A Silicon Detector for the HERA Transverse Polarimeter

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Polarization at HERA Fabio Metlica (Penn
State University) ZEUS Students
Seminar 06/06/03
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Polarization at HERA

• Physics with Polarized Lepton Beams
• The HERA Upgrade gt HERA II
• Polarization at HERA
• HERA Polarimeters TPOL and LPOL
• Summary

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HERA II Physics with Polarized Lepton Beams

• The study of polarized lepton (e/e-) proton
  deep inelastic scattering at high will be one
  of the main physics topics at HERAII. Accurate
  polarization (P?0.7) measurement together with
  high luminosity opens a new field for HERA
  physics at high . Possibility to study the
  chiral structure of the SM
• -Charged currents only LH particles
  interact
• -Neutral currents LH, RH particles
  interact with Z unequally
• Electroweak physics study EW interactions using
  the 4 possible combinations e-, e with Plt0 and
  Pgt0. (Future Physics at HERA workshop).
• -Polarized NC,CC cross section
  measurements
• -Light quark couplings to the Z-boson
  
• -Precise measurements of EW
  parameters.
• QCD G2 structure function
• Search for new physics physics beyond SM
• -Right handed charged currents
• -Leptoquarks

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Polarized NC, CC Cross Sections

• Investigate EW physics NC and CC cross
  section measurements at high with Pgt0,
  Plt0.
• Cross
  sections for NC and CC

X(p)
X(p)
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Light Quark Couplings to the Z Boson
Neutral Current

• Polarization effect is large on cross sections
• for .
• Different behavior at high due to Z
  exchange.
• Split up to factor 2, at .
  
• Exploit difference to extract the light
  quark(u,d) couplings to the Z-boson.

x0.2
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Light Quark Couplings to the Z Boson

• Measurement of polarized NC,CC? disentangle the
  light quark(u,d) couplings to Z.
• Done by using all four charge/polarization
  combinations.
• Unpolarized DIS with e-/e beams ? axial
  couplings.
• Polarized DIS with the 4 charge/ polarization
  combinations ? vector couplings.

1? Contours for NC Couplings for Different
Polarizations
u coupling fixed
d coupling fixed
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Light Quark Couplings to the Z Boson

• With of data equally divided between
  e/e- and P0.7
• Based on full MC simulation detector
  improvements would improve precision.
• Complementary to LEP measurements with heavy
  quarks and similar to LEP precision for heavy
  quarks.

u,d couplings to Z-boson
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Measurements of EW Parameters

• Free parameters in SM
  
• Measure constrain ?
• data from NC/CC cross sections with
• beam polarization ?
• Test of EW universality.
• LH gives highest precision cross section
  largest.
• High Pol. is worth a factor 4 in Lumi for NC.

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Measure G2 Structure Function

• 
  structure function arising from ?Z
  interference.
• G2 can be measured from the neutral current
  parity violating asymmetry
• 
  
  ? degree of polarization (P)
  
  
  
  
  
  axial charge of electron
• Simulation for P?50 with for
  each polarization setting G2 can be well
  measured at high x (M. Klein DIS2001 Bologna).

x?1
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Search for Right Handed CC

• Standard model only LH CC exists
• Cross section vanishes for purely
• RH electron (for P1)

RHCC
LHCC
For e reverse sign
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Search for Right Handed CC

• Set a limit on the mass of the hypothetical
  boson (model dependent)
• (250
  data).
• Direct search.
• Need high polarization ?50, and
• precise polarization measurement.

e-p SM MC
ep SM MC
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New Physics Beyond SM

• Polarization is a useful tool for searching new
  physics signals.
• SM backgrounds can be reduced (turned off) by
  varying the degree of polarization. If new
  physics have different couplings, the S/B will be
  increased.
• Helps determine properties of newly discovered
  particles.
• Study chirality of new particles e.g.
  leptoquarks.

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HERA II

• HERA UPGRADE increase luminosity and provide
  longitudinal lepton beam polarization to the
  colliding experiments. Long shutdown 2000-2001,
  to modify IP and install spin rotators.
• 
  LUMINOSITY UPGRADE
• Strong focusing at IP major changes of machine
  lattice near IP. Sets of superconducting
  quadrupoles installed close to the H1/ZEUS IP,
  inside the detectors.
• Design luminosity HERA I (achieved)
• Delivered by HERA I
• (during 1992-2000)
• Design luminosity HERA II
• To be delivered by HERA II by end 2006
• (original plan,
  now?)
• Likely startup value for lumi

IP AREA
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POL2000 Group Polarization Upgrade

• ZEUS TPOL position sensitive detector (silicon
  microstrip fiber detector) IC-London and Tokyo
  Metropolitan University
• H1 LPOL cavity and fast DAQ for TPOL
• HERMES LPOL operation
• HERA polarized lepton beam studies and operation

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HERA II Polarization

• In HERA leptons become transversely polarized
  through the emission of synchrotron radiation
  (spin flips) Sokolov-Ternov effect.
• The transverse polarization is converted into
  longitudinal polarization near the interaction
  points by Spin Rotators (HERA I HERMES HERA II
  also H1/ZEUS) .
• The lepton beam transverse
• polarization is measured by the
• TPOL polarimeter, and the
• longitudinal polarization is
• measured at the LPOL
• polarimeter, independently.

