LEADING BARYON PRODUCTION at HERA

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LEADING BARYON PRODUCTION at HERA Lorenzo
Rinaldi On behalf of H1 and ZEUS Collaborations
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Motivations

• Large fraction of events with a Leading Baryon
  (LB) in final state carrying high fraction of the
  proton beam momentum
• LB produced at small angle in forward direction
  difficult detection
• Production mechanism still not clear
• Interest in LB study for next experiments _at_ LHC
  (absorptive corrections for diffractive Higgs,
  pile-up background...)

• Experimental results discussed in this talk
• Leading Proton (LP) spectra in DIS
• Leading Neutron (LN) spectra in DIS and
  photoproduction
• Dijet production with a Leading Neutron
• Latest developments in theory
• Comparison with models

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Leading baryon production in ep collisions
LB production affected by absorption and
rescattering effects evidences of vertex
factorization violation
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Leading baryon detectors

• 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 lt 1 spT2 few MeV2 (better than p-beam
  spread 50 - 100 MeV)

• ZEUS Forward Neutron Calorimeter (FNC)
• 10l lead-scintillator sandwich
• s/E 0.65/vE, ?Eabs2
• Acceptance qnlt0.8 mrad, azimuthal coverage 30

• ZEUS Forward Neutron Tracker (FNT)
• Scint. hodoscope _at_ 1?int, sx,y0.23cm, s?22µrad

• H1 Forward Neutron Calorimeter (FNC)
• Lead-scintillator calorimeter _at_ 107m from I.P.
  veto hodoscopes
• s(E)/E20, neutron detection eff. 935

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Leading Proton cross section vs xL

• Montecarlo samples (standard fragmentation)
• Herwig (cluster model)
• MEPS (parton shower,SCI)
• Ariadne (CDM)
• Bad description of xL spectrum

• Flat below diff. peak ?(1-xL)a, a0
• Good description by reggeon-exchange model

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LP cross section vs pT2
Data distribution exponential
Fit to exponential in each xL bin
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LP b-slopes vs xL

• Different slopes in LEPTO
• Better HERWIG
• b-slope not well simulated by fragmentation
  models

• No strong dependence observed on xL
• observed fluctuations due to fit range
• Good agreement with prediction from
  Reggeon-exchange model

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LP summary

• leading proton quantities have been measured
  with high precision
• flat cross section vs xL below the diffractive
  peak
• approximate exponential fall of pT2 cross
  section
• no visible dependence of pT2 slopes vs xL
• Good description by reggeon-exchange model
• Fragmentation models fail to describe LP
  production
• Accurate measurements available for MC tunings

And what about the leading neutrons? ? ? ?
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Leading Neutron One-Pion-Exchange model
O.P.E. partially explains the LN production
a(t) and F2(xL,t) model dependent
Longitudinal momentum spectrum and pT2 slopes
discriminate between different parametrizations
of fluxes
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Rescattering model and absorption 1
Model 1 One pion exchange in the framework of
triple-Regge formalism
Nikolaev,Speth Zakharov Re-scattering
processes via additional pomeron exchanges
(Optical Theorem) (hep-ph/9708290)
(Kaidalov,) Khoze, Martin, Ryskin (KKMR) Enhanced
absorptive corrections (? exclusive Higgs _at_ LHC),
calculation of
migrations, include also r and
a2 exchange (different xL pT dependences) (hep-p
h/0602215, hep-ph/0606213)
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Rescattering model and absorption 2
Model 2 calculations from DAlesio and Pirner
in the framework of target fragmentation (EPJ
A7(2000) 109)

• more absorption when photon size larger (small
  Q2) ? less neutrons detected in photoproduction
• more absorption when mean p-n system size
  (rnp) smaller at low xL ? less neutrons
  detected at low xL
• more absorption ? fewer neutrons detected with
  higher pT2 ? larger b-slope expected in
  photoproduction

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LN longitudinal momentum spectrum
PHP DIS
DIS
DAlesio Pirner

• LN yield increases with xL due to increase in
  phase space pT2 lt 0.476 xL2
• LN yield decreases for xL?1 due to kinematic limit

sWa, a(sgp) ? a(s gp) Wp2(1-xL)Wp2 ? (1-xL)
-0.1

• LN yield in PHP lt yield in DIS ?
  factorization violation
• Models in agreement with data !

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LN longitudinal momentum spectrum 2

• KKMR predictions Including migrations and other
  iso-vector exchanges, like r and a2
• Pure p exchange too high
• Absorption and migrations effects reduces the LN
  yield and fit the data better
• Additional r and a2 exchanges enhance the LN yield

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LN cross section vs pT2 in xL bins
DIS

• Exponential behavior with slope b
• Intercept and exponential slope fully
  characterize the pT2 spectra

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LN intercepts and b-slopes vs xL

• LN slope increases up to xL0.8
• LP slope almost flat
• Similar values in the range 0.7ltxLlt0.85 when p
  exchange dominates

• intercept cross section integrated over all pT2
  ? rise towards xL0

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LN b-slopes vs xL Models

• OPE models
• Dominant at 0.6ltxLlt0.9
• (non-) Reggeized flux, different form factors
  with different parameters
• none of the models seem to decribe the data well

KKMR model good description of the data
considering absorption effects and r,a2 exchange
contributions
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LN b-slopes DIS photoproduction
p
p,r,a2
slopes different in PHP and DIS in general
agreement with expectation from absorption
? more absorption _at_ small n-p size ?
depletion _at_ large pT
?
steeper slope in PHP
DIS
PHP
Fit to exponential
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LN b-slopes DIS photoproduction from KKMR model
p
p,r,a2
Other exchanges flatten the pT2 distributions in
both, a bit more in PHP than in DIS
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LN production compared to MC models
Assuming the leading baryon production proceeds
via the standard fragmentation process ? do
standard MC generators describe the data ?

• LeptoMEPS best for xL spectra, but flat b-slope,
• LeptoAriadne, RAPGAP in standard mode, CASCADE
  cannot describe any of the distributions too few
  neutrons, too low xL, b-slopes too flat.

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Dijet production with a LN
DIS ? direct PHP
resolved PHP
x?obs
Re-scattering processes expected for resolved
PHP photon acts hadron-like ? additional
interactions between remnants and scattered
partons Relevant variable momentum fraction
of the photon entering the hard sub-process.
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Dijet production with a LN

• DIS
• gp?jjXn

• DIS
• gp?jjXn

Lower values _at_ higher xL
Comparable slopes
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Dijet production with a LN sjjn/sjj
ZEUS RAPGAP/HERWIG-MI ( )
(Nucl.Phys.B596,3(2001))
ok
Not ok
H1 RAPGAP/PYTHIA-MI ( ) (Eur.
Phys. J. C41 (2005) 273-286 )
Not ok
ok
No possibility to decide on factorization
breaking due to re-scattering processes in
resolved PHP (xglt1) with hadron-like photon.
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Summary

• High precision measurements of leading baryon
  production available from HERA
• LP spectra measured and well described by
  Reggeon-exchange model
• LN production characterized by rescattering
  effects (observed also in LP, not shown here)
  available models give a general good description
  of the data
• MC generators in general fail to reproduce the
  measured quantities ? need to understand the
  process of leading baryon production and the
  implementation of its mechanism in the
  generators.
• theory provides now a lot of predictions,
  experimental measurements fundamental for model
  tuning
• Leading baryon study remains an important
  topic in HEP with a direct impact on next
  experiments _at_ LHC

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LP absorption effects