Elastic Scattering and Diffraction at D Ø

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

2. About 40% of the total pp cross-section is elastic scattering and diffraction • 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 Colour singlet exchange • 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 • Quantum numbers of the vacuum: • no charge • no colour • often referred to as Pomeron exchange

3. either p or p intact • p and p intact, with no momentum loss • no other particles produced Searches for colour singlet exchange • Two types of analysis discussed in this talk: • Single Diffraction • search for rapidity gap in forward regions of DØ • Luminosity Monitor • Calorimeter rapidity gap • Elastic Scattering proton track • search for intact protons in beam pipe • Forward Proton Detector proton track

4. South (η>0) North (η<0) p p 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

5. Calorimeter Liquid argon/uranium calorimeter Cells arranged in layers: • electromagnetic (EM) • fine hadronic (FH) • coarse hadronic (CH) • Sum E of Cells in EM and FH layers above threshold: EEM > 100 MeV EFH > 200 MeV 2.7 LM range 4.4 2.6 Esum range 4.1 - 5.3 LM EM CH FH

6. Calorimeter energy sum • Use energy sum to distinguish proton break-up from empty calorimeter: Log(energy sum) on North side: Areas are normalised to 1 empty events physics samples 10 GeV • Esum cut of 10GeV was chosen for current study • Final value will be optimised using full data sample • 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

7. Efficiency and backgrounds Considerations to convert detector signal into physics: • 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 These studies are currently underway, and are required for a measurement of the ratio of diffractive to non-diffractive events

8. DØ Run II preliminary Summer 2003 • 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 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)

9. 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 WORK IN PROGRESS cosmics shape expected from inclusive sample

10. 89.8 ± 0.1 GeV 89.6 ± 1.0 GeV 90.2 ± 1.3 GeV 89.3 ± 2.0 GeV Z Mass of rapidity gap candidates • Add Esum requirement: • 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 Gap North &Gap Southcombined

11. Diffractive Z→μμ candidate outgoing proton side outgoing anti-proton side muon muon muon 11 muon

12. 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 12

13. Forward Proton Detector Forward Proton Detector (FPD) - a series of momentum spectrometers that make use of accelerator magnets in conjunction with position detectors along the beam line • Quadrupole Spectrometers • surround the beam: up, down, in, out • use quadrupole magnets (focus beam) • 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. ξ = 1 - pLf/pLi t = (pf - pi)2 Forward Proton Detector Forward Proton Detector • scintillating fiber tracker • can be brought within a few millimetres of the beam • 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 Reconstructed track is used to calculate kinematic variables of the scattered proton: t - four-momentum transfer ξ - the fraction of longitudinal momentum lost by the proton t ~ θ2, where θ is scattering angle where pi(f) = inital (final) momentum

15. p P A2U A1U p p P1D P2D Elastic Scattering • Elastic scattering: ξ = 0 • Quadrupole acceptance: • t > 0.8 GeV2 (requires sufficient scattering angle to leave beam) • all ξ (no longitudinal momentum loss necessary) • Measure dN/dt for elastic scattering using preliminary and incomplete FPD: • antiproton side: • quadrupole ‘up’ spectrometer • trigger only • proton side: • quadrupole ‘down’ spectrometer • full detector read-out

16. Preliminary Elastic Scattering Results ξdistribution

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.

18. Elastic Scattering & Diffraction: Summary • 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 • 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 • The future: study many diffractive physics channels using rapidity gaps and full Forward Proton Detector system