Electron and Ion Polarimetry for EIC Wolfgang Lorenzon (Michigan) Electron-Ion Collider Workshop Hampton University 20 May 2008 Thanks to Yousef Makdisi

EIC Objectives e-p and e-ion collisions c.m. energies: 20 - 100 GeV 10 GeV (~3 - 20 GeV) electrons/positrons 250 GeV (~30 - 250 GeV) protons 100 GeV/u (~50-100 GeV/u) heavy ions (eRHIC) / (~15-170 GeV/u) light ions (3He) Polarized lepton, proton and light ion beams Longitudinal polarization at Interaction Point (IP): ~70% or better Bunch separation: 3 - 35 ns Luminosity: L(ep) ~1033 - 1034 cm-2 s-1 per IP Goal: 50 fb-1 in 10 years 2

Electron Ion Collider ELIC Addition of a high energy polarized electron beam facility to the existing RHIC [eRHIC] Addition of a high energy hadron/nuclear beam facility at Jefferson Lab [ELectron Ion Collider: ELIC] will drastically enhance our ability to study fundamental and universal aspects of QCD ELIC 3

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 > 1 GeV laser photons scatter off lepton beam Goal: measure DP/P ≈ 1% (realistic ?)

How to measure polarization of p beams? For transverse beam polarization: 1. p - hydrogen: p-p elastic scattering 10 – 100 GeV AN (2%-10%) at low t(0.1-0.3): drops with 1/Ep 2. p - hydrogen: inclusive pion production 12 – 200 GeV AN <50% for p+/ p- at xF ~0.8, but is it large over entire EIC energy range? 3. p - carbon: p-C elastic (CNI region) 24 – 250 GeV AN <5% (“calculable”), but high cross section & weak dependence on Ep 4. p - hydrogen: p-p elastic (CNI region) 24 – 250 GeV AN <5% (“calculable”), but high cross section & weak dependence on Ep Goal: measure DP/P ≈ 2-3% (challenging) Note: unlike e-/e+ polarimeters (where QED processes are calculable), proton polarimeters rely on experimental verifications (especially at high energies).

e-/e+ Polarimeter Roundup Laboratory Polarimeter Relative precision Dominant systematic uncertainty JLab 5 MeV Mott ~1% Sherman function Hall A Møller ~2-3% target polarization Hall B Møller 1.6% (?) 2-3% (realistic ?) target polarization, Levchuk effect Hall C Møller 1.3% (best quoted) 0.5% (possible ?) target polarization, Levchuk effect, high current extrapolation Hall A Compton 1% (@ > 3 GeV) detector acceptance + response HERA LPol Compton 1.6% analyzing power TPol Compton 3.1% focus correction + analyzing power Cavity LPol Compton ? still unknown MIT-Bates Mott ~3% Sherman function + detector response Transmission >4% Compton ~4% SLAC 0.5%

The “Spin Dance” Experiment (2000) 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 Systematics shown: Mott Møller C 1% Compton Møller B 1.6% Møller A 3% Even including systematic errors, discrepancy still significant

Lessons Learned Providing/proving precision at 1% level challenging Including polarization diagnostics/monitoring in beam lattice design crucial Measure polarization at (or close to) IP Measure beam polarization continuously protects against drifts or systematic current-dependence to polarization Flip electron and laser polarizations fast enough to protect against drifts 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 absolute measurement does not have to be fast 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 New ideas

Dominant Challenge: determine Az Best tool to measure e- polarization → Compton e- (integrating mode) Traditional approach: use a dipole magnet to momentum analyze Compton e- accurate knowledge of ∫Bdl must calibrate the electron detector fit the asymmetry shape or use Compton Edge

Electron Polarimetry Kent Paschke 9/14/2007 W. Lorenzon PSTP 2007

e-/e+ Polarimetry at EIC 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? Design efforts and simulations have started 11

EIC Compton Polarimeter 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 12

Possible Compton IP Location (ELIC) ~85 m available for electron polarimetry ~20 m needed for chicane simulations started for IP location at s=161 m location can be shifted due to cell structure (8.2m) of lattice design Alex Bogacz 13

Compton Polarimetry Pair Spectrometer - Geant simulations with pencil beams (10 GeV leptons on 2.32 eV photons) - including beam smearing (a, b functions): resolution (2%-3.5%) Plans: - fix configuration (dipole strength, length, position, hodoscope position and sizes, … - estimate efficiencies, count rates 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 beam 2.32 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% 14 14 additional smearing: 15%

