1 Percent-level Polarimetry in JLab Hall C Dave Gaskell Jefferson Lab EIC14 – March 20, 2014 Outline 1.Hall C Møller Polarimeter 2.Hall C Compton Polarimeter 3.Results and comparisons (Q- Weak) 4.EIC Considerations

2 JLab Polarimetry Techniques Three different processes used to measure electron beam polarization at JLab –Møller scattering:, atomic electrons in Fe (or Fe-alloy) polarized using external magnetic field –Compton scattering:, laser photons scatter from electron beam –Mott scattering:, spin-orbit coupling of electron spin with (large Z) target nucleus Each has advantages and disadvantages in JLab environment MethodAdvantageDisadvantage ComptonNon-destructiveCan be time consuming, systematics energy dependent MøllerRapid, precise measurementsDestructive, low current only MottRapid, precise measurementsDoes not measure polarization at the experiment

3 Møller Polarimetry Møller polarimetry benefits from large longitudinal asymmetry  -7/9  Asymmetry independent of energy  Relatively slowly varying near θ cm =90 o  Large asymmetry diluted by need to use iron foils to create polarized electrons  Rates are large, so rapid measurements are easy  The need to use Fe or Fe-alloy foils means measurement must be destructive P e ~ 8% Making measurements at high beam currents challenging  foil depolarization -7/9

4 Hall C Møller Polarimeter 2 quadrupole optics maintains constant tune at detector plane “Moderate” acceptance mitigates Levchuk effect  still a non- trivial source of uncertainty Target = pure Fe foil, brute-force polarized out of plane with 3-4 T superconducting magnet Target polarization uncertainty = 0.25% [NIM A 462 (2001) 382]

5 Hall C Møller Target Fe-alloy, in-plane polarized targets typically result is systematic errors of 2-3% –Require careful measurement of magnetization of foil Pure Fe saturated in 4 T field –Spin polarization well known  0.25% –Temperature dependence well known –No need to directly measure foil polarization Effect Ms[B]Ms[B] error Saturation magnetization (T  0 K,B  0 T)2.2160± Saturation magnetization (T=294 K, B=1 T)2.177±0.002 Corrections for B=1  4 T0.0059± Total magnetization2.183±0.002 Magnetization from orbital motion0.0918± Magnetization from spin2.0911±0.004 Target electron polarization (T=294 K, B= 4 T) ±

6 Hall C Møller Acceptance Møller events Detectors Optics designed to maintain similar acceptance at detectors independent of beam energy Collimators in front of Pb:Glass detectors define acceptance One slightly larger to reduce sensitivity to Levchuk effect

7 Møller Systematic Uncertainties Systematic error table from Q-Weak (2 nd run)  Some uncertainties larger than “usual” due to low beam energy (1 GeV)  Levchuk effect, target polarization same at all energies Total uncertainty less than 1%

8 Møller Systematic Studies Q-Weak afforded the opportunity for several systematic studies Scan of solenoid field to check for saturation and/or foil mis-alignment Scan of quad currents to check for sensitivity to spectrometer optics

9 Compton Polarimetry Compton polarimetry is non- destructive – allows polarization measurements without affecting the experiment Two main challenges for Compton polarimetry at JLab  Relatively low beam currents (~100 μA) - need novel laser technology  Relatively small asymmetries (compared to colliders) Asymmetry has a strong dependence on backscattered photon energy  Understanding detector response is crucial

10 Hall C Compton Polarimeter Installed – commissioned and used during Q-Weak Components: 1.4-dipole chicane: Deflect electron beam vertically 57 cm 2.Laser system: Low gain Fabry-Perot cavity pumped by 10 W green DPSS laser – provided kW CW power 3.Photon detector: PbWO4 operated in energy-weighted integrating mode 4.Electron detector: 4 planes of diamond strip detectors (96 strips)

11 Low Gain Fabry-Perot Cavity Laser EOM Cavity ~ ~ Oscillator Phase shifter Mixer Low-pass filter Servo amp Optical isolator Photodiode Error signal Transmitted Reflected Coherent Verdi V10 High power CW laser ( nm locked to low gain, external Fabry-Perot cavity via Pound-Drever-Hall technique

