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Carl Gagliardi – WWND – Trans Spin at RHIC 1
Transverse Spin Physics in pp Collisions at RHIC
Carl A. GagliardiTexas A&M University
Outline
• Introduction• Forward rapidity measurements• Mid-rapidity measurements• Looking ahead
Carl Gagliardi – WWND – Trans Spin at RHIC 2
RHIC: the Relativistic Heavy Ion Collider
• Search for and study the Quark-Gluon Plasma• Explore the partonic structure of the proton• Determine the partonic structure of nuclei
Carl Gagliardi – WWND – Trans Spin at RHIC 3
From PDFs to polarized PDFs
• There are really three different sets of PDFs for the proton
q(x)g(x)
Δq(x)Δg(x)
δq(x)
Proton spin
+
Proton spin
Unpolarized PDF
“Polarized” distribution
“Transversity” distribution
Consider a proton moving toward the right
Carl Gagliardi – WWND – Trans Spin at RHIC 4
Current knowledge of the polarized distributions
• Quarks and antiquarks only carry ~30% of total proton spin• Proton “spin crisis”
gz
qz
pz LLGS
2
1
• Know very little about orbital motion
• Gluon and anti-quark polarizations have large uncertainties
RHIC transverse spin program
DSSV, PRL 101, 072001
Anselmino et al, arXiv:0807.0173
• Transversity also has large uncertainties
Carl Gagliardi – WWND – Trans Spin at RHIC 5
RHIC: the world’s first polarized hadron collider
• 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 if any depolarization
BRAHMS
PHENIX
AGS
BOOSTER
Spin Rotators(longitudinal polarization)
Solenoid Partial Siberian Snake
Siberian Snakes
200 MeV PolarimeterAGS Internal Polarimeter
Rf Dipole
RHIC pC PolarimetersAbsolute Polarimeter (H jet)
AGS pC Polarimeters
Strong Helical AGS Snake
Helical Partial Siberian Snake
Spin Rotators(longitudinal polarization)
Spin flipper
Siberian Snakes
STAR
PHOBOS
Pol. H- SourceLINAC
Carl Gagliardi – WWND – Trans Spin at RHIC 6
Transverse single-spin asymmetries
• Definition:
• dσ↑(↓) – cross section for scattering to the left when incoming proton has spin up(down)
Two methods of measurements: • Single arm calorimeter:
R – relative luminosity
Pbeam – beam polarization
• Two arm (left-right) calorimeter:
Less sensitive to instrumental effects
dd
ddAN
L
LR
RNN
RNN
PA
beamN
1
LRRL
LRRL
BeamN
NNNN
NNNN
PA
1
π0, xF<0
π0, xF>0
Left
Right
p p
Positive AN: more 0 going to
left of the polarized beam
Carl Gagliardi – WWND – Trans Spin at RHIC 7
Transverse single-spin asymmetries at forward rapidity
• Large single-spin asymmetries at CM energy of 19.4 GeV
TqSN /pmαA
• Weren’t supposed to be there in naïve pQCD
FNAL E704
Carl Gagliardi – WWND – Trans Spin at RHIC 8
Forward π0 production at ISR energies
Bourrely and Soffer, EPJ C36, 371: NLO pQCD calculations underpredict the data at ISR energies Maybe the E704 results arise from soft physics?
√s=23.3GeV √s=52.8GeV
xF xF
Ed
3 d
p3 [b
/GeV
3 ]
Ed
3 d
p3 [b
/GeV
3 ]
NLO calculations with different
scales:
pT and pT/2
Data-pQCD differences
at pT=1.5GeV
Carl Gagliardi – WWND – Trans Spin at RHIC 9
First AN measurement at RHIC
PRL 92, 171801 (2004)
Sivers: spin and kT correlation in parton distribution functions (initial state)
Collins: spin and kT correlation in fragmentation function (final state)
Qiu and Sterman (initial state) / Koike (final state): twist-3 pQCD calculations, multi-parton correlations
Can be described by several models:
Similar to result from E704 experiment (√s=19.4 GeV, 0.5 < pT < 2.0 GeV/c)
√s=200 GeV, <η> = 3.8
STARSTAR
Carl Gagliardi – WWND – Trans Spin at RHIC 10
Forward pp π0 + X cross sections at 200 GeV
NLO pQCD calculations by Vogelsang, et al.
