Elastic Scattering and Diffraction at D

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Elastic Scattering andDiffraction at DØ
Tamsin Edwards for the DØ collaboration 14th -
18th April, 2004 XII International Workshop on
Deep Inelastic Scattering, trbské Pleso,
Slovakia
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Colour singlet exchange

• The Tevatron collides protons and antiprotons at
  vs 1.96 TeV at an average rate of 1.7 MHz

• Elastic and diffractive processes involve the
  exchange of a colour singlet

• Colour singlet exchange

• Quantum numbers of the vacuum

• no charge
• no colour

• often referred to as Pomeron exchange

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Searches for colour singlet exchange

• Two types of analysis discussed in this talk

• Single Diffraction

• search for rapidity gap in forward regions of DØ
• Luminosity Monitor
• Calorimeter

rapidity gap

• Elastic Scattering

proton track

• search for intact protons in beam pipe
• Forward Proton Detector

proton track
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Luminosity Monitor

• Luminosity Monitor (LM)
• Scintillating detector
• 2.7 lt ? lt 4.4
• Charge from wedges on one side are summed
  Detector is on/off on each side, North and South

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Calorimeter
Liquid argon/uranium calorimeter

• Cells arranged in layers
• electromagnetic (EM)
• fine hadronic (FH)
• coarse hadronic (CH)

• Sum E of Cells in
• EM and FH layers
• above threshold
• EEM gt 100 MeV
• EFH gt 200 MeV

2.7 LM range 4.4
2.6 Esum range 4.1 - 5.3
LM
FH
EM
CH
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Calorimeter energy sum

• Use energy sum to distinguish proton break-up
  from empty calorimeter

Log(energy sum) on North side
Areas are normalised to 1
empty events
physics samples
10 GeV

• Esum cut of 10GeV was chosen for current study
• Final value will be optimised using full data
  sample

• Compare 'empty event' sample with physics
  samples
• Empty event sample random trigger. Veto LM
  signals and primary vertex, i.e. mostly empty
  bunch crossings
• Physics samples minimum bias (coincidence in
  LM), jet and Z?µµ events

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Efficiency and backgrounds
Considerations to convert detector signal into
physics

• Contamination from fake interactions
• rapidity gap selection may favour non-physics
  events

• Contamination from non-diffractive events
• proton break-up not detected
• acceptance
• efficiency

• Efficiency for diffractive events
• gap filled by
• backscatter
• beam losses
• noise
• pile-up effects
• multiple interactions

These studies are currently underway, and are
required for a measurement of the ratio of
diffractive to non-diffractive events
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Search for diffractive Z?µµ

• Inclusive Z?µµ sample well understood

• di-muon (?lt2) or single muon (?lt 1.6)
  trigger

• 2 muons, pT gt 15GeV, opposite charge

• at least one muon isolated in tracker and
  calorimeter

• anti-cosmics cuts based on tracks
• displacement wrt beam
• acolinearity of two tracks

Mµµ (GeV)
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First step towards gap LM only

• Separate the Z sample into four groups according
  to LM on/off

• Expect worst cosmic ray contamination in
  sample with both sides of LM off
• no evidence of overwhelming cosmics
  background in LM off samples

WORK IN PROGRESS
cosmics shape expected from inclusive sample
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Z Mass of rapidity gap candidates

• Add Esum requirement

• Invariant mass confirms that these are all
  Drell-Yann/Z events
• Will be able to compare Z boson kinematics
  (pT, pz, rapidity)

WORK IN PROGRESS
Gap North Gap Southcombined
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Diffractive Z?µµ candidate
outgoing proton side
outgoing anti-proton side
muon
muon
muon
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muon
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Z?µµ with rapidity gaps Summary

• Preliminary definition of rapidity gap at DØ Run
  II
• Study of Z?µµ- events with a rapidity gap
  signature (little or no energy detected in the
  forward direction)
• Current status
• Evidence of Z events with a rapidity gap
  signature
• Quantitative studies of gap definition,
  backgrounds, efficiency in progress (effects
  could be large)
• No interpretation in terms of diffractive
  physics possible yet
• Plans
• Measurement of the fraction of diffractively
  produced Z events
• Diffractive W?µ?, W/Z?electrons, jets and other
  channels
• Use tracks from Forward Proton Detector

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Forward Proton Detector

• Forward Proton Detector (FPD)

- a series of momentum spectrometers that make
use of accelerator magnets in conjunction with
position detectors along the beam line

• Quadrupole Spectrometers
• surround the beam up, down, in, out
• use quadrupole magnets (focus beam)

• Dipole Spectrometer
• inside the beam ring in the horizontal plane
• use dipole magnet (bends beam)

• also shown here separators (bring beams
  together for collisions)

A total of 9 spectrometers composed of 18 Roman
Pots
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Forward Proton Detector
Forward Proton Detector

• scintillating fiber tracker
• can be brought within a few millimetres of the
  beam

• six layers to minimise ghost hits and
  reconstruction ambiguities
• diagonal U, U
• opposite diagonal V, V
• vertical X, X
• trigger scintillator

• primed layers offset from unprimed
• read out by PMTs

Reconstructed track is used to calculate
kinematic variables of the scattered proton
t - four-momentum transfer
? - the fraction of longitudinal momentum
lost by the proton
t ?2, where ? is scattering angle
where pi(f) inital (final) momentum
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Elastic Scattering

• Elastic scattering ? 0

• Quadrupole acceptance
• t gt 0.8 GeV2 (requires sufficient scattering
  angle to leave beam)
• all ? (no longitudinal momentum loss necessary)

• Measure dN/dt for elastic scattering using
  preliminary and incomplete FPD

• antiproton side
• quadrupole up spectrometer
• trigger only

• proton side
• quadrupole down spectrometer
• full detector read-out

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Preliminary Elastic Scattering Results
? distribution
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Preliminary Elastic Scattering Results

• The ds/dt data collected by different
  experiments at different energies
• A factor of 10-2 must be applied to each
  curve
• New DØ dN/dt distribution has been normalized
  by E710 data
• Compare slope with model Block et al, Phys.
  Rev. D41, pp 978, 1990.

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Elastic Scattering Diffraction Summary

• Study of Z?µµ- events with a rapidity gap
  signature
• Evidence of Z events with a rapidity gap
  signature
• Quantitative studies of gap definition,
  backgrounds, efficiency in progress

• Proton-antiproton elastic scattering was
  measured by the DØ Forward Proton Detector

• dN/dt was measured in the range 0.96 lt t lt
  1.34 GeV2

• The future study many diffractive physics
  channels using rapidity gaps and full Forward
  Proton Detector system