GEANT4 Simulation of MOLLER Experiment

Transcription

1 GEANT4 Simulation of MOLLER Experiment Mark Dalton UVa (for Dustin McNulty, Luis Mercado and the MOLLER Collaboration)

2 Outline Introduction to the MOLLER experiment Simulation Overview Spectrometer Design Generators Collimation

3 Møller Scattering A PV = me G F 2πα 16 sin 2 Θ (3 + cos 2 Θ) 2 Qe W Derman and Marciano (1978) Purely leptonic reaction A PV " m e E lab (1# 4sin 2 $ W ) Small, well-understood dilution " # 1 E lab "(sin 2 # W ) sin 2 # W $.5 "(A PV ) A PV Figure of Merit rises linearly with E lab SLAC: Highest beam energy with moderate polarized luminosity JLab 11 GeV: Moderate beam energy with LARGE polarized luminosity September 18, 29 Overview and Meeting Goals Slide: 3 K Kumar

4 MOLLER Experiment Measurement Of Lepton-Lepton Electroweak Reaction Ebeam = 11 GeV APV = 35.6 ppb δ(apv) =.73 ppb δ(q e W) = ± 2.1 (stat.) ± 1. (syst.) % 75 μa 8% polarized δ(sin 2 θw) = ±.26 (stat.) ±.12 (syst.) ~.1% ~ 38 weeks (~ 2 yrs)

5 Spectrometer Concept e cross sec4on (mb) Asymmetry (ppb) e e e iden4cal par4cles Lab Angle (mrad) Highly boosted lab frame Center of Mass Angle Backward Lab Energy (GeV) Forward Lab Energy (GeV) Forward Backward Center of Mass Angle Center of Mass Angle Figure of merit highest at θ CM = 9 o... are collected as θ CM =[6,9] over here! All of those rays of θ CM =[9,12] that you don t get here... Slide: K Paschke

6 Two Toroid Spectrometer z=9. m y (m) y (m) z=6. m z=14.5 m x (m) z=2. m.18 y (m).4 1 meter radial focus, 3 meters from target Clean separa4on from backgrounds x (m).18 y (m) x (m) x (m).6 z=28.25 m y (m).18.8 y (m) z=24. m Designed by UVa (Clayton Davis) x (m) x (m).6 Radial Fields (edge effect) creates azimuthal defocussing which populates the full ring at the detector Slide: K Paschke

7 Toroid design concept meters ee ep More complicated magnet geometry used to control integral Bdl without extensive defocusing Pre-bender magnet pushes highest angle tracks above high field region, and focuses other tracks Focus at ~32.5 meters Overlap between neighboring sectors 1.5 meter target, full range of theta and phi meters ep ee, 6 o 75 o ee, 75 o 15 o ee, 15 o 12 o meters Slide: K Paschke meters

8 Simulation Overview Proposal level (proof of principle) simulation Geant4 used for tracking, calculation of acceptance and collimation planning A work in progress

9 Simulation Schematic Event Generators Beam on Target Geant Tracking (Magnetic field) Killing Collimation Collimator Rescattering Perfect Detection

10 Specifics Geant 4 framework based on a distribution example Developed spectrometer design based on UVa in-house code. Magnetic field incorporated using QWeak interpolation code of the fieldmap. Collimators kill all particles, no showering, rescattering. Detectors perfectly detect all energy.

11 Spectrometer Design (written/used by Clayton Davis, UVa) 3-D Biot-Savart calculation using current segments. Text file defined straight current paths of finite extent. Parameters prescribed density of current segments used to model each current section. Calculation for single coil, with all seven added after appropriate rotation/translation. 3-D Ray tracing optics rays are tracked through the system (Runge-Kutte). Step-size and coil granularity errors are estimated through additional calculations. Simpler to use (and faster?) than GEANT. 3-D Field Map Field map output used for GEANT MC code. Optics agrees between GEANT and homebuilt tracking. (Field map takes as long to generate as the ray traces do.)

12 traj4up Nested toroidal coils 7 coils returning current to central line current % azimuth line current near beamline degree azimuthal fill for coil limbs 38 cm height 775 A/coil 16 A/coil 1686 A/coil 39 A/coil 2916 A/coil 528 A/coil 1.7 cm 369,6 A total 1 m 11 m 12 m 13 m 14 m 14.5m 16.5m

13 Generators Implemented by Dustin McNulty Møller Event Generator Moller Cross Section (Ebeam = 11GeV) Elastic ep Event Generator Elastic ep Cross Section (Ebeam =11GeV) d#/d" (barns) 1-2 d#/d" (barns) ! cm (degrees) (degrees)! lab Inelastic ep cross section based on tables from Peter Bosted.

14 Additional details Incoming radiation done in generator with genercone Outgoing radiation done in Geant4 Uniformly rastered beam 5 x 5 mm z-target vertex uniformly sampled; realistic vertex distribution obtained by cross section weighting events

15 Acceptance

16 Future developments Aluminum Target window scattering Realistic target z-vertex sampling Realistic beam angle and offset at the target Multiple scattering for incoming (pre-vertex) beam Internal radiative vertex correction Beam - target - spectrometer misalignments

17 Collimation Studies by Luis Mercado Block line-of-sight photons. Done with main collimator at 6 meters and several sequential collimators to block onebounce photons. Avoid blocking the signal. Estimate heating in collimation elemets.

18 Collimation Simulation done with internal Geant cross sections using beam and target approach. Only 3 Moller events per 1M beam electrons. Out of 1 Moller events generated, about 7% hit the detector. 6 photons get through the collimators. Photon energies are mostly a few hundred MeV.

19 1 beam electrons Plot of photons only

20 Energy profile for 1 events.

21 Summary Simulation used to produce proposal. Work is ongoing, additional collaborators welcome and encouraged.