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Polarization

• Polarization is defined as
• Polarization has to be known at the same level of
  precision as Total Luminosity, because it
  enters linearly in the cross section for many
  processes e.g.
• Polarization precision goal for HERA II
  0.01 ( 0.02-0.04 for HERA I),
• needed for polarization physics.
• The absolute value of the degree of lepton
  polarization is the same along the whole ring.
  The actual location of the polarization
  measurement is not confined to the experiment IP.
• Precise measurement from the TPOL/LPOL
  polarimeters together with machine lattice
  simulations, will provide confidence of having an
  accurate P measurement at IP.

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Polarization

• Polarization builds up and settles asymptotically
  to an equilibrium value.

HERA I

• Theoretical max polarization ? 92, but reduced
  by counteracting depolarizing effects (dependence
  on ring parameters),
• each spin rotator pair reduces polariz 3.
  
• HERA I ? 55-65 polarization
• HERA II startup ? 50 polarization (aim for
  more ?60). Spin rotators and different lattice
  setups reduces polarization.

HERA I
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Polarization Measurement

• The polarimeters make use of the spin-dependent
  cross section for Compton scattering of polarized
  photons on polarized leptons(e/-)
• Alternatively L/R (100Hz) circularly polarized
  laser light is scattered off leptons . The
  produced Compton photons are backscattered (in
  lab frame) into a narrow cone centered around the
  initial lepton direction and are detected by the
  polarimeter (LPOL or TPOL).
• The Compton beam size at the TPOL is
  
  

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Polarization Measurement

• The polarization at HERA is determined by
  measuring asymmetries.

LPOL
TPOL
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Polarization Measurement

• Need accurate per bunch per minute polarization
  measurement ? LPOL and TPOL upgrade for HERA2.

• The polarization of lepton colliding and
  non-colliding bunches differs (measured by LPOL
  at HERA I). Depends on the machine polarization
  tuning and varies in time (beam-beam effects).

HERA I LPOL
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Polarization TPOL

• TPOL upgrade completed
• Fast DAQ to measure the polarization of
  individual bunches per minute.
• Radhard silicon position sensitive detector to
  allow in-situ eta-y calibration.
• TPOL ready for polarization measurement.

Laser room 9th floor HERA B
TPOL Setup (similar setup for LPOL)
HERA Tunnel
HERA B
TPOL
Lepton direction
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TPOL-Spatial Asymmetry

• Spatial asymmetry
• The typical total average vertical shift is about
  0.15 mm.
• In practice is obtained by measuring the
  up-down calorimeter energy asymmetry of the
  Compton photons
• determined by the
  calibration MC test beam data.
• transformation is of
  primary importance (main source of systematic
  error).

test beam data
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TPOL HERA I

• TPOL Tungsten scintillator sampling calorimeter,
  10W CW laser alternating LCP, RCP at 90Hz .
• Operated in Single -photon mode 1 photon per
  200 bunch crossings

Compton energy spectrum of TPOL (HERA1)
Compton spatial distribution at TPOL (HERA1)
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Upgraded TPOL

• TPOL calo measures total energy
  
  . ,vertical position
  Y of the Compton ? by the transformation.
• Silicon detector improve position resolution
  from 1 mm (calo) to better than 50 ?m
  (silicon) intrinsic silicon resol. is 24?m.
• Transformation determine
  accurately in-situ in real time, do not have to
  rely on test beam data.
• Silicon position sensitive detector prototype
  (1cm1cm), final setup (6.3cm6.3cm)
  (IC-London).
• A single scintillating fiber detector for
  position calibration (?5??m) and monitoring of
  the Si-detector (Tokyo Metro. Uni).

TPOL calo Tungsten scintillator sampling
calorimeter. Laser 10W CW laser alternating
LCP,RCP 90Hz .
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Prototype Silicon Detector for TPOL

• Prototype silicon detector1cm1cm,
• APV25 readout chip.
• Tested with DESY T22 test beam
• (6 GeV electrons).

Landau distr.

