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Parton distributions and Physical Measurements

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Title: Parton distributions and Physical Measurements


1
Parton distributions and Physical Measurements
  • Hung-Liang Lai
  • Taipei Municipal University of Education, TAIWAN

In collaboration with J. Huston, S. Mrenna, P.
Nadolsky, J. Pumplin, D. Stump , W.K. Tung, C.-P.
Yuan
_at_QCD08, July 7, 2008
2
Outline
  • An overview of latest CTEQ PDFs
  • Impact on LHC predictions
  • Analysis of Correlations due to PDFs
  • PDFs for LO Monte Carto Generators (Preliminary)

3
CTEQ 6.6 series (arXiv0802.0007 hep-ph)
  • Full implementation of the General-mass (GM) QCD
    formalism
  • Factorization scheme (Collins)
  • PDFs and their evolution
  • Mass-dependence of hard cross section (SACOT)
  • Final States, on-shell kinematics
  • Rescaling (ACOTc), Phase space,
  • Free strange PDFs, i.e. independent shape
    parameters from (ubar dbar)
  • ? CTEQ6.6M 44 uncertainty sets
  • Exploration of PDFs with intrinsic charm ?
    CTEQ6.6C
  • Alternative sets for variousaS(MZ)0.112.125?
    CTEQ6.6A

4
Suppression of charm contribution to F2 in the GM
scheme results in larger light quark distribution
at small x ? CTEQ6.1(dashed) u d are below
CTEQ6.6 by 2-5 at x 10-3
5
Very different strange distribution (fixed vs.
free strange shape)
6
Impact of new CTEQ PDFs on LHC Predictions
7
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W Z production at LHC x 10-3
-10-2 Difference of 3 in quark distributions
results in 6 q-qbar luminosity and W Z cross
sections
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Analysis of correlations due to PDFs
14
For 2N PDF uncertainty eigensets, the uncertainty
for an observable X due to PDF is given by
Correlation cosine for observables X and Y
15
Role of the correlation angle
(0) measurement of X imposes tight (loose)
constraints on Y
16
Cosf0.956
Cosf-0.80
17
Z vs. PDFs
W vs. PDFs
Z,W are strongly correlated with g,b,c at
x0.005, and anti-correlated at x0.1 due to
momentum conservation of gluon.
18
Higgs vs. PDFs
t-tbar vs. PDFs
19
PDFs for LO Monte Carlo Generators (preliminary)
  • Problem in consistency of using LO matrix
    elements, parton showering, with usual LO/NLO
    PDFs in Monte Carlo generators.
  • Prescriptions
  • (a) relax momentum sum rule (Sherstnev Thorne)
  • (b) adding selected pseudo-data sets in the
    fits with free expt. normalizations
    factorization scales

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Summary
  • Our knowledge of PDFs and its uncertainty is
    crucial for hadron collider physics.
  • Heavy quark mass effects are noticeable, even at
    high scale, and thus have impact on LHC study.
  • Analysis of correlations in PDF parameter space
    is a powerful method to understand relations
    among physical observables as well as PDFs.
  • Modified LO PDFs could be more suitable for LO
    MC generators.
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