the spin of the proton - jlab.org proton spin crisis & the melosh-wigner rotation • it is...
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3
Bo-Qiang Ma (马伯强)
Peking Univ (北京大学)
The 7th Workshop on Hadron Physics in China and Opportunities Worldwide
August 3-7, 2015, Duke Kunshan University
?
Collaborators: Enzo Barone, Stan Brodsky, Jacques Soffer, Andreas Schafer, Ivan Schmidt, Jian-Jun Yang, Qi-Ren Zhang
and students: Bowen Xiao, Zhun Lu, Bing Zhang, Jun She, Jiacai Zhu, Xinyu Zhang,Tianbo Liu
The spin of the proton
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It has been 30 years
of the proton “spin crisis” or “spin puzzle”
• Spin Structure: experimentally
spin “crisis” or “puzzle”: where is the proton’s missing spin?
3.0 sdu
0.020u d s
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The Proton “Spin Crisis”
3.0 sdu
In contradiction with the naïve quark
model expectation:
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Why there is the proton spin puzzle/crisis?
• The quark model is very successful for the
classification of baryons and mesons
• The quark model is good to explain the magnetic
moments of octet baryons
• The quark model gave the birth of QCD as a theory
for strong interaction
So why there is serious problem with spin of the proton
in the quark model?
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• photon scatters incoherently off
massless, pointlike, spin-1/2 quarks
• probability that a quark carries fraction of parent proton’s momentum is q(), (0< < 1)
The parton model (Feynman 1969)
•the functions u(x), d(x), s(x), … are called parton distribution functions (pdfs) - they encode information about the proton’s deep structure
...)(9
1)(
9
1)(
9
4
)()()()( 2
,
21
0,
2
xsxxdxxux
xqxexqedxF q
q
Infinite
Momentum
Frame
•Parton model is established under the collinear approxiamtion: The transversal motion of partons is neglected or integrated over.
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How to get a clear picture of nucleon?
• PDFs are physically defined in the IMF
(infinite-momentum frame) or with space-
time on the light-cone.
• Whether the physical picture of a nucleon
is the same in different frames?
A physical quantity defined by matrix element is frame-independent, but its physical picture is frame-dependent.
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The improvement to the parton model?
• What would be the consequence by taking
into account the transversal motions of
partons?
• It might be trivial in unpolarized situation. However
it brings significant influences to spin dependent
quantities (helicity and transversity distributions)
and transversal momentum dependent quantities
(TMDs or 3dPDFs).
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22
The Notion of Spin
• Related to the space-time symmetry of the Poincaré group
• Generators
0
( , ), space-time translator
infinitesimal Lorentz transformation
1 angular momentum
2
boost generator
1Pauli-Lubanski vertor
2
k ij
ijk
k k
P H P
J
J J J
K K J
w J P
2 2
2 2
Casimir operators: mass
spin
P P P m
w w w s
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The Wigner Rotation
for a rest particle ( ,0) (0, ) w
for a moving particle L( ) ( ,0) (0, ) L( ) /
L( ) ratationless Lorentz boost
Wigner Rotation
, ,
( , )
( , ) L( )
w
w
m p s
p p m s p w m
p
s p s p
s R p s p p
R p p
-1L ( ) a pure rotationp
E.Wigner, Ann.Math.40(1939)149
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Melosh Rotation for Spin-1/2 Particle
The connection between spin states in the rest frame
and infinite momentum frame
Or between spin states in the conventional equal time
dynamics and the light-front dynamics
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What is Δq measured in DIS
• Δq is defined by
• Using light-cone Dirac spinors
• Using conventional Dirac spinors
5
5
s , | | ,
, | | ,
q p s q q p s
q p s q q p s
1
0d ( ) ( )q x q x q x
3
22
0 3
0 3 0
d ( ) ( )
( )
2( )( )
q
q
q pM q p q p
p p m pM
p p p m
-
Thus Δq is the light-cone quark spin or quark spin in the infinite momentum frame,
not that in the rest frame of the proton
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The proton spin crisis
& the Melosh-Wigner rotation
• It is shown that the proton “spin crisis” or “spin puzzle” can
be understood by the relativistic effect of quark transversal
motions due to the Melosh-Wigner rotation.
• The quark helicity Δq measured in polarized deep inelastic
scattering is actually the quark spin in the infinite momentum
frame or in the light-cone formalism, and it is different from the
quark spin in the nucleon rest frame or in the quark model.
