Elastic Scattering andDiffraction at DØ

Tamsin Edwardsfor the DØ collaboration

14th - 18th April, 2004

XII International Workshop on Deep Inelastic Scattering,

Štrbské Pleso, Slovakia

15th April 2004 2

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

• experimental signatures:• rapidity gap - absence of particles or energy above threshold in some region of rapidity in the detector• intact proton - p or p scattered at small angle from the beam

• About 40% of the total pp cross-section is elastic scattering and diffraction

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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:

• either p or p intact

• p and p intact, with no momentum loss• no other particles produced

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Luminosity Monitor

North(η<0)

South(η>0)

pp

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

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Calorimeter

FHEM CHLM

2.7

LMrange

4.4

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

2.6

Esumrange

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

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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:

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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

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Search for diffractive Z→μμ

DØ Run II preliminarySummer 2003

• 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

• RunI publication ”Observation of diffractively produced W and Z bosons in pp Collisions at sqrt(s)=1.8 TeV”, Phys. Lett. B 574, 169 (2003) Nine single diffractive Z→e+e- events. No result in muon channel.

• RunII: first search for forward rapidity gaps in Z→μ+μ- events

Mμμ (GeV)

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cosmics shape expected frominclusive sample

WORK IN PROGRESS

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

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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)

89.8 ± 0.1 GeV 89.6 ± 1.0 GeV

89.3 ± 2.0 GeV90.2 ± 1.3 GeV

WORK IN PROGRESS

• Add Esum requirement:

Gap North &Gap Southcombined

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Diffractive Z→μμ candidate

outgoing proton side

outgoing anti-proton side

muon

muon

muon

muon

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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

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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

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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

Forward Proton Detector

• 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

ξ = 1 - pLf/pL

it = (pf - pi)2

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

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Elastic Scattering

A2U A1U

P2DP1D

p

pp

P

• 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

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Preliminary Elastic Scattering Results

ξ distribution

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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.

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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