1. Elastic Scattering and Diffraction at DØ Tamsin Edwards for the DØ collaboration 14 th - 18 th April, 2004 XII International Workshop on Deep Inelastic Scattering, Štrbské Pleso, Slovakia

2. 15th April 20042 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

3. 15th April 20043 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

4. 15th April 20044 Luminosity Monitor North (η<0) South (η>0) p p 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

5. 15th April 20045 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: E EM > 100 MeV E FH > 200 MeV Liquid argon/uranium calorimeter

6. 15th April 20046 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:

7. 15th April 20047 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

8. 15th April 20048 Search for diffractive Z → μμ DØ Run II preliminary Summer 2003 Inclusive Z→μμ sample well understood: di-muon (|η|<2) or single muon (|η|< ~1.6) trigger 2 muons, p T > 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 M μμ (GeV)

9. 15th April 20049 cosmics shape expected from inclusive 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

10. 15th April 200410 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 (p T, p z, rapidity) 89.8 ± 0.1 GeV 89.6 ± 1.0 GeV 89.3 ± 2.0 GeV 90.2 ± 1.3 GeV WORK IN PROGRESS Add Esum requirement: Gap North & Gap South combined

11. 15th April 200411 Diffractive Z→μμ candidate outgoing proton side outgoing anti-proton side muon

12. 15th April 200412 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

13. 15th April 200413 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

14. 15th April 200414 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 - p L f /p L i t = (p f - p i ) 2 where p i(f) = inital (final) momentum Reconstructed track is used to calculate kinematic variables of the scattered proton: t ~ θ 2, where θ is scattering angle

15. 15th April 200415 Elastic Scattering A2UA2UA1UA1U P2DP1D p p p P Measure dN/dt for elastic scattering using preliminary and incomplete FPD: Quadrupole acceptance: t > 0.8 GeV 2 (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

16. 15th April 200416 Preliminary Elastic Scattering Results ξ distribution

17. 15th April 200417 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.

18. 15th April 200418 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 GeV 2 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