The Hadronic Final State at HERA

1. (Some Highlights from) The Hadronic Final State at HERA Rainer Mankel DESY for the ZEUS & H1 collaborations C2CR Conference Prague 9-Sep-2005

2. Typical Structure of Hadronic Final States at HERA Current jet Diffractive system [gap] Proton remnant Q2 > 1 GeV2 : deep-inelastic scattering (DIS) Q2 < 1 GeV2 : photo-production (PHP) Diffractive process R. Mankel: The Hadronic Final State at HERA

3. hadron-hadron interaction Quark jet Proton remnant Proton remnant Quark jet ep interaction Quark jet Proton remnant Comparison of Final State Structure e+e- interaction • contains main features of energetic hadron interaction (proton remnant) • less complex than hadron-hadron interaction • clean reconstruction of kinematic variables • ideal laboratory for studying QCD Quark jet Anti-quark jet R. Mankel: The Hadronic Final State at HERA

4. current jet e p proton remnant scattered electron Colliding Beam Detector • Colliding mode detectors can generally measure current jet & scattered electron very well (“central region”) • in these areas, also theoretical models are tested & tuned best • The proton remnant emerges close to beam pipe & is less accessible • these areas also pose big challenges to theory • Cosmic ray experiments cannot distinguish between proton (nucleus) remnants & jets R. Mankel: The Hadronic Final State at HERA

5. Some Questions Related to Hadronic Final State • How well do we understand the workings of QCD in the forward area? • How strong is the diffractive component at high energies • At which accuracy can we describe production of heavy flavors & resulting leptons R. Mankel: The Hadronic Final State at HERA

6. Outline • Introduction • Leading baryon production • Jets in the forward area • Diffraction at high Q2 • Heavy flavor production • Summary R. Mankel: The Hadronic Final State at HERA

7. Leading Baryon Production • Sizeable fraction of events with leading baryons • Production mechanism not entirely understood • At HERA, special forward detectors allow precision measurements (p, n) • FPS, FNC (H1) • LPS, FNC (ZEUS) • Example: Leading Proton Spectrometer • 6 stations of roman pots in downstream curve of proton beam • each station with 6 Si detector planes • acceptance extends in range 0.4 < xL <1 (xL= ELP / Ep) pt2 < 0.5 GeV2 R. Mankel: The Hadronic Final State at HERA

8. Typical Production Mechanisms • Hadronisation of proton remnant • Herwig (cluster model) • MEPS (parton shower,SCI) • Ariadne (CDM) p,IR,IP N,P N,P p’ p’ • Exchange of virtual particles • leading protons: 0, Pomeron, Reggeon • leading neutrons: +, +, … R. Mankel: The Hadronic Final State at HERA

9. Data Diffractive peak Herwig MEPS (1-xL)1.0 Flat spectrum for xL<0.95 Ariadne (1-xL)1.0 (1-xL)1.4 Leading Proton Spectrum (DIS) • Cross section vs. xL = ELP / Ep. Very precise data. • Standard fragmentation models fail to describe flat part between 0.6-0.95 Theory R. Mankel: The Hadronic Final State at HERA

10. Leading Proton pT Spectra • Fit transverse momentum spectrum with • Slope b hardly dependent on xL • Well-established models with standard hadronization fail to describe leading baryon production R. Mankel: The Hadronic Final State at HERA

11. neutrons protons Leading Neutron Production • Leading neutrons show entirely different behavior • steep increase of pT slope with increasing xL • equally inexplicable with proton remnant fragmentation • In case of neutrons, one non-fragmentation process (+exchange) is expected to dominate • ideal process to test validity of exchange model R. Mankel: The Hadronic Final State at HERA

12. same model with three different parameter sets Leading Neutron Production (cont’d) • Factorize cross section into pion flux from proton and pion-photon cross section • Precise data allow to compare various parameterizations of pion flux • constrains parameters on some models • excludes other models R. Mankel: The Hadronic Final State at HERA

13. Leading Neutrons in Di-Jet Events • Comparison of di-jet events with & without leading neutrons allows further tests of models • Elaborates further differences in production mechanisms. Pion exchange models able to describe the data • Is the production of the leading neutron independent of the photon virtuality (factorization)? • Di-jet events in photo-production have lower leading neutron rates than those in DIS • factorization violation • Difference is most pronounced at lower neutron energies R. Mankel: The Hadronic Final State at HERA

14. Leading Neutron in Di-Jet Events (cont’d) • Smooth transition between photo-production & DIS regime • Depletion of neutrons at low Q2 may be indicative of absorption / rescattering processes at work Low Q2 High Q2 R. Mankel: The Hadronic Final State at HERA

15. Leading Baryons: Conclusions • HERA experiments provide precise measurements of leading baryon production using dedicated forward detectors • General purpose models fail to describe leading baryon production via standard fragmentation of proton remnant • Virtual particle exchange processes improve the picture. Powerful constraints on model parameters from HERA data. R. Mankel: The Hadronic Final State at HERA

16. Forward Jets • Forward area is particularly sensitive to details in evolution of parton cascade • At low x, we do not probe the valence structure of the proton, but rather see universal structure of QCD radiation at work • signature: forward jet • This allows us to examine different mechanisms of parton cascade evolutions R. Mankel: The Hadronic Final State at HERA

17. Dynamics of Parton Evolution DGLAP Dokshitzer-Gribov-Lipatov-Altarelli-Parisi BFKL Balitsky-Fadin-Kuraev-Lipatov CCFM Ciafaloni-Catani-Fiorani-Marchesini  • Evolution in powers of ln Q2 • Strongly orderered in kT • Well established at high x and Q2, but expected to break down at low x • Evolution in powers of ln 1/x • Strongly orderered in x • May be applicable at low x • Evolution in both ln Q2 and ln 1/x • Bridge between DGLAP and BFKL • Angular ordering • May be applicable at low x R. Mankel: The Hadronic Final State at HERA

