# Presentation: 281827

## Elastic Scattering andDiffraction at DØ

- Tamsin Edwards
- for the DØ collaboration
- 14th - 18th April, 2004
- XII International Workshop on Deep Inelastic Scattering,
- Štrbské Pleso, Slovakia

## Colour singlet exchange

-  no charge
-  no colour

-  often referred to as **Pomeron** exchange

-  Quantum numbers of the vacuum:

-  The Tevatron collides protons and antiprotons at √s = 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**

- 15th April 2004

## Searches for colour singlet exchange 

-  Single Diffraction

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

-  Elastic Scattering

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

- proton track

- rapidity gap

- proton track

-  Two types of analysis discussed in this talk:

- 15th April 2004

## Luminosity Monitor

**Luminosity Monitor (LM)**
-  Scintillating detector
-  2.7 < |η| < 4.4
-  Charge from wedges on one side are summed:  Detector is **on/off** on each side, North and South

- 15th April 2004

## Calorimeter

**FH**

**EM**

**CH**

**LM**

**2.7**
**LM**
**range**
**4.4**

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

**2.6**
**Esum**
**range**
**4.1 - 5.3**

-  Sum E of Cells in
-     EM and FH layers
-     above threshold:
-  EEM > 100 MeV
-  EFH > 200 MeV

- Liquid argon/uranium calorimeter

- 15th April 2004

## Calorimeter energy sum

- Areas are normalised to 1

- empty events

- physics samples

-  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

- Log(energy sum) on North side:

- 10 GeV

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

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

- 15th April 2004

## Efficiency and backgrounds

-  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

- Considerations to convert detector signal into physics:

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

- 15th April 2004

## Search for diffractive Z→μμ

-  Inclusive Z→μμ sample well understood:

-  di-muon (|η|<2) or   single muon (|η|< ~1.6) trigger

-  2 muons, pT > 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)

- 15th April 2004

## First step towards gap: LM only

- cosmics shape
- expected from
- inclusive sample

**WORK IN PROGRESS**

-  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

- 15th April 2004

## Z Mass of rapidity gap candidates

-   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**

-  Add Esum requirement:

- Gap North &Gap Southcombined

- 15th April 2004

## Diffractive Z→μμ candidate

- Diffractive Z→μμ candidate

- outgoing proton side

- outgoing
- anti-proton side

- muon

- muon

- muon

- muon

- 15th April 2004

## Z→μμ with rapidity gaps: Summary

- Z→μμ with rapidity gaps: Summary

-  Preliminary definition of rapidity gap at DØ Run II
-  Study of Z→μ+μ- 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→μν, W/Z→electrons, jets and other channels
-  Use tracks from Forward Proton Detector

- 15th April 2004

## Forward Proton Detector

**Forward Proton Detector (FPD)**

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

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

-  **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

- 15th April 2004

## Forward Proton Detector

-  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

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

- ξ - the fraction of longitudinal      momentum lost by the proton

- t - four-momentum transfer

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

- Reconstructed track is used to calculate kinematic variables of
- the scattered proton:

- t ~ θ2, where θ is scattering angle

- 15th April 2004

## Elastic Scattering

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

-  Quadrupole acceptance:
-  t > 0.8 GeV2 (requires sufficient scattering angle to leave beam)
-  all ξ (no longitudinal momentum loss necessary)

-  proton side:
-  quadrupole ‘down’ spectrometer
-  full detector read-out

-  antiproton side:
-  quadrupole ‘up’ spectrometer
-  trigger only

-  Elastic scattering: ξ = 0

- 15th April 2004

## Preliminary Elastic Scattering Results

- ξ distribution

- 15th April 2004

## Preliminary Elastic Scattering Results

-  The dσ/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.

- 15th April 2004

## Elastic Scattering & Diffraction: Summary

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

-  dN/dt was measured in the range 0.96 < |t| < 1.34 GeV2

-  Study of Z→μ+μ- events with a rapidity gap signature
-  Evidence of Z events with a rapidity gap signature
-  Quantitative studies of gap definition, backgrounds, efficiency in progress

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

- 15th April 2004