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Prospects for and Status of the Measurement of
High x Parton Distributions at HERA
Z. Zhang LAL, Orsay On behalf of the H1 and
ZEUS Collaborations
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Outline of the talk

• Past HERA-I overview of cross section and
• structure function
  results
• Impact of HERA data on PDFs
• With emphasis on high x region
• Present First results from HERA-II
• Future Prospects

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HERA-I Data Samples (1992-2000)
Lpeak 2 1031cm-2s-1, Ldesign1.5 1031cm-2s-1
All 3 datasets have been used by H1 and ZEUS
to measure double differential NC, CC cross
sections single differential NC, CC cross
sections total (CC) cross sections in full
kinematic domain
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From Cross Sections to Structure Functions
NC Cross Section
NC Reduced cross section
Dominant contribution Sizeable
only at high y (ygt0.6)
Contribution only important
at high Q2
CC Cross Section
CC Reduced cross section
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HERA Kinematic Range vs. Fixed Target

• F2 at low x, Q2
• data drive global PDF fits
• since a decade
• - the main source of constraint
• on gluon/sea density at low x
• Indirect FL measurements
• consistent with QCD (scaling
• violation) prediction
• F2, xF3 at high x
• also at high Q2
• thus still statistical limited
• HERA data free from nuclear,
• heavy target mass corrections
• required for fixed target data

HERA input crucial for predictions of new physics
measurements at LHC
x
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Precision HERA F2 Data
Cover a huge kinematic range gt4 orders
magnitude in x Q2 Strong scaling violation
-- due to g?qqbar q?qg ? provides indirect
constraint on gluon density Best precision
down to 2 _at_low Q2 syst dominates
already Smooth transition in Q2 between fixed
target data and HERA data
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First Structure Function xF3 _at_ HERA
xF3 constrains thus uv,dv at high x/Q2
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CC Reduced Cross Sections e-p vs. ep
The CC e-p cross section dominated by u quark
density the CC ep cross section dominated
by d quark density CC ep (e-p) cross
sections are unique for constraining d (u) quark
density
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PDF Fit with H1 Data Only (H1 2000 PDF Fit)
Eur. Phys. J. C30(2003)1, hep-ex/0304003

• Use all H1 NC CC cross section data (ep
  e-p) for Q2min3.5GeV2
• - Parameterize 5 PDF sets
• with a functional form
• 
  Fit in MSbar,
• 
  massless scheme
• low x medium x and high
  x behavior
• at Q204GeV2, with in total 10 free parameters
  c2/ndf540/(621-10)
• (number of parameters in Pn(x)
  differs for different PDF sets)
• Momentum sum rule and u, d quark flavor counting
  applied
• Correlated systematic errors properly treated

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u d Quark Densities
xu xd

• From H1 2000 PDF Fit
• Best optimum precision of PDF
• without local fluctuation
• From Local Extraction Method
• Free from nuclear corrections
• Two methods are complementary
• HERA data start to constrain
• the u, d quark densities at

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PDF Fits with ZEUS Data Only
For ZEUS fixed target, see Phys. Rev.
D67(2003)012007, hep-ex/0208023
1) ZEUS s(NC,CC) only ZEUS-O ZEUS NC, CC
ep, e-p cross sections preliminary 2)
ZEUS s(NC,CC) Jet ZEUS-JETS ZEUS NC,
CC ep, e-p cross sections Jet data
preliminary - Different PDF sets simpler
functional form w.r.t. H1 only fit - Q207GeV2,
Q2min2.5GeV2 - Some of the parameters fixed
externally - Use Thorne-Roberts variable flavor
number scheme
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PDF Precision in a Global Fit or HERA Only Fit
Global fit HERA data fixed target data
Fit with HERA (ZEUS) data only

• Uncorrelated uncertainty (statunc. syst)
  dominates in the latter fit
• (thus improvement expected with HERA-II data)
• Precision better in a global fit but nontrivial
  to treat syst inconsistent
• data additional nuclear corrections for some
  of the fixed target data.

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ZEUS Fit with or w/o DIS gp Jet Data
Including DIS inclusive jet photoproduction
di-jet data improves the PDF precision at medium
large x For the future Including further
charm and beauty structure function data should
also help in the medium x range
gp
DIS
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Comparing PDFs from H1 versus ZEUS only Fit

• Gluon sea scaled by x20
• PDF difference between
• ZEUS and H1 may be
• explained by
• different parametric
• forms
• different heavy flavor
• treatments
• inclusion or not jet data
• - consistency of data sets

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HERA-II Performance
HERA delivered
H1 accumulated

• 3 fold increase achieved
• w.r.t. HERA-I by
• finely focused beams
• higher beam currents
• Slow start-up 2002-03
• due mainly to high beam
• related background
• HERA now has best ever
• performance

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Polarization at HERA-II
Spin rotators
e beam naturally transversely polarized
(Sokolov-Ternov effect)
Longitudinally polarized e beam
Feb. 2003 a world first Helically
polarized e colliding with a high
energy proton beam with 3 pairs of spin
rotators
Typical Pe55 Rise-time 22min Two
independent measurements by LPOL TPOL
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First HERA-II Results to Moriond 2004
A fraction of data used Now more data available
at different polarization values Results will
be updated soon Differential NC and CC cross
sections will follow High luminosity runs are
really needed _at_high x, Q2
Extrapolation _at_Pe-1 tests the absence
of right-handed current
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Projected HERA-II Performance
The current scenario
Four different running scenarios considered
pe, ee- All
scenarios give similar total luminosity
700pb-1 before the end of HERA-II running
in June 2007
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Expected Precision at HERA-II High Lumi Running
(A)
NC ep, e-p cross section asymmetry ? xF3 xF3
sensitive to 2uvdv
NC sensitive to sum of all q, qbar
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HERA-II High Lumi Running (B)
From NC polarization asymmetry sensitive to d/u
ratio at high x Simulation with L200pb-1,
pe0.5
CC ep, e-p cross sections constrain uv and dv at
high x Simulation 250pb-1
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HERA-II Low Proton Energy Running
Simulation with 30pb-1, Ee27GeV,
Ep460GeV Access to higher x and
lower Q2 Direct FL measurement also needs
data _at_different center-of-mass energies
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Summary
HERA-I data have - large kinematic coverage
rich data samples (NC, CC, ep, e-p) - been
driving the global QCD fits since a decade -
greatly improved our knowledge on PDFs (in
particular gluon) _at_low x - enabled PDF
extraction w/o external input - started to
constrain PDFs at high x High luminosity run
with polarization _at_ HERA-II gives a good
prospect for gaining knowledge precision _at_ high
x There is strong interest in the community to
continue beyond HERA-II but need solution for
a new injector replacing PETRA and funding