Flavor Symmetry of Parton Distributions and Fragmentation Functions

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Flavor Symmetry of Parton Distributions and
Fragmentation Functions
Jen-Chieh Peng
University of Illinois at Urbana-Champaign
Workshop on Future Prospects in QCD at High
Energy BNL, July 17-22, 2006
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Outline
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Is in the proton?

Test of the Gottfried Sum Rule
New Muon Collaboration (NMC) obtains
SG 0.235 0.026
( Significantly lower than 1/3 ! )
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Drell-Yan Measurements
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Unpolarized Semi-Inclusive DIS
Use SIDIS to extract parton distribution functions
Universality of fragmentation functions
(hep-ph/0011155)
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Flavor structure of sea-quark distribution via
SIDIS

• Assuming factorization is valid

• Require knowledge on the fragmentation function
  D(z)

SIDIS from HERMES
Drell-Yan vs SIDIS
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Future experiments to measure at large
x
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Future SIDIS for at small x ?
HERMES Semi-Inclusive DIS
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Independent of nuclear effect in deuteron!
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Garvey and Peng, nucl-ex/0109010
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Models for asymmetry
Meson Cloud Models
Chiral-Quark Soliton Model
Instantons

• Quark degrees of freedom in a pion mean-field
• nucleon chiral soliton
• expand in 1/Nc

Theses models also have implications on

• asymmetry between and

• flavor structure of the polarized sea

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Meson cloud model
Analysis of neutrino DIS data
Signal and Thomas Brodsky and Ma Melnitchouk and
Malheiro Christiansen and Magnin
Barone et al.
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Result from NuTeV
s(x)
s-bar(x)
D. Mason, DIS 2005 talk
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Spin and flavor are closely connected

• Meson Cloud Model

• Pauli Blocking Model

A spin-up valence quark would inhibit the
probability of generating a spin-down antiquark

• Instanton Model

• Chiral-Quark Soliton Model

• Statistical Model

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JCP, Eur. Phys. J. A18 (2003) 395
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Flavor Structure of the Helicity Distributions

• No evidence for

• Measurement of W production at RHIC-spin would
  provide new information

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Drell-Yan decay angular distributions
T and F are the decay polar and azimuthal angles
of the µ in the dilepton rest-frame
Collins-Soper frame
A general expression for Drell-Yan decay angular
distributions
In general
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Drell-Yan decay angular distributions
T and F are the decay polar and azimuthal angles
of the µ in the dilepton rest-frame
Collins-Soper frame
A general expression for Drell-Yan decay angular
distributions
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Decay angular distributions in pion-induced
Drell-Yan
NA10 p- W
Z. Phys. 37 (1988) 545
Dashed curves are from pQCD calculations
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Decay angular distributions in pion-induced
Drell-Yan
Is the Lam-Tung relation violated?
140 GeV/c
194 GeV/c
286 GeV/c
Data from NA10 (Z. Phys. 37 (1988) 545)
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Boer-Mulders function h1-
?10.47, MC2.3 GeV
Boer, PRD 60 (1999) 014012
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Motivation for measuring decay angular
distributions in pp and pd Drell-Yan

• No proton-induced Drell-Yan azimuthal decay
  angular distribution data
• Provide constraints on models explaining the
  pion-induced Drell-Yan data. (h1- is expected to
  be small for sea quarks. The vacuum effects
  should be similar for pN and pN)
• Test of the Lam-Tung relation in proton-induced
  Drell-Yan
• Compare the decay angular distribution of pp
  versus pd

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Decay angular distributions for pd Drell-Yan at
800 GeV/c
pd at 800 GeV/c
lt?gt 1.070.07
ltµgt 0.040.013
lt?gt 0.030.01
lt2?-1?gt -0.130.07
E866 Preliminary
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Comparison of NA10 versus E866
Sea-quark Boer-Mulders function is relatively
small
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Comparison of pd versus pp
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Many Quark Distributions Are Probed in
Semi-Inclusive DIS



Unpolarized
Boer-Mulders
Transversity
Polarized target

Sivers
Polarzied beam and target
SL and ST Target Polarizations ?e Beam
Polarization
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for large x
SU(6) symmetry d/u ?1/2
SZ 0 dominance d/u?1/5
S 0 dominance d/u?0
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Can one probe the meson cloud directly?
Scattering of electron off virtual pion was
studied at HERA by tagging forward-going
proton/neutron
Pion structure functions at 10-4 lt x lt 10-2 were
measured
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BONUS at Hall-B
Tagging slow-moving spectator protons
This setup is ideal for detecting the scattering
off the meson cloud a) ep?epX (p0 cloud)
b) ep?e?X (K cloud)
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Connections between parton distribution functions
and fragmentation functions
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(No Transcript)
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Ma, Schmidt, Soffer, Yang assumes the following
relations between fragmentation function and
parton distributions
(hep-ph/0208122)
CTEQ5 PDF
BSB PDF
Precise ee- data at large z from Belle could
shed light on the connection between PDF and
fragmentation functions
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Connection between PDF and FF for mesons?
SMRS pion PDF
Kretzer pion fragmentation function
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Connection between PDF and FF for mesons?
Kretzer pion fragmentation function
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Flavor structure of the fragmentation functions
1) Hyperons
S contains uus valence quarks
(Alberg et al. hep-ph/9809243)
Can be measured with Drell-Yan using S beam.
Difficult experiment!
X
2) Pions
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Flavor structure of the fragmentation functions
3) Kaons
Difficult to measure sea-quark distributions in
K !
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What is known about kaon fragmentation functions?
KKP global fit
This implies
Lingyan Zhu (PR-04-114)
Connections between the parton distribution and
fragmentation functions?
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Summary

• The flavor asymmetry of light sea quarks provides
  insight on nucleon sea. Future measurements of
  the sea quarks at higher and lower x and
  polarized sea are anticipated.
• Flavor and spin structure of the nucleon are
  connected.
• Complementarity between electron and hadron
  probes.
• Connection between the parton distribution
  function and the parton fragmentation functions
  need to be better understood.
• The flavor structure of nucleon and meson
  fragmentation functions could be an interesting
  area for further investigations.