Transcript The RHIC SPIN Program Achievements and Future Opportunities The RHIC Spin Program Achievements and Future Opportunities Carl Gagliardi Texas A&M University on behalf of the RHIC.

2012
The RHIC SPIN Program
Achievements and Future Opportunities
The RHIC Spin Program
Achievements and Future Opportunities
Carl Gagliardi
Texas A&M University
on behalf of the RHIC Spin Collaboration
What contributes to the proton spin?
Consider a proton moving
toward the right
Proton spin 


Helicity
Δq(x)
Δg(x)
Polarized DIS: ~ 0.3
Poorly constrained
Spin sum rule:
Proton spin 
Transversity
δq(x)

RHIC spin program:

Chiral odd; very little data
The RHIC Spin Program – DNP 2012 – Gagliardi
•
•
•
•
What is ΔG?
Role of antiquarks in ΔΣ?
How large is transversity?
Move toward a 2- or 3-D
picture of the proton?
2
Gluon polarization before RHIC data
Leader et al, PRD 75, 074027
ΔG = 0.13 ± 0.16
ΔG = -0.20 ± 0.41
ΔG ~ 0.01
Kinematic region of
polarized measurements
• Unpolarized parton distributions are determined precisely from DIS
– Gluon distribution obtained indirectly from Q2 dependence
• Kinematics of polarized DIS measurements far more limited
2 lever arm provides only weak constraints on Δg(x)
–
Q
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DSSV – first global NLO analysis with RHIC pp data
de Florian et al., PRL 101, 072001
• The first global NLO analysis to include inclusive DIS, SIDIS, and
RHIC pp data on an equal footing
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Improved precision from 2006 to 2009
STAR
• Substantially larger figure of merit (P4 x L) than in all previous
runs combined
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New global analysis with 2009 RHIC data
Special thanks to the DSSV group!
• DSSV++ is a new, preliminary global analysis from the DSSV group
that includes 2009 ALL measurements from PHENIX and STAR

0.2
0.05
06
g ( x, Q 2  10 GeV 2 ) dx  0.1000..07
• First experimental evidence of non-zero gluon polarization in the
RHIC range (0.05 < x < 0.2)
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Where from here?
Increased precision
for inclusive π0 and
jets, including 510
GeV
Di-jets and dihadrons will
constrain the
functional form
of Δg(x)
Extrapolation of ΔG over all x
gives ~60% of proton spin,
but with huge uncertainties
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Forward rapidity
measurements
sensitive to lower x
Toward end of decade,
upgrades will enable dijets at forward rapidity
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What about the sea quarks?
What role do antiquarks play in determining the proton properties?
arXiv:0904.3821
• Large flavor asymmetry in the
antiquark sea of the nucleon
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Many models that describe the flavor
asymmetry also predict a spin
asymmetry in the antiquark sea
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Probing sea quark polarization through Ws
u  d W   l  
u  d W   l  
• Weak interaction process
– Only left-handed quarks
– Only right-handed anti-quarks
• Perfect spin separation
W
L
A
Parity violating single helicity asymmetry AL
d(x1)u(x 2 )  u(x1)d(x 2 )
W
L
A
u(x1)d (x2 )  d (x1)u(x2 )
• Complementary to SIDIS measurements
– High Q2 ~ MW2

