825 GeV2). We do see a 25%
improvement in the width of the neutron peak (see
columns 4 and 5).
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. 5).
Fig.5. Profile Histogram of ΔqB/p vs. qB/pc
with fitted line.
Table 1. Centroid and width of uncorrected and
corrected data by sector