Leading Neutron Energy and p T Distributions from ZEUS

1. Leading Neutron Energy and pT Distributions from ZEUS A. Solano Univ. of Torino and INFN On behalf of the ZEUS Collaboration • Outline: • Introduction and models • Data-sets • Leading neutrons in DIS • Leading neutrons in photoproduction • Leading neutrons & leading protons • Summary

2. Leading neutron production Neutrons often carry a large fraction of the proton beam energy xL= En/Ep Test of production models: e’ • `Standard` fragmentation: • compare to usual MC generators e’ • Particle exchange: • compare toOne Pion Exchange (OPE) models • probe the structure function of the exchange particle • test vertex factorization: many models predict factorization violation due to rescattering on the photon (neutron absorption)

3. Absorption • d’Alesio and Pirner, EPJ A7 (2000) 109 • the larger the photon, the fewer the n’s • detected (more absorption in PHP than DIS) • the smaller the nπ system, the fewer the n’s • detected (more absorption at high pT in PHP • vs. DIS) p Absorption from additional pomeron exchange: recently Kaidalov, Khoze, Martin, Ryskin (Nikolaev, Speth, Zakharov, hep-ph/9708290) hep-ph/0602215; Khoze, Martin, Ryskin hep-ph/0606213 A • evaluate correction due to enhanced • absorptivediagrams B (~ 15%) • show importance of migration due to • rescattering for xL< 0.8 • include effects of ρ, a2 exchange • estimate the gap survival factor (important • for LHC!) which takes into account that • rescattering may populate the rapidity gap • with secondary particles carrying away • energy from the leading neutron B

4. Leading baryon detectors y • Forward Neutron Calorimeter (FNC): • 10 λlPb-scintillator sandwich • σ/E = 0.65/√E, energy scale accuracy ±2% • Forward Neutron Tracker (FNT): • scintillator hodoscope at 1 λl (position detector) • σX,Y = 0,23 cm σθ = 22 μrad (xo , yo): neutron 0o point • Leading Proton Spectrometer (LPS): • 6 stations of silicon μstrips detectors (S1-S6) • σXL< 1% pT resolution is dominated by pT spread of the proton beam (50-100 MeV)

5. Data samples For all samples:neutron:0.2 < xL < 1 θn < 0.75 mrad pT2 < 0.476 xL2 GeV2 (due to magnet apertures) Deep Inelastic Scattering (DIS): 40 pb-1, Q2 > 2 GeV2 Photoproduction (PHP): 6 pb-1, Q2 < 0.02 GeV2 Dijets in photoproduction: 40 pb-1, Q2 < 1 GeV2, 130 < W < 280 GeV ETjet1 > 7.5 GeV, ETjet2 > 6.5 GeV -1.5 < ηjet1,2< 2.5 DIS and PHP have very different inclusive cross sections σinc for sensible comparisons look at σLN/σinc Additional benefit: systematic uncertainties of central ZEUS cancel

6. xLdistribution in DIS Below xL ~ 0.7 yield drops due to decreasing pT2 range • Systematic uncertainties from: • 0o point • FNC energy scale • dead material before FNC

7. pT2 distributions in DIS • Note varying pT2 ranges in different xL bins • Data are well described by exponential distributions: get intercepts and slopes

8. pT2 distributions in DIS: intercepts & slopes Intercepts a(xL) Slopes b(xL)

9. b(xL) in DIS: comparison with OPE models Numerous parametrizations of the pion flux fπ/p(xL,pT2) in literature A. Martin, 2nd HERA-LHC Workshop Best agreeing models shown here More refinement needed: absorption, migration, more exchanges

10. LN in DIS: comparison with non-OPE MCs • Compare with several popular MC models w/o OPE, all with default settings: • LEPTO ~ ok in shape and magnitude for the xL distribution • All models fail in reproducing the slopes

11. xL distribution in PHP vs. DIS ρ Curves from model by Kaidalov, Khoze, Martin, Ryskin • Data in agreement with absorption hypothesis • Gap survival factor from xL distribution: • (A. Martin, 2nd HERA-LHC Workshop) S2 ~ 0.5 (~ 0.4 including ρ, a2)

12. pT2 distributions in PHP vs. DIS normalized to 1 at pT2 = 0 π π,ρ,a2 Comparison with model by A. Martin, 2nd HERA-LHC Workshop • Clear difference: • b(PHP) > b(DIS) for 0.6 < xL < 0.9 • Qualitatively consistent with absorption: • more abs. at small rnπ , i.e. large pT, in PHP

13. LN in dijet photoproduction vs. DIS Neutron energy spectra suggest phase space limitation: with energetic dijets in the final state little room is left for leading neutron Slopes have similar magnitude, statistics limits any conclusion on possible differences Hard to draw any conclusion on absorption

14. Leading neutrons & leading protons in DIS • Clear different trends • LN: main contribution is π exchange • LP: contribution of other trajectories • Similar magnitude for xL~ 0.7-0.8 • π exchange LN ~ LP

15. Summary • Precise measurements of leading neutron xL and pT2 distributions were presented • Pure OPE models do not describe the data • MCs with `standard` fragmentation do not describe the data, LEPTO is promising • The b-slopes in DIS are better reproduced including ρ, a2 exchanges • Comparing PHP with DIS shows that leading neutron production is suppressed • in photoproduction at low xL, high pT, in agreement with absorption hypothesis • The neutron energy spectrum in photoproduction is compatible with effects of • absorption and migration as calculated by Kaidalov, Khoze, Martin and Ryskin, • by which a gap survival factor S2 ~ 0.4 has been evaluated • Leading neutrons in dijet photoproduction have similar slopes but a different • energy spectrum than in DIS • The b-slopes of protons and neutrons have different behaviours but agree at • xL ~ 0.7-0.8 where π exchange is dominant in both cases