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

PrimEx collaboration meeting 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  d ’ 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 D. Sober et al., CLAS-NOTE

PrimEx collaboration meeting 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. C i =E c /E i.  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 Experimental setup e+e+ e-e- MS PS1 & PS2 MS – microstrip detectors from photon polarimeter: 2X and 2Y planes, 50mm pitch. Single counters of PS1 on each side. PS2 – full plane. Each pair of (XY) planes cover 20x20 mm 2 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 Analysis of hits in the microstrip detectors 50  m For adjacent hits the position is calculated as a weighted average using the ADC values Plane # # of hits X1 Y1 X2 Y # of hits X1 Y1 X2 # of non-adjacent hits

PrimEx collaboration meeting Tagger energy corrections B c - field value at the center of the magnet, l p - distance from the center of the magnet to the MS plane x - distance between hit position and the beam center.

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

PrimEx collaboration meeting Difference between measured and TOSCA maps Integral is taken along the Z axis for different transverse positions X. 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 E c.  The maximum variations of the r for a single X is used as an error for F(B), +/ X=18.7cm; Y=0cm

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

PrimEx collaboration meeting Final corrections

PrimEx collaboration meeting After correcting for swapped cables

PrimEx collaboration meeting Final results, reaction  d ’ pK + K - (n) G10, 3375 A Simulations

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

PrimEx collaboration meeting Measured PS dipole field