1. W. LorenzonEIC, 8 November 20021 The Longitudinal Polarimeter at HERA W. Lorenzon (Michigan) Collaboration Polarization at HERA Laser System & Calorimeter Statistical and Systematic Precision Laser Cavity Project Summary and Outlook

2. W. LorenzonEIC, 8 November 20022 Electron Polarization at HERA Self polarization of electrons by Synchrotron radiation in the curved sections (“Sokolov- Ternov effect”)

3. W. LorenzonEIC, 8 November 20023 Sokolov Ternov Effect P ST = 0.924 for an ideal, flat machine, independent of any machine parameter.  ST = 37 min for HERA @ 27.5 GeV (prop.  , R 3 ) Depolarization effects (  D ) in a real machine can substantially reduce P max : and effective build-up time constant  : P ST and  ST calculable from first principles a simultaneous measurement of P max and  provides a calibration method:

4. W. LorenzonEIC, 8 November 20024 HERMES Spin Rotators  spin = a  orbit  spin  orbit (mrad) 45 0 12.5 90 0 25 180 0 50 Spin Tune at 27.5 GeV a  = E/(0.440 GeV) = 62.5

5. W. LorenzonEIC, 8 November 20025 Principle of the P e Measurement with the Longitudinal Polarimeter Compton Scattering: e+  e’+  Cross Section: d  /dE  = d  0 /dE  [ 1+ P e P A z (E  ) ] d  0, A z : known (QED) P e : longitudinal polarization of e beam P : circular polarization (+-1) of laser beam e (27.5 GeV)  (2.33 eV)  EE Calorimeter Back Scattered Compton photon

6. W. LorenzonEIC, 8 November 20026 Experimental Setup - Overview Overview Calorimeter position NaBi(WO 4 ) 2 crystal calorimeter

7. W. LorenzonEIC, 8 November 20027 Experimental Setup – Laser System - M1/2 M3/4 M5/6: phase-compensated mirrors - laser light polarization measured continuously in box #2

8. W. LorenzonEIC, 8 November 20028 Experimental Setup - Details

9. W. LorenzonEIC, 8 November 20029 Laser Control - COP Automatic Control - Synchronizing laser and electron beam - optimize luminosity - optimize polarization - center  beam on calorimeter - control & readjust all parameters: … laser spots on all mirrors using CCD cameras

10. W. LorenzonEIC, 8 November 200210 Polarimeter Operation I Single-Photon Mode Advantages: - large asymmetry: 0.60 (max) - easy comparison to d  /dE Disadvantage: - dP/P = 0.01 in 2.5 h (too long) A s (E  ) = (   –    (   +     = P c P e A z (E  )

11. W. LorenzonEIC, 8 November 200211 Polarimeter Operation II Multi-Photon Mode Advantages: - eff. independent of brems. Bkg and detector cutoff energy - dP/P = 0.01 in 1 min Disadvantage: - no easy monitoring of calorimeter performance A m = (   –    (   +       = P c P e A p A p = (   –    (   +     = 0.184 (if detector is linear)

12. W. LorenzonEIC, 8 November 200212 Polarization Determination A p = (   –    (   +     = 0.193 (for crystal calorimeter) Question: Is response function linear over full single to multi- photon range? 0.9% syst. uncertainty 0.8% syst. uncertainty DESY CERN

13. W. LorenzonEIC, 8 November 200213 Polarization Performance HERA: 220 bunches separated by 96 ns 174 colliding + 15 non-colliding = 189 filled bunches 20 min measurement dP/P = 0.03 in each bunch time dependence: helpful tool for tuning

14. W. LorenzonEIC, 8 November 200214 “Non-Intrusive” Method Possible to ‘empty’ an electron bunch with a powerful laser beam at HERA

15. W. LorenzonEIC, 8 November 200215 A large Systematic Effect - Solved In multi-photon mode: 1000  ’s 6.8 TeV in detector Protect PMT’s from saturation insert Ni foil in 3mm air gap Problem: NBW crystals are 19 X 0 small shower leakage with large analyzing power A p (w/ foil) = 1.25 x A p (w/o foil) No light attenuators are used since early 1999

16. W. LorenzonEIC, 8 November 200216 Systematic Uncertainties Source  P e /P e (%) (2000) Analyzing Power A p - response function - single to multi photon transition A p long-term stability Gain mismatching Laser light polarization Pockels Cell misalignment Electron beam instability +- 1.2  (+-0.9) (+-0.8) +- 0.5 +- 0.3  +- 0.2 +- 0.4  +- 0.8  Total+-1.6  new sampling calorimeter built and tested at DESY and CERN  statistics limited

17. W. LorenzonEIC, 8 November 200217 New Sampling Calorimeter Wave length shifters Tungsten platesScintillator plates PMT’s Test beam results DESY CERN ‘r(E) = 1’ better energy resolution & linearity than NBW calorimeter

18. W. LorenzonEIC, 8 November 200218 New Sampling Calorimeter - Details beam view side view - sampling & NBW calorimeters sit on movable table (x-y) fast switching possible - one calorimeter is always protected from synchrotron and bremsstrahlung radiation