PRL 97, 152302STARSTAR
The error bars are statistical plus point-to-point systematic
Consistent with NLO pQCD calculations at 3.3 < η < 4.0
Data at low pT trend from KKP fragmentation functions toward Kretzer.
Carl Gagliardi – WWND – Trans Spin at RHIC 11
Sivers and Collins effects in pp collisions
Collins mechanism: final-state asymmetry in the forward jet fragmentation
Sivers mechanism: initial-state kT dependence in the parton distribution
SPkT,q
p
p
SP
p
p
Sq kT,πSensitive to proton spin – parton transverse motion correlations
Sensitive to transversity
Observed transverse single-spin asymmetries could arise from the Sivers effect or Collins
effect, or from a linear combination of the two
Carl Gagliardi – WWND – Trans Spin at RHIC 12
CollinsSivers
Sivers and Collins effects in deep-inelastic scattering
• Semi-inclusive DIS can distinguish the Sivers and Collins effects• HERMES finds both are non-zero
Carl Gagliardi – WWND – Trans Spin at RHIC 13
Recent π0 results at 200 GeV from STAR
• Sivers fit to HERMES SIDIS describes η = 3.3; overpredicts η = 3.7• Twist-3 fit to E704 plus preliminary STAR Runs 3 and 5 data
describes η = 3.7; underpredicts η = 3.3
STARSTAR PRL 101, 222001
Carl Gagliardi – WWND – Trans Spin at RHIC 14
Charged pion measurements at 200 GeV from BRAHMS
• Sign dependence of charged pion asymmetries seen in FNAL E704 persists to 200 GeV
BRAHMS
2.3 deg (~3.4)4 deg (~3)
Carl Gagliardi – WWND – Trans Spin at RHIC 15
3.0<<4.0
p+p0+X at s=62.4 GeV/c2 p+p0+X at s=62.4 GeV/c2
0 results at 62.4 GeV from PHENIX
• Asymmetries are comparable or larger at 62.4 GeV than they are at 200 GeV
Carl Gagliardi – WWND – Trans Spin at RHIC 16
Charged-hadron results at 62.4 GeV from BRAHMS
BRAHMS
Limitation of the BRAHMS measurements:Very strong correlation between xF and pT
from small acceptance
PRL 101, 042001
• Very large asymmetries!• K- (= su ) asymmetry is a surprise. Sea-quark Sivers
effect or disfavored fragmentation?
Carl Gagliardi – WWND – Trans Spin at RHIC 17
Inclusive π0 AN(pT) in xF bins
• Combined data from three runs at <η>=3.3, 3.7 and 4.0
• Measured AN is not a smooth decreasing function of pT
as predicted by theoretical models
Kouvaris et al,PRD 74, 114013
STARSTAR PRL 101, 222001
Carl Gagliardi – WWND – Trans Spin at RHIC 18
A potential fly in the ointment?
