Objectives

1. Objectives • Determine Detector offsets in hall B reference frame and thus absolute beam position at Hycal • Examine Flux calculation and see if beam trips can be eliminated more effectively by changing parameters within existing software.

2. Beam Position Mathew Reece & Dustin Woolford

3. Beam Position • Ideally to find the absolute beam position two BPMs can be used and then offsets determined from the projection of that line to other detectors. • However there is a magnetic field present between the two BPMs in hall B. Is it negligible? • There is a single run (4943) where the beam position is known to change abruptly midway through the run at BPM1 and at Hycal but very little at BPM2.

4. Whats known • BPM data suggests that readouts are in the hall B frame • HYCAL, x = 0.02 mm beam right, y = 0.09 mm high relative to CLAS center line. • Gamma Profiler needs to be determined

5. Linearity • By using only data from (4349), the unknown detector offsets do not affect the calculations. • We assume that the beam is linear between the BPMs and compare every other event in the run with the first event. • The angles are determined by the arctangent of the difference between the measured points on a detector for the first event and a later event divided by the distance between that detector and BPM 2. As shown on the previous slide, α1 is for BPM 1 and α2 is for the γ profiler. • By our conventions, the ratio α2/α1 will be -1 if the beam is linear [since arctan x = -arctan (-x)]

6. Important note • It is important to verify that the beam position on BPM 2 does not vary significantly in order for this method to be valid. As the graph shows, the location where the beam passes through BPM 2 does not move more than 0.2 mm during the run.

7. Now that the constancy of BPM 2 has been verified, let us look at the α2/α1 ratio:

8. Determining offsets • A Java program that will use BPM2 and Hycal to determine (x,y) at the gamma profiler is written. • Average offsets for each run must be determined and then entered into the MySQL database • The same program must be re run for BPM2 and Gamma Profiler to determine (x,y, cos(x), cos(y) • Yet to be completed

9. Luminosity Dustin Woolford

10. General Idea • Tagged Yield = Cross Section x target thickness x solid angle x Nγtagged (exp) • Nγtagged (exp) = Ne (exp) x Tagging ratio • Tagging ratio = Nγtagged (cal) / Ne (cal) • Given a Tagged yield, target thickness, solid angle, and the flux the Pi0 cross section can be extracted

11. Flux Nγtagged (exp) • (Flux) Tagged photons per run per T channel • Nγί= Neί x Rί where N and R are the number of electrons per T channel and the tagging ratio respectively. • Rίis determined during the TAC runs • Neί= ( ηeί / w x ηtrigs) x live 1 -ηeί = # e- in a T channel in a given window - w = size of the TDC window - ηtrigs = # of trigger events • Basically average e- rate for a T channel x live time

12. Live Time of DAQ • Dead time = Live 1 / Live 2 • Both Live 1 and Live 2 are driven by a 195.316 +/- 0.0045 kHz internal clock. • Live 1 is gated, Live 2 is free • TDC start is attached to the tagger and the stop is initiated by a Pi0 event • Trigger is also activated by a Pi0 event in hycal • There is a 25 ns internal dead time for the TDC separate from the trigger dead time. • Don’t need to correct for dead time because only tagged Pi0 event go to elastic scattering.

13. Considerations • Two types of tagging ratios 1) Absolute – uses lead glass solid block and is assumed to have 100% efficiency at low photon intensities (TAC) - given by Rabs = N γTAC · e-/ Ne- 2) Relative – uses the pair spectrometer to monitor the tagging ratio during runs but has 0.06% efficiency. -given by Rrel = N pse+e- - e-/ Ne- • Method 1 must be used to calibrate 2.

14. Considerations cont’d • Effects that may reduce the Absolute tagging ratio from 1, three primary factors 1) photon is produced but absorbed before reaching the TAC ( effect is reduced by helium bag…but not corrected for ) 2) electron decelerates in the target without producing a photon 3) extra electrons in the tagger (effect gets large at high beam intensities so Rrel must be corrected for )

15. Corrections (brief list, non-exhaustive) • Relative tagging ratio has back ground that must be accounted for (background = Integral of events / w) • Rrel is intensity independent at the at low beam intensity ( 0.1 – 100 nA

16. Corrections cont’d • To correct for dependence at high beam intensity the relative tagging ratio per T counter was averaged for all runs. Then for each affected run, Rrel was normalized to its corresponding average per T counter. • Collimators 8.6 mm and 12.7 mm were found to cut 4 % and 1% of the beam respectively • Run to run stability ( good up to run 5100) • Live time problem ( implemented algorithm to solve )

17. Tagger Detector Rate • 3 ways ( Integral, exponential, and Poisson method) • Integral is primary means • The time distribution is integrated over a range which excludes the triggering events and depletion from LIFO limits. The integral is divided by the product of integration interval and # of events over which the distribution was accumulated. (Aram’s luminosity monitoring, Primex notes) • Independent of TDC dead time due to using Live 1 • Can be used for high and low rate detectors • Tosses a lot of data and abstract coincidence detectors are affected subtly by the LIFO limit and dead time

18. Pflux package • Prim_ana has Pflux package that links data analysis with the luminosity • Attempts to identify and eliminate beam trips. • Run is pre-segmented into 5 second intervals. Pflux Uses the integral method to calculate tagging rate and live time for each segment. • Averages live time for all segments and fits Gaussian to the “avg. histogram” • Identifies everything outside 3 σ as a beam trip. • cuts 2 five second intervals following identified beam trip. • Configurations - beam_trip : activates beam trip cuts - num_bad : # of 5 second intervals cut after beam trip - livetime_sigma : number of standard deviations that contain “good” data

19. Live Time Study( courtesy of Eric Clinton and Aram Teymurazyan) • Uses Pflux package to output flux per run per T channel, per E channel, live time and average flux and live time for each run • Standalone from prim_ana • Use Root and excel to graph flux as a function of sigma and # of cut intervals.