1. Fiducial Cuts for the CLAS E5 Data Set K. Greenholt (G.P. Gilfoyle) Department of Physics University of Richmond, Virginia INTRODUCTION The purpose of the Thomas Jefferson National Accelerator Facility (JLab) is to understand the fundamental properties of matter in terms of quarks and gluons. We describe here how data is collected at Jefferson Lab and how we determine the electron fiducial volume of one of the end station detectors called CLAS (CEBAF Large Acceptance Spectrometer) located in Hall B. We do this by focusing on data where the efficiency of the detector is well understood. CEBAF The Continuous Electron Beam Accelerating Facility (CEBAF) at JLab in Newport News, Virginia, is used to study the properties of quark matter. 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 near the speed of light. The beam is sent to one of three halls where the beam collides with a target and the debris is measured. These data were collected in Hall B with CLAS [Fig. 1]. CLAS CLAS is located in Hall B and is used to detect pions, electrons, protons and other subatomic particles. The detector is able to detect most particles created in a nuclear reaction, because of its unique nearly-full-solid- angle structure. There are six different layers of CLAS [see Fig. 2] which produce electrical signals, providing us with information on velocity, momentum, and energy, and allow us to identify different subatomic particles. The drift chambers make up the first three layers, and determine the paths of different particles. The next layer is the Cerenkov counters which separate electrons from pions. The following layer is made of the time of flight scintillators to determine time of flight and hence velocity. The calorimeters, used to measure the energy of the particles, make up the final layer. Also in CLAS is a toroidal magnet that causes charged particles to bend as they pass through the drift chambers. This bending is used to determine momentum. This magnetic interaction is of particular interest to us, as we attempt to define the fiducial volume of the detector, because it affects the regions of stable efficiency. Fig. 1 JLab Accelerator and Halls A, B, and C Fig. 2 CLAS Event Display(CED), displays signals received from each layer of CLAS. Acceptance (often referred to as efficiency) is the ratio of the events measured in the detector versus the actual events produced in the nuclear reaction. In plain English, “how much of the good stuff do we actually catch in the detector?” Stable Acceptance is a focus on the flat, smooth regions of data. What have we done so far? Last summer, we explored the effects of fiducial cuts on electron data from CLAS at 2.56 GeV, normal polarity. For this particular energy setting, we found that fiducial cuts were an effective means of isolating regions of stable efficiency. What is Acceptance? Results: Generating fiducial cuts on electrons at 2.56 GeV reversed polarity and 4.23 GeV, normal polarity. Challenges posed by different energy and polarity settings: Stage 3: Conclusions WHAT’S THE CHALLENGE? In regions of the azimuthal scattering angle  near the current-carrying coils that produce the CLAS magnetic field  the efficiency, or acceptance, of CLAS is not well known. To prevent the inclusion of these events in our sample, we generate constraints [fiducial cuts] on electron and proton scattering angles to exclude the regions of the magnetic field near the coils. Hall B Fig. 4. Fiducial cut in terms of events plotted against  angle, showing the region of stable efficiency in the  distribution for the electrons in the labeled momentum and  bin. Hall C Hall A Fig 3. Data plot from CLAS showing  versus  for the electron. Note: six sector orientation. 1)We generated fiducial cuts for five settings of CLAS, including both proton and electron data at 2.56 GeV normal and reversed polarity, as well as electron data at 4.23 GeV, normal polarity. 2)We removed points which were statistically low, and isolated regions of stable efficiency. 3)We created a central fiducial in the 2.56 GeV reversed polarity data, and excluded this region of low acceptance. 4)We observed reasonably smooth dependence on electron momentum for all fit parameters. References: 'Fiducial Cuts for electrons in the CLAS/E2 data at 4.4 GeV', D. Protopopescu, F. W. Hersman, M. Holtrop, UNH, S. Stepanyan, CNU, and CLAS/E2 run group, CLAS-Note 2000-007, November 27, 2000. One way to measure the effects of the fiducial cuts was to generate data with the cuts on and contrast it to data with the cuts off. We show the results of this below: Because of the way the detector was designed, when we have a reversed field, and consequently out-bending electrons, especially at low theta angles, we see a gap in the middle of our data. This is caused by the structure of the Winston Cones, and the positioning of the Photo-Multiplier Tubes. This is seen clearly below, in a plot [Fig. 5] of phi_e_vs.theta_e_vs.p_e_xy for Sector, momentum bin 9. As you can see, the data near the edges of the acceptance are often cut out, when the fiducials were used, giving us regions of predictable and flat acceptance. PROCEDURES: Stage 1: First Generation Fit  We plot the number of events versus the  angle for a particular momentum bin and  angle bin. We then use a CERN program called Minuit to fit a trapezoidal curve to the data points. The fiducial cut is defined as the edge of the plateau in Fig. 4. Stage 2: Second Generation Fit  We fit the upper and lower sector edges defined by the first generation fits, and plots them against the  electron angle. We then use Minuit to fit another curve to these data points. While often this fit is symmetrical, the procedure does not require symmetry. Stage 3: Third Generation Fit  We plot the results generated by the first and second generation fits against the momentum of the electron (ascertained when the particle passes through the toroidal magnet), and fit these data with a polynomial function. Left: 4.23 GeV, normal polarity; Right: 2.56 GeV, reversed polarity. Sector 1, Generation 3, 2.56 GeV, Reversed Polarity Sector 2, Generation 3, 2.56 GeV, Reversed Polarity We look for a symmetry between the upper and lower edges, as well as a flat t0, [initial theta angle]. Sector 3, Momentum bin 14, theta bin 20 Sector 4, Momentum bin 14, theta bin 20 Goal: To expand our knowledge of regions of CLAS in which the acceptance of the detector is well understood by generating electron and proton fiducial cuts at higher energies and with different polarity settings. Sector 4, 2.56 Reversed Polarity data, momentum bins 8 & 14 The repercussions of this gap can be more clearly seen in first generation plots, such as those to the right, in figures 6 and 7. Figure 3 Figure 4 Figure 6 Figure 5 Figure 7