Transcript Global Analysis of Parton Distributions

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
The two Topcite papers that started
this journey in 1984:
Q**2 DEPENDENT PARAMETRIZATIONS OF
PARTON DISTRIBUTION FUNCTIONS.1092 citations
By D.W. Duke, J.F. Owens (Florida State U.),. FSUHEP-831115, Nov 1983. Phys.Rev.D30:49,1984
SUPER COLLIDER PHYSICS.
By E. Eichten, I. Hinchliffe, Kenneth D. Lane , C.
Quigg,. Feb 1984. 550pp.
1667 citations
Rev.Mod.Phys.56:579,1984
How far have we come along?
What still remains unclear?
How far do we still to go?
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
To reveal the difference in both large and small x regions
The Valence u Quark:
progression of improvements
LO fits to early fixed-target DIS data
To view small and large x in one plot
NLO fits to more fixedtarget DIS data sets
The beginning of the Hera era
….
Refinements …
All in the details now?
Time to move on to
something else?
D quark, the other twin:
Early LO fits
NLO, no dramatic changes
The impact of Hera
The old and the new
Does the happy story continue?
The story about the gluon is more interesting,
and not as happy …
Evolving …
Hera again …
Small-x’s gain is large-x’s loss!
consolidation
What goes up must come down?
Does gluon go negative at small x
and low Q?
(MRST)
Uncertainties of PDFs:
CTEQ6
Theory uncertainties
not included
Thus, only lower bounds on
the uncertainties
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 exist–but not always consistent with each
other (WA70 and E706);
 Theoretical uncertainties in NLO QCD
overwhelming; Resummed QCD promising, but
has not delivered so far.
The non-strange sea quarks:
do they observe isospin symmetry?
Theorists: Sure, why not ?
Isn’t the gluon flavor neutral?
Experiments: Let Nature speaks for him/her self!
n
p
Measurement of F2 -F2 in
NC DIS experiments
Surprised, you
theorists?
No, there is no
physics reason
for db=ub !
More experimental inputs: (mostly DY asymmetry)
Caution:
“Modern fit” without DY and Collider input:
New DY data (E866) have raised new
questions about the large x region
Comparing the Valence Quarks of the Nucleon:
Odd man out?
Strange Content of the Nucleon Structure
SU(3) flavor symmetric sea quarks? Why not?
Experimental input: (low statistics) data on Dimuon
(charm) production in Neutrino-Nucleus scattering.
No qualitatively new development
CCFR-NuTeV (high statistics) data for
dimuon production from n N and anti-n N
scattering.
Odd man out?
All together:
Issue: Uncertainties on PDFs
• The statistical principles and methods for uncertainty
analyses are well established:
Likelihood, c2, … etc.---all textbook stuff, nothing
extraordinary in principle.
• The devil is, not mainly in the details, rather:
 Unknown theoretical uncertainties
 Unknown experimental uncertainties
• What’s needed?
Reality #1 : compatibility of experiments
(Giele etal, 2001)
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.
c2
L -1
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?
Case study: consequences on as analysis in the GKK
approach (likelihood)
Uncertainties of Physical Predictions:
What is the true uncertainty? (GKK)
Ds ~
0.4
Ds~
0.1
Case study: CTEQ global analysis of sW (c2 method)
Estimate the uncertainty
on the predicted cross
section for ppbar  W+X
at the Tevatron collider.
global c2
local c2’s
Each experiment defines a “prediction” and a “range”.
This figure shows the Dc2 = 1 ranges.
This figure shows broader ranges for each experiment based on the “90%
confidence level” (cumulative distribution function of the rescaled c2).
“Uncertainty” in 3 scenarios
(either directly measured or indirectly inferred physical quantity )
c2
c2
L -1
c2
L -1
L -1

D






Uncertainty dominated by:

D
D
 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
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.
“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
Tension between CDF/D0 data sets?
• CTEQ6 Analysis: Eigenvector 15 in the Hessian
approach is particularly sensitive to jet data:
+ direction: D0=1.24 CDF=1.60
- direction:
0.435
2.04
Collins and Pumplin Study - hep-ph/0106173, and …
Pumplin – Ringberg03:
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 c2‘s of other data
sets by amounts >> 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”.
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”
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