Leading Baryons at HERA

1. Leading Baryons at HERA Graziano Bruni INFN – Bologna On behalf of H1 and ZEUS Collaborations

2. Outline • Physics case • Recent and new results on: • Leading protons (new) • Leading neutrons • Comparison with models • Leading neutron + 2 jets in gp • Summary Graziano Bruni - INFN (Bologna)

3. h= LB = p, n X = hadronic state Current jet DIS regime Scale for secondary particle production decreases from Q2 (currentregion) to a soft hadronic scale (proton fragmentation region) Photoproduction (gp) regime Hadronic component of the photon. Can re-introduce hard scale (e.g. requiring high-pT jets) Proton remnant Q2 > 1 GeV2 : deep-inelastic scattering (DIS) Q2 < 1 GeV2 : photo-production (gp) Semi-inclusive reaction Graziano Bruni - INFN (Bologna)

4. Some approaches • Fracture Functions( L.Trentadue, G.Veneziano Phys. Lett. B323, 201 (1994) ) - Not discussed here • MC fragmentation models • Dynamical particle-exchange models Graziano Bruni - INFN (Bologna)

5. Q2, xBj, y, W LB xL=ELB/Ep pT2 t=(p-p’)2 p p’ Production models • Hadronisation of proton remnant • Herwig (cluster model) • MEPS (parton shower,SCI) • Ariadne (CDM) • Exchange of virtual particles • leading protons: 0, IR, IP (isoscalar + isovector) • leading neutrons: +, +, … (isovector) Graziano Bruni - INFN (Bologna)

6. DIS gp Rescattering and absorption e.g. LN production via p-exchange Neutron absorption through rescattering • D’Alesio and Pirner • (EPJ A7(2000) 109) • Neutron rescatters on g hadronic component. Absorption enhanced when p-n system size rpnsmall w.r.to the g: rpn ~ 1/pT . Dependence on the pion-flux from xL: <rpn> increases with xL more absorption at low xL . - n kicked to lower xL & higer pT (migration) • may escape detection (absorption loss) • more absorption in photoproduction than DIS(g “size” larger) Graziano Bruni - INFN (Bologna)

7. Nikolaev,Speth and Zakharov(NSZ) (hep-ph/9708290) Re-scattering processes via additional pomeron exchanges + Optical Theorem (  Uncertainties in p structure function extraction ) (Kaidalov,) Khoze, Martin, Ryskin (KKMR) (hep-ph/0602215, hep-ph/0606213) Enhanced absorptive corrections, calculation of migrations, include also rand a2 exchange (different xL & pT dependences) • Measure xL and pT2 distributions • Study dependence on Q2 • Compare gp and DIS • Look for effects due to absorption Graziano Bruni - INFN (Bologna)

8. Experimental tools Graziano Bruni - INFN (Bologna)

9. 20 m 107 m Lower xL accessible ZEUS Leading Proton Spectrometer (LPS) • 6 stations each made by 6 Silicon-detector planes • Stations inserted at 10sbeam from the proton beam during data taking • sxL < 1%, spT2~ few MeV2 (better than p-beam spread ~ 50 - 100 MeV) ZEUS Forward Neutron Calorimeter (FNC) + Forward Neutron Tracker (FNT) • 10l lead-scintillator sandwich • σ/E =0.65/√E, Energy scale 2% • Acceptance qn<0.8 mrad, azimuthal coverage 30% • FNT: Scint. hodoscope @ 1λint, σx,y=0.23 cm, σθ=22 μrad H1 Forward Proton Spectrometer (FPS) • 2 stations each made by 4 planes of sci-fiber hodoscopes + trigger scintillators • sx = sy = 100 mm, s(E) < 8 GeV, |Escale| = 10 GeV, <etrack> ~ 50% H1 Forward Neutron Calorimeter (FNC) • Lead-scintillator calorimeter @ 107m from I.P. + veto hodoscopes • s(E)/E≈20%, neutron detection eff. 93±5% Graziano Bruni - INFN (Bologna)

10. Fully inclusive: ep  eX Semi-inclusive: ep  epX Leading Protons (DIS-regime) Graziano Bruni - INFN (Bologna)

11. Cross section normalised to inclusive DIS cross section Flat below diffractive peak pT2<0.5 GeV2 DIS and low Q2 data are compatible Non diffractive region NEW ZEUS Results 12.8 pb-1 xL cross-section Graziano Bruni - INFN (Bologna)

12. Different Q2 PT2 cross-section in bins of xL Exponential behaviour Fit to ~ exp(-b pT2) No strong dependence of b on xL Graziano Bruni - INFN (Bologna)

13. Rates to inclusive DIS Q2 Structure function ratio 4.2 – 237 GeV2 Measurement for 0.32<xL<0.92 (diffractive peak excluded) rLP(3) almost independent of x and Q2 with average value ~0.4 xBj 9.6x10-5 – 3x10-3 Graziano Bruni - INFN (Bologna)

14. ~24% of DIS events have a LP with 0.32<xL<0.92, almost independently of x and Q2 Low Q2 0.32 < xL< 0.92 0.60 < xL< 0.97 Slight increase with Q2 not excluded Graziano Bruni - INFN (Bologna)

15. Flux-factor Elementary cross-section (notice xL dependence) Broken if absorption Leading neutrons • One Pion Exchange is the leading contribution at large xL • (Regge)-factorization of the cross section g*p  nX Limiting Fragmentation(in the proton target region the production of particles is independent on the incident particle)  Vertex Factorization Graziano Bruni - INFN (Bologna)

