The A2 recoil nucleon polarimeter  Daniel Watts University of Edinburgh, UK

Why nucleon polarimetry?  Would add a unique capability to the MAMI setup – Valuable compliment to circularly and linearly polarised photon beams and polarised target systems.  Observables from recoil group will allow MAMI to make the first complete measurement in pion/eta photoproduction (Nucleon polarimetry proposal approved by last MAMI-ELSA PAC)

Double-polarisation in pseudo-scalar meson photoproduction Polarisation of  target recoil Observable

Nucleon Scattering and polarisation Analysing power of scatterer Polar angle distribution for unpolarised nucleons x and y (transverse) components of nucleon polarisation Number of nucleons scattered In the direction  n() =n o (){1+A()[P y cos()–P x sin()] 

 Simulate p(   )p channel – realistic beam/target & detector parameters  New routines written for GEANT3 – introduced modulation of  for hadronic interactions (take A=1)  All other processes left in. e.g. coulomb scattering, nuclear de-excitations …  Explore possible designs for polarimeter New GEANT simulations incorporating polarimetry  00 p/p/ p   x p defines  plane

Reconstructed Phi in incident nucleon frame (deg) E  (MeV) A = (  + -  - ) / (  + +  - ) Design 1: Graphite at CB exit ~32% reduction in A  pol ~ 2.4% (E  = GeV,  scat >20)   (cm) > 130 Graphite scatterer TAPS Target CB skirt Analyser efficiency (7cm graphite) Yield (a.u)    COM  (Deg) Yield (a.u)

Design 2: Graphite in CB tunnel ~45% reduction in A  pol ~ 3% (E  = GeV,  scat >15)   (cm) > 130 Graphite scatterer Target Analyser efficiency (7cm graphite) Reconstructed Phi in incident nucleon frame (deg) A = (  + -  - ) / (  + -  - ) Yield (a.u) E  (MeV)    COM  (Deg)

Design 3: Graphite Near Target ~46% reduction in A  pol ~ 3 % (E  = GeV,  scat >15) Graphite scatterer Target Analyser efficiency (7cm graphite)   (cm) > 90 Reconstructed Phi in incident nucleon frame (deg) A = (  + -  - ) / (  + -  - ) Yield (a.u)    COM  (Deg)E  (MeV)

~35% dilution of analysing power Acceptance X% If proves worth can move more upstream to greatly increase acceptance Design 4: Graphite Near Target + subsequent CB detection!!   (cm) > 60 o ~53% reduction in A  pol ~ 2.6% (E  = MeV,  scat >20) Graphite scatterer Target Analyser efficiency (7cm graphite) Reconstructed Phi in incident nucleon frame (deg) A = (  + -  - ) / (  + -  - ) Yield (a.u)    COM  (Deg)E  (MeV)

Test polarimeter Polarimeter with adjustable thickness and hole diameter Will fit in “orange pipe” used in PID tests Polarimeter presently being machined in Edinburgh Ready for use in tests from late Oct

Tracker detector(s)  First polarimetry measurements on proton target do not need tracker – BUT tracker necessary for neutron target measurements (Fermi motion)  Need to finalise polarimeter design before can finalise tracker design – need test beam time Tracker Possibilities - Si detectors on face(s) of graphite - Wire chambers - Scintillating fibre  Money already available – Edinburgh £120k GWU £50k Also Mainz, UCLA, …

Conclusion  Simulations give good indication that we can start testing nucleon polarimeter (and getting first data!) now.  Test polarimeter module ready this month - need test beamtime with prototype to move the project forward  Forward angle tracker pre-requisite to allow neutron target measurements in the longer term

~35% dilution of analysing power Acceptance X% If proves worth can move more upstream to greatly increase acceptance Reconstructed Phi in incident nucleon frame Egamma (MeV) A = (  + -  - ) / (  + -  - ) Graphite CB tunnel Design 4: Graphite Near Target + subsequent CB detection!!   (cm) > 60 ~50% reduction in A  pol ~ X%

Reconstructed Phi in incident nucleon frame Egamma (MeV) A = (  + -  - ) / (  + -  - ) Graphite CB tunnel Design 1: Graphite at CB exit ~32% reduction in A  pol ~ 2.4%   (cm) > 130