B.-Q. Ma, J.Phys. G 17 (1991) L53
B.-Q. Ma, Q.-R. Zhang, Z.Phys.C 58 (1993) 479-482
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Quark spin sum is not a Lorentz invariant quantity
Thus the quark spin sum equals to the proton in the rest
frame does not mean that it equals to the proton spin in
the infinite momentum frame
in the rest frame
does not mean that
in the infinite momentum frame
q p
q
q p
q
s S
s S
Therefore it is not a surprise that the quark spin sum measured in DIS does not equal to the proton spin
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An intuitive picture to understand the spin puzzle
B.-Q. Ma, J.Phys.G 17 (1991) L53-L58
B.-Q. Ma, Q.-R. Zhang, Z.Phys.C 58 (1993) 479-482
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A general consensusThe quark helicity Δq defined in the infinite momentum frame is generally not the same as the constituent quark spin component in the proton rest frame, just like that it is not sensible to compare apple with orange.
H.-Y.Cheng, hep-ph/0002157,Chin.J.Phys.38:753,2000
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Other approaches with same conclusion
Contribution from the lower component of Dirac spinors in the rest frame:
B.-Q. Ma, Q.-R. Zhang, Z.Phys.C 58 (1993) 479-482
D.Qing, X.-S.Chen, F.Wang, Phys.Rev.D58:114032,1998.
P.Zavada, Phys.Rev.D65:054040,2002.
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The Spin Distributions in Quark Model
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Relativistic Effect due to Melosh-Rotation
from
We obtain
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Relativistic SU(6) Quark Model
Flavor Symmetric Case
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Relativistic SU(6) Quark Model
Flavor Asymmetric Case
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Relativistic SU(6) Quark Model
Flavor Asymmetric Case + Intrinsic Sea
More detailed discussions, see, B.-Q.Ma, J.-J.Yang, I.Schmidt,
Eur.Phys.J.A12(2001)353Understanding the Proton Spin “Puzzle” with a New “Minimal” Quark Model
Three quark valence component could be as large as 70% to account for the data
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A relativistic quark-diquark model
B.-Q. Ma, Phys.Lett. B 375 (1996) 320-326. B.-Q. Ma, I.Schmidt, J.Soffer, Phys.Lett. B 441 (1998) 461-467.
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A relativistic quark-diquark model
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A relativistic quark-diquark model
B.-Q. Ma, Phys.Lett. B 375 (1996) 320-326. B.-Q. Ma, I.Schmidt, J.Soffer, Phys.Lett. B 441 (1998) 461-467.
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The Melosh-Wigner rotation in pQCD based parametrization of quark helicity distributions
“The helicity distributions measured on the light-cone are
related by a Wigner rotation (Melosh transformation) to the
ordinary spin Siz of the quarks in an equal-time rest-frame
wavefunction description. Thus, due to the non-collinearity
of the quarks, one cannot expect that the quark helicities
will sum simply to the proton spin.”
S.J.Brodsky, M.Burkardt, and I.Schmidt,
Nucl.Phys.B441 (1995) 197-214, p.202
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pQCD counting rule
h (1 )
2 1 2 | |
p
z z q N
q x
p n s s s s
-+
• Based on the minimum connected tree graph of hard
gluon exchanges.
• “Helicity retention” is predicted -- The helicity of a
valence quark will match that of the parent nucleon.
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Parameters in pQCD counting rule analysis
B.-Q. Ma, I. Schmidt, J.-J. Yang, Phys.Rev.D63(2001) 037501.
1
32i
3
1
52i
5
(1 )
(1 )
i
i
q
q
Aq x x
B
Cq x x
B
In leading term
New Development: H. Avakian, S.J.Brodsky, D.Boer, F.Yuan, Phys.Rev.Lett.99:082001,2007.
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Two different sets of parton distributions
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Different predictions in two models
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The proton spin in a light-cone chiral quark model
Y.Zhang, B.-Q. Ma, PRD 85 (2012) 114048.