12 Low Gain Fabry-Perot Cavity Locked cavity from development tests at UVa Final system routinely achieved stored laser powers larger than 1.5 kW Reflected Transmitted Error signal

13 Laser Polarization - the Transfer Function Knowledge of the laser polarization inside cavity is a key systematic uncertainty  Polarization usually inferred from measurements of beam transmitted through cavity, after 2 nd mirror P laser ? Typically a “transfer function” is measured with cavity open to air Possible complications due to:  Change in birefringence due to mechanical stresses (tightening bolts)  Change in birefringence when pulling vacuum

14 Laser Polarization – the “Entrance” Function Propagation of light into the Fabry-Perot cavity can be described by matrix, M E  Light propagating in opposite direction described by transpose matrix, (M E ) T  If input polarization (ε 1 ) linear, polarization at cavity (ε 2 ) circular only if polarization of reflected light (ε 4 ) linear and orthogonal to input* Laser MEME MEME MTMT MTMT Exit-line polarization monitoring Steering mirrors, vacuum entrance window, half and quarter wave plates (M E ) T Steering mirrors, vacuum exit window ε1ε1 ε2ε2 ε3ε3 ε4ε4 ε 2 =M E ε 1 ε 4 =(M E ) T ε 3 ε 4 =(M E ) T M E ε 1 *J. Opt. Soc. Am. A/Vol. 10, No. 10/October 1993 JINST 5 (2010) P06006

15 Cavity Polarization via Reflected Power “If input polarization (ε 1 ) linear, polarization at cavity (ε 2 ) circular only if polarization of reflected light (ε 4 ) linear and orthogonal to input”  In the context of the Hall C system, this means that the circular polarization at cavity is maximized when retro-reflected light is minimized Circular polarization in cavity  Above statement was verified experimentally (with cavity open) by directly measuring circular polarization in cavity while monitoring retro-reflected power  Additionally, by fitting/modeling the entrance function we can determine the degree of circular polarization by monitoring the reflected power – even for the case when system is not optimized

16 Reflected Power Scans Using a combination of half and quarter wave plates, we can build an arbitrary polarization state  Scanning this polarization phase space and monitoring the retro-reflected power, we can build a model for the entrance function, M E  Free parameters include variations to HWP and QWP thicknesses, arbitrary element with non-zero birefringence Using this entrance function, we can determine the laser polarization inside the cavity for an arbitrary input state

17 Laser Polarization Systematic Uncertainty Cavity polarization optimization scans performed with cavity unlocked  No measureable difference in laser polarization when comparing to locked cavity Cavity locked Cavity unlocked Additional sources of potential uncertainty due to transmission through input cavity mirror and potential laser depolarization  Both constrained by measurement to be very small Overall systematic error on laser polarization in cavity ~ 0.1%

18 Compton Electron Detector Diamond microstrips used to detect scattered electrons  Radiation hard  Four 21mm x 21mm planes each with 96 horizontal 200 μm wide micro-strips.  Rough-tracking based/coincidence trigger suppresses backgrounds

19 Compton Electron Detector Measurements Polarization analysis:  Yield for each electron helicity state measured in each strip  Background yields measured by “turning off” (unlocking) the laser  Asymmetry constructed in each strip Strip number corresponds to scattered electron energy  Endpoint and zero-crossing of asymmetry provide kinematic scale  2-parameter fit to beam polarization and Compton endpoint

20 Preliminary Systematic Uncertainties Systematic UncertaintyUncertainty ΔP/P (%) Laser Polarization 0.1%0.1 Dipole field strength ( T)0.02 Beam energy 1 MeV0.09 Detector Longitudinal Position 1 mm0.03 Detector Rotation (pitch) 1 degree0.04 Asymmetry time averaging 0.15% Asymmetry fit 0.3% DAQ – dead time, eff. Under study?? Systematic uncertainties still under investigation, but final precision expected to be better than 1%  DA- related systematics likely the most significant remaining issue to study