~35% dilution of analysing power Acceptance X% If proves worth can move more upstream to greatly increase acceptance Reconstructed Phi in incident nucleon frame Egamma (MeV) A = (  + -  - ) / (  + -  - ) Graphite CB tunnel Design 3: Graphite Near Target ~46% reduction in A  pol ~ X %

Reconstructed Phi in incident nucleon frame Egamma (MeV) A = (  + -  - ) / (  + -  - ) Design 2: Graphite in CB tunnel Graphite CB tunnel ~35% reduction in A  pol ~ 3%   (cm) > 130

Nucleon polarimetry concept Graphite sheet TAPS Crystal Ball  beam Hydrogen target cell Useful scattered event Select events with scattering angles larger than ~10 degrees : arising from nuclear interaction n() =n o (){1+A()[P y cos()–P x sin()]

Design 3 – 7cm Graphite 8cm from target  ~30% dilution of analysing power  Acceptance X%  If proves worth can move more upstream to greatly increase acceptance Reconstructed Phi in incident nucleon frame Egamma (MeV) A = (  + -  - ) / (  + -  - )

P T Previous experimental data – SAID database Data for all CM breakup angles O x’ C x’ Recent JLAB data not in database

GEANT simulation of polarimeter No Graphite With Graphite scatterer Simulation includes realistic smearing of energy deposits due to experimental energy resolution and proper cluster finding algorithms Finite target size and E  resolution included Angle between  N (E ,   ) and TAPS hit

~30% dilution of analysing power Acceptance X% Reconstructed Phi in incident nucleon frame Egamma (MeV) A = (  + -  - ) / (  + -  - ) Design 1: Graphite in CB tunnel Graphite CB tunnel

   CM) >~130 o E=150 MeV E=200 Eg=300 E=500 E=750 E=1000 E=1500 Polarimeter acceptance Nucleon angle in lab (deg) Pion angle in CM (deg) Kinematic acceptance of polarimeter p(  )N

More forward recoils than for pion production. Almost all recoils are incident on polarimeter up to ~0.8 GeV Eg=720 Eg=820 Eg=920 Eg=1520 Lab nucleon angle (degrees) CM  angle (degrees) Polarimeter acceptance Kinematic acceptance of polarimeter p(  )N

MAID predictions and expected data accuracy - p(  )N 300 hrs MAMI B 500 hrs MAMI C

New GEANT simulations  Simulate New routines added to GEANT – introduced  modulation for hadronic interactions (take A=1)  Simulated p(,p) 0 data. Run through AcquRoot analysis. Accurate description of target size, beam properties, CB & mini TAPS. E= MeV  All other processes left in.  Explore possible designs for polarimeter without need for tracker p

MAID predictions and expected data accuracy - p(  )N 300 hrs MAMI B Full MAID No P 11 (1440)

Cx’ – Extraction and expected accuracy Plot difference in  distributions for two helicity states (cut on region of  with reasonable A()) Left with simple sin() Dependence. Extract Px Photon energy (MeV) Cx’  P  =0.7, E=±25MeV,   =130±10  ~ 1 b/sr → Cx ~  ~ 0.1 b/sr → Cx ~0.05  Greatly improved data quality -

Expected data accuracy Common parameters: Photon beam: 2.5x10 5  sec -1 MeV -1 Bin ±12.5 MeV Target: 2.1  nuclei / cm 2 Meson:    Bin ±10 o Polarimeter: 3% probability for a (detected) nuclear scatter Average analysing power ~0.4

Principles of nucleon polarimetry  Well established technique – relies on spin-orbit interaction in Nucleon-Nucleon interaction  Polarimeters - exploited nucleon or nuclear targets ( 2 H, 4 He, 12 C, 28 Si) – tended to use materials with well known analysing powers pomme A1 FPP G En Polarimeter Kent state

 Measure direction of nucleon before and after the scatterer with sufficient accuracy to determine an analysing reaction has taken place. Polarimetry basics For incident protons also have multiple (coulomb) scattering  scat =5-20 o  scat

Scattered nucleon detection in TAPS  1 TAPS block ~ position resolution for hit  TAPS~0.9m from scatterer N  Straight through 10 o scatter 20 o scatter