Leading Baryon Production at HERA

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Leading Baryon Production at HERA
HERA

• Review of Measurements
• New preliminary Results

Kerstin Borras Small x and
Diffraction Workshop FNAL Sep 2003
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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
• Vertex factorization can be tested

p,n
p,n

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

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

• Proton vertex
• Leading baryon variables
• xL z xp or xn
• t (p p)2 - pT2 / xL

Cross section dependence on baryon
variables independent of particle production at
photon vertex
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Leading Proton Production
Publication xL pT2 Structure Vertex Factor.
H1 semi-incl. DIS (LPLN,96/97) (Eur.Phys.J.C6,587(1999)) (X) X
H1 semi-incl. PHP DIS(96) (Nucl.Phys.B619,3(2001)) X X
ZEUS prel. ? in p-diss. PHP(97) (EPS 2001 652 ) X X
ZEUS semi-incl. DIS(94/95) (Nucl.Phys.B685,3(2002)) X X X X
ZEUS prel. semi-incl. DIS(96/97) (EPS 2003 544) X X
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LP Cross Section as a function of XL
update with 96-97 data four times higher
statistic

• Data are very precise
• Flat for xLlt0.95

Non-diss. diffraction in xLgt0.97 ,

proton-diss. diffraction (ep?eXN) included in
xLlt0.97
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LP Cross Section as a function of pT2

• High precision
• line exp(-6.7 pT2)

LPS acceptance in xL and pT2 (stations 4,5,6)
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LP b-slopes as function of xL

• New data consistent with published 95 data
• Variation of b-slopes ? variation of pT2
  acceptance
• No dependence on xL visible

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Leading Neutron Production
Publication XL pT2 Structure Vertex Factor.
H1 semi-incl. DIS (LPLN,96/97) (Eur.Phys.J.C6,587(1999)) (X) X (pion)
H1 prel. dijets in PHP DIS(96/97) (ICHEP 2002 988) X X (pion) X
ZEUS prel. semi-incl. DIS (99) (ICHEP 2000 882) X
ZEUS prel. ? in p-diss. PHP (97) (EPS 2001 652 ) X X
ZEUS dijets in PHP (95) (Nucl.Phys.B596,3(2001)) X X (pion) X
ZEUS semi-incl. PHPDIS (95-97) (Nucl.Phys.B637,3(2002)) X (X) X (pion) X
ZEUS prel. semi-incl. PHPDIS (00) (ICHEP 2002 824) X X
ZEUS prel. D in PHP (98-00) (EPS 2003 567) X X X
ZEUS prel. semi-incl. PHP(00) (EPS 2003 546) X(pion) X(pion)
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LN xL spectrum for basic sample
Test the One-Pion-Exchange picture
Look at En spectra, cross sections
dependences on Q2, xp, x?,
xL gt 0.5 agreement in shape and rate with
One-Pion-Exchange MC models in both PHP and DIS
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LN Dijet differential cross sections
PHP
DIS
One-Pion-Exchange simulation with different pion
structures describe data in shape and in
normalization No preference for any structure
function yet.
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LN Pion trajectory
Pion flux
xL independent
Cross section for neutron production
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LN Pion trajectory
(1-xL) distributions behave power law like in
different t ranges
powers aeff(t)
Powers aeff(t) are linear dependent on t
aeff(t)1.060.080.52-0.42 (2.780.330.30-0.42
GeV-2) t
One-Pion-Exchange (ap(t)t-mp2) aIP(0)
1.060.080.52-0.42 ap 1.380.170.15-021
GeV-2
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LN Vertex Factorization
No dependence on particle production at upper
vertex compare ?p with pp
Factor of two different ! New developments with
recent measurements from NA49 (EPS 2003)
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LN Vertex Factorization II
RHO production in PHP
Dijet production in PHP and DIS
? No significant violation
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Summary

• A lot of precise leading baryon measurements at
  high energies available now
• Knowledge on xL spectra, pT2 distributions,
  b-slopes, structures and vertex factorization
  greatly increased
• Phenomenological and theoretical input for
  describing the data and comparisons to TeVatron
  measurements needed
• Tune available (new?) models to high energy
  behavior
• t-distribution in LP at HERA comparable to
  TeVatron ?
• Clarify Pion flux models
• Vertex factorization violation or holding
  depending on final state ?

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NA49 preliminary EPS 2003