NLO QCD fits to polarized semi-inclusive DIS data
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Transcript of NLO QCD fits to polarized semi-inclusive DIS data
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NLO QCD fits to polarized NLO QCD fits to polarized semi-inclusive DIS datasemi-inclusive DIS data
Rodolfo SassotRodolfo SassotUniversidad de Buenos AiresUniversidad de Buenos Aires
JLAB MAY 5, 2005
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R. Sassot JLAB, May 5, 2005 2
30 years studying the short distance structure of the proton:
from hints on point-like structure, to parton densities.
QCD + DIS high precision PDFs
predictions and tests of ‘new physics’
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R. Sassot JLAB, May 5, 2005 3
why spin? • inessential complication?
indispensable tool for unveiling hadron structure,
search for the unexpected.
why polarized DIS?
• best benchmark for hadron structure,• top cited spin crisis,
• 90’s experimental programs.
why polarized SIDIS?
• flavor and quark-antiquark discriminatrion,• increasing statistics,
• universality and precision of fragmentation functions (FF).
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R. Sassot JLAB, May 5, 2005 4
Outline:
• QCD improved parton model description of polarized DIS and SIDIS
• global QCD fits to polarized data.
• uncertainties in pPDFs
• uncertainties in future/ongoing measurements
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R. Sassot JLAB, May 5, 2005 5
Polarized DIS (pDIS)
‘structure function’
‘parton distribution’ (pPDF)
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R. Sassot JLAB, May 5, 2005 6
pDIS flavor decomposition
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R. Sassot JLAB, May 5, 2005 7
QCD improved parton model: LO and NLO
‘naïve’
‘LO’
‘NLO’ ‘add a gluon’
W R. Mertig, W. L. van Neerven, Z.Phys.C70 637 (1996) W. Vogelsang, Phys.Rev.D54 20023 (1996)
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R. Sassot JLAB, May 5, 2005 8
Polarized SIDIS (pSIDIS)
‘fragmentation function’ (FF)
D. de Florian, C. Garcia Canal, R.S. Nuc.Phys.B470 195 (1996)
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R. Sassot JLAB, May 5, 2005 9
Flavor decompositon in pSIDIS
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R. Sassot JLAB, May 5, 2005 10
NLO combined analysis:
• and non perturbative quantities to be measured, • QCD predicts Q -dependence, not boundary conditions
1. make ansatz about the x-dependence at some scale
2
2. evolve from to and compute
3. compare with data
4. fit to minimize
• data obtained for different values of x and Q 2
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R. Sassot JLAB, May 5, 2005 11
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R. Sassot JLAB, May 5, 2005 12
borrowed from W. Vogelsang
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R. Sassot JLAB, May 5, 2005 13
NLO combined analysis: parameterizations
pDIS can probe:
pSIDIS give access to:
positivity relative to MRST02: 20 parameters
A.D. Maritn et al. Eur.Phys.J.C28 (2002) 455
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R. Sassot JLAB, May 5, 2005 14
NLO combined analysis: data
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R. Sassot JLAB, May 5, 2005 15
Fragmentation functions input:
KRE: S. Phys.Rev.D62 054001 (Kretzer 2000)
KKP: B. A. Kniehl, G. Kramer, B. Potter, Nucl.Phys.B582 514 (2000)
flavor separation:
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R. Sassot JLAB, May 5, 2005 16
Results:
478-20=458 d.o.f313 pDIS165 pSIDIS
Interplay between pDIS and pSIDIS
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R. Sassot JLAB, May 5, 2005 17
Uncertainties:
J. Pumplin et al. Phys.Rev.D65 014011 (2002)
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R. Sassot JLAB, May 5, 2005 18
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R. Sassot JLAB, May 5, 2005 19
Future/ongoing measurements:
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R. Sassot JLAB, May 5, 2005 20
Future/ongoing measurements:
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R. Sassot JLAB, May 5, 2005 21
Conclusions:
• pPDFs have evolved dramatically:
• pSIDIS opens a window to sea quarks and helps to constrain other densities
• dependence on FFs as a caveat & opportunity:
• future prospects:
successfull experimental programs
tools for the analysis (NLO)
from ‘scenarios’ to ‘constraints’
• overall picture combined NLO global fits:
perfect consistency pDIS/pSIDIS
improved constraints on pPDFsbest fits favor SU(3) in the sea
tackle both problems together?
encouraging
PHENIX:
COMPASS & CLAS, E04-113 and FFs
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R. Sassot JLAB, May 5, 2005 22
Acknowledgments:
Xiaodong Jiang, fruitfull collaboration,
JLAB, hospitality and support.
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R. Sassot JLAB, May 5, 2005 23
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R. Sassot JLAB, May 5, 2005 24
Role of pDIS and pSIDIS in the extraction of pPDFs:
• pDIS as the main source of information on pPDFs.
• hadronic decays data & assumptions: flavor symmetry
• e.m. interactions can not discriminate quarks from antiquarks:
large uncertainties in sea quark and gluon pPDFs
• pSIDIS as a way out:
SMC @ CERN: first steps; encouragingHERMES @ DESY: from consistency checks to constraintsCOMPASS @ CERN: …..E04-113 @ JLAB: …..
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R. Sassot JLAB, May 5, 2005 25
Uncertainties: