# Presentation: 281818

## Elastic & Diffractive Scatteringat the DØ Experiment

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

## Hadron scattering at the Tevatron

- 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

- The Tevatron accelerator collides protons and antiprotons at a
- centre-of-mass energy of √s = 1.96 TeV at an average rate of 1.7 MHz

- T.Edwards          APS Meeting

- 6th April 2003

## Elastic Scattering

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

-  no charge
-  no colour (colour-singlet)

- Dominantly exchanged object at high energies
- given the name ”_Pomeron_”

-  structure not well understood

- What could be passed between p and pthat transfers momentum without either breaking up?

- Exchanged object must have certain properties:

- “quantum numbers of the vacuum”

- Elastic scattering events

- T.Edwards          APS Meeting

- 6th April 2003

## 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  η = - ln tan (θ/2)
-   where θ is angle of particle with respect to beam

- Elastic scattering event

- The experimental view of the Pomeron is
- “the thing that causes rapidity gaps”

- T.Edwards          APS Meeting

- 6th April 2003

## 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

- T.Edwards          APS Meeting

- 6th April 2003

## Rapidity Gaps

- Finding rapidity gaps:

-  _double gap trigger_
-  1 jet with pT > 25 GeV
-  no hits in either LM

-  _single gap triggers_
-  1 jet with pT > 25 GeV
-  gap south: no hits in south LM
-  gap north: no hits in north LM

- Triggers for gap events:

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

- Analysing rapidity gaps:

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

- Look at energy in forward region of calorimeter:

- T.Edwards          APS Meeting

- 6th April 2003

## Energy in forward calorimeter

- North

- South

- Gap North
- events

- Shows expected diffractive behaviour: peak at very low energy on gap side

- _Sum of energy of cells_ above threshold (100MeV) in first four layers of calorimeter (“electromagnetic”) in forward/backward regions:

- _South_: 2.6 < η < 4.1

- _North_: - 4.1 < η < -2.6

- EM Energy (Gev)

- EM Energy (Gev)

- T.Edwards          APS Meeting

- 6th April 2003

## 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 Quadrupole_spectrometers

- _Proton Quadrupole_spectrometers

- _Dipole_spectrometer

- T.Edwards          APS Meeting

- 6th April 2003

## 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

- T.Edwards          APS Meeting

- 6th April 2003

## The Forward Proton Detector

- With momentum measurement we can obtain:

- Momentum fraction taken by the pomeron, ξ
-  elastic: ξ =0
-  pomeron exchange dominates for ξ < 0.05

- Four-momentum transfer, t
-  t ~ θ2, where θ is scattering angle

- where pi(f) = inital (final)momentum

- Acceptance

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

-  Quadrupole: requires sufficient scattering angle

- ξ > 0.04

-  t > 0.8 GeV2

- all t

- all ξ

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

- T.Edwards          APS Meeting

- 6th April 2003

## FPD Preliminary Results

- ξ and t for elastic scattering events

-  ξ = 0
-  exponential  t distribution

- Expected elastic behaviour:

- t (GeV2)

-  ξ

- Preliminary plots:

- T.Edwards          APS Meeting

- 6th April 2003

## Summary

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

-  Forward Proton Detector will tell us even more

-  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

- T.Edwards          APS Meeting

- 6th April 2003