Leading Baryon Production at HERA

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Leading Baryon Production at HERA
HERA

• Recent Measurements
• Developments in Theory
• Comparisons to Monte Carlo Simulations

Kerstin Borras
ICHEP 2006
RAS Moscow (DESY)
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Motivation

• Significant fraction of ep scattering events
  contain a leading baryon in the final state
  carrying a substantial proportion of the energy
  of the incoming proton
• Production models not yet completely understood

p,n
p,n

• Standard target fragmentation
• complex final-state with p or n in proton remnant
• or p stays intact

Particle exchange (à la Regge) p iso-scalar,
iso-vector (IR, ?, a2 )
n iso-vector (p, ?, a2 )
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Kinematics and Vertex Factorization
Photon vertex lepton variables Q2, x , W , y

• Proton vertex
• leading baryon variables
• xL Ep,n / Ep
• t (p p)2 ? - pT2 / xL

Cross section dependence on leading baryon
variables independent of kinematics at photon
vertex
W
Violation of factorization in LB production
similar as in diffraction
re-scattering processes can lead to
absorption and migrations
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Theoretical models (I)
Production of leading neutrons or protons during
the fragmentation process. ? check, if standard
settings of the MC generators reproduce the
measured xL and pT2 distributions (see plots at
the end of the talk).
p,n
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Theoretical models (II)
2) One pion exchange in the framework of
triple-Regge formalism
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Leading Baryon Detection
Forward Neutron Calorimeter (FNC)
10 ?int Pb-Sc
sandwich, s/E 0.65/vE, ?Eabs2 Forward Neutron
Tracker (FNT)
Sc hodoscope _at_ 1?int, sx,y0.23cm,
s?22µrad Leading Proton Spectrometer (LPS)
Six
stations with silicon µ-strip detectors
sxL lt 1, spT? 5 MeV H1 similar
devices
pT resolution limited by beam spread 50 MeV
horizontal, 100MeV vertical

• Data samples
• neutron 0.2ltxLlt1, ?nlt0.75mrad ? pT2 lt 0.476 xL2
  GeV2
• proton 0.56ltxL, pT2 varying with xL between pT2
  lt0.15 and lt0.5 GeV2
• DIS (ep ? Xn) 40pb-1, Q2gt2GeV2
• PHP (?p ? Xn) 6pb-1, Q2lt0.02 GeV2
• di-jets in PHP (?p ? jjXn) 40pb-1, Q2lt1GeV2,
  130ltWlt280GeV,
  ETj1gt7.5GeV, ETj2gt6.5, -1.5lt?j1,2lt2.5

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Leading neutrons xL spectrum
DIS

• 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

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Leading Neutrons pT2 spectra in DIS
DIS

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

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Leading Neutrons pT2 spectra intercept
pT20
DIS

• intercept cross section integrated over all pT2
  ? rise towards low xL

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Leading Neutrons b - slopes
One Pion Exchange Model
xL
p
,n
s (cm energy of ep system )2 fp/p pion flux
factor
exponential fit
b-slopes in data not described by the b-slopes in
the models.
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Leading Neutrons b-slopes in PHP DIS
normalized _at_ pT20
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Leading Neutrons Protons b-slopes

• slopes almost flat for LP
  (different additional exchanges)
• visible decrease for xL?1 in LP can be due to
  changing acceptance of LPS in pT2 for increasing
  xL (for
  small pT2 slope seems steeper as for larger pT2)

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Predictions from Theory (KMR)

• Including other iso-vector exchanges, like ? and
  a2,
• additional exchanges increase the cross section
  (e.g. for xL)
• but they decrease the b-slopes
• ? difficult to describe all dependencies
  simultaneously

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Predictions from Theory (KMR)
p
p,r,a2
Other exchanges flatten the pT2 distributions in
both, a bit more in PHP than in DIS
In summary adjustment of all available
parameters with the precise leading baryon data
gives not only valuable information for the
absorptive effects at work in exclusive
Higgs-production _at_ LHC, but also determines the
fluxes and the measurement of F2p.
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Leading Neutrons Comparisons to MC
Assuming the leading baryon production proceeds
via the standard fragmentation process ? do
standard MC generators describe the data ?
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Factorization in PHP di-jets with LN ?
resolved PHP
DIS ? direct PHP
x?obs
Re-scattering processes expected for resolved
PHP photon acts
hadron-like ? additional interactions between
remnants and scattered partons ? absorption (see
also diffractive di-jets in PHP, previous
talk) Relevant variable x? momentum fraction
of the photon entering the hard sub-process.
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Factorization in PHP di-jets with LN ?
ZEUS RAPGAP/HERWIG-MI (Nucl.Phys.B596,3(2
001))
not ok
ok
No possibility to decide on factorization
breaking due to re-scattering processes in
resolved PHP (x?lt1) with hadron-like photon.
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Summary

• a lot more, new and precise, leading baryon data
  available from HERA
• precise input for the determination the pion-flux
  and the pion structure function
• indications for absorption/migration observed
• theory provides now a lot of predictions ? can be
  used to further tune the absorption factors
  expected for the discovery channel of exclusive
  Higgs production _at_ LHC
• MC generators in general not describing the data
  ? need to understand the process
  of leading baryon production and the
  implementation of its mechanism in the generators.

? Even in this old and traditional high energy
physics topic a lot remains still to be
understood which has direct impact on the physics
_at_ LHC ?