Status of the recoil nucleon polarimeter Dan Watts, Derek Glazier, Mark Sikora (SUPA PhD student) (University of Edinburgh, UK)

Outline Physics motivation Polarimeter operation Beam test - proof of polarimeter concept First results - beam helicity transfer observabes (Cx) Outlook

Physics motivation: Nucleon excitation spectrum Excitation spectrum is fundamental to nucleon structure - but not firmly established Particularly disappointing given the potential advances from theory Lattice QCD Holographic dual of QCD Constituent quark models QCD models

  + N →  m Polarisation observables Linear Polarisation Circular polarisation Recoil polarimeter - enable the first complete measurement of observables Fully constrain the reaction amplitudes Longitudinally polarised proton target Transversely polarised  just one of 16 observables in pseudo scalar meson photoproduction Complete measurement requires 8 well chosen observables Only possible with double polarisation measurements

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

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()]  Nucleon scattering and polarisation

The polarimeter setup

Test data results - p(   ) yield E e =1.5 GeV

Test data analysis – p(  0 )p C x’ 2 x 3 day beam times (E e =0.85 and 1.5 GeV) - First data for C x !! Photon energy (MeV) Cx’Cx’ Cx’Cx’

2  0 – test of helicity bit Single spin beam helicity asymmetry

Test data analysis - p(  ) First measurement of beam helicity transfer in  photoproduction!! Azimuthal scatter angle in polarimeter E  <0.9 geV All   E  = GeV All  

Summary and outlook Succesful nucleon polarimeter test - now ready for production beamtime Formalism for extraction of Ox, T we developed for CB proposal used succesfully to extract observables in JLAB kaon photoproduction measurement New Edinburgh PhD student to work on project poised to provide unique measurements of double-polarisation observables for for  and  meson photoproduction

MAID predictions and expected data accuracy - p(  )N 300 hrs MAMI B 500 hrs MAMI C  cm =120 o ±10

For events with nuclear scatter in polarimeter Tagged nucleon events

Present PWA solutions indicate sensitivity of observables to specific resonances Sensitivity to Roper P 11 (1440) MAID PWA No Roper Cross section Linear Polarisation Asymmetry Linear Polarisation + RECOIL E  = 500 MeV Recoil observables give large sensitivities to poorly established resonances e.g. Roper P 11 (1440)

High quality meson photoproduction data with polarisation observables can be expected from MAMI Determination of beam, target and recoil polarisation will give a “complete” measurement of observables Commissioning data for nucleon polarimeter expected in 2007 Summary

Double polarisation in meson photoproduction Many overlapping resonances are a problem Double polarisation observables give new constraints on resonance properties and reaction mechanisms Polarised  beams  + p → N + meson Polarised targets  target recoil

For events Scattered in polarimeter

Present knowledge of the spectrum “Roper” Resonance  Mass ~ ±20 MeV  Width ~ ±100 MeV!! Large discrepancies between analyses of same experimental data with different amplitude analysis methods  (1232) P 11 (1440) D 13 (1520) F 15 (1680)

Intense tagged photon beam, circularly or linearly polarised Longitudinally polarised proton and neutron targets Approved programme of measurements

4 complex amplitudes - 16 observables in meson photoproduction To fix the 4 amplitudes unambiguously need to measure 8 real quantities d  + 3 single polarisation + 5 double polarisation Cannot choose from same set Need recoil polarisation measurements  target recoil Why measure double polarisation observables?

Photon Tagger upgrade

Predicted sensitivity to poorly established resonances Resonance parameters from quark model (Capstick and Roberts) Solid – SAID Dashed – background + **** Dotdash- background + **** +N - 3/2 (1960) Dutta, Gao and Lee, PRC 65, (2002) Cx’ (  + recoil) – theoretical predictions

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

First determination p(,p) 0 in 2002 Hall A JLab MAID & SAID poor description of new data Recent C x’ measurement at JLab Polarisation transfer C x’ Photon energy (MeV)

The proposed experimental setup Graphite sheet TAPS Crystal Ball  beam Hydrogen target cell Initial path of proton Polarimeter acceptance : ±20 o polar angle (target at centre) Most events suffer only coulomb scattering 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()]

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

   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

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

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

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

Summary The different sensitivities offered by recoil polarisation observables will give new constraints on the excitation spectrum of the nucleon. Data will be complimentary to the beam-target measurement programmes in place at MAMI and other facilities UK EPSRC grant already awarded to help setup the facility (including 2 year postdoc and graphite)

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

Ox’ – linearly polarised  and recoil One measurement : p(  + )n Yerevan 80’s  P~2/√(A 2 N) P  =0.4, E  =±25 MeV,  m =130±10  ~ 1  b/sr →  Ox ~ 0.04  ~ 0.1  b/sr →  Ox ~0.12 Polarimeter - full  acceptance - determine T as the y component. Periodically change polarisation direction by ±45 o - eliminate detector effects.

Lx (Longitudinally polarised Target + recoil) No previous measurements Mainz target: ~80% polarisation P T =0.7, E  =±25MeV,  m =130±10  ~ 1  b/sr →  Lx ~  ~ 0.1  b/sr →  Lx ~0.05 BUT: Limitations in beam intensity and dilution from polarised target Must measure background contribution from non-proton events. Prompt to background 1:1 worsens error by √2  Transversely polarised (Tx)?

Cross sections Eg bin +-25 MeV Pion bin +-10 degrees, 500 hrbeamtime p( ,N)  Cross sections as low as 1  b/sr (>2*106 n bin1-) p( ,N)  p( ,N)  Cross sections as low as 0.1  b/sr (0.2*106 nucleons per bin) Assume 1% of nucleons undergo nuclear interaction in proposed graphite sheet (select high analysing power with theta cut)

Estimate of polarimetry accuracy Take d  d  ~1  b/sr,   =130±10  DA ~  CB-TAPS~0.7, N  =2.5x10 5  sec -1 MeV -1 N Nucleons = N T x N  x  DA x  CB-TAPS x   2222 day -1 MeV -1  500 hour beamtime have 2.3x10 6 nucleons in E  =±25MeV bin  Polarimeter efficiency 2% gives 4.6x10 4 useful nucleons  Absolute error in polarisation  P~√(2/A 2 N) ~ 0.02 (A~0.4 for 12C)  For 0.1  b/sr absolute uncertainty in polarisation  P~0.06  For double polarisation must divide error by beam(target) polarisation  p( 0 )p p()p   =130

d  d  ~1  b/sr,   =130±10,  DA ~0.7;  CB-TAPS ~0.5, N  =2.5x10 5  s -1 MeV -1  N Nucleons = N T x N  x  DA x  CB-TAPS x   day -1 MeV -1  20 days beam, E  =±25MeV → 5.5x10 6 nucleons  polarimeter) ~2 % → 11.1x10 4 useful nucleons  Analysing power A~0.4 for 12 C  ~1  b/sr →  P~√(2/A 2 N) ~ (abs. error)  ~0.1  b/sr →  P~0.026  For double polarisation must include further effects of degree of beam(target) polarisation Estimate of polarimetry accuracy p( 0 )p p()p   =130   =130

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

Detrimental side-effects of scatterer material  To hit polarimeter T N >100 MeV in (p,)N above the   Proton energy loss 100 MeV.  Multiple scattering 100 MeV  0.37 radiation lengths  conversion ~ 30% T p incident proton (MeV) T p exit proton (MeV) T p after graphite Energy loss Coulomb scattering Proton energy (MeV) FWHM scattering angle (deg)