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

9-Sep-2005R. Mankel: The Hadronic Final State at HERA2 Typical Structure of Hadronic Final States at HERA Current jet Proton remnant [gap] Q 2 > 1 GeV 2 : deep- inelastic scattering (DIS) Q 2 < 1 GeV 2 : photo- production (PHP) Diffractive process Diffractive system

9-Sep-2005R. Mankel: The Hadronic Final State at HERA3 Comparison of Final State Structure  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 e + e  interaction Quark jet Proton remnant hadron-hadron interaction Proton remnant Quark jet e  p interaction

9-Sep-2005R. Mankel: The Hadronic Final State at HERA4ep Colliding mode detectors can generally measure current jet & scattered electron very well (“central region”) Colliding mode detectors can generally measure current jet & scattered electron very well (“central region”) in these areas, also theoretical models are tested & tuned best in these areas, also theoretical models are tested & tuned best The proton remnant emerges close to beam pipe & is less accessible The proton remnant emerges close to beam pipe & is less accessible these areas also pose big challenges to theory these areas also pose big challenges to theory Cosmic ray experiments cannot distinguish between proton (nucleus) remnants & jets Cosmic ray experiments cannot distinguish between proton (nucleus) remnants & jets Colliding Beam Detector scattered electron current jet protonremnant

9-Sep-2005R. Mankel: The Hadronic Final State at HERA5 Some Questions Related to Hadronic Final State How well do we understand the workings of QCD in the forward area? How well do we understand the workings of QCD in the forward area? How strong is the diffractive component at high energies How strong is the diffractive component at high energies At which accuracy can we describe production of heavy flavors & resulting leptons At which accuracy can we describe production of heavy flavors & resulting leptons

9-Sep-2005R. Mankel: The Hadronic Final State at HERA6 Outline Introduction Introduction Leading baryon production Leading baryon production Jets in the forward area Jets in the forward area Diffraction at high Q 2 Diffraction at high Q 2 Heavy flavor production Heavy flavor production Summary Summary

9-Sep-2005R. Mankel: The Hadronic Final State at HERA7 Leading Baryon Production Sizeable fraction of events with leading baryons Sizeable fraction of events with leading baryons Production mechanism not entirely understood Production mechanism not entirely understood At HERA, special forward detectors allow precision measurements (p, n) At HERA, special forward detectors allow precision measurements (p, n) FPS, FNC (H1) FPS, FNC (H1) LPS, FNC (ZEUS) LPS, FNC (ZEUS) Example: Leading Proton Spectrometer Example: Leading Proton Spectrometer 6 stations of roman pots in downstream curve of proton beam 6 stations of roman pots in downstream curve of proton beam each station with 6 Si detector planes each station with 6 Si detector planes acceptance extends in range 0.4 < x L <1 (x L = E LP / E p ) p t 2 < 0.5 GeV 2 acceptance extends in range 0.4 < x L <1 (x L = E LP / E p ) p t 2 < 0.5 GeV 2

9-Sep-2005R. Mankel: The Hadronic Final State at HERA8 Typical Production Mechanisms Hadronisation of proton remnant Hadronisation of proton remnant Herwig (cluster model) Herwig (cluster model) MEPS (parton shower,SCI) MEPS (parton shower,SCI) Ariadne (CDM) Ariadne (CDM) ,IR,IP N,P p’ Exchange of virtual particles Exchange of virtual particles leading protons:  0, Pomeron, Reggeon leading protons:  0, Pomeron, Reggeon leading neutrons:  +,  +, … leading neutrons:  +,  +, …

9-Sep-2005R. Mankel: The Hadronic Final State at HERA9 Leading Proton Spectrum (DIS) Cross section vs. x L = E LP / E p. Very precise data. Cross section vs. x L = E LP / E p. Very precise data.  Standard fragmentation models fail to describe flat part between Herwig Ariadne MEPS (1-x L ) 1.0 (1-x L ) 1.4 Theory Diffractive peak Flat spectrum for x L <0.95Data

9-Sep-2005R. Mankel: The Hadronic Final State at HERA10 Leading Proton p T Spectra Fit transverse momentum spectrum with Fit transverse momentum spectrum with Slope b hardly dependent on x L Slope b hardly dependent on x L Well-established models with standard hadronization fail to describe leading baryon production Well-established models with standard hadronization fail to describe leading baryon production

9-Sep-2005R. Mankel: The Hadronic Final State at HERA11 Leading Neutron Production Leading neutrons show entirely different behavior Leading neutrons show entirely different behavior steep increase of p T slope with increasing x L steep increase of p T slope with increasing x L equally inexplicable with proton remnant fragmentation equally inexplicable with proton remnant fragmentation In case of neutrons, one non- fragmentation process (  + exchange) is expected to dominate In case of neutrons, one non- fragmentation process (  + exchange) is expected to dominate ideal process to test validity of exchange model ideal process to test validity of exchange model protons neutrons