RHIC Polarized Collider Absolute Polarimeter (H jet) RHIC pC Polarimeters BRAHMS & PP2PP PHOBOS Siberian Snakes Siberian Snakes Goal: DPb/Pb = 5% PHENIX STAR Spin Rotators (longitudinal polarization) Spin Rotators (longitudinal polarization) Pol. H- Source LINAC BOOSTER Helical Partial Siberian Snake AGS Show layout & point of polarimeters and experimental locations Emphasize bunch polarizations prepared at source going through AGS etc. to RHIC Time spacing between them ~100 ns Transverse polarization in ring, longitudinal at experiments (optional) using spin rotators Emphasize none of these techniques existed just 4-5 years ago, all developed here at RHIC with international collaborations led by US and Japanese teams. 200 MeV Polarimeter AGS pC Polarimeter Strong AGS Snake Source: Lamb Shift Polarimeter Linac (200 MeV): p-C scattering (calibrated with p-D elastic scattering) Ap-X ≈ 0.50

p-p and p-C elastic scattering in CNI region The asymmetry is “calculable”: J. Schwinger, Phys. Rev. 69,681 (1946) Weak beam momentum dependence Analyzing power is few percent (≤ 5%) Cross section is high The single-flip hadronic amplitude is unknown, estimated at ~15 % uncertainty → absolute calibration necessary A simple apparatus (detect the slow recoil protons or carbon @ ~ 900) PLB 638 (2006) 450 |r5|=0 RHIC @ 100 GeV Concept test: first at IUCF and later at the AGS C targets survive RHIC beam heating

Hyperfine state (1),(2),(3),(4) The RHIC Polarized Hydrogen Jet Target pumps 1000 l/s compression 106 for H nozzle temperature 70K sextupoles 1.5T pole field and 2.5T/cm grad. RF transitions SFT (1.43GHz) WFT (14MHz) holding field 1.2 kG B/B = 10-3 vacuum 10-8 Torr (Jet on) / 10-9 Torr (Jet off) molecular hydrogen contamination 1.5% overall nuclear polarization dilution of 3% Jet beam intensity 12.4 x 1016 H atoms /sec nuclear polarization (BRP): 95.8% ± 0.1% Jet beam polarization measured (after corrections): 92.4% ± 1.8% Jet beam size 6.6 mm FWHM In 2006 the Jet measured the beam to jet polarization ratio to 10% per 6-hr store Hyperfine states (1),(2),(3),(4) (1),(2) Pz+ : (1),(4) SFT ON (2)(4) Pz- : (2),(3) WFT ON (1)(3) Pz0: (1),(2),(3),(4) (SFT&WFT ON ) Hyperfine state (1),(2),(3),(4)

p-C polarimeter vs Hydrogen Jet (2006) p-C CNI data Fill Number H-Jet calibration data p-C CNI data 100 GeV 32 GeV

Issues with p Polarimetry at RHIC Beam Polarization: desired goal for RHIC {5%} → DPb/Pb = 4.2% largest syst uncertainties: beam polarization profile {5%} improvement in C target mechanism is expected to eliminate this uncertainty molecular H fraction {1.8%} residual gas background {2.1%} H-Jet Pb measurements per fill {10% (stat) in 6 hr} increase Si t-range acceptance open up the holding field magnet aperture p-C polarimeter {2-3% (stat) per min} replace Si strips with APDs (better energy resolution) improve beam profile and polarization profile measurements Molecular H component molecular H fraction is 1.5% → 3% nuclear dilution (if H2 is unpolarized) H2 content confirmed with electron beam ionizing jet beam and analyzing it with magnet repeat those measurements using proton beam luminescence and a CCD camera → H lines seen, but not H2 lines: more work needed DPsyst/Psyst = 2.8% 19

e-/e+ & p/ion Polarimetry at EIC No serious obstacles are foreseen to achieve 1% precision for electron beam polarimetry at the EIC (3-20 GeV) JLAB at 12 GeV will be a natural testbed for future EIC e-/e+ Polarimeter tests evaluate new ideas/technologies for the EIC There are issues that need attention (crossing frequency 3-35 ns; beam-beam effects at high currents; crab crossing effect on polarization) Proton beam polarimetry between 24 GeV (injection) – 250 GeV (top energy) seems possible at 2-3% level (but not easy) if goal is at 1-2% level: there is a long way to go major challenges are closer bunch spacing at the EIC and reducing the H jet molecular fraction to below 2% Studies for 3He beams have started Design efforts and simulations have started for e-/e+ & p/ion polarimetry 20