18. Forward Jet Measurements (DIS) Cuts designed to enhance BFKL effects xBj<0.004, 7o<jet<20o, xjet>0.035 DGLAP • leading order suppressed by kinematics • even with NLO, factor 2 below data at low x CCFM • distribution too hard • comparatively poor description of the data CDM (similar to BFKL) • generally good DGLAP with resolved virtual photonsimilar to CDM, but fails to describe forward+dijet sample R. Mankel: The Hadronic Final State at HERA

19. Forward Jets Summary • Limitations of the pure DGLAP approach clearly seen in the forward area • higher order parton emissions break ordering scheme • Calculations which include such processes (CDM) provide better description R. Mankel: The Hadronic Final State at HERA

20. Diffractive Final States at High Q2 • Hard diffractive process is characterized by rapidity gap near outgoing proton • caused by colorless exchange • It is an interesting question if & how far diffraction extends to the large Q2 region • clean final states at LHC, e.g. for Higgs? Large rapidity gap • Look for rapidity gaps in neutral current events • Comparison of charged current / neutral current events  universal behavior? R. Mankel: The Hadronic Final State at HERA

21. Rapidity Gaps in NC Events Rapiditygap Forward Plug Calorimeter (FPC) with FPC veto (at beam pipe) • “Normal” DIS MC (Ariadne) clearly insufficient at low max • Need Ariadne+RAPGAP (diffractive MC) to describe the data R. Mankel: The Hadronic Final State at HERA 5

22. For comparison: Low-Q2 Data Rapidity Gaps in NC: Q2 Dependence High -Q2 NC x P<0.05 • Sizable diffractive contribution to NC cross section • drops with rising Q2 • still 2% at Q2=1500 GeV2 • NC and CC compatible R. Mankel: The Hadronic Final State at HERA

23. - D B Interaction vertex - B - D + - -(D) Muons from Heavy Flavor Decays • Apart of weak decays of pions and other light mesons, heavy flavor final states contribute in particular to the muon rates at high transverse momentum • Main challenge tagging of quark flavors • decay impact parameters • pT relative to jet • di-muon events ( correlation) • Study of di-muon event signatures allows to use low ptμ thresholds, measure the total bb cross section - R. Mankel: The Hadronic Final State at HERA

24. non-isolated, ET>8 GeV Di-Muons: Data vs MC • Same-charge combinations used to normalize light-flavor background • Good overall description with MC • bb contribution ~2000 events, purity ~43% R. Mankel: The Hadronic Final State at HERA

25. bb Cross Section from Di-Muon Events • NLO QCD predictions:PHP: 5.8 nb (FMNR,CTEQ5M)DIS: 1.0 nb (HVQDIS,CTEQ5F4)  6.8 nb • NLO prediction lower than the data, though not entirely incompatible within errors • Compare with recent H1 measurement of visbb in PHP using D* correlations H1: pT(D*)>1.5 GeV, |(D*)|<1.5. p()>2 GeV, |()|<1.7, 0.05<y<0.75, Q2<1 GeV2 +3.0 –1.7 • similar trend R. Mankel: The Hadronic Final State at HERA

26. Summary • Wealth of measurements from HERA on structure of hadronic final state • only a small selection presented • Leading baryons & forward jets probe QCD dynamics in vicinity of proton remnant • allows accurate distinctions between different models • Hard diffraction reaches up to high Q2 • Measurement of open beauty cross section  leptons at high pT R. Mankel: The Hadronic Final State at HERA

27. The End R. Mankel: The Hadronic Final State at HERA

28. Backup Slides R. Mankel: The Hadronic Final State at HERA

29. Direct Comparison of Global Phase Space and BFKL-Sensitive Regime • Global phase space: • CDM (BFKL) works well • MEPS (DGLAP) slightly worse • fixed-order QCD underestimates data at high jet (missing higher orders) Q2 > 25 GeV2 y > 0.04 Ee’>10 GeV ETjet>6 GeV -1<jet<3 in addition: had>90o 0<jet<3 0.5<(ETjet)2/Q2<2 Global Phase Space BFKL Phase Space • BFKL-sensitive phase space: • Steep falloff with jet(hcut) • MEPS (DGLAP) fails to describe data • CDM (BFKL) works well • NLO QCD is better than LO (t-channel gluon exchange) R. Mankel: The Hadronic Final State at HERA

30. More Pieces to Pentaquark Puzzle (1520): both in forward & backward hemisphere +: only in forward hemisphere • +signal mainly from forward pseudo rapidity region • unlike regular baryons (1520) and c • predominantly at medium Q2 • similar to c • no sign of decuplet partners seen in –– and –+ ( NA49) R. Mankel: The Hadronic Final State at HERA

31. same B (charm cascade) + J/ different B’s (signal!) + cc + light flavor BG low mass high mass low mass high mass mainly light flavor BG different B’s (charm cascade + BB mixing) + light flavor BG Di-Muon Mass Spectra (Data vs MC) • Good description with MC • bb contribution ~2000 events R. Mankel: The Hadronic Final State at HERA

32. bb from Di-Muons: Normalization of BG-MC • cc: normalize to D*mu analysis • Bethe-Heitler, elastic charmonium: normalize to data under isolation cut • Light flavor: use like sign spectrum (minus bb MC) R. Mankel: The Hadronic Final State at HERA

33. R. Mankel: The Hadronic Final State at HERA