The Hadronic Final State at HERA

1
The Hadronic Final State at HERA
(Some Highlights from)

• 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 gt 1 GeV2 deep-inelastic scattering (DIS) Q2
lt 1 GeV2 photo-production (PHP)
Diffractive process
3
Comparison of Final State Structure
ee- interaction
Quark jet
Anti-quark jet

• 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

4
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

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

6
Outline

• Introduction
• Leading baryon production
• Jets in the forward area
• Diffraction at high Q2
• Heavy flavor production
• Summary

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 lt xL lt1
  (xL ELP / Ep) pt2 lt 0.5 GeV2

8
Typical Production Mechanisms
p,IR,IP
N,P
N,P
p
p

• Hadronisation of proton remnant
• Herwig (cluster model)
• MEPS (parton shower,SCI)
• Ariadne (CDM)

• Exchange of virtual particles
• leading protons ?0, Pomeron, Reggeon
• leading neutrons ?, ?,

9
Leading Proton Spectrum (DIS)
Theory

• Cross section vs. xL ELP / Ep. Very precise
  data.
• Standard fragmentation models fail to describe
  flat part between 0.6-0.95

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

11
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

12
Leading Neutron Production (contd)

• 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

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

14
Leading Neutron in Di-Jet Events (contd)

• 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
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.

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

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

18
Forward Jet Measurements (DIS)
Cuts designed to enhance BFKL effects
xBjlt0.004, 7olt?jetlt20o, xjetgt0.035

• 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 photon similar to
CDM, but fails to describe forwarddijet sample
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

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?

21
Rapidity Gaps in NC Events
Rapidity gap
Forward Plug Calorimeter (FPC)
with FPC veto (at beam pipe)

• Normal DIS MC (Ariadne) clearly insufficient at
  low ?max
• Need AriadneRAPGAP (diffractive MC) to describe
  the data

5
22
Rapidity Gaps in NC Q2 Dependence
High -Q2 NC x Plt0.05

• Sizable diffractive contribution to NC cross
  section
• drops with rising Q2
• still 2 at Q21500 GeV2
• NC and CC compatible

23
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

-
24
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

25
bb Cross Section from Di-Muon Events

• NLO QCD predictionsPHP 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

pT(D)gt1.5 GeV, ?(D)lt1.5. p(?)gt2 GeV,
?(?)lt1.7, 0.05ltylt0.75, Q2lt1 GeV2
H1
3.0 1.7

• similar trend

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

27

• The End

28

• Backup Slides

29
Direct Comparison of Global Phase Space and
BFKL-Sensitive Regime
Q2 gt 25 GeV2 y gt 0.04 Eegt10 GeV ETjetgt6
GeV -1lt?jetlt3
in addition ?hadgt90o 0lt?jetlt3 0.5lt(ETjet)2/Q2lt2
Global Phase Space
BFKL Phase Space

• Global phase space
• CDM (BFKL) works well
• MEPS (DGLAP) slightly worse
• fixed-order QCD underestimates data at high ?jet
  (missing higher orders)

• BFKL-sensitive phase space
• Steep falloff with ?jet (?h cut)
• MEPS (DGLAP) fails to describe data
• CDM (BFKL) works well
• NLO QCD is better than LO (t-channel gluon
  exchange)

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)

31
Di-Muon Mass Spectra (Data vs MC)

• Good description with MC
• bb contribution 2000 events

32
bb from Di-Muons Normalization of BG-MC

• cc normalize to Dmu analysis
• Bethe-Heitler, elastic charmonium normalize to
  data under isolation cut
• Light flavor use like sign spectrum (minus bb MC)

33
(No Transcript)