Leading Baryons at HERA

1
Leading Baryonsat HERA
32nd International Conference on High Energy
Physics Beijing, China, Aug. 16-22, 2004 Heuijin
Lim(DESY) on behalf of the H1 and ZEUS
Collaborations

• Introduction
• Diffractive structure function measured in
  events with a leading proton (See also M.
  Kapishins talk)
• D photoproduction with a leading neutron
• Dijet with a leading neutron
• Summary

2
Introduction

• A significant fraction of ep scattering events
  contains a leading baryon produced at small t
  (soft process).

, IP
p, n
p, n

• Standard fragmentation
• p or n appears in fragmentation of target jet

• In exchange models
• p from exchange of neutral iso-scalar or
  iso-vector (p0, IR, IP)
• n from exchange of charged iso-vector (p, r,
  )

3
Diffractive structure function measured with a
leading proton
Kinematics of ep geXp
Diffractive structure function of proton
4
t dependence from LPS

• H1 b-value considerably below ZEUS result.
• Regge phenomenology predicts
• shrinkage of the diffractive peak
• Additional b dependence expected in models.
• Data not yet precise enough to show
  presence/absence of aIP term.

• Fit t distribution to
• ? ds/dt shows steep fall-off with t as in
  elastic hadron-hadron scattering.

5
Q2 dependence of cross section with LPS

• At high Q2, ds/dMX falls with Q2.
• As Q2 g0, ds/dMX depends weakly on Q2.
• Data are compared with the color dipole model
  (BEKW parametrization).
• At high Q2 in high MX bins,,
• Data above BEKW.
• Expect this region (xIPgt0.01) to be dominated by
  Reggeon exchange.

(Bartels, Ellis, Kowalski and Wüsthoff)
For medium b,
For small b,
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Diffractive structure function of the proton
xIPF2D(3) (xIP,b,Q2) with LPS

• xIPgt0.01
• xIPF2D(3) increases as xIPg1
• ? Reggeon contributions
• xIPlt0.01
• xIPF2D(3) increases as xIPg0
• ? Parton evolution as xIPg0

• Regge fit (xIP lt 0.01) with common Pomeron flux
  factor

with
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Diffractive hard scattering factorization
Collins (1998) Trentadue, Veneziano (1994)
Berera, Soper (1996)
Universal partonic cross section
Diffractive parton distribution function
fi/pD(z,Q2,xIP,t) Probability to find in a
proton, with a probe of resolution Q2, parton i
with momentum fraction z, under the condition
that the proton remains intact and emerges with
small energy loss, xIP, and momentum transfer, t.
NLO QCD fits to the ZEUS data

• Perform fit to the results from LPS and from
  diffractive charm in DIS.
• xIP lt 0.01 and Q2gt2 GeV2
• Regge factorisation assumption,
• Donnachie and Landshoff Pomeron flux
• Parametrise PDFs(quark flavour singlet and
  gluon) using zf(z) (a1a2za3z2)(1-z)a4
• For light quark distribution, assuming
• Charm quarks were treated in Thorne-Roberts
  variable flavor number (TRVFN) scheme with
  mc1.45 GeV.
• NLO evolution QCDNUM

8
NLO QCD fit on LPScharm data

• QCD fit describes data with
• c2/ndf 37.8/36
• Fraction of the t-channel momentum carried by
  gluons
• ZEUS at Q22 GeV2
• H1(2002) at Q210 GeV2.
• consistent with H1.

9
Comparison of LPS and H1-FPS

• Good agreement between the two experiments.
• H1 QCD fit (from large rapidity gap data, see M.
  Kapishins talk) describes the results of
  ZEUS(LPS) and H1(FPS).

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Forward neutron calorimeter
Leading Proton Spectrometer
FNC
P beam hole
Z 106 m
Window for n-acceptance qn lt 0.8 mrad
H1
107
ZEUS

• ZEUS FNC consists of 10 lI Pb-scintillator
  sandwich calorimeter.

• H1 FNC consists of Pb-scintillator fibers.

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Factorization hypothesis

• Study relationship between soft and hard
  interactions
• Hard scale mc2 for charm production
• ET for dijet events
• Q2 for DIS.
• Soft scale pT of the neutron
• Study vertex factorization hypothesis.
• Ratios (ds/dY neutron tagged)/(ds/dY inclusive)
  are flat with Y kinematic variables of the event.

Independent vertices
with ss(1-xL)
n
pion form-factor
parton distribution in pion
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D photoproduction with a leading neutron
Q2 lt 1 GeV2
X
p
n

• tagged in FNC
• 0.2 lt xL lt 1
• qn lt 0.8 mrad

13
Differential cross sections of D production with
a leading neutron
Different pion PDFs

• Standard fragmentation (HERWIG, PYTHIA,
  RAPGAP(inc)) and RAPGAP(OPE) describe W, pT and h
  dist.
• ? Only OPE describes xL dist.
• Data is not sensitive to the different variants
  of the pion structure function.
• Light-cone(f1) and exponential(f3) pion form
  factors are compatible with data in shape.

Q2 lt 1 GeV2
Different pion form factors
14
Ratio if D yield with and without a neutron
Q2 lt 1 GeV2

• Ratio is flat with respect to W, pT(D) and
  h(D).

Agreement with factorization hypothesis.
15
Dijet with a leading neutron
Jet
Dijet cross sections compared to pion exchange
MC (POMPYT) with different pion PDFs. ? Not
sensitive.
Jet

• fLN
• flat with Etjet
• ? Support factorization hypothesis
• rising with xgjet

16
Summary

• Diffractive structure function measured with a
  leading proton in DIS
• Recent data from ZEUS with improved precision
  and extended kinematic range.
• Slope of ds/dt is compatible with soft
  interaction at the proton vertex.
• Data can be described by color dipole model
  (e.g. BEKW parameterisation).
• Data described by a NLO QCD fit

FPS-H VFPS FPS-V

• D and dijet with a leading neutron
• Data described by one-pion-exchange model.
• Data agree with the factorization hypothesis.

• H1 Very Forward Proton Spectrometer (VFPS)
• Tag the scattered proton at HERA II for high
  rate studies of diffraction
• Large acceptance for low xIP and 0lttlt0.5 GeV2
• Expect physics data soon.

Acceptance Red 90-100 , Blue0-10