Introduction The Thomas Jefferson National Accelerator Facility JLab

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 E 5 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 Ge. V. 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, q. B/pm, event by event, where q is particle charge, B is the ratio of the torus current to 3860 A, 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 q. B/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: Δq. B/p = α + β(q. B/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 W 2 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 W 2 vs. φ e Conclusions Fig. 4. 2 D Histogram of Δq. B/p vs. q. B/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 Ge. V 2). 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 Δq. B/p vs. q. B/pc with fitted line. Table 1. Centroid and width of uncorrected and corrected data by sector