Proton%20spin%20structure%20from%20longitudinally%20polarized%20pp%20collisions%20from%20PHENIXat%20RHIC

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Proton spin structure from longitudinally
polarized pp collisions from PHENIXat RHIC
Alexander Bazilevsky BNL The 6th
Circum-Pan-Pacific Symposium on High Energy Spin
Physics July 30 August 2, 2007 Vancouver BC,
Canada
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Nucleon Spin Structure
Naïve parton model
Determination of ?G and ?q-bar is the main goal
of longitudinal spin program at RHIC
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Parton Distribution Functions (PDF)

• Quark Distribution

unpolarised distribution
q(x,Q2)

helicity distribution
Dq(x,Q2)
transversity distribution
dq(x,Q2)
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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!

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To polarized pp collider
Utilizes 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 probes
• Access to different gluon momentum fraction x
• Different probes different systematics
• Use different energies ?s
• Access to different gluon momentum fraction x

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RHIC as polarized proton collider
Absolute Polarimeter (H jet)
RHIC pC Polarimeters
Siberian Snakes
Spin Rotators
2 ? 1011 Pol. Protons / Bunch e 20 p mm mrad
Partial Siberian Snake
LINAC
BOOSTER
Pol. Proton Source 500 mA, 300 ms
AGS
AGS Internal Polarimeter
200 MeV Polarimeter
Rf Dipoles
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PHENIX for Spