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W. Lorenzon EIC, 8 November 2002 1
The Longitudinal Polarimeterat HERA
W. Lorenzon (Michigan) Collaboration
• Polarization at HERA
• Laser System & Calorimeter
• Statistical and Systematic Precision
• Laser Cavity Project
• Summary and Outlook
W. Lorenzon EIC, 8 November 2002 2
Electron Polarization at HERA
Self polarization of electrons by Synchrotron radiation in the curved sections (“Sokolov-Ternov effect”)
)1()( /tePtP
W. Lorenzon EIC, 8 November 2002 3
Sokolov Ternov Effect
• PST = 0.924 for an ideal, flat machine, independent of any machine parameter.
• ST = 37 min for HERA @ 27.5 GeV (prop. , R3)
• Depolarization effects (D) in a real machine can substantially reduce Pmax:
and effective build-up time constant :
• PST and ST calculable from first principles a simultaneous measurement of Pmax and provides a calibration method:
DST
DSTPP
max
DST
DST
measτP PkPST
ST max
W. Lorenzon EIC, 8 November 2002 4
HERMES Spin Rotators
spin = aorbit
spin orbit
(mrad)
450 12.5 900 25 1800 50
Spin Tune at 27.5 GeVa = E/(0.440 GeV) = 62.5
W. Lorenzon EIC, 8 November 2002 5
Principle of the Pe Measurement withthe Longitudinal Polarimeter
Compton Scattering: e+ e’+
Cross Section: d/dE = d0/dE[1+ PePAz(E)]
d0, Az: known (QED) Pe: longitudinal polarization of e beam P: circular polarization (+-1) of laser beam
e (27.5 GeV) (2.33 eV)
ECalorimeterBack Scattered
Compton photon
W. Lorenzon EIC, 8 November 2002 6
Experimental Setup - Overview
Overview
Calorimeter position
NaBi(WO4)2 crystal calorimeter
W. Lorenzon EIC, 8 November 2002 7
Experimental Setup – Laser System
- M1/2 M3/4 M5/6: phase-compensated mirrors- laser light polarization measured continuously in box #2
W. Lorenzon EIC, 8 November 2002 8
Experimental Setup - Details
W. Lorenzon EIC, 8 November 2002 9
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
W. Lorenzon EIC, 8 November 2002 10
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)
As(E) = (–(+= Pc Pe Az(E)
W. Lorenzon EIC, 8 November 2002 11
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
Am = (–(+= Pc Pe Ap
Ap = (–(+
= 0.184 (if detector is linear)
W. Lorenzon EIC, 8 November 2002 12
Polarization Determination
Ap = (–(+
= 0.193 (for crystal calorimeter)
dEErEdEdE
Eii )()/(
max
min
Question: Is response function linear over full single to multi-photon range?
0.9% syst. uncertainty 0.8% syst. uncertainty
DESY
CERN
W. Lorenzon EIC, 8 November 2002 13
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
W. Lorenzon EIC, 8 November 2002 14
“Non-Intrusive” Method
Possible to ‘empty’ an electron bunch with a powerful laser beam at HERA
W. Lorenzon EIC, 8 November 2002 15
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 X0
small shower leakage with large analyzing power
Ap(w/ foil) = 1.25 x Ap(w/o foil)No light attenuators are used since early 1999
W. Lorenzon EIC, 8 November 2002 16
Systematic Uncertainties
Source Pe/Pe (%)
(2000)
Analyzing Power Ap- response function - single to multi photon transition
Ap long-term stability
Gain mismatchingLaser light polarizationPockels Cell misalignmentElectron 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 CERNstatistics limited
W. Lorenzon EIC, 8 November 2002 17
New Sampling Calorimeter
Wave length shifters
Tungsten plates
Scintillator plates
PMT’sTest beam results
DESY
CERN
‘r(E) = 1’
better energy resolution & linearity than NBW calorimeter
W. Lorenzon EIC, 8 November 2002 18
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
W. Lorenzon EIC, 8 November 2002 19
Systematic Uncertainties
Source Pe/Pe (%)
(2000)
Pe/Pe (%)
(>2002)
Analyzing Power Ap- response function - single to multi photon transition
Ap long-term stability
Gain mismatchingLaser light polarizationPockels Cell misalignmentElectron 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+-0.2+-0.2+-0.3
Total +-1.6 +-1.0
new sampling calorimeter built and tested at DESY and CERNstatistics limited expected precision (multi-photon mode)
W. Lorenzon EIC, 8 November 2002 20
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)
W. Lorenzon EIC, 8 November 2002 21
Precision Measurements (QCD):
Neutral Current (NC) and Charged Current (CC) cross-sections very sensitive to Pe at high Q2
Parton densities at large x
Electroweak Physics:
NC qZ couplings: vq, aq
CC W boson (propagator) mass
Physics Beyond Standard Model:
Right-handed CC interaction Enhanced sensitivity for searching for Leptoquarks, Rp violating SUSY
Necessities of having Fast & Precise Pe MeasurementThe Physics Consideration
Example: NC cross-section
Up to Q2 = s = 105
W. Lorenzon EIC, 8 November 2002 22
A Comparison of Different Polarimeters
Expected precision0.7W
Fabry-Perot Cavity
PL e- rate rate (n) (Pe)stat (Pe)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)
W. Lorenzon EIC, 8 November 2002 23
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
HERA e beam
(A proven technology at Jefferson Lab)
Linearly polarizedphotons
Circularlypolarizedphotons
W. Lorenzon EIC, 8 November 2002 24
Final
cavity Mount for travel
Fabry-Perot Cavity
W. Lorenzon EIC, 8 November 2002 25
beam pipe
‘laser axis’
bellows
Fabry-Perot Cavity - Details
W. Lorenzon EIC, 8 November 2002 26
beam scanmeasurement
x
y
Intensity
Beam intensity after cavity (Gaussian in principle)
W. Lorenzon EIC, 8 November 2002 27
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)
W. Lorenzon EIC, 8 November 2002 28
New LPOL (few-photon mode): Existing LPOL (multi-photon mode):
Compton and Bremsstrahlung edges Up to 1000 produced per pulseclearly visible Signal/background ratio improvedBackground determination and > 5TeV measured in the detector!Calibration easy Calibration difficult(Pe)syst: per mill level expected Non-linearity main syst. error
Photon Detection and Systematic Uncertainty of Pe
S=+1
S=-1
S=-1 S=+1
W. Lorenzon 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
W. Lorenzon EIC, 8 November 2002 30
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?
W. Lorenzon EIC, 8 November 2002 31
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
W. Lorenzon EIC, 8 November 2002 32
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 Ebeam > 1 GeV
– large asymmetries: A ~ 2*E
– polarization measured/monitored continuously