GEANT Simulations and Track Reconstruction for the Super-Bigbite Spectrometer Project SBS Technical Review L.Pentchev Jan 22, 2010

Outline GEANT simulations of GEp(5) background and GEM photon detection efficiency Experimental verification of MC simulations Description of track search algorithm Efficiency of track reconstruction

GEANT3 Simulations of Background Rates SBS high background rates require GEM capability Anticipated GEM rates are orders of magnitude lower than the ones demonstrated at CERN DAQ and tracking require knowledge of background rates for design

GEANT3 Simulations of Background Rates GEANT3 code with 100 keV threshold used for background simulations Includes description of target, scattering chamber, magnet, SBS detectors (with COMPASS type GEM), BigCal, beamline, beam dump Magnetic fields from separate calculations included with magnet volumes Several configurations tested to study background contributions Several configurations studied: Target only Target + scattering chamber + magnet, field clamps, lead shielding + beam line and beam dump

GEANT3 Simulations of Background Rates Low energy (<1 GeV) charged particles swept by the magnet: rates on First Tracker dominated by photon flux, while rear tracker rates dominated by e- produced by target photons in analyzers Two step simulations for photons: background rates simulations with 100 keV threshold and GEM photon detection simulations with 10 keV threshold 5

GEM Photon Detection Probability Separate g efficiency study by E.Cisbani using Geant4 with 10 keV threshold “standard” COMPASS-type GEM Photon Energy, MeV Chamber <g eff> (%) 1 0.31 2 0.54 3 0.63 4 0.73 5 0.76 6 0.79 Photon spectrum on front GEM chamber From GEANT3 Material thickness (~photon eff.) will be reduced by about 30% compared to COMPASS GEMs 6

MC Background Results: First GEM chamber Initial soft electrons swept by the magnet Photons originate from the target or from electrons hitting material in front of the magnet First GEM chamber: Configuration Ebeam = 11 GeV, 75 mA rates, MHz/cm2 g induced hits, Charged rates, MHz/cm2 Target 114 0.350 1,010 Target + Scatt. chamber 118 0.361 Target + Scatt. Ch. + Magnet 143 0.437 0.119

BigCal Coordinate Detector Background Results: All Trackers For full configuration of GEp(5): Hit rate, kHz/cm2 First Tracker 556 Second Tracker 359 Third Tracker 125 BigCal Coordinate Detector 130 2nd tracker CH2 CH2 1st tracker 3rd tracker BigCal Coordinate Detector Al absorber

MC and experimental Background Rate Comparison Transversity Experiment E06-010 5.9 GeV beam, BigBite at 300, 40cm 3He polarized target 3 MWDC behind the BigBite magnet Luminosity with H2: 1.5 1036 cm-2 s-1; 11.8 mA current GEANT3 Calculation Simplified target description 100 keV threshold Hadron rates from DINREG Differential measurements of MWDC rates with - without H2 in target cell results: Data Simulation (g + hadron) 6.1 MHz 4.8 MHz 20% of rate for filled target configuration are “room” background MC explains 80% of the experimental rates This is encouraging level of agreement Dedicated studies with simpler geometry and detector shielding could provide more confidence in MC

Tracking Detector Configuration Lead (X,Y) x2 (U,V) x2 (X,Y) x2 (U,V) x2 (X,Y) x2 (X,Y) x2 HCAL CH2 CH2 (U,V) x2 1st tracker 2nd tracker 3rd tracker CH2 Number of Layers Area (cm2) Pitch (mm) Channels Front Tracker 6 40x150 0.4 49k 2nd Tracker 4 50x200 1.6 13.5K 3rd Tracker 13.5k BigCal 2 80x300 1.0 12k (X) x2 Al absorber BigCal

Track Reconstruction: Step 1 From the BigCal hit, reconstruct e- position (vertical only) on BigCal Coordinate Detector Elastic kinematics correlate e- and p tracks 3rd MC for elastic events 2nd 1st tracker p e BigCal beam

Track Reconstruction: Step 1 BigCal hit constrains proton search region to 0.2 * 18cm2 (x * y) and 0.7 * 30 mrad2 (Qx * Qy)

Track Reconstruction: Step 2 Tracking in First Tracker: For each possible hit on each GEM plane, starting from the front, by using angular constraints, small region on next GEM chamber is defined

Track Reconstruction: Step 3 Proton is likely to be only scattered in first or second analyzer HCAL + front tracker constrains 75 cm2 search region in 3rd tracker 14

Track Reconstruction: Step 4 Front and 3rd tracker constrains 2nd tracker search region to 2 x 0.9 cm2 areas

Track Reconstruction Inefficiency due to random rates Using calculated full setup rates: Tracker Search Area Rate Pitch Occupancy Pseudo-Track [cm2] [kHz/cm2] [mm] [%] [per event] Front 3.6 556 0.4 14.5 0.007 2nd 0.8 359 1.6 18.8 0.015 3rd 72.4 125 7.1 0.001 BigCal 4.5 130 1.0 2.3 0.026 Pseudo-tracks represents tracking inefficiency Total inefficiency about 5% combining all trackers Future steps: full simulations including dead zones, amplitude distribution of signals, MC simulation of tracking

Summary MC demonstrates that rates are well below the maximum capability of GEMs The trackers are capable of operation at projected luminosity with 5% losses due to pseudo-tracks Presently, there is agreement at 20% level, between MC and experimental rates in a BigBite experiment Additional experimental studies and MC simulations will be performed