Energy calibration of the Hall B bremsstrahlung tagging system using magnetic pair spectrometer

1. Energy calibration of the Hall B bremsstrahlung tagging system using magnetic pair spectrometer S. Stepanyan (JLAB) PrimEx collaboration meeting

2. Tagger energy variations • The first observation of the nonlinearities in the tagger energy spectrum in the search for pentaquarks in the g2a data (S. Stepanyan et al.). Empirical corrections were derived using the exclusive reaction gd’pp+p-(n). • Later similar results have been obtained by M. Williams et al. from the analysis of g1c data (higher statistics, full focal plane). • These was explained by the effects of gravitational sag and various possible misalignmets of the tagger focal plane (D. Sober et al.). S. Stepanyan CLAS Analysis Note 03-105 D. Sober et al., CLAS-NOTE 2004-019 PrimEx collaboration meeting

3. Experimental measurements • The tagged photon energy spectrum was measured in coincidence with e+e- pairs detected in the pair spectrometer at several different values of the PS dipole field. • The average ratio of the photon energies, reconstructed in PS and defined by the tagger for the E-counter “i”is defined as relative energy correction for that E-counter. Ci=Ec/Ei. • Data are taken during the g10 run. Pair spectrometer was equipped with microstrip detectors for better position determination of e+ and e-, and thus better energy resolution. • DAQ configuration: tagger + PS with microstrip detectors. • New EPICS control of the PS magnet. Automated procedure for scans. • Total of 10 scans, 115 stettings of PS field. • Measurements at field settings close to the end point energy were conducted to set the absolute energy scale. PrimEx collaboration meeting

4. MS e+ • Single counters of PS1 on each side. • PS2 – full plane. e- PS1 & PS2 • MS – microstrip detectors from photon polarimeter: 2X and 2Y planes, 50mm pitch. Experimental setup • Each pair of (XY) planes cover 20x20 mm2 area. • Distance between two X planes was 450+/-0.05mm, centered on the beam within ~1mm. • Field in the center of the PS dipole magnet was measured with few x10-3 precision. PrimEx collaboration meeting

5. 50mm X1 Y1 X2 Y2 9 8 7 6 5 4 3 2 1 0- # of hits X1 Y1 X2 # of non-adjacent hits 1 2 3 1 2 3 1 2 3 1 2 3 4 Plane # 1 2 3 # of hits Analysis of hits in the microstrip detectors • For adjacent hits the position is calculated as a weighted average using the ADC values. PrimEx collaboration meeting

6. Tagger energy corrections Bc- field value at the center of the magnet, lp-distance from the center of the magnet to the MS plane x- distance between hit position and the beam center. PrimEx collaboration meeting

7. Determination of the effective field length • Integral is calculated using trajectories simulated from the target center to the center of the microstrip X-plane. • Trajectories are simulated using Runge-Kutta-Nystroem method (ray-tracing program from B. Mecking). • TOSCA generated field distribution (from A. Glamazdin). PrimEx collaboration meeting

8. In the ideal case of accurate knowledge of the field distribution photon energy is reconstructed with accurace much betetr than 0.1%. Correction for the finite detector size, G(DX) DX is the distance between e+e- Ec uses Leff from above PrimEx collaboration meeting

9. Difference between measured and TOSCA maps Integral is taken along the Z axis for different transverse positions X. X=18.7cm; Y=0cm X=13.5cm; Y=0cm X=8.6cm; Y=0cm X=0cm; Y=0cm • Shape of the dependence at X=0;Y=0 was used to correct Ec. • The maximum variations of the r for a single X is used as an error for F(B), +/-0.05. PrimEx collaboration meeting

10. End point energy measurement • Normalized yield of e+e- as a function of Ec. • For 4 different acceptances of e+e- detection the ratio of beam energy to the defined “end point” energy was within 0.1%. Ee.p. • EB for this measurements was 3.7765 GeV (from accelerator and Hall A beam energy measurements). • Ee.p. is defined as a mid point of the falling edge of the e+e- coincidence rate, and is 3.784 GeV. PrimEx collaboration meeting

11. Final corrections PrimEx collaboration meeting

12. After correcting for swapped cables PrimEx collaboration meeting

13. Final results, reaction gd’pK+K-(n) Simulations G10, 3375 A PrimEx collaboration meeting

14. Summary • The tagger energy corrections were derived from the measurements of the tagged photon spectrum in coincidence with e+e- pairs in the PS at several different values of the PS dipole field. • For calculation of the effective field length, and the correction for the finite detector sizes TOSCA generated field distributions (maps) were used. • Calculated energy was corrected for the difference between the generated and the real field distributions. • Estimated error on obtained corrections ~0.1%. • Final energy scale is defined from measurements of the e+e- coincidence rate close to the “end point” energy. PrimEx collaboration meeting

15. MS e+ Pair Spectrometer e- SC1 & SC2 Photon energy calibration • Single scintillator counter SC1 plane on each side. • Full SC2 plane. • MS – microstrip detectors : X and Y planes, 50mm pitch. 115 settings of PS field Reaction gd’pK+K-(n) MM of the neutron PrimEx collaboration meeting

16. Measured PS dipole field PrimEx collaboration meeting