Elastic

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Elastic Diffractive Scatteringat the DØ
Experiment

• Tamsin Edwards
• University of Manchester, UK
• APS Meeting5th - 8th April 2003

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Hadron scattering at the Tevatron
The Tevatron accelerator collides protons and
antiprotons at a centre-of-mass energy of vs
1.96 TeV at an average rate of 1.7 MHz

• 60 of total pp cross-section is Inelastic
  Scattering
• p and p break up
• Main part of DØ experimental analysis
• rare hard scale processes production of jets,
  W/Z bosons, b top quarks etc

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Elastic Scattering
Elastic scattering events

• p and p scatter with no momentum loss
• beam particles remain intact, scattered at very
  small angle
• no other particles produced

What could be passed between p and pthat
transfers momentum without either breaking up?
Exchanged object must have certain properties

• no charge
• no colour (colour-singlet)

quantum numbers of the vacuum
Dominantly exchanged object at high energies
given the name Pomeron

• structure not well understood

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Elastic Scattering

• Colour-singlet exchange

• gives rise to absence of particles or energy
  above threshold in some region of rapidity a
  Rapidity Gap

• rapidity pseudorapidity ? - ln tan (?/2)
• where ? is angle of particle with respect to
  beam

The experimental view of the Pomeron is the
thing that causes rapidity gaps
Elastic scattering event
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Diffractive Scattering

• Diffractive Scattering takes some momentum

Single Diffractive Scattering
Double Pomeron Exchange

• To use perturbative predictions, look at hard
  diffractive processes
• jets, heavy particles also produced

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Rapidity Gaps

• Finding rapidity gaps

• Calorimeter
• liquid-argon/uranium
• Luminosity Monitors (LM)
• scintillators close to beam pipe

Triggers for gap events

• single gap triggers
• 1 jet with pT gt 25 GeV
• gap south no hits in south LM
• gap north no hits in north LM

• double gap trigger
• 1 jet with pT gt 25 GeV
• no hits in either LM

Analysing rapidity gaps
Look at energy in forward region of calorimeter

• how much energy is deposited close to beam pipe
  on the gap side?
• on the non-gap side?

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Energy in forward calorimeter
Sum of energy of cells above threshold (100MeV)
in first four layers of calorimeter
(electromagnetic) in forward/backward regions
South 2.6 lt ? lt 4.1
North - 4.1 lt ? lt -2.6
North
South
Gap North events
EM Energy (Gev)
EM Energy (Gev)
Shows expected diffractive behaviour peak at
very low energy on gap side
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The Forward Proton Detector

• FPD measures track of scattered p or p in
  magnetic field to determine momentum and
  scattering angle

D - Dipole magnet, bends beam around ring Q -
Quadrupole magnets, focus beam S - Separators,
bring beams together to collide
Antiproton Quadrupolespectrometers
Proton Quadrupolespectrometers
Dipolespectrometer
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The Forward Proton Detector

• Each spectrometer consists of 2 Roman Pots,
• vessels containing scintillating fiber tracking
  detector

Proton Up quadrupolespectrometer
Detector

• U, U diagonal wires
• X, X horizontal wires
• trigger scintillator
• V, V opposite diagonal wires

- primed layers offset from unprimed - read out
by PMTs
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The Forward Proton Detector
With momentum measurement we can obtain

• Momentum fraction taken by the pomeron, ?
• elastic ? 0
• pomeron exchange dominates for ? lt 0.05

where pi(f) inital (final)momentum

• Four-momentum transfer, t
• t ?2, where ? is scattering angle

Much more information is available with FPD than
with gap analysis alone
Acceptance

• Dipole requires sufficient momentum loss for
  proton to be bent into detector

all t
? gt 0.04

• Quadrupole requires sufficient scattering angle

t gt 0.8 GeV2
all ?
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FPD Preliminary Results
Preliminary plots
? and t for elastic scattering events
Expected elastic behaviour

• ? 0
• exponential t distribution

?
t (GeV2)
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Summary

• Many types of interesting events hard double
  pomeron exchange, diffractive production of
  jets, W/Z bosons, heavy quarks...
• previous papers from UA8, Hera, DØ Run I have
  driven current experimental interest

• Gap analysis can already tell us a lot about
  these events

• Forward Proton Detector will tell us even more