Electron Polarimetry Working Group Update Wolfgang Lorenzon (Michigan) EIC Collaboration Meeting Stony Brook Dec 7-8, W. Lorenzon SBU Dec-2007

EIC Electron Polarimetry Workshop August 23-24, 2007 hosted by the University of Michigan (Ann Arbor) (A. Deshpande, W. Lorenzon) 2W. Lorenzon SBU Dec-2007

Workshop Participants First NameLast NameAffiliation Kieran *BoyleStony Brook AbhayDeshpandeRIKEN-BNL / Stony Brook ChristophMontagBNL Brian * BallMichigan Wouter * DeconinckMichigan Avetik * HayrapetyanMichigan WolfgangLorenzonMichigan EugeneChudakovJefferson Lab DaveGaskellJefferson Lab JosephGramesJefferson Lab JeffMartinUniversity of Winnipeg Anna * MicherdzinskaUniversity of Winnipeg KentPaschkeUniversity of Virginia YuhongZhangJefferson Lab WilburFranklinMIT Bates BNL: 3 / HERA: 4 / Jlab: 7 / MIT-Bates: 1 Accelerator/Source: 3 / Polarimetry: 12 / students/postdocs (*): 5 3W. Lorenzon SBU Dec-2007

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. 4W. Lorenzon SBU Dec-2007

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 – GeV atomic electron in Fe (or Fe-alloy) polarized by external magnetic field 3. e  - photons: Compton scattering > GeV laser photons scatter off lepton beam 5W. Lorenzon SBU Dec-2007

LaboratoryPolarimeterRelative precisionDominant systematic uncertainty JLab5 MeV Mott~1%Sherman function Hall A Møller~2-3%target polarization Hall B Møller1.6% (quoted) 2-3% (realistic ?) target polarization, Levchuk effect Hall C Møller1.3% (best quoted) 0.5% (possible ?) target polarization, Levchuk effect, high current extrapolation Hall A Compton1% > 3 GeV)detector acceptance + response HERALPol Compton1.6% (~2%)analyzing power TPol Compton3.1%focus correction + analyzing power Cavity LPol Compton?still unknown MIT-BatesMott~3%Sherman function + detector response Transmission>4%analyzing power Compton~4%analyzing power SLACCompton0.5%analyzing power Polarimeter Roundup 6W. Lorenzon SBU Dec-2007

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

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? 8W. Lorenzon SBU Dec-2007

532 nm HERA (27.5 GeV) EIC (10 GeV) Jlab HERA EIC -7/9 Compton edge: Compton vs Møller Polarimetry 9 Detect  at 0 °, e - < E e Strong  need <<1 at E e < 20 GeV P laser ~ 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  CM ~ 90 °  good systematics beam energy independent ferromagnetic target P T ~ 8% beam heating (I e < 2-4  A), Levchuck eff. invasive measurement syst. error 2-3% typically 0.5% (1%?) at high magn. field rad. corr. to Born < 0.3% W. Lorenzon SBU Dec-2007

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: P e ~ , density ~ 3 ∙ cm -3, stat. 1% in 10 min at 100  A ) – expected depolarization for 100  A CEBAF < – 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 mA, transverse – stat 20% in 8 minutes (5 ∙ e - /cm 2, 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 pulses at 499 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 A z – 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 10

Hybrid Electron Compton Polarimeter with online self-calibration W. Deconinck, A. Airapetian chicane separates polarimetry from accelerator scattered electron momentum 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

A 2 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 12 W. Lorenzon SBU Dec-2007

Longitudinal Polarimetry Pair Spectrometer Geant simulations with pencil beams (10 GeV leptons on 2.32 eV photons) Coincidence Mode: - acceptance (from 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 (  functions) - fix configuration (dipole strength, length, position, hodoscope position and sizes, … - estimate efficiencies, count rates e + e – coincidence mode e + e – single arm mode single hodo channels all 18 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 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% 7.5 GeV beam 2.32 eV laser

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 cm 2 will subtend relevant region for asymmetry at lowest energy for 50 m flight path 15

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

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

W. Lorenzon SBU Dec-2007 Conclusions Electron Polarimetry working group has been formed –kick-off at A 2 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) 18