16. Phase space coverage xL spectrum • LN yield decreases for xL1 due to kinematic limit • Below xL ~ 0.7 the yield drops due to decreasing pT2 range: pT2 < 0.476 xL2 DIS regime • pT2 < 0.476 xL2 Graziano Bruni - INFN (Bologna)

17. Good description through a single exponential in each xL bin b ~ 0 for xL < 0.3 DIS cross section vs pT2 in bins of xL Falling with xL Plateau 0.4 < xL < 0.8 Graziano Bruni - INFN (Bologna)

18. 3 Q2 bins + gp gp Q2 – dependence: xL spectra • Yield increases with Q2: large increase from gpto DIS • Smaller Q2 dependence at intermediate values • Violation of vertex factorization Graziano Bruni - INFN (Bologna)

19. Ratio gp/DIS • ~ 70% intermediate xL • Rises above 1 as xL 1 • Consistent with absorption: • separation p-n decreases at low xL • smaller separation  more absorption at low xL Ratio gp/DIS p2T < 0.476 x2L Q2 – dependence: xL spectraComparison gp vs DIS Graziano Bruni - INFN (Bologna)

20. gp Q2 – dependence: p2T slopes • DIS – slopes: ~ no Q2 dependence • gp – slopes: higher in 0.6 < xL < 0.9 Graziano Bruni - INFN (Bologna)

21. Normalized to 1 gp-DIS DIS gp Q2 – dependence: p2T slopes. Comparison gp vs DIS bgp > bDIS for 0.6 < xL < 0.9 Depletion at large pT Consistent with vertex factorization violation from absorption: - more absorption at small rpn  large pT - loss of LN at high pT larger slope Graziano Bruni - INFN (Bologna)

22. Same pT2 range: pT2 < 0.04 GeV2 LP LN: rise with xL LP: ~ flat LN Pure isovector exchange: LP = ½ LN  Other IR contributions in LP Similar slopes in 0.7 < xL < 0.85 Comparison LP - LN Graziano Bruni - INFN (Bologna)

23. Comparison to models Graziano Bruni - INFN (Bologna)

24. IP IR pN pD Leading Protons Standard fragmentation MC models fail to describe the data DJANGOH+SCI+MEPS ~ ok b(xL) shape Reasonable description by p, IR, IP exchange model Graziano Bruni - INFN (Bologna)

25. Leading neutrons MC models generally fail to fully reproduce the data • Shapes ~ ok by RAPGAP with standard fragmentation+ p-exch. • LEPTO+SCI: xL shapes ~ ok, not slopes Graziano Bruni - INFN (Bologna)

26. Models differ by a and F Reasonable agreement in shape, not in rate One-Pion Exchange Models Graziano Bruni - INFN (Bologna)

27. KKMR model (p – exchange) KMR model + secondary IR (r,a2) Slopes - DIS (K)KMR model Absorption & migration included in the predictions: not sufficient to describe the slopes Good description of data when additional IR included Graziano Bruni - INFN (Bologna)

28. XL - gp KMR model + secondary IR (r,a2) Pure p-xch. + inclusion migrations in xL and p2T after rescattering Reasonable description of shape and normaliz. Add (r,a2)-xch: again reasonable agreement in gp KKMR model (p – exchange) Graziano Bruni - INFN (Bologna)

29. Models: OPE + absorption gp / DIS ( different cms energy dependence) Good agreement with the data Comparison DIS - gp Graziano Bruni - INFN (Bologna)

30. Absorption effects seen going from hard  soft scale High Q2  Low Q2  gp ( ) Photon momentum fraction that enters in the hard scattering Jet ET > 7.5 GeV Introduce a hard scale in gp xg = 1 direct PHP, DIS xg < 1 resolved PHP ( hadron-like photon ) Jet ET > 6.5 GeV Still absorption? Leading n in gp + dijets Graziano Bruni - INFN (Bologna)

31. Factorization tests H1-DATA - (Eur. Phys. J. C41 (2005) 273-286 ) RAPGAP/PYTHIA-MI ZEUS-DATA - Nucl.Phys.B596,3(2001) RAPGAP / HERWIG MI • Ratio almost independent on ETjet: factorization • Strong dependence on xg: breaking of factorization • Fewer neutrons in the resolved region Not yet conclusions on factorization breaking in resolved gp (xγ<1) Graziano Bruni - INFN (Bologna)

32. gp DIS LN+jj (gp) vs LN (DIS) b-slopes similar in magnitude and shape in DIS and gp+dijet  Same production mechanism • gp without jets: suppression at low xL • gp with jets: suppression at high xL Not yet firm conclusions on absorption Graziano Bruni - INFN (Bologna)

33. Summary • New data from HERA on LB production and properties, new ZEUS LP results • Observation of factorization breaking effects going from hard to soft scales, mostly in LN • gp + hard jets: more work needed • LP needs isoscalar-IR contributions to explain the data • Standard fragmentation MC-models fail to describe the data • Improvement in MC models with particle-exchange implemented • LN: pure p exchange not sufficient (slopes not described) • Recent calculations with p exchange and absorption/migration effects improve the agreement in magnitude and shape of the xL spectra • Additional exchanges (r, a2) improve further Graziano Bruni - INFN (Bologna)

34. Additional slides Graziano Bruni - INFN (Bologna)

35. h Production in the current region xL Fragmentation function Production in the target region: F.F. xL Current jet Fracture function h s = scurrent + starget - F.F. are non-perturbative universal functions - Q2 dependence governed by evolution equations Dynamical models allow to link them to concepts like structure-functions of exchanged objects (e.g. IR trajectories) Proton remnant Fracture Functions approach L.Trentadue, G.Veneziano Phys. Lett. B323, 201 (1994) Graziano Bruni - INFN (Bologna)