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Elastic Scattering and Diffraction at D Ø. Tamsin Edwards for the D Ø collaboration 14 th - 18 th April, 2004 XII International Workshop on Deep Inelastic Scattering, Š trbsk é Pleso, Slovakia. About 40% of the total pp cross-section is elastic scattering and diffraction. - PowerPoint PPT Presentation
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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
15th April 2004 3
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
15th April 2004 4
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
15th April 2004 5
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
15th April 2004 6
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 7
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 8
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)
15th April 2004 9
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
15th April 2004 10
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
15th April 2004 1111
Diffractive Z→μμ candidate
outgoing proton side
outgoing anti-proton side
muon
muon
muon
muon
15th April 2004 1212
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 13
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 14
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
15th April 2004 15
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
15th April 2004 16
Preliminary Elastic Scattering Results
ξ distribution
15th April 2004 17
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 18
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
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