Electron Polarimetry Working Group Update

1. Wolfgang Lorenzon (Michigan) EIC Collaboration MeetingStony Brook Dec 7-8, 2007 Electron Polarimetry Working GroupUpdate W. Lorenzon SBU Dec-2007

2. EIC Electron Polarimetry Workshop August 23-24, 2007 hosted by the University of Michigan (Ann Arbor) http://eic.physics.lsa.umich.edu/(A. Deshpande, W. Lorenzon) W. Lorenzon SBU Dec-2007

3. Workshop Participants BNL: 3 / HERA: 4 / Jlab: 7 / MIT-Bates: 1 Accelerator/Source: 3 / Polarimetry: 12 / students/postdocs (*): 5 W. Lorenzon SBU Dec-2007

4. Goals of Workshop Which design/physics processes are appropriate for EIC? What difficulties will different design parameters present? What is required to achieve sub-1% precision? What resources are needed over next 5 years to achieve CD0 by the next Long Range Plan Meeting (2012) → Exchange of ideas among experts in electron polarimetry and source & accelerator design to examine existing and novel electron beam polarization measurement schemes. W. Lorenzon SBU Dec-2007

5. How to measure polarization of e-/e+ beams? Three different targets used currently: 1. e- - nucleus: Mott scattering 30 – 300 keV (5 MeV: JLab)spin-orbit coupling of electron spin with (large Z) target nucleus 2. e - electrons: Møller (Bhabha) scat. MeV – GeVatomic electron in Fe (or Fe-alloy) polarized by external magnetic field 3. e - photons: Compton scattering > GeVlaser photons scatter off lepton beam W. Lorenzon SBU Dec-2007

6. Polarimeter Roundup W. Lorenzon SBU Dec-2007

7. Phys. Rev. ST Accel. Beams 7, 042802 (2004) Results shown include statistical errors only → some amplification to account for non-sinusoidal behavior Statistically significant disagreement The “Spin Dance” Experiment (2000) Systematics shown: Mott Møller C 1% Compton Møller B 1.6% Møller A 3% Even including systematic errors, discrepancy still significant W. Lorenzon SBU Dec-2007

8. Lessons Learned Include polarization diagnostics and monitoring in beam lattice design minimize bremsstrahlung and synchrotron radiation Measure beam polarization continuously protects against drifts or systematic current-dependence to polarization Providing/proving precision at 1% level very challenging Multiple devices/techniques to measure polarization cross-comparisons of individual polarimeters are crucial for testing systematics of each device at least one polarimeter needs to measure absolute polarization, others might do relative measurements Compton Scattering advantages: laser polarization can be measured accurately – pure QED – non-invasive, continuous monitor – backgrounds easy to measure – ideal at high energy / high beam currents disadvantages: at low beam currents: time consuming – at low energies: small asymmetries – systematics: energy dependent Møller Scattering advantages: rapid, precise measurements – large analyzing power – high B field Fe target: ~0.5% systematic errors disadvantages: destructive – low currents only – target polarization low (Fe foil: 8%) – Levchuk effect New ideas? W. Lorenzon SBU Dec-2007

9. Compton vs Møller Polarimetry HERA EIC Jlab -7/9 • Detect g at 0°, e-< Ee • Strong  need <<1 • at Ee< 20 GeV • Plaser~100% • non-invasive measurement • syst. Error: 3 → 50 GeV (~1 → 0.5%) hard at < 1 GeV: (Jlab project: ~0.8%) • rad. corr. to Born < 0.1% • Detect e-at qCM~90° •  good systematics • beam energy independent • ferromagnetic target PT~8% • beam heating (Ie < 2-4 mA), Levchuck eff. • invasive measurement • syst. error 2-3% typically0.5% (1%?) at high magn. field • rad. corr. to Born < 0.3% W. Lorenzon SBU Dec-2007

10. New Ideas Polarized Hydrogen in a cold magnetic trap (E. Chudakov et al., IEEE Trans. Nucl. Sci. 51, 1533 (2004)) use ultra-cold traps (at 300 mK: Pe~ 1-10-5, density ~ 3∙1015 cm-3 , stat. 1% in 10 min at 100 mA) expected depolarization for 100 mA CEBAF < 10-4 limitations: beam heating → “continuous” beam & complexity of target advantages: expected accuracy < 0.5% & non-invasive, continuous, the same beam Problem: very unlikely to work for high beam currents for EIC (due to gas and cell heating) Jet Target: avoids these problems VEPP-3 100 mA, transverse stat 20% in 8 minutes (5 ∙ 1011 e- /cm2 , 100% polarization) What is electron polarization in a jet? New fiber laser technology (Jeff Martin for Hall C) Gain switched fiber laser huge luminosity boost when locked to Jlab beam structure (30 ps pulsesat499 MHz) lower instantaneous rates than high power pulsed lasers external to beam line vacuum → easy access in-house experience (Jlab source group) excellent stability, low maintenance Compton e- analysis (Kent Paschke for PV-DIS experiments) dominant challenge: determination of analyzing power Az zero-crossing e- analysis: two points of well-defined energy (Compton edge, zero crossing) linear fit of zero crossing: integrate between two points absolute calibration (only input is QED) weak dependence of energy resolution & no need to calibrate calorimeter