9-Sep-2005R. Mankel: The Hadronic Final State at HERA12 Leading Neutron Production (cont’d) Factorize cross section into pion flux from proton and pion-photon cross section Factorize cross section into pion flux from proton and pion-photon cross section Precise data allow to compare various parameterizations of pion flux Precise data allow to compare various parameterizations of pion flux constrains parameters on some models constrains parameters on some models excludes other models excludes other models same model with three different parameter sets

9-Sep-2005R. Mankel: The Hadronic Final State at HERA13 Leading Neutrons in Di-Jet Events Comparison of di-jet events with & without leading neutrons allows further tests of models 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 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)? 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 Di-jet events in photo-production have lower leading neutron rates than those in DIS factorization violation factorization violation Difference is most pronounced at lower neutron energies Difference is most pronounced at lower neutron energies

9-Sep-2005R. Mankel: The Hadronic Final State at HERA14 Leading Neutron in Di-Jet Events (cont’d) Smooth transition between photo-production & DIS regime Smooth transition between photo-production & DIS regime Depletion of neutrons at low Q 2 may be indicative of absorption / rescattering processes at work Depletion of neutrons at low Q 2 may be indicative of absorption / rescattering processes at work High Q 2 Low Q 2

9-Sep-2005R. Mankel: The Hadronic Final State at HERA15 Leading Baryons: Conclusions HERA experiments provide precise measurements of leading baryon production using dedicated forward detectors 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 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. Virtual particle exchange processes improve the picture. Powerful constraints on model parameters from HERA data.

9-Sep-2005R. Mankel: The Hadronic Final State at HERA16 Forward Jets Forward area is particularly sensitive to details in evolution of parton cascade 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 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 signature: forward jet This allows us to examine different mechanisms of parton cascade evolutions This allows us to examine different mechanisms of parton cascade evolutions

9-Sep-2005R. Mankel: The Hadronic Final State at HERA17 Dynamics of Parton Evolution DGLAP Dokshitzer-Gribov-Lipatov-Altarelli- Parisi Evolution in powers of ln Q 2 Evolution in powers of ln Q 2 Strongly orderered in k T Strongly orderered in k T Well established at high x and Q 2, but expected to break down at low x Well established at high x and Q 2, but expected to break down at low x Evolution in powers of ln 1/x Evolution in powers of ln 1/x Strongly orderered in x Strongly orderered in x May be applicable at low x May be applicable at low x BFKLBalitsky-Fadin-Kuraev-LipatovCCFMCiafaloni-Catani-Fiorani-Marchesini  Evolution in both ln Q 2 and ln 1/x Evolution in both ln Q 2 and ln 1/x Bridge between DGLAP and BFKL Bridge between DGLAP and BFKL Angular ordering Angular ordering May be applicable at low x May be applicable at low x

9-Sep-2005R. Mankel: The Hadronic Final State at HERA18 Forward Jet Measurements (DIS) DGLAP leading order suppressed by kinematics leading order suppressed by kinematics even with NLO, factor 2 below data at low x even with NLO, factor 2 below data at low x x Bj CCFM distribution too hard distribution too hard comparatively poor description of the data comparatively poor description of the data CDM (similar to BFKL) generally good generally good DGLAP with resolved virtual photon similar to CDM, but fails to describe forward+dijet sample Cuts designed to enhance BFKL effects

9-Sep-2005R. Mankel: The Hadronic Final State at HERA19 Forward Jets Summary Limitations of the pure DGLAP approach clearly seen in the forward area Limitations of the pure DGLAP approach clearly seen in the forward area higher order parton emissions break ordering scheme higher order parton emissions break ordering scheme Calculations which include such processes (CDM) provide better description Calculations which include such processes (CDM) provide better description

9-Sep-2005R. Mankel: The Hadronic Final State at HERA20 Diffractive Final States at High Q 2 Hard diffractive process is characterized by rapidity gap near outgoing proton Hard diffractive process is characterized by rapidity gap near outgoing proton caused by colorless exchange caused by colorless exchange It is an interesting question if & how far diffraction extends to the large Q 2 region It is an interesting question if & how far diffraction extends to the large Q 2 region clean final states at LHC, e.g. for Higgs? 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?