– No fragmentation
function effects
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W measurements through 2011 RHIC run
2009 run
stot
× BR(W ® l n ) (pb)
W
104
Theory: FEWZ and MSTW08 NLO PDFs
3
10
pp ® W
102
pp ® W
+
pp ® W
-
STAR
Phenix
ATLAS
CMS
UA1
UA2
CDF
D0
tot
sZ/
× BR(Z/ g * ® ll) (pb)
g*
10
3
10
102
pp ® Z/ g *
10
pp ® Z/ g *
3
10
s (GeV)
2011 run
• Important proof-of-principle studies
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High precision W asymmetry era
Δu
PHENIX and STAR
through 2013 run
Δd
• First preliminary results from 2012 already provide substantial sensitivity
• Future results will provide a dramatic reduction in the uncertainties
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Implications for antiquark polarizations
PHENIX and STAR
through 2013 run
• W asymmetry measurements from RHIC will provide a substantial
reduction in the current uncertainties for the antiquark polarizations
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Transverse spin asymmetries in high energy p+p
Left
Right
• Large transverse single-spin asymmetries over a very wide range of √s
• Naïve collinear pQCD predicts AN ~ αs mq / √s ~ 0
• May arise from
– Sivers effect / twist-3
– Transversity + Collins fragmentation function
• No evidence to date of a fall-off at high pT
– Something we haven’t thought of yet ?
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Sivers and Collins effects in pp collisions
Sivers/twist-3 mechanism:
asymmetry in jet or γ production
SP
Collins mechanism:
asymmetry in jet fragmentation
SP
kT,q
p
p
p
Sensitive to proton spin –
parton transverse motion
correlations
• Signatures:
– AN for jets or direct photons
• NOT universal
– Sign change from SIDIS to
Drell-Yan
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p
Sensitive to
transversity
Sq
kT,π
• Signatures:
– Collins effect
– Interference
fragmentation functions
• Believed to be universal
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Isolating the underlying dynamics
SIVERS
Transversity x Collins
Rapidity dependence of
 AN for π0 and eta with increased pt coverage
 π+/- π0 azimuthal distribution in jets
 Interference fragmentation function
 AN for jets
 AN for direct photons
 AN for heavy flavor  gluon
Collins Asymmetry A º 2 <sin( f
AN
0.1
0.12
Expected Asymmetries of Prompt Photons
S
f )> vs. z
h
Collins Asymmetry A º 2 <sin(f
S
f )> vs. jT
h
STAR
Preliminary
TransversityxInterference
FF
p+ Asymmetry
p data horizontally offset for clarity
p- Asymmetry
0.05
0.08
0.06
0.04
2 <sin(f
S
h
f )>
0.1
arXiv: 1208.1962
Tq,F KQVY
Tq,F SIDIS new
Tq,F SIDIS old
0
systematic uncertainties
systematic uncertainties
-0.05
RHIC 2006
Systematic effects contributing to <1% of total uncertainty excluded
s = 200 GeV
±
p­p ® jet( p ) ; jet p T>10 GeV
-0.1
0
0.02
0.1
Thu Oct 18 08:33:04 2012
0.2
0.3
0.4
0.5
z
0.6
0.7
0.8
0.9
10
-1
j (GeV) 1
10
T
0
-0.02
-0.04
-0.06
-0.08
s = 200 GeV, P=60%, Ldt=50 pb
0.4
0.5
0.6
0.7
-1
0.8
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Sivers effect sign change
Intermediate QT
Q>>QT/pT>>LQCD
Transverse
momentum
dependent
Q>>QT>=LQCD
Q>>pT
Collinear/
twist-3
Q,QT>>LQCD
pT~Q
Efremov, Teryaev;
Qiu, Sterman
Sivers fct.
LQCD
<< QT/PT <<
Q
QT/PT
Critical test for our understanding of TMD’s and TMD factorization
QCD:
DIS:
attractive FSI
Drell-Yan:
repulsive ISI
SiversDIS = - SiversDY or SiversW or SiversZ0
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What can RHIC do today ?
Delivered luminosity of 500pb-1 with existing detectors
STAR AN(W):
-1.0 < y < 1.5
W-fully reconstructed
-
s = 510 GeV
0 < q < 3 GeV/c
0.15
T
0.35
Projection, Ldt=500 pb
T
-1
Projection, Ldt=500 pb
0.1
STAR
0.3
AN
s = 510 GeV
0 < q < 3 GeV/c
0.4
Expected asymmetries for W + -bosons
AN
AN
Expected asymmetries for W -bosons
PHENIX AN(DY):
1.2<|y|<2.4
Muon-Arms+FVTX  S/B ~ 0.2
Expected asymmetries in Drell-Yan
s = 510 GeV
0 < q < 1 GeV/c
T
0.02
4 < mg * < 8 GeV/c
-1
STAR
arXiv: 0903.3629
0.04
2
0
arXiv: 0903.3629
0.25
0.2
0.05
-0.02
0
-0.04
0.15
-0.06
-0.05
0.1
Projection, Ldt=500 pb -1
-0.08
0.05
-0.1
PHENIX
arXiv: 0912.1319
-0.1
0
-2
-1
0
1
2
y
-0.15
-2
-1
0
W
1
2
y
-4
-2
0
2
4
y
W
g*
Plus extremely clean measurement of AN(Z0)
to +/-10% for <y> ~0
Caveat:
potentially large evolution effects on AN for DY, W, Z0
not yet theoretically full under control and accounted for
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Forward upgrades late in the decade
• Both PHENIX and STAR are planning substantial forward upgrades
• Full suite of forward photon, hadron, jet, and Drell-Yan measurements
– High precision measurements where transverse spin effects are the
largest
• Only opportunity to measure the Sivers sign change if evolution is as
rapid as some calculations imply
– Δg(x) down to x ~ 0.001 with fully reconstructed di-jets
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Additional measurements in write-up
• Polarized p+A
– TMDs are closely related to unintegrated parton distributions like
xg(x,kT)
– Transverse asymmetries vs. A and pT provide an alternative
method to determine the saturation scale
– See next talk (John Lajoie)
• Generalized parton distribution Eg
– Sensitive to gluon spin-orbit correlations
– Measured via transverse spin dependence of exclusive J/ψ
production in ultra-peripheral p+A collisions
• Polarized 3He
– Access to polarized neutrons
• Tagging spectator protons will enhance signal:background
– Determine the flavor dependence of transverse spin asymmetries
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Conclusions
• RHIC is making unique contributions to our understanding of the
proton spin
–
–
–
–
Gluon polarization
Flavor-separated quark and anti-quark polarizations
Transversity
TMDs and twist-3 correlations
• The rest of this decade will bring further substantial improvements
in all of these
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The RHIC Spin Program – DNP 2012 – Gagliardi
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RHIC: the world’s first (and only!) polarized hadron collider
RHIC pC Polarimeters
Absolute Polarimeter (H jet)
BRAHMS
PHOBOS
Siberian Snakes
Siberian Snakes
PHENIX
STAR
Spin Rotators
(longitudinal polarization)
-
Pol. H Source
Spin flipper
LINAC
Helical Partial Siberian Snake
BOOSTER
AGS
200 MeV Polarimeter
AGS Internal Polarimeter
AGS pC Polarimeters
Rf Dipole
•
•
•
•
Spin Rotators
(longitudinal polarization)
Solenoid Partial Siberian Snake
Strong Helical AGS Snake
Spin varies from rf bucket to rf bucket (9.4 MHz)
Spin pattern changes from fill to fill
Spin rotators provide choice of spin orientation
Billions of spin reversals during a fill with little depolarization
The RHIC Spin Program – DNP 2012 – Gagliardi
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Exploring gluon polarization at RHIC
       f a f b
ALL   

aˆ LL

 
f a fb
f: polarized parton distribution functions
cos 
For most RHIC kinematics, gg and qg
dominate, making ALL for π0 and jets
sensitive to gluon polarization.
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