Probing Gluon Polarization
with J/Psi in Longitudinally
Polarized pp Collisions
Ming X. Liu
Los Alamos National Lab
The Challenge: the spin “crisis”
• Understanding the
nucleon sub-structure
1 1
dx 1 ( x) g ( x) LZ
2 0 2
1
G Lz
2
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Polarized PDFs
Asymmetry Analysis
Collaboration
M. Hirai, S. Kumano and N.
Saito, PRD (2004)
Gluon polarization
(G) remains
poorly constrained
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SIDIS and Gluon Polarization Measurements
• Semi-inclusive DIS
– HERMES @ DESY
• high-pT hadron pairs
– SMC @ CERN
• high-pT hadron pairs
– COMPASS @ CERN
• high-pT hadron pairs
• open charm (projection)
lepton beam
g*
or
heavy flavor
gluon
or
nucleon target
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@RHIC-SPIN
• Polarized hadron collisions
– double longitudinal spin asymmetry
d d
ALL
d d
– leading-order gluon interactions
• direct-photon production
• heavy-flavor production
proton beam proton beam
or or
gluon gluon
heavy flavor
photon gluon
or
or
proton beam proton beam
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Advantage of Heavy Quarks
Sensitive to gluon polarization: g(x)
Gluon Fusion dominates at LO
PHYTHA estimate: GeV Charm Beauty
200 95:5 85:15
( gg QQ ) : (qq QQ ) 500 97:3 92:8
For open charm production
g ( x1 ) g ( x2 ) gg QQ
ALL a LL
g ( x1 ) g ( x2 )
Karliner & Robinett, Phys.Lett.B324 (1994)
aLL ˆ
s
= 1.1
2M
Decay modes:
1.5
e+e-, +-, e, eX, X cceX
2
bbeX
J/ 5
x
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cos
6
The PHENIX Muon Arms
μ+
μ-
South Muon arm : -1.2 > h > -2.0 North Muon arm : 2.4 > h > 1.2
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ALL from J/+-
• Analysis approach
– Polarization=47% (average)
– R is from BBC_in (GL1P)
– N_Jpsi Yield
1 N J /Psi R N J /Psi
ALL
PB P N J / Psi R N J / Psi
Y
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J/psi Yield Measurements
1. Fill by fill {Charge(+-) –
charge(++,--)}
• No need fill by fill muon
efficiency correction
+- ++,--
• Have Open charm, DY
and other background
• Fix mass range to
extract N_Jpsi
• We used this one for the
final results.
2. Gaussian+Exponential
background
• Used to estimate the
background under J/Psi
peak Dimuon mass distributions
( M M J ? psi ) 2 ( M M Psi ' ) 2
dN 1 1
A e K M N e 2 2
N ' e 2 2
dM 2 2
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Raw asymmetry per Fill:
N_J/Psi > 10 per fill
2 sigma cut:
2.80 = 1.00892
ALL (raw)= -0.007 0.013
Raw J/Psi yield per MB
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Signals
Raw Asymmetry Per Fill:
N_Jpsi > 10
Two pT bins: 0.0-1.4 GeV and 1.4 – 10 GeV
2.80 < Mass < 3.44, 2 sigma cut
ALL(raw) = +0.0011 0.0175
ALL(raw) = -0.0145 0.0179
0 < pT < 1.4 1.4 < pT < 10
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Background
Background Raw Asymmetry:
+-
Left of the peak: 2.0 < Mass < 2.5
ALL(raw)= 0.0107 0.0220
N = 2068 45.5
Under the peak:
ALL(raw) = -0.0351 0.0318
N = 988 31 ++, --
ALL ( RAW ) 0.0041 0.018
BG
ALL r ALL
incl BG
AJ / Psi
1 r
LL
(ALL ) 2 r 2 (ALL ) 2
incl BG
A J / Psi
LL
1 r Dimuon mass distributions
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ALL vs pT
g ( x1 ) g ( x2 ) gg J / X
ALL a LL
g ( x1 ) g ( x2 )
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About Theoretical Calculations
• LO NRQCD: ~
2
s
3 s2 1
dx1
( pp J / ( ))
27 s m 2
4 m2
x1
{
s
2
f q ( x1 ,2m) f q (
4m
,2m) ( 0 1) O8J / ( ) (3S1 ) mix of various
x1 s processes
15 4m 2 9 1
f g ( x1 ,2m) f g ( ,2m) [ 2 (1 0 ) O8J / ( ) (3P0 ) O8J / ( ) (1S0 ) ]}
32 x1 s m 2
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Theoretical Calculations (cont.)
Partonic asymmetry for ggJ/ X
• NLO RNQCD: ~ 3
s aLL J /X (h 4mc / s 2 , xT pT / pm ax )
gg 2
g q {c, c } q
q q {c, c } g
g g {c, c } g
“final state” mixture ratios: fixed from other data
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NLO NRQCD and PHENIX data
PHENIX, PRL 92, 051802 (2004)
Theoretical predictions of J/Ψ production at RHIC are in good
agreement with the PHENIX data: COM process dominant
– PRD 68 (2003) 034003 G. Nayak, M. Liu, F. Cooper
– PRL 93 (2004) 171801 F. Cooper, M. Liu, G. Nayak
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NRQCD Calculations of ALL
• NRQCD LO (hep-ph/0509335 G. Nayak et al.)
GRSV
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Summary and Outlook
• First measurement of double spin asymmetry
with J/Psi from longitudinally polarized p-p
collisions
– Almost pure gluon fusion
– 10k J/psi, statistically limited
• Theoretical work in progress (G. Nayak et al)
– No gluon polarization
– Maximum(+,-) polarization
– Excellent QCD test ground
• Open charm with prompt muons with a much
larger statistics from run5
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