21 Compton Photon Detector Preliminary Integrated Asymmetry Hall C Compton used lead- tungstate for photon detection  Operated in integration mode, similar to Hall A system (see previous talk by G. Franklin)  Smaller signal (due to laser and crystal) results in larger statistical errors  Linearity and absolute analyzing power still under investigation

22 Polarization Measurements Q-Weak Run 2 – November 2011 to May 2012 P Moller +/- stat (inner) +/- point-to-point systematic (0.54%) P Compton +/- stat +/- preliminary systematic (0.6%) Photocathode re-activation 0.64% normalization unc. not shown Preliminary

23 Hall C Møller vs. Compton Comparison of Møller and Compton results for periods of “stable” polarization Overall agreement looks good – difference of more than 1- sigma from average for 2 nd sample

24 Møller-Compton Cross Calibration Møller measurements typically made at 1 μA, Compton measurements at 180 μA  Performed a direct comparison at the same beam current  4.5 μA Møller analysis required extra corrections for beam heating, dead time Compton analysis slightly more sensitive to noise Preliminary

25 Polarization at Low and High Currents Møller and Compton give same results at same current  Does beam polarization depend on beam current? Compared low current Compton results to measurements just before and after low current test – agreement is excellent 4.5 μA180 μA Preliminary

26 Summary Hall C (JLab) houses two polarimeters capable of systematic uncertainties better than 1% –Compton polarimeter measures beam polarization with minimal impact on main experiment –Møller polarimeter allows rapid, high statistics measurements enabling easy systematic investigations Hall C polarimeters display excellent agreement Multiple measurements crucial for proving high precision and aids in monitoring system performance

27 EIC Considerations – Møller Polarimetry Møller polarimetry is an excellent candidate for precision polarimetry over the 1-10 GeV range –Application at EIC would require a new target – iron foil too destructive –Hydrogen jet target likely technically feasible, but precision on target polarization inadequate –Hybrid system with pure iron foil for calibration of “relative” target? –2 3 S 1 metastable helium also suggested as possible target* Required space not (necessarily) large: Hall C system uses about 5 meters of dedicated beamline * C. Sinclair at PSTP13

28 EIC Considerations – Compton Polarimetry In principle, high electron currents at EIC suggest that Compton polarimetry should be relatively simple Potential issues: –Large synchrotron backgrounds create problems for (integrating) photon detectors –Large currents also suggest potentially large backgrounds from other sources: beam halo and Bremsstrahlung –Counting-mode in electron detector may not be feasible Fabry-Perot cavity may be required to achieve sufficient precision  1% measurement for each “pulse-train”? –Mirror protecting apertures in FP cavity can create still more background! Care is needed

29 JLab Polarimetry  EIC JLab experience provides a valuable road-map for achieving precision polarimetry at EIC Fixed-target environment means our experience is not always directly applicable –Need to have all polarimeters be non-destructive limits options High beam currents provide unique challenges – high rates and large backgrounds

30 Acknowledgements Hall C Compton Group: –JLab, Manitoba, MIT, Mississippi State, UVa, William and Mary, Winnipeg, Uva Q-Weak Møller Group: –JLab, William and Mary, Ohio U. Work funded by NSERC, NSF, DOE

31 Extra

32 Precision Electron Beam Polarimetry Polarized electron beam at Jefferson Lab used to measure a variety of observables In general, most precise knowledge of the electron beam polarization required by experiments measuring parity violating (PV) electron scattering Examples: –PREX (neutron distribution in Pb)  1% at 850 MeV –Q Weak (Weak charge of the proton)  1% at 1 GeV –MOLLER (e-e scattering)  0.5% at 11 GeV –SOLID (PV DIS)  0.5% at 11 GeV

33 Møller Response vs. Beam Position At low energies (~1 GeV), Møller polarimeter shows increased sensitivity to absolute beam energy  Tested our understanding of variation of analyzing power with dedicated beam position scan – simulated response shows very good agreement with data

34 Low Gain Fabry-Perot Cavity Final system routinely achieved stored laser powers larger than 1.5 kW Locked cavity from development tests at UVa

35 Laser Table Layout