• Results in ZEUS Note ZEUS-01-019
• Strips S/N ? 20, Efficiency ? 97
• Position resolution lt 50 ?m (beam telescope).
• Good Eta-Y measurement

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Silicon Detector for TPOL

• CERN test beam 6-50 GeV
• Si detector(6.3cm6.3cm),readout by 6 APV25, 80?m
  pitch
• Spare TPOL calo. (tungsten-scintillator
  sampling)
• Scintillating fiber Detector (7cm,1mm?)
• precision stage motor (reso. 1 ?m).

CERN test beam results and upgraded TPOL setup
in ZEUS Note ZEUS-02-019

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CERN Test Beam Results
Good eta-y
Non-Linearitylt1
Fiber calib. Precis50?m
Resolution 25/sqrt(E)
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TPOL HERAII Data
Silicon event
Silicon Raw data
signal
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Silicon Detector for TPOL
Silicon 80?m
Pitch Adaptor 44?m-gt80?m
Silicon
APV25 44?m
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TPOL in HERA Tunnel
Open TPOL( CaloSiliconFiber)
TPOL BOX
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LPOL

• No major change Original LPOL operational
• Two calorimeters (Crystal and Sampling ) which
  may be used alternatively (movable support
  tables). Change to sampling better energy
  linearity.
• Pulsed laser operates in Multi-photon mode
  approx. 600 to1000 Compton photons produced
  per laser pulse (100 Hz) shot on a lepton bunch.
  
• Measure shifts of energy spectra for the two
  light polarizations.
• Large difference in the spectra close to the
  Compton edge.
• LPOL ready for polarization measurements.

RCP
LCP
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LPOL Cavity Upgrade in Progress

• LPOL upgrade New more powerful laser
  system ( Fabry-Perot cavity new laser) to
  operate in single photon mode (1-2 photons per
  bunch crossing).
• Large laser photon flux increase
  probability of Compton scattering.
• 1) direct calibration with Compton edge (define
  energy scale of calo)
• 2) improve statistical error by having a high
  average laser power 10kW (not commercially
  available).

LPOL Cavity IP
HERMES
Lepton direction
LPOL
IP
CW laser
Original LPOL IP
Linear polarized laser light
Circular polarized laser light
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LPOL Cavity
(Orsay)

• The cavity has been successfully installed
  during shutdown in HERA tunnel (near HERMES).
• Cavity is locked and stable (gain factor
  ), should be ready by end of shutdown.
• Startup plan start with original LPOL setup,
  then move to laser cavity setup.

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LPOL Cavity Installation in HERA Tunnel, 2003
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HERA Polarization Tuning 2003

• High polarization , P50, with all 3
• spin rotators on and colliding beams.
• Both LPOL and TPOL working, and in reasonable
  agreement. TPOL minor technical problems low
  TPOL luminosity.

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Polarimeter Online Java Display
Displays various polarization info
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Polarization Data for ZEUS

• All polarimeter info is stored into the ORACLE
  database. Accessible to all HERA experiments.
• Software developed (Arafat Gabareen) to read
  polarization data from ORACLE and integrate it
  with the rest of the ZEUS data.
• Polarization data integration during ZEUS event
  reconstruction, typically a few days or a week
  after the data is taken.
• Two ADAMO tables
• 1)TPOL and LPOL values P,?P for all 220
  bunches.
• 2)TPOL and LPOL values P,?P average for
  colliding and non-colliding bunches.

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Extra Info

• e?e- 4-6 weeks. Orbits differ, magnets need to
  be moved by 6mm.
• Longitudinal Polarization flip 1-2 days. HERA1
  (LPOL) done once a month during access day
  (mechanical movement of magnets) plus time for
  beam polarization tuning.
• Longitudinal Polarization direction (Plt0 or Pgt0)
  each experiment can choose independently.
• Polarimeters no background problem seen. No
  lattice modification carried out at polarimeter
  locations, backgrounds similar to HERA1.
• Lepton transverse polarization did not affect the
  measurements at HERAI (H1/ZEUS).
  suppressed to order compared to
  .
• (Does Transverse Electron Polarization
  Affect Measurements at HERA? M. Kraemer et. al.)

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Summary

• HERA has been upgraded?HERAII
• - luminosity increase
• - lepton beam polarization P?50 with
  ?P/P 1 (upgraded polarimeters).
• New field of HERA physics is opened with
  polarized lepton beams.
• Two polarimeters LPOL(cavity upgrade to be
  completed) and TPOL to provide accurate
  polarization measurements to the HERAII
  experiments.
• Polarimeters will operate independently and cross
  check one another together with HERAII machine
  lattice simulations will provide confidence in
  the IP polarization value.
• TPOL/LPOL working, measured 50 polarization
  before start of shutdown.
• Discussion between experiments and DESY to
  organize running and maintenance of the
  polarimeters.