1984

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1984 2004 20 Years of Global QCD Analysis of
the Parton Structure of Nucleon A survey of
open issues through the historical perspective

• DIS04 Strbske Pleso Tung

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The two Topcite papers that started this journey
in 1984
Q2 DEPENDENT PARAMETRIZATIONS OF PARTON
DISTRIBUTION FUNCTIONS. 1092 citations By D.W.
Duke, J.F. Owens (Florida State U.),.
FSU-HEP-831115, Nov 1983. Phys.Rev.D3049,1984
SUPER COLLIDER PHYSICS.By E. Eichten, I.
Hinchliffe, Kenneth D. Lane , C. Quigg,. Feb
1984. 550pp. 1667 citations Rev.Mod.Phys.56579,
1984
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How far have we come along?
What still remains unclear?How far do we still
to go?
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Agenda

• The Valence quarks
• The Gluon
• The Sea quarks
• Breaking of Iso-spin Symmetry
• Breaking of flavor SU(3)
• Strangeness Asymmetry?
• Iso-spin Violation?
• Heavy Quark Parton Distributions

• Uncertainties of
• Parton Distributions, and
• Their Physical Predictions

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The Valence u Quark progression of improvements
LO fits to early fixed-target DIS data
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The beginning of the Hera era .
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Refinements
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All in the details now?
Time to move on to something else?
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D quark, the other twin
Early LO fits
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NLO, no dramatic changes
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The impact of Hera
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The old and the new
Does the happy story continue?
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The story about the gluon is more interesting,
and not as happy
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Evolving
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Hera again
Small-xs gain is large-xs loss!
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consolidation
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What goes up must come down?
Does gluon go negative at small x and low
Q? (MRST)
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Uncertainties of PDFs CTEQ6
Theory un-certainties not included
Thus, only lower bounds on the uncertainties
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Two potential Direct Measurements of the Gluon
Distribution

• Measurement of the longitudinal Structure
  Function in DIS.
• Crucial. Still possible at Hera?
• Direct Photon Production in Hadron Collisions
• Data existbut not always consistent with each
  other (WA70 and E706)
• Theoretical uncertainties in NLO QCD
  overwhelming Resummed QCD promising, but has not
  delivered so far.

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The non-strange sea quarks do they observe
isospin symmetry?
Theorists Sure, why not ?Isnt the gluon flavor
neutral?
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Experiments Let Nature speaks for him/her self!
Surprised, you theorists? No, there is no physics
reason for dbub !
Measurement of F2n-F2p in NC DIS experiments
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More experimental inputs (mostly DY asymmetry)
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Caution Modern fit without DY and Collider
input
New DY data (E866) have raised new questions
about the large x region
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Comparing the Valence Quarks of the Nucleon
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Odd man out?
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Strange Content of the Nucleon Structure
SU(3) flavor symmetric sea quarks? Why not?
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Experimental input (low statistics) data on
Dimuon (charm) production in Neutrino-Nucleus
scattering.
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No qualitatively new development
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Odd man out?
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All together
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Issue Uncertainties on PDFs

• The statistical principles and methods for
  uncertainty analyses are well establishedLikelih
  ood, c2, etc.---all textbook stuff, nothing
  extraordinary in principle.

• The devil is, not mainly in the details, rather
• Unknown theoretical uncertainties
• Unknown experimental uncertainties
• Whats needed?

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Reality 1 compatibility of experiments
(Giele etal, 2001)
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Basic dilemma What is the real uncertainty on a
measured quantity due to incompatible
experimental results?
Imagine that two experimental groups have
measured a quantity ? , with the results shown.
q
What is the value of ? ?
What do confidence levels mean?
(This is common occurrence in the real world.)
Are all experimental errors understood? Should
the errors be taken at face value?
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Case study consequences on as analysis in the
GKK approach (likelihood)
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Uncertainties of Physical Predictions What is
the true uncertainty? (GKK)
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Case study CTEQ global analysis of sW (c2 method)
Estimate the uncertainty on the predicted cross
section for ppbar ? WX at the Tevatron collider.
global c2
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Each experiment defines a prediction and a
range. This figure shows the Dc2 1 ranges.
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This figure shows broader ranges for each
experiment based on the 90 confidence level
(cumulative distribution function of the rescaled
c2).
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Uncertainty in 3 scenarios
(either directly measured or indirectly inferred
physical quantity q)
Uncertainty dominated by

• Only case I is textbook safe but II and III are
  real.
• There are commonly used prescriptions for dealing
  with II and III but none can be rigorously
  justified.
• Over time, inconsistencies are eliminated by
  refined experiments and analyses

This is the Source of large tolerance, Dc2
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Mimi-Summary

• The important issue is not about methodology
  likelihood vs. c2 or Monte Carlo sampling or
  Hessian approximation,
• They are essentially equivalent, given consistent
  theoretical and experimental input.
• The challenges concern
• Catalog, define, and quantify theoretical
  uncertainties
• Learn to live and work with imperfect and
  incompatible data sets---there is no unique
  procedure, only intuition
• Learn to agree to disagree
• Learn to compromise, forge consensus (e.g. choice
  of sensible schemes), while also emphasize
  distinctiveness, hence diversity and integrity of
  the physics results.

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Tension between different physical processes
and experiments?

• Intra-process tension
• BCDMS / NMC / HERA ? (cf. GKK as analyses)
• CC (CCFR) / NC ? (nuclear vs. nucleon targets,
  ..)
• CDF / D0 (both prefer large-x gluons but there
  are more subtle tensions)
• Inter-process tension
• DIS / Jets ? (MRST2003)
• DY / Jets ? (MRST2001 ?)

How do we systematically address these potential
incompatibilities? Likelihood method of GKK
Collins and Pumplin
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Tension between CDF/D0 data sets?

• CTEQ6 Analysis Eigenvector 15 in the Hessian
  approach is particularly sensitive to jet data
• direction D01.24 CDF1.60
• - direction 0.435 2.04

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Collins and Pumplin Study - hep-ph/0106173, and
Pumplin Ringberg03
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• Lessons learned, so far, are not surprising
• The scale of acceptable changes of c2 must be
  large. Adding a new data set and refitting may
  increase the c2s of other data sets by amounts
  gtgt 1.
• Global analysis requires compromises the PDF
  model that gives the best fit to one set of data
  does not give the best fit to others.
• But it provides a systematic way of investigating
  the relevant problems, and quantifying the
  incompatibilities.

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A critical technical advance in the Hessian
approach which enabled the CTEQ uncertainty
studies

• The Hessian method for c2 analysis has always
  been the standard, but uncertainty estimates in
  global QCD analysis by standard tools had been
  known to be extremely unreliable due to two
  practical problems
• .extreme range of eigenvalues (flat vs. steep)
• numerical fluctuations of theory predictions

An iterative method by Jon Pumplin solved both of
these technical difficulties, provided the means
to generate reliable eigenvectors in parton
parameter space, hence allow the systematic
exploration of this space, particularly the a
priori unknown flat directions
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CTEQ agenda for studying Nucleon Structure and
Collider Physics

• Large x behavior of G(x,Q), u(x,Q) and d(x,Q)

• New frontiers on detailed flavor structure of the
  nucleon
• Pinning down the strangeness sector of nucleon
  structure
• Understanding the charm content of the nucleon

• Precision W/Z phenomenology at the Tevatron and
  LHC
• Predictions by and feedback to global analysis
• Transverse momentum, resummation and W-mass

• NNLO analysis

• Higgs, Top, and Beyond SM Phenomenology