19. W. LorenzonEIC, 8 November 200219 Systematic Uncertainties Source  P e /P e (%) (2000)  P e /P e (%) (>2002) Analyzing Power A p - response function - single to multi photon transition A p long-term stability Gain mismatching Laser light polarization Pockels Cell misalignment Electron beam instability +- 1.2  (0.9) (0.8) +- 0.5 +- 0.3  +- 0.2 +- 0.4  +- 0.8  +- 0.8 (+-0.2) (+-0.8) +- 0.5 +-0.2 +-0.3 Total+-1.6+-1.0  new sampling calorimeter built and tested at DESY and CERN  statistics limited expected precision (multi-photon mode)

20. W. LorenzonEIC, 8 November 200220 Polarization-2000 HERMES, H1, ZEUS and Machine Group Goal: Fast and precise polarization measurements of each electron bunch Task: major upgrade to Transverse Polarimeter (done) upgrade laser system for Longitudinal Polarimeter Update on Laser Cavity System (courtesy F. Zomer)

21. W. LorenzonEIC, 8 November 200221 Precision Measurements (QCD): Neutral Current (NC) and Charged Current (CC) cross-sections very sensitive to P e at high Q 2  Parton densities at large x Electroweak Physics: NC  qZ couplings: v q, a q CC  W boson (propagator) mass Physics Beyond Standard Model: Right-handed CC interaction Enhanced sensitivity for searching for Leptoquarks, R p violating SUSY Necessities of having Fast & Precise P e Measurement The Physics Consideration Example: NC cross-section Up to Q 2 = s = 10 5

22. W. LorenzonEIC, 8 November 200222 A Comparison of Different Polarimeters Expected precision 0.7W Fabry-Perot Cavity P L e-  rate  rate (n  ) (  P e ) stat (  P e ) syst LPOL: 33MW 0.1KHz 1000  /pulse 1%/min ~2% pulse laser multi-  mode (all bunches) i.e. 0.01  /bc 1%/(>30min) (bc=bunch crossing) (single bunch) TPOL: 10W 10MHz 0.01  /bc 1-2%/min ~4%  cw laser single-  mode (all bunches) <2% (cw=continuous wave) (upgrade) New LPOL: 5kW 10MHz 1  /bc 0.1%/6s per mill cw laser few-  mode (all bunches) 1%/min (single bunch)

23. W. LorenzonEIC, 8 November 200223 A High Gain Fabry-Perot Cavity Use Fabry-Perot cavity to amplify laser power: All optical components are fixed rigidly on an optical table SS HERA e beam (A proven technology at Jefferson Lab) Linearly polarized photons Circularly polarized photons

24. W. LorenzonEIC, 8 November 200224 Final cavity Mount for travel Fabry-Perot Cavity

25. W. LorenzonEIC, 8 November 200225 beam pipe ‘laser axis’ bellows Fabry-Perot Cavity - Details

26. W. LorenzonEIC, 8 November 200226 beam scan measurement x y Intensity Beam intensity after cavity (Gaussian in principle)

27. W. LorenzonEIC, 8 November 200227 Laser Cavity Timelines Mechanics: – cavity arrived beginning July at Orsay August-Sept: vacuum tests – optical mounts and cavity mirror mounts Being done in LAL workshop (finished in Sept.) Feedback and cavity gain – still low, investigations being done & wait the final system for more more tests (mirror alignment syst. variations) Laser polarisation – per mill level almost reached after 1 year of work… Test of the final setup: – started in September Installation in HERA tunnel: – during March shutdown (16 weeks)

28. W. LorenzonEIC, 8 November 200228 New LPOL (few-photon mode): Existing LPOL (multi-photon mode): Compton and Bremsstrahlung edges Up to 1000  produced per pulse clearly visible Signal/background ratio improved Background determination and > 5TeV measured in the detector! Calibration easy Calibration difficult (  P e ) syst : per mill level expected Non-linearity  main syst. error Photon Detection and Systematic Uncertainty of P e S  =+1 S  =-1 S  =+1

29. W. LorenzonEIC, 8 November 200229 Summary on Laser Cavity New polarimeter with a Fabry-Perot cavity: High statistics precision: Large gain in cw laser power (~10 4 ) 0.1% every 6s (all bunches) 1% every minute (single bunch) Small systematic uncertainties: Few-photon mode Per mill level Fast and precise measurement of P e : Valuable feedback for HERA machine to achieve maximum polarization Necessary to fully exploit the physics potential at HERA after upgrade Cavity prototype being tested at LAL, Orsay Final set-up will be installed during the shutdown in spring 2002

30. W. LorenzonEIC, 8 November 200230 Polarization after Lumi Upgrade “old” configuration: spin rotators for ZEUS & H1 off First dedicated spin tuning: encouraging progress What is polarization w/ all spin rotators turned on?

31. W. LorenzonEIC, 8 November 200231 TPOL/LPOL Ratio LPOL: operate at beam current > 5 mA to prevent signal to be too close to pedestal Still under investigation TPOL: various analysis codes give different answers! Strategy: investigate ratio for good and bad fills and look for any dependencies good fill

32. W. LorenzonEIC, 8 November 200232 Conclusions Longitudinal Polarization is currently measured with – 1% statistical uncertainty per minute (for all bunches) – 1.6% systematic uncertainty (with NBW calorimeter) – ~1% systematic uncertainty (with new Sampling calorimeter) After upgrade to optical cavity is finished, – systematic uncertainty: 1% or less. – statistical uncertainty: 1-2% for each individual bunch Longitudinal Polarimeter measures rate (energy) differences Compton Scattering is ideal for E beam > 1 GeV – large asymmetries: A ~  2 *E – polarization measured/monitored continuously