NLO QCD fits to polarized semi-inclusive DIS data

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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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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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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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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Polarized DIS (pDIS)

‘structure function’

‘parton distribution’ (pPDF)

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pDIS flavor decomposition

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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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Polarized SIDIS (pSIDIS)

‘fragmentation function’ (FF)

D. de Florian, C. Garcia Canal, R.S. Nuc.Phys.B470 195 (1996)

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Flavor decompositon in pSIDIS

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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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borrowed from W. Vogelsang

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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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NLO combined analysis: data

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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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Results:

478-20=458 d.o.f313 pDIS165 pSIDIS

Interplay between pDIS and pSIDIS

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Uncertainties:

J. Pumplin et al. Phys.Rev.D65 014011 (2002)

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Future/ongoing measurements:

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Future/ongoing measurements:

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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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Acknowledgments:

Xiaodong Jiang, fruitfull collaboration,

JLAB, hospitality and support.

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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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Uncertainties: