E906 at FNAL: Drell-Yan Measurements of Light Antiquarks
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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
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
3
J.I. Friedman, Rev. Mod. Phys. Vol. 63, 615 (1991)
4
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 2s
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
5
J.I. Friedman, Rev. Mod. Phys. Vol. 63, 615 (1991)
6
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 ! )
8
• 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 !
9
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)
11
Naïve Assumption:
NMC (Gottfried Sum Rule)
NA51 (Drell-Yan, 1994)
E866/NuSea (Drell-Yan, 1998)
12
F2np ( x) 2x[d ( x) u ( x) s( x) c ( x)]
F2nn ( x) 2x[u( x) d ( x) s( x) c ( x)]
13
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)
15
x(s+s)
( s( x) s( x)) (u( x) d ( x))
HERMES, Phys. Lett. B 666, 446 (2008)
16
17
http://www.physics.adelaide.edu.au/theory/staff/leinweber/VisualQCD/QCDvacuum/welcome.html
18
Pauli blocking
guu is more suppressed than gdd 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.
19
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.
20
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
21
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
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.
24
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)
25
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.
26
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 .
27
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
33
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)
34
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
35
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.
37
38
39
p+p at sqrt(s)=500 GeV
Yang, Peng, and Groe-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
42
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.
43
Using DIS, Drell-Yan and SIDIS processes, the
structure of sea quarks in the nucleon are
explored.
A large asymmetry between d and u was found at
intermediate-x regions.
No large asymmetry was observed between s and
s.
44
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.
45
The sea quarks are connected with the nonperturbative feature of QCD. They could be
the key to understand the confinement!
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