Transcript E906 at FNAL: Drell-Yan Measurements of Light Antiquarks

8th Circum-Pan-Pacific Symposium on High Energy Spin Physics
June 20-24, 2011 in Cairns, QLD, Australia
Wen-Chen Chang
Institute of Physics, Academia Sinica
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Evidences for the Existence of Sea Quarks
Flavor Asymmetry of Sea Quarks
Theoretical Interpretations
Intrinsic Sea Quark & Light-cone 5q Model
Current & Future Experiments
Conclusion
2
Q2 :Four-momentum transfer
x : Bjorken variable (=Q2/2Mn)
n : Energy transfer
M : Nucleon mass
W : Final state hadronic mass
d 2
  Mott [W2 (n , Q 2 )  2W1 (n , Q 2 ) * tan 2 ( / 2)]
'
d dE
  Mott [ F2 ( x, Q 2 ) / n  2 F1 ( x, Q 2 ) / M * tan 2 ( / 2)]
•Scaling
•Valence quarks
•Quark-antiquark pairs
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J.I. Friedman, Rev. Mod. Phys. Vol. 63, 615 (1991)
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
Axial vector current matrix elements:
2s q  p, s | q  5q | p, s  2s (q  q  q   q  )

Scalar density matrix elements:
 p | qq | p 
F (3)
F (u )  F ( d )
F ( q) 
,

 p | uu  dd  ss | p  F (8) F (u)  F (d )  2 F ( s)
sQM
exp
u   d
u  d  2s
5/3
1.26
1
0.6
F (3) / F (8)
1/3
0.23
The simplest interpretation of these failures is that the sQM lacks a quark sea.
Hence the number counts of the quark flavors does not come out correctly.
- Ling-Fong Li and Ta-Pei Cheng, arXiV: hep-ph/9709293
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J.I. Friedman, Rev. Mod. Phys. Vol. 63, 615 (1991)
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1
SG   [( F2p ( x )  F2n ( x )) / x ] dx
0
1 2 1
   (u p ( x )  d p ( x )) dx
3 3 0
1

( if u p  d p )
3
Assume an isotopic quark-antiquark sea,
GSR is only sensitive to valance quarks.
7
New Muon Collaboration (NMC), Phys. Rev. D50 (1994) R1
SG = 0.235 ± 0.026
( Significantly lower than 1/3 ! )
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• Uncertain extrapolation for 0.0 < x < 0.004
• Charge symmetry violation (un  d p, dn  u p )
• u ( x)  d ( x) in the proton
1
 (d ( x)  u ( x)) dx  0.148  0.04
0
Need independent methods to check the d  u asymmetry,
and to measure its x-dependence !
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Acceptance in Fixed-target Experiments
d
4 2 1
2

e
[q( xt )q( xb )  q( xt )q( xb )]

dxbdxt 9 xb xt s
 pd
|x
pp
2
b
 x t


d ( xb )
1


1
4u( xb )   d ( xt )  1  d ( xt ) 

1 
  1 

2
d ( xb ) d ( xt )   u( xt )  2  u( xt ) 
 1  4u( x ) u( x ) 
b
t 

10

Naïve Assumption:
 NMC (Gottfried Sum Rule)
 NA51 (Drell-Yan, 1994)
NA 51 Drell-Yan
confirms
d(x) >u(x)
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
Naïve Assumption:
 NMC (Gottfried Sum Rule)
 NA51 (Drell-Yan, 1994)
 E866/NuSea (Drell-Yan, 1998)
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F2np ( x)  2x[d ( x)  u ( x)  s( x)  c ( x)]
F2nn ( x)  2x[u( x)  d ( x)  s( x)  c ( x)]
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n   N     c  X ; c  s    n 
(s  s )  0.5 * (u  d )
s ( x)  s ( x)
CCFR, Z. Phys. C 65, 189 (1995)
14
NuTeV, PRL 99, 192001 (2007)
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
x(s+s)
( s( x)  s( x))  (u( x)  d ( x))
HERMES, Phys. Lett. B 666, 446 (2008)
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17
http://www.physics.adelaide.edu.au/theory/staff/leinweber/VisualQCD/QCDvacuum/welcome.html
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
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Pauli blocking


 guu is more suppressed than gdd in the proton
since p=uud (Field and Feynman 1977)
 pQCD calculation (Ross, Sachrajda 1979)
 Bag model calculation (Signal, Thomas, Schreiber 1991)
Chiral quark-soliton model (Pobylitsa et al. 1999)
Instanton model (Dorokhov, Kochelev 1993)
Statistical model (Bourrely et al. 1995; Bhalerao 1996)
Balance model (Zhang, Ma 2001)
The valence quarks affect the gluon splitting.
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


