proton spin structure from longitudinally polarized pp collisions from phenixat rhic
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Proton spin structure from longitudinally polarized pp
collisions from PHENIXat RHIC
Alexander BazilevskyBNL
The 6th Circum-Pan-Pacific Symposium on High Energy Spin Physics
July 30 – August 2, 2007Vancouver BC, Canada
Nucleon Spin Structure
Naïve parton model:
1989 EMC (CERN):=0.120.090.14
Spin Crisis sdusdu
vv du 21
21
Determination of G and q-bar is the main goal of longitudinal spin program at RHIC
Gluons are polarized (G) Sea quarks are polarized:
Gqq 21
21
For complete descriptioninclude parton orbital angular momentum LZ:
ZLGqq 21
21
Parton Distribution Functions (PDF)• Quark Distribution
q(x,Q2)=
q(x,Q2)=
q(x,Q2)=
=unpolarised distribution
helicity distribution
g(x,Q2)=
g(x,Q2)=No Transverse Gluon Distribution in 1/2
• Gluon Distributionstransversity distribution
Polarized PDF from DISAsymmetry Analysis Collaboration
M. Hirai, S. Kumano and N. Saito, PRD (2004)
• Valence distributions well determined
• Sea Distribution poorly constrained
• Gluon can be either positive, 0, negative!
… To polarized pp colliderUtilizes strongly interacting probes
Probes gluon directly Higher s clean pQCD interpretation Elegant way to explore guark and anti-
quark polarizations through W production
Polarized Gluon Distribution Measurements (G(x)): Use a variety of probesAccess to different gluon momentum fraction xDifferent probes – different systematics
Use different energies s Access to different gluon momentum fraction x
RHIC as polarized proton collider
BRAHMS & PP2PP (p)
STAR (p)PHENIX (p)
AGS
LINAC BOOSTER
Pol. Proton Source500 A, 300 s
GeVs
L
50050
onPolarizati%70cms102 2132
max
Spin Rotators
Partial Siberian Snake
Siberian Snakes
200 MeV Polarimeter AGS Internal PolarimeterRf Dipoles
RHIC pC PolarimetersAbsolute Polarimeter (H jet)
2 1011 Pol. Protons / Bunch = 20 mm mrad
PHENIX for Spin
Electromagnetic Calorimeter
Drift ChamberRing Imaging Cherenkov Counter
JMuon Id/Muon Tracker
Relative LuminosityBeam Beam Counter (BBC) Zero Degree Calorimeter (ZDC)
Local Polarimetry - ZDC
Philosophy (initial design):
High rate capability & granularityHigh rate capability & granularity Good mass resolution & particle IDGood mass resolution & particle ID Sacrifice acceptanceSacrifice acceptance
PHENIX Long. Spin runs
Year s [GeV] Recorded L Pol [%]FOM (P4L)
2003 (Run 3) 200 .35 pb-1 32 3.7 nb-1
2004 (Run 4) 200 .12 pb-1 45 4.9 nb-1
2005 (Run 5) 200 3.4 pb-1 50 200 nb-1
2006 (Run 6) 200 7.5 pb-1 60 1000 nb-1
2006 (Run 6) 62.4 .08 pb-1 ** 48 4.2 nb-1 **
Unpol. Cross Section in pppp0 X : hep-ex-0704.3599 pp X: PRL 98, 012002
Good agreement between NLO pQCD calculations and data confirmation that pQCD can be used to extract spin dependent pdf’s from RHIC data.