An upgrade of previous work by including Melosh-Wigner rotation: T. P. Cheng and L. F. Li, PRL 74 (1995) 2872
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Chances:New Research Directions
• New quantities:Transversity, Generalized
Parton Distributions, Collins Functions,
Silver Functions, Boer-Mulders Functions,
Pretzelosity, Wigner Distributions
• Hyperon Physics:The spin structure of
Lambda and Sigma hyperons
B.-Q. Ma, I. Schmidt, J.-J. Yang, PLB 477 (2000) 107, PRD 61 (2000) 034017B.-Q. Ma, J. Soffer, PRL 82 (1999) 2250
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The Melosh-Wigner Rotation in
Transversity
I.Schmidt&J.Soffer, Phys.Lett.B 407 (1997) 331
B.-Q. Ma, I. Schmidt, J. Soffer, Phys.Lett. B 441 (1998) 461-467.
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The Melosh-Wigner Rotation in
Quark Orbital Angular Moment
Ma&Schmidt, Phys.Rev.D 58 (1998) 096008
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Spin and orbital sum in light-cone formalism
Ma&Schmidt, Phys.Rev.D 58 (1998) 096008
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Transverse Momentum Dependent Quark Distributions
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The Melosh-Wigner Rotation in “Pretzelosity”
J.She, J.Zhu, B.-Q.Ma, Phys.Rev.D79 (2009) 054008
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New Sum Rule of Physical Observables
J.She, J.Zhu, B.-Q.Ma, Phys.Rev.D79 (2009) 054008
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The Melosh-Wigner Rotation in five 3dPDFs
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76
Names for New (tmd) PDF:
COMPASS pion p Drell-Yan process
can also measure
the pretzelosity distributions of the nucleon.
横纵度
纵横度
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The Necessity of Polarized p pbar Collider
The polarized proton antiproton Drell-Yan process
is ideal to measure
the pretzelosity distributions of the nucleon.
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Probing Pretzelosity in pion p Drell-Yan Process
COMPASS pion p Drell-Yan process
can also measure
the pretzelosity distributions of the nucleon.
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Three QCD spin sums for the proton spin
X.-S.Chen, X.-F.Lu, W.-M.Sun, F.Wang, T.Goldman, PRL100(2008)232002
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Angular momentum of quarks and gluons
from generalized form factors
X. Ji, PRL78(1997)611
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Angular momentum of quarks and gluons on the light-cone
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Angular momenta of quarks and gluons on the light-cone
S.J. Brodsky, D.S. Hwang, B.-Q. Ma, I. Schmidt, Nucl. Phys. B 593 (2001) 311
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Sum rules of quarks and gluons on the light-cone
S.J. Brodsky, D.S. Hwang, B.-Q. Ma, I. Schmidt, Nucl. Phys. B 593 (2001) 311
A and B are called gravitational form factors
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Angular momentum of quarks on the light-cone
• We start from a quark model with total angular momentum from
quarks, but we don’t have a correct sum of angular momenta from
generalized form factors.
• The definition of quark angular momentum as from generalized form
factors is artificial.
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Arbitrary in defining angular momenta: what is C?
as
so
so
But A(0) is the momentum fraction, not angular momentum
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89
?
A simple QED system as thought experiment
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90
?
A simple QED system as thought experiment: an electron
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91
?
A simple QED system as thought experiment: an electron
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92
?
A simple QED system as thought experiment: an electron
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93
?
A simple QED system as thought experiment: an electron
Therefore is unjustified.
T.Liu and B.-Q.Ma, Phy.Rev.D91 (2015) 017501, arXiv:1412.7775
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94
?
Spectator diquark model calculation
T.Liu and B.-Q. Ma, Phys.Lett.B741 (2015) 256, arXiv:1501.00062
We cannot identify the canonical angular momentums with half the sum of gravitational form factors:
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The Melosh-Wigner rotation
is not the whole story
• The role of sea is not addressed
• The role of gluon is not addressed
It is important to study the roles played by the sea
quarks and gluons. Thus more theoretical and
experimental researches can provide us a more
completed picture of the nucleon spin structure.
Gluons are hidden in the spectators in our quark-diquark model
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Conclusions
• The relativistic effect of parton transversal motions plays an
significant role in spin-dependent quantities: helicity and
transversity, five 3dPDFs or TMDs, GPDs, the Wigner
distributions.
• It is still challenging to measure the quark orbital angular
momentum:
1. The pretzelosity with quark transversal motions is an important
quantity for the spin-orbital correlation of the nucleon
2. The sum rule between helicity and transversity pdfs can serve as an
estimate of quark orbital angular momentum.
• It is necessary to push forward theoretical explorations and
experimental measurements of new quantities of the nucleon.