Meson cloud in the nucleons (Thomas 1983, Kumano 1991):
Sullivan process in DIS.
p  N ,   :  0 :    2 : 1 : 0
n
 Chiral quark model (Eichten et al. 1992; Wakamatsu 1992):
Goldstone bosons couple to valence quarks.
q  q ,   :  0 :    4 : 3 : 2


The pion cloud is a source of antiquarks in the protons and it lead to d>u.
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s(x)=s(x)?

Meson Cloud Model (Signal and Thomas, 1987)
s( x)  s ( x) at large x

Chiral Field (Burkardt and Warr , 1992)
s( x)  s ( x) at large x

Baryon-Meson Fluctuation (Brodsky and Ma , 1996)
s( x)  s ( x) at large x

Perturbative evolution (Catani et al., 2004)
s( x)  s ( x) at large x
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1
I    [u( x)  d ( x)]dx
0
J.C. Peng, Eur. Phys. J. A 18, 395–399 (2003)
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Is u = d ?

HERMES (PRD71, 012003 (2005))
0.3

( u  d )dx  0.048  0.057  0.028 at Q2 =2.5 GeV2
0.023

COMPASS (NPB 198, 116, (2010))
Light quark sea helicity densities
are flavor symmetric.
0.3

( u  d )dx  0.052  0.035  0.013 at Q2  3 GeV2
0.004

DSSV2008 (PRL 101, 072001 (2008))
1
 (u  d )dx  0.117  0.036
at Q2  10 GeV2
0
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Extrinsic
Intrinsic
Gluon splitting in leading twist
Gluon fusion & light quark scattering
(higher-twist)
Perturbative radiation
Non-perturbative dynamics
CP invariant
Possible CP non-invariant
Fast fluctuation
With a longer lifetime
Of small x
Of large x (valence like)
Strong Q2 dependent
Small Q2 dependent
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It is generally agreed that the observed flavor asymmetry mostly
resulted from the intrinsic sea quarks.
For further investigation, it will be good to separate their
contributions.
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d ( x)  u( x)
• d ( x)  u( x) is a flavor-nonsinglet (FNS) quantity.
• Extrinsic sea quarks vanish at
1st order in s .
• Non-perturbative models are
able to describe the trend.
• Greater deviation is seen at
large-x valence region.
• No model predicts d ( x)  u( x)
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Select a non-perturbative model with a
minimal set of parameters.
Construct the x distribution of flavor nonsinglet quantities: d  u , d  u  s  s , at the
initial scale.
After a QCD evolution with the splitting
function PNS to the experimental Q2 scale,
make a comparison with the data.
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In the 1980’s Brodsky et al. (BHPS) suggested the existence of
“intrinsic” charm (PLB 93,451; PRD 23, 2745).
| p  P3q | uud   P5q | uudQQ  .....
2
m
P( x1,..., x5 )  N5 (1   xi ) / [m2p   i ]2
i 1
i 1 xi
5


5
Dominant Fock state configurations have the minimal
invariant mass, i.e. the ones with equal-rapidity constituents.
The large charm mass gives the c quark a larger x than the
other comoving light partons, more valence-like.
The intrinsic charm in | uudcc  can contribute to the charm production at large x F .
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arXiv:hep-ph/9706252
ISR
Still No Conclusive Evidence…..
CTEQ Global Analysis
PRD 75, 054029
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 In principle, the probability of 5q state ~1/MQ2 .
So the probability is larger for | uudQQ  of light Q.
 We consider the flavor asymmetry of sea quark
as the experimental evidences for the intrinsic
| uuduu , | uudd d , | uudss  5-quark states.
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| uudcc 
In the limit of a large mass for quark Q (charm):
1 ~ 2
P( x5 )  N 5 x5 [(1  x5 )(1  10 x5  x52 )  2 x5 (1  x5 ) ln(1 / x5 )]
2
mc=1.5, ms=0.5, mu, md=0.3 GeV
P( xQ ; uudQQ ) is obtained numerically.
P5uudQQ   P( xQ ; uudQQ )dx
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
uudd d
5
P
P
uuduu
5

 0.118
W.C. Chang and J.C. Peng, arXiv: 1102.5631
The shapes of the x

distributions of d(x)
and 
u(x) are the same
in the 5-q model and
thus their difference.
 Need to evolve the 5-q
model prediction from
the initial scale  to the
experimental scale at
Q2=54 GeV2.