• Same comparison fails at lower energies
||<0.35
Probing G in pol. pp collisions pp hX
hf
fXff
baba
hf
fXffLL
fXff
baba
LL Ddff
Dadff
ddddA
ba
baba
ˆ
ˆˆ
,
,
Double longitudinal spin asymmetry ALL is sensitive to G
Measuring ALL
LLR
RNNRNN
PPddddALL ;
||1
21
(N) Yield (R) Relative Luminosity
BBC vs ZDC(P) Polarization
RHIC Polarimeter (at 12 o’clock)Local Polarimeters (SMD&ZDC)
Bunch spin configuration alternates every 106 ns Data for all bunch spin configurations are collected at the same time
Possibility for false asymmetries are greatly reduced
ALL: 0
pT(GeV)
Run3,4,5: PRL 93, 202002; PRD 73, 091102; hep-ex-0704.3599
5 10
GRSV model:“G = 0”: G(Q2=1GeV2)=0.1“G = std”: G(Q2=1GeV2)=0.4
Stat. uncertainties are on level to distinguish “std” and “0” scenarios? …
PHENIX Preliminary Run6 (s=200 GeV)
From soft to hard
exponential fit
Exponent (e-pT) describes our pion cross section data perfectly well at pT<1 GeV/c (dominated by soft physics):
=5.560.02 (GeV/c)-1
2/NDF=6.2/3
Assume that exponent describes soft physics contribution also at higher pTs soft physics contribution at pT>2 GeV/c is <10%
For G constrain use pi0 ALL data at pT>2 GeV/c
hep-ex-0704.3599
From pT to xgluon
Log10(xgluon)NLO pQCD: 0 pT=29 GeV/c xgluon=0.020.3
GRSV model: G(xgluon=0.020.3) ~ 0.6G(xgluon =01 ) Each pT bin corresponds to a wide range in xgluon, heavily overlapping with other pT bins
These data is not much sensitive to variation of G(xgluon) within our x range
Any quantitative analysis should assume some G(xgluon) shape
From ALL to G (with GRSV)Calc. by W.Vogelsang and M.Stratmann
“std” scenario, G(Q2=1GeV2)=0.4, is excluded by data on >3 sigma level: 2(std)2
min>9Only exp. stat. uncertainties are included
(the effect of syst. uncertainties is expected to be small in the final results)
Theoretical uncertainties are not included
Extending x range is crucial!Gehrmann-Stirling models
GSC: G(xgluon= 01) = 1 G(xgluon= 0.020.3) ~ 0
GRSV-0: G(xgluon= 01) = 0 G(xgluon= 0.020.3) ~ 0
GRSV-std: G(xgluon= 01) = 0.4 G(xgluon= 0.020.3) ~ 0.25
GSC: G(xgluon= 01) = 1
GRSV-0: G(xgluon= 01) = 0
GRSV-std: G(xgluon= 01) = 0.4
Current data is sensitive to G for xgluon= 0.020.3
G: what’s next
Improve exp. (stat.) uncertainties and move to higher pT More precise G constrain in probed x range Probe higher x and constrain G vs x
Different s Different x
Different channels Different systematics Different x gqg sensitive to G sign
GG
GG
gq g
GG
QQgg
Improve exp. uncertaintiesNeed more FOM=P4 L (stat. uncertainty ~ FOM)
0: expectations from Run-8
Higher pT measurements probe higher x constrain G vs x
Different channelsNeed more FOM=P4 L (stat. uncertainty ~ FOM)
Different sensitivities of charged pions to u and d provide more sensitivity to sign of G through qg scattering
Predictions are sensitive to fragmentation functions
Xpp
Xpp
Different channelsNeed more FOM=P4 L (stat. uncertainty ~ FOM)
XJpp /Xpp
Complementary to 0 measurements Need fragmentation functions
Probe G with heavy quarksOpen charm will come soonNeed more theoretical input
pp + jet
Theoretically clean (no fragmentation at LO) Gluon Compton dominates sensitive to sign of G Requires substantial FOM=P4 L
PHENIX Projection
Different ss=62 GeV 0 cross section described by NLO pQCD within theoretical uncertainties
Sensitivity of Run6 s=62 GeV data collected in one week is comparable to Run5 s=200 GeV data collected in two months, for the same xT=2pT/s
s=500 GeV will give access to lower x; starts in 2009
Flavor decomposition
Measured through longitudinal single spin asymmetry AL in W production at s=500 GeV
First data expected in 2009-2010
Wdu
Wud
Wdu
Wud
Other measurements
Helicity correlated kT from PHENIX
May be sensitive to orbital angular momentum
PHENIX UpgradesSilicon Tracking
VTX (barrel) by 2009FVTX (forward) by 2011
Electromagnetic CalorimetryNCC by 2011MPC, already installed!
