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The Longitudinal Polarimeter at HERA

This document summarizes the latest advancements in the longitudinal polarimeter employed at HERA, focusing on the statistical and systematic precision of electron polarization measurements. Key developments include the implementation of a Fabry-Perot cavity to amplify laser power and achieve high precision in polarization determination. The report also discusses the methodology for measuring the longitudinal polarization (Pe) and addresses systematic uncertainties encountered during experiments. These enhancements are crucial for maximizing the physics potential at HERA, especially in the context of upcoming upgrades and future experiments.

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The Longitudinal Polarimeter at HERA

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  1. 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 EIC, 8 November 2002

  2. Electron Polarization at HERA EIC, 8 November 2002

  3. Sokolov Ternov Effect EIC, 8 November 2002

  4. HERMES Spin Rotators EIC, 8 November 2002

  5. Principle of the Pe Measurement with the Longitudinal Polarimeter Eg l (2.33 eV) e (27.5 GeV) a Back Scattered Compton photon Calorimeter Compton Scattering: e+lg e’+ Cross Section: ds/dEg = ds0/dEg[1+PePlAz(Eg)] ds0, Az: known (QED) Pe:longitudinal polarization ofe beam Pl: circular polarization (+-1) of laser beam EIC, 8 November 2002

  6. Experimental Setup - Overview EIC, 8 November 2002

  7. Experimental Setup – Laser System EIC, 8 November 2002

  8. Experimental Setup - Details EIC, 8 November 2002

  9. Laser Control - COP EIC, 8 November 2002

  10. Polarimeter Operation I EIC, 8 November 2002

  11. Polarimeter Operation II EIC, 8 November 2002

  12. Polarization Determination EIC, 8 November 2002

  13. Polarization Performance EIC, 8 November 2002

  14. “Non-Intrusive” Method EIC, 8 November 2002

  15. A large Systematic Effect - Solved EIC, 8 November 2002

  16. Systematic Uncertainties EIC, 8 November 2002

  17. New Sampling Calorimeter EIC, 8 November 2002

  18. New Sampling Calorimeter - Details EIC, 8 November 2002

  19. Systematic Uncertainties EIC, 8 November 2002

  20. Polarization-2000 EIC, 8 November 2002

  21. Necessities of having Fast & Precise Pe Measurement The Physics Consideration Precision Measurements (QCD): Neutral Current (NC) and Charged Current (CC) cross-sections very sensitive to Pe at high Q2 g Parton densities at large x Electroweak Physics: NC g qZ couplings: vq, aq CC g W boson (propagator) mass Physics Beyond Standard Model: Right-handed CC interaction Enhanced sensitivity for searching for Leptoquarks, Rp violating SUSY Example: NC cross-section Up to Q2=s=105 EIC, 8 November 2002

  22. A Comparison of Different Polarimeters PLe-g rateg rate(ng)(dPe)stat (dPe)syst LPOL: 33MW 0.1KHz1000 g/pulse1%/min~2% pulse lasermulti-g mode(all bunches) i.e. 0.01 g/bc1%/(>30min) (bc=bunch crossing)(single bunch) TPOL: 10W 10MHz0.01 g/bc1-2%/min~4% g cw lasersingle-g mode(all bunches)<2% (cw=continuous wave)(upgrade) New LPOL: 5kW 10MHz1 g/bc0.1%/6sper mill cw laserfew-g mode(all bunches) 1%/min (single bunch) Fabry-Perot Cavity Expected precision 0.7W EIC, 8 November 2002

  23. A High Gain Fabry-Perot Cavity Sg Use Fabry-Perot cavity to amplify laser power: (A proven technology at Jefferson Lab) HERA e beam Linearly polarized photons Circularly polarized photons All optical components are fixed rigidly on an optical table EIC, 8 November 2002

  24. Fabry-Perot Cavity Final cavity Mount for travel EIC, 8 November 2002

  25. Fabry-Perot Cavity - Details beam pipe bellows ‘laser axis’ EIC, 8 November 2002

  26. Beam intensity after cavity (Gaussian in principle) Intensity beam scan measurement x y EIC, 8 November 2002

  27. Laser Cavity Timelines EIC, 8 November 2002

  28. Photon Detection and Systematic Uncertainty of Pe New LPOL (few-photon mode):Existing LPOL (multi-photon mode): Compton and Bremsstrahlung edges Up to 1000 g produced per pulse clearly visibleSignal/background ratio improved Background determination and > 5TeV measured in the detector! Calibration easy Calibration difficult (dPe)syst: per mill level expectedNon-linearity g main syst. error Sg=-1 Sg=+1 Sg=+1 Sg=-1 EIC, 8 November 2002

  29. Summary on Laser Cavity New polarimeter with a Fabry-Perot cavity: High statistics precision: Large gain in cw laser power (~104) 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 Pe: 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 EIC, 8 November 2002

  30. Polarization after Lumi Upgrade EIC, 8 November 2002

  31. TPOL/LPOL Ratio EIC, 8 November 2002

  32. Conclusions EIC, 8 November 2002

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