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

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’


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).


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


Polarized DIS (pDIS)

‘structure function’

‘parton distribution’ (pPDF)


pDIS flavor decomposition


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)


Polarized SIDIS (pSIDIS)

‘fragmentation function’ (FF)

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


Flavor decompositon in pSIDIS


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


borrowed from W. Vogelsang


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


NLO combined analysis: data


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:


Results:

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

Interplay between pDIS and pSIDIS


Uncertainties:

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


Future/ongoing measurements:


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


Acknowledgments:

Xiaodong Jiang, fruitfull collaboration,

JLAB, hospitality and support.


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: …..


Uncertainties:

NLO QCD fits to polarized semi-inclusive DIS data

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