Muon trigger upgradeBy 2009Momentum selectivity in the LVL-1 trigger
G from heavy flavor, photon-tagged jetsExpanded reach in x
Flavor separation of spin asymmetriesW physics at 500GeV
Transverse Spin Physics (see talk by M.Liu)
rapidity
See talk by I.Nakagawa
Summary RHIC is the world’s first and the only facility which provides collisions of
high energy polarized protons Allows to directly use strongly interacting probes (parton collisions) High s NLO pQCD is applicable
Inclusive 0 accumulated data for ALL has reached high statistical significance to constrain G in the limited x range (~0.020.3) G is consistent with zero Theoretical uncertainties might be significant
Extending x coverage is crucial Other channels from high luminosity and polarization Different s
PHENIX upgrades strengthen its capability in nucleon spin structure study Larger x-range and new channels (e.g. heavy flavor)W measurements for flavor decomposition
PolarimetryUtilizes small angle elastic scattering in the coulomb-nuclear interference (CNI) region
Pbeam N
ANpC
N NL NR
NL NR
Fast relative polarization measurements with proton-Carbon polarimeter
Single measurement for a few seconds
(Relatively) slow absolute polarization measurements with polarized atomic hydrogen jet target polarimeter
Used to normalize pC measurementstargetNtarget
beamNbeam
PAPA
targettarget
beambeam PP
Beam polarization in Run6: P ~ 60-65%
Polarization measurements in Run5: P/P~6% and (PBPY)/(PBPY)~9%
p p or C12
Backup: Rel. Lum. In PHENIX
Year [GeV] R ALL
2005 * 200 1.0e-4 2.3e-4
2006 * 200 3.9e-4 5.4e-4
2006 * 62.4 1.3e-3 2.8e-3
Backup: s=62 vs 200 GeV
Partonic Orbital Angular Momentum
Peripheral Collisions
Larger
Integrate over b, leftwith some residual kT
2Tk
Net pT kick
Central Collisions
Smaller 2TkJet 2
Jet 1
Jet 2 w/<kT>=0
Peripheral Collisions
Larger Jet 1
Jet 2 w/<kT>=0
Jet 2
Like helicities: Beam momenta
• Partonic orbital angular momentum leads to rotation of partons correlated with the proton spin vector
• This leads to different pT imbalances (pT-kicks) of jet pairs in semiclassical models
• Can be measured by measuring helicity dependence of <kT
2>
Partonic Orbital Angular Momentum
• Partonic orbital angular momentum leads to rotation of partons correlated with the proton spin vector
• This leads to different pT imbalances (pT-kicks) of jet pairs in semiclassical models
• Can be measured by measuring helicity dependence of <kT
2>
Integrate over b, leftwith different residual kT
Net pT kick
Central Collisions
Larger 2TkJet 2
Jet 1
Jet 2 w/<kT>=0
2Tk
Peripheral Collisions
Smaller Jet 1
Jet 2 w/<kT>=0
Jet 2
Unlike helicities: Beam momenta
Backup: W
W production» Produced in parity violating V-A process
— Chirality / helicity of quarks defined» Couples to weak charge
— Flavor almost fixed
W a b a bL
a b a b
u(x )d(x ) d(x )u(x )Au(x )d(x ) d(x )u(x )
xa>>xb: AL(W+) → u/u(x) - -xb>>xa: AL(W+) → d/d(x)
Backup: SIDIS for G
HERMES preliminary
Backup: GFrom M. Stratmann
Backup: from G to ALL
GRSV: G(Q2=1GeV2)= 1.76 +1.89
By Marco&Werner
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