A compact pair polarimeter and spectrometer Ken Livingston, University of Glasgow, Scotland 1 NPE Seminar October 2016 Motivation Pair production and polarimetry Compact polarimeter design 1

The tagged photon facilities Motivation* Baryons: N* Energy (GeV) Mesons: GlueX 1.5 1.5 3 11 12 The tagged photon facilities Hardon physics set for major breakthroughs in both Pol observables + Ampitude analysis – require high precision measurements Beam and target polarisation need to be known very accuarately. 2

Polarization observables Linear Polarisation  Circular polarisation  Nucleon recoil polarimeter x +N → Π → Y Hyperons are “self analysing”  Transverse pol targets p,n Longitudinally pol targets p,n 3

Polarization observables – Measuring asymmetries: a simple example,  4

Polarization observables – Measuring asymmetries: a simple example,  Systematics of detector acceptance cancel out. “Only” need to know Plin, the degree of linear polarization. 5

Linearly polarised photons Tagged bremsstrahlung photons up to 6GeV. Timing resolution < 1 beam bucket. Circularly polarized up to ~80% now standard Linearly polarized coherent bremsstrahlung up to >90%. 6

Linearly polarised photons Tagged bremsstrahlung photons up to 6GeV. Timing resolution < 1 beam bucket. Circularly polarized up to ~80% now standard Linearly polarized coherent bremsstrahlung up to 90%. 7

Pair production Photon incident on a thin scattering foil. Pair production – recoil is a Nucleus < 20% analysing power, high cross section ] Triplet production – recoil is an atomic electron. > 20% analysing power in recoil electron angle, easy to detect, low cross section The distribution of azimuthal angle of the pair gives the degree of linear pol. Selecting symmetric pairs increases the analysing power Looks easy ! 8

Pair production – why it's not easy dX 9

Pair production – why it's not easy X dX 10

Pair production For symmetric events (x+ ~ x- = x) x >> dX: Measure Eg. Use only dY for polarimetry dx/x ~ dE/E: Eg from tagger. Use dX and dY for polarimetry Dx/x ~ d(B.dl)/B.dl 11

Halbach dipole array Timepix3 Polarimeter design Halbach dipole array Timepix3 12

Simulation 13

Simulation Worst case Best case Based on Mainz PS rates with ~10nA beam Best case (High Eg resolution from Tagger.) <2% error per tagger bin in 20minutes. Worst case (No Eg from tagger, dY only) <2% error per tagger bin in 5Hrs 14

Simulation Worst case Best case Based on Mainz PS rates with ~10nA beam Best case (High Eg resolution from Tagger.) <2% error per tagger bin in 20minutes. Worst case (No Eg from tagger, dY only) <2% error per tagger bin in 5Hrs 15

Conclusion Looks polarised to me 16

STFC1 Motivation – spectroscopy. Baryon – CLAS, MAMI, Bonn, Meson, GlueX Pol observables – Lin pol coherent brem - polarimetry 3) Current method – distribution of tagged electrons. Good stats. Limit is systematics (%5) Not detecting the photon. Infer pol from dist of degraded electrons. Only improve by measuring the pol of the photon. 4) PS process. Detect the photon. 5) In beam or Mag field. 6) Why is this not in use. Rate calc. Size. 7) Design. Result. 8) Cost. 17