• To date, the η meson has looked like a “high-mass, low-yield π0” in all measurements at RHIC
• AN for the η mass region is much larger at high xF
STAR 2006 PRELIMINARY
η ~ 3.66
STARSTAR
Carl Gagliardi – WWND – Trans Spin at RHIC 19
Mid-rapidity inclusive pion AN
• Mid-rapidity pion yields are gluon-dominated at these pT
• No Collins effect for gluons• May help to constrain the gluon Sivers function
PRL 95, 202001
Carl Gagliardi – WWND – Trans Spin at RHIC 20
Mid-rapidity inclusive jet AN
• Gluon-dominated at low pT
• Dominated by qg scattering at higher pT
• Within uncertainties, AN is consistent with zero
STAR Preliminary
STAR PreliminarySTAR Preliminary
STAR Preliminary
PT (GeV/c)
PT (GeV/c) PT (GeV/c)
PT (GeV/c)
-0.5< <0.0-0.9< <-0.5
0.0< <0.5 0.5< <0.9
STARSTAR
Carl Gagliardi – WWND – Trans Spin at RHIC 21
Sivers effect in di-jet production
180open
1
2
spin
di-jetbisector
kTx
>
180
for
k T
x > 0
Sivers effect:
• Left/right asymmetry in the kT of the partons in a polarized proton
• Spin dependent sideways boost to di-jets
• Measure the di-jet opening angle as a function of proton spin
• Requires parton orbital angular momentum
Carl Gagliardi – WWND – Trans Spin at RHIC 22
Mid-rapidity di-jet Sivers effect measurementPRL 99, 142003
• Observed asymmetries are an order of magnitude smaller than seen in semi-inclusive DIS by HERMES
• Detailed cancellations of initial vs. final state effects and u vs. d quark effects, coupled with very small gluon Sivers effect?
STARSTAR
Carl Gagliardi – WWND – Trans Spin at RHIC 23
Mid-rapidity di-hadron Sivers effect measurement
• PHENIX is performing a similar measurement• Back-to-back correlations between a trigger π0 and an away-side
charged hadron
Carl Gagliardi – WWND – Trans Spin at RHIC 24
Separating Sivers and Collins effects in pp collisions
Collins mechanism: asymmetry in the forward jet fragmentation
Sivers mechanism: asymmetry in the forward jet or γ production
SPkT,q
p
p
SP
p
p
Sq kT,πSensitive to proton spin – parton transverse motion correlations
Sensitive to transversity
• Need to go beyond inclusive hadrons to measurements of jets or direct γ
Carl Gagliardi – WWND – Trans Spin at RHIC 25
Large-acceptance forward detectors
• Both PHENIX and STAR have installed large-acceptance electromagnetic calorimeters in the forward direction
PHENIX Muon Piston Calorimeters3.1 < |η| < 3.7
STAR Forward Meson Spectrometer2.5 < η < 4.0
Carl Gagliardi – WWND – Trans Spin at RHIC 26
First look at jet-like events with the STAR FMS
• Comparisons of the jet profile and effective mass in data vs. PYTHIA + GEANT simulations
Carl Gagliardi – WWND – Trans Spin at RHIC 27
Transverse spin forward + mid-rapidity jet
Bacchetta et al., PRL 99, 212002
• Conventional calculations predict the asymmetry to have the same sign in SIDIS and +jet
• Calculations that account for the repulsive interactions between like color charges predict opposite sign
• Critical test of our basic theoretical understanding
Carl Gagliardi – WWND – Trans Spin at RHIC 28
In the further future: Drell-Yan
• In SIDIS, final-state interaction of outgoing quark with proton remnant involves opposite color charges – attractive
• In Drell-Yan, initial-state interaction of the incoming quark with the spectator components of the proton involves like color charges – repulsive
• Sign of AN should reverse
Carl Gagliardi – WWND – Trans Spin at RHIC 29
Conclusions
• RHIC has observed large transverse single-spin asymmetries for forward particle production
• These asymmetries may provide evidence for parton orbital angular momentum and/or quark transversity
• Measurements to identify the underlying cause(s) are underway
• Future measurements will provide a direct illustration of attractive vs. repulsive color-charge interactions
• RHIC, the world’s first polarized hadron collider, is generating a wealth of new data regarding the spin structure of the proton
• Stay tuned!
Carl Gagliardi – WWND – Trans Spin at RHIC 30
Carl Gagliardi – WWND – Trans Spin at RHIC 31
Low-pT forward transverse single-spin asymmetries
• Rotator tuning for longitudinal polarization requires local polarimetry
STAR BBCs
PHENIX ZDCs