11. W. Deconinck, A. Airapetian Hybrid Electron Compton Polarimeterwith online self-calibration chicane separates polarimetry from accelerator scattered electronmomentum analyzed in dipole magnet measured with Si or diamond strip detector pair spectrometer (counting mode) e+e– pair production in variable converter dipole magnet separates/analyzes e+ e– sampling calorimeter (integrating mode)count rate independent Insensitive to calorimeter response 11

12. A2 Workshop Summary Electron beam polarimetry between 3 – 20 GeV seems possible at 1% level: no apparent show stoppers (but not easy) Imperative to include polarimetry in beam lattice design Use multiple devices/techniques to control systematics Issues: crossing frequency 3–35 ns: very different from RHIC and HERA beam-beam effects (depolarization) at high currents crab-crossing of bunches: effect on polarization, how to measure it? measure longitudinal polarization only, or transverse needed as well? polarimetry before, at, or after IP dedicated IP, separated from experiments? Workshop attendees agreed to be part of e-pol working group coordination of initial activities and directions: W. Lorenzon members: A. Airapetian, D. Gaskell (long. polar.), W. Franklin (trans. polar.), E. Chudakov (Møller targets) Design efforts and simulations just starting W. Lorenzon SBU Dec-2007 12

13. Longitudinal Polarimetry Pair Spectrometer Geant simulations with pencil beams (10 GeV leptons on 2.32 eV photons) Coincidence Mode: - acceptance (from <1.51 GeV (“zero crossing”) to >2.63 GeV (Compton edge) - resolution (2%-3.5%) Single Arm Mode: - analyzing magnet relates momentum and position of pair produced e - e+ - provide well defined e - or e+ beams to calibrate the Compton photon calorimeter Plans: - include beam smearing (a, b functions) - fix configuration (dipole strength, length, position, hodoscope position and sizes, … - estimate efficiencies, count rates e+e– coincidence mode all 18 hodo channels e+e– single arm mode single hodo channels 13

14. Longitudinal Polarimetry (II) Compton electron detection - using chicane design, max deflection from e- beam: 22.4 cm (10 GeV), 6.7 cm (3 GeV) deflection at “zero-crossing”: 11.1 cm (10 GeV), 3.3 cm (3 GeV) →e- detection should be easy Plans: - include realistic beam properties →study bkgd rates due to halo and beam divergence - adopt Geant MC from Hall C Compton design - learn from Jlab Hall C new Compton polarimeter 7.5 GeV beam2.32 eV laser • Compton photon detection • Sampling calorimeter (W, pSi) modeled in Geant • based on HERA calorimeter • study effect of additional energy smearing No additional smearing additional smearing: 5% additional smearing: 10% additional smearing: 15% 14

15. Transverse Polarimetry Energy Dependence - analyzing power as function of scattered photon energy - large variation in energy of peak analyzing power 20 GeV studies - using pencil beams - peak asymmetry in gamma spectrum at ~6 GeV for 20 GeV electron beam of - resolution of ~1 m needed in vertical centroid for 1% polar. measurement for 50 m flight path 3 GeV studies - peak asymmetry in gamma spectrum at ~200 MeV for 3 GeV electron beam - position sensitive detector of 10*10 cm2 will subtend relevant region for asymmetry at lowest energy for 50 m flight path 15

16. Transverse Polarimetry (II) • Plans: • Asymmetries appear adequate for transverse polarimetry, even at low energies. • Inclusion of transverse electron polarimetry within IP polarimeter appears feasible with compact position-sensitive detector in photon arm. Flight path greater than 50 m desirable. • Next steps: • Include beta functions and emittance at IP • Projection of asymmetry vs. position for asymmetry for EIC energies • Begin simulation to determine effective analyzing power of calorimeter • Use of electron vertical information? 16

17. Møller Polarimetry • Hydrogen Atomic Jet • Just started investigations • Several problems to address: • Breit-Rabi measurement analyzes only part of jet • → uniformity of jet has to be understood • large background from ions in the beam: most of them associated with jet (hard to measure) • origin of background observed in Novosibirsk still unclear (in contact with them) • clarification of depolarization by beam RF needed • → might be considerable 17

18. Conclusions • Electron Polarimetry working group has been formed • kick-off at A2 Workshop in Aug 2007 • design efforts and simulations have started • dialog with accelerator groups at BNL / JLab • There are issues that need attention (crossing frequency 3-35 ns; beam-beam effects at high currents; crab crossing effect on polarization) • JLAB at 12 GeV will be a natural testbed for future EIC Polarimeter tests • evaluate new ideas/technologies for the EIC • No serious obstacles are foreseen to achieve 1% precision for electron beam polarimetry at the EIC (3-20 GeV) W. Lorenzon SBU Dec-2007 18