9-Sep-2005R. Mankel: The Hadronic Final State at HERA21 Rapidity Gaps in NC Events Forward Plug Calorimeter (FPC) Rapidity gap “Normal” DIS MC (Ariadne) clearly insufficient at low  max “Normal” DIS MC (Ariadne) clearly insufficient at low  max  Need Ariadne+RAPGAP (diffractive MC) to describe the data with FPC veto (at beam pipe) 5

9-Sep-2005R. Mankel: The Hadronic Final State at HERA22 Rapidity Gaps in NC: Q 2 Dependence  Sizable diffractive contribution to NC cross section drops with rising Q 2 still 2% at Q 2 =1500 GeV 2  NC and CC compatible For comparison: Low-Q 2 Data High -Q 2 NC x P <0.05

9-Sep-2005R. Mankel: The Hadronic Final State at HERA23 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 p T relative to jet di-muon events (  correlation) Study of di-muon event signatures allows to use low p t μ thresholds,  measure the total bb cross section B B -   D D - Interaction vertex   (D) - -

9-Sep-2005R. Mankel: The Hadronic Final State at HERA24 Di-Muons: Data vs MC  Good overall description with MC  bb contribution ~2000 events, purity ~43% Same-charge combinations used to normalize light-flavor background Same-charge combinations used to normalize light-flavor background non-isolated, E T >8 GeV

9-Sep-2005R. Mankel: The Hadronic Final State at HERA25 bb Cross Section from Di-Muon Events NLO QCD predictions: PHP: 5.8 nb NLO QCD predictions: PHP: 5.8 nb (FMNR,CTEQ5M) DIS: 1.0 nb (FMNR,CTEQ5M) DIS: 1.0 nb (HVQDIS,CTEQ5F4) (HVQDIS,CTEQ5F4)  6.8 nb  6.8 nb  NLO prediction lower than the data, though not entirely incompatible within errors +3.0–1.7 Compare with recent H1 measurement of  vis bb in PHP using D*  correlations Compare with recent H1 measurement of  vis bb in PHP using D*  correlations p T (D*)>1.5 GeV, |  (D*)| 2 GeV, |  (  )|<1.7, 0.05<y<0.75, Q 2 <1 GeV 2 H1 :  similar trend

9-Sep-2005R. Mankel: The Hadronic Final State at HERA26 Summary Wealth of measurements from HERA on structure of hadronic final state Wealth of measurements from HERA on structure of hadronic final state only a small selection presented only a small selection presented Leading baryons & forward jets probe QCD dynamics in vicinity of proton remnant Leading baryons & forward jets probe QCD dynamics in vicinity of proton remnant allows accurate distinctions between different models allows accurate distinctions between different models Hard diffraction reaches up to high Q 2 Hard diffraction reaches up to high Q 2 Measurement of open beauty cross section  leptons at high p T Measurement of open beauty cross section  leptons at high p T

9-Sep-2005R. Mankel: The Hadronic Final State at HERA27 The End

9-Sep-2005R. Mankel: The Hadronic Final State at HERA28 Backup Slides

9-Sep-2005R. Mankel: The Hadronic Final State at HERA29 Direct Comparison of Global Phase Space and BFKL-Sensitive Regime Global phase space: Global phase space: CDM (BFKL) works well CDM (BFKL) works well MEPS (DGLAP) slightly worse MEPS (DGLAP) slightly worse fixed-order QCD underestimates data at high  jet (missing higher orders) fixed-order QCD underestimates data at high  jet (missing higher orders) BFKL-sensitive phase space: BFKL-sensitive phase space: Steep falloff with  jet (  h cut) Steep falloff with  jet (  h cut) MEPS (DGLAP) fails to describe data MEPS (DGLAP) fails to describe data CDM (BFKL) works well CDM (BFKL) works well NLO QCD is better than LO (t-channel gluon exchange) NLO QCD is better than LO (t-channel gluon exchange) Q 2 > 25 GeV 2 y > 0.04 E e ’>10 GeV E T jet >6 GeV -1<  jet <3 in addition:  had >90 o 0<  jet <3 0.5<(E T jet ) 2 /Q 2 <2 Global Phase Space BFKL Phase Space

9-Sep-2005R. Mankel: The Hadronic Final State at HERA30 More Pieces to Pentaquark Puzzle  (1520) : both in forward & backward hemisphere  + : only in forward hemisphere signal mainly from forward pseudo rapidity region  + signal mainly from forward pseudo rapidity region unlike regular baryons unlike regular baryons  (1520) and  c predominantly at medium Q 2 similar to  c no sign of decuplet partners seen in  –  – and  –  + (  NA49)

9-Sep-2005R. Mankel: The Hadronic Final State at HERA31 Di-Muon Mass Spectra (Data vs MC) low mass high mass same B (charm cascade) + J/  different B’s (signal!) + cc + light flavor BG mainly light flavor BG different B’s (charm cascade + BB mixing) + light flavor BG  Good description with MC  bb contribution ~2000 events

9-Sep-2005R. Mankel: The Hadronic Final State at HERA32 bb from Di-Muons: Normalization of BG-MC cc: normalize to D*mu analysis cc: normalize to D*mu analysis Bethe-Heitler, elastic charmonium: normalize to data under isolation cut Bethe-Heitler, elastic charmonium: normalize to data under isolation cut Light flavor: use like sign spectrum (minus bb MC) Light flavor: use like sign spectrum (minus bb MC)

9-Sep-2005R. Mankel: The Hadronic Final State at HERA33