Leading Baryon Production at HERA

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Leading Baryon Production at HERA
Ioannis Gialas University of Aegean

• Talk given at
• CIPANP 2003 New York
• May 20, 2003

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Contents

• Leading Baryons in HERA (ZEUS, H1)
• Physics Motivation
• Forward Detectors FNC, LPS,
• Event selection
• Leading Baryon spectra. Slopes in pT2
• Semiinclusive structure function F2LB
• F2LB/ F2 ratio. Dependence on xBJ, Q2
• Pion structure function F2p
• Conclusions
• Will not discuss diffraction in this talk

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HERA layout

• Ep 820 GeV
• Ee 27.5 GeV
• Lpeak 1?10 31 cm-2 s-1
• Bunch crossing time 96 ns

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ZEUS detector
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Leading Baryon Production
Regge picture

• Proton in final state (Neutral exchange)
• Diffraction (Pomeron)
• p0, f0 exchange
• Neutron in final state (Charged exchange)

Fragmentation picture
Leading baryons are produced in the hadronization
of the proton remnant
Projectile/Target Factorization
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Leading Baryon Kinematics
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Why do it?
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Leading Proton Spectrometer

• Roman pots
• Collected Luminosity
• 2.5 pb-1 in 1995
• 6 stations along beam
• Used in 1995 S4,S5,S6 double pot vertical
• 6 Si strip planes/pot
• Orientations 45, 0, -45
• The detector planes follow the beam
  profile

Measure pT,xL

• Resolution
• ? 0.3 in longitudinal momentum pz (or xL
  pz/pbeam)
• ? 3 in transverse momentum pT
• Measurement area 0.6ltxLlt1.0 0 lt pT2lt
  0.5 GeV2

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Forward Neutron Calorimeter
Pb-Sci Calorimeter (10?) Veto counters in
front Energy Scale 1 Energy Resolution
Measure Neutron Energy En
Acceptance determined by beam-pipe apertures
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Event selection

• DIS event selection
• 3 lt Q2 lt 254 GeV2
• 1.2?10-4 lt xBJ lt 4? 10-2
• 45 lt W lt 225 GeV
• -50 lt VTXz lt 50 cm
• Ee gt 10 GeV
• BPC event selection
• 0.11 lt Q2 lt 0.65 GeV2
• 45 lt W lt 225 GeV
• -50 lt VTXz lt 50 cm
• 10 lt Ee lt 28 GeV

• LPS track
• 0.6 lt xL lt 1.0
• 0 lt pT2 lt 0.5 GeV2
• FNC track
• Neutral hadronic shower
• 0.6 lt xL lt 1.0
• ? lt 0.8 mrad

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Neutron and proton xL spectra
Both distributions use ? cut Both distributions
are acceptance corrected Color Dipole Model as
implemented in Ariadne fail to describe the
data If the mechanism at work is isovector
exchange it is expected NLP 0.5 NLN Instead
it is observed NLP ? 4 NLN
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Events with a LP vs xL
0.6 lt xL lt0.91 and pT2 lt0.5 GeV2

• NLP/NDIS12.7 ?0.3?0.9
• NLP/NBPC13.0 ?0.5?0.7
• Lepto (Soft Color Interaction) does not describe
  the observed proton rates

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LP events
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LP
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H1
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H1
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H1
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Measurement of pT2 slopes
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pT2 slopes vs xL
xL
The pT2 slopes are sensitive to the reaction
mechanism. Model by Szczurek, Nikolaev, Speth
approximates the data by a summation of
contributions of various exchange mechanisms, ?,
IP, and f0 (see also the xL spectrum)
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Semi-inclusive Structure Functions
The DIS semi-inclusive cross section is written
In an analogous way in the case of Leading
proton production
Assuming and
Factorization of cross sections would support
ratios independent of xL, pT2
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F2LP/ F2DIS vs xBJ, Q2 0.85lt xLlt 0.97
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versus Q2 (xLgt0.6)
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LP ratio
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LN ratio
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F2LP
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F2LP
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F2LN
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p structure function
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F2LN
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Summary

• Measurements of leading n and p production in a
  wide Q2 range (0.1 - 250 GeV2) were made
• The data can be described by a sum of various
  particle exchanges
• F2LP was measured by determining the ratio
  F2LP/F2DIS (minimizing systematic uncertainties)
• The F2LP/F2DIS and runcLN dependence on Q2 is
  flat indicating that F2LP and F2LN exhibit the
  same xBJ behavior and Q2 scaling as F2DIS.
• The flatness of F2LP/F2DIS and ratio
  with respect to Q2 and xBJ is an indication of
  factorization

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H1 detector
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Cross section parametrization

• F2 Structure function
• FL Longitudinal structure function
• F3 Parity violating term arising from the Z0
  exchange

• F2em Contribution to F2 due to photon
  exchange
• F2wk Contribution to F2 due to Z0 exchange
• F2int Contribution to F2 due to ?Z0
  interference

corrections
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Kinematics reconstruction
Electron method
Jacquet-Blondel method
Double Angle method
Combined methods, eg PT method