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
P5uudss  0.024
W.C. Chang and J.C. Peng, arXiv: 1105.2381
 are
The x(s(x)+s(x))
from HERMES kaon
SIDIS data at <Q2>=2.5
GeV2.
 Assume data at x>0.1
are originated from the
 5intrinsic |uudss>
quark state.

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

P5uudd d  0.240
P5uuduu  0.122




The d(x)+u(x) from
CTEQ 6.6.

 The s(x)+s(x) from
HERMES kaon SIDIS
data at <Q2>=2.5 GeV2.
uudss
 Assume P5
 0.024
 Probabilities of 5-q
states associated with
the light sea quarks are
extracted.
W.C. Chang and J.C. Peng, arXiv: 1105.2381,1102.5631
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
 P(dd)
P(uu)

P(ss)

P(cc)
0.198 0.148
0.093
0.011 Bag model;
Donoghue and Golowich, PRD15, 3421 (1977)
Reference
0.003 Light-cone 5q model;
Hoffmann and Moore, ZPC 20, 71 (1983)
0.250 0.250
0.122 0.240
0.050
0.009 Meson cloud model;
Navarra et al., PRD 54, 842 (1996)
0.10 0.15
Constituent 5q model;
Riska and Zou, PLB 636, 265 (2006)
0.024
Light-cone 5q model;
Chang and Peng, this work (2011)
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It is surprising that many FNS quantities can be
reasonably described by such a naïve model with
very few parameters (mass of quarks and the
initial scale).
 For completeness, this model should be
extended to take into account:





Anti-symmetric wave function
Chiral symmetry breaking effect
Spin structure
Higher configuration of Fock states
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Fermilab E866/NuSea
 Data in 1996-1997
 1H, 2H, and nuclear targets
 800 GeV proton beam
Fermilab E906/SeaQuest
 Data taking planned in 2010
 1H, 2H, and nuclear targets
 120 GeV proton Beam
d
4 2 1
2

e
[q( xt )q( xb )  q( xt )q( xb )]

dxbdxt 9 xb xt s
 Cross section scales as 1/s
– 7x that of 800 GeV beam
 Backgrounds, primarily from J/
decays scale as s
– 7x Luminosity for same detector
rate as 800 GeV beam
50x statistics!!
Tevatron
800 GeV
Main
Injector
120 GeV
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 
Ratio of Drell-Yan cross sections
(in leading order—E866 data analysis confirmed in NLO)
Global NLO PDF fits which
include E866 cross section
ratios agree with E866 results
 Fermilab E906/Drell-Yan will
extend these measurements
and reduce statistical
uncertainty.
 E906 expects systematic
uncertainty to remain at
approx. 1% in cross section
ratio.

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p+p at sqrt(s)=500 GeV
Yang, Peng, and Groe-Perdekam, Phys. Lett. B 680, 231 (2009)
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Kensuke’s talk on Monday
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20 GeV PT Results
●
Caveats
●
●
●
J. Mans :: CMS EWK Measurements
Very
preliminary,
not part of
publication on
the topic
Only muons
(no electrons)
Uncertified
systematic
errors
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COMPASS Polarized -induced DY experiment
at CERN: spin structure of sea quark.
 MINERνA at FNAL: x-dependence of nuclear
effects for sea and valance quarks.
 JLAB-12 GeV: transverse spatial distribution of
partons.
 
 (Polarized) DY experiment at J-PARC: d/u at
very large-x region.
 EIC at RHIC: sea quark distributions and their
spin dependence.

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
Using DIS, Drell-Yan and SIDIS processes, the
structure of sea quarks in the nucleon are
explored.

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 A large asymmetry between d and u was found at
intermediate-x regions.
 No large asymmetry was observed between s and

s.
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
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The observed large flavor asymmetry mostly
resulted from the non-perturbative
effects.
 

The measured x distributions of (d-u), (s+s)
 

and (u+d-s-s) could be reasonably described
by the light-cone 5q model. The probabilities
of the intrinsic 5q states of light sea quarks
are extracted.
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
The sea quarks are connected with the nonperturbative feature of QCD. They could be
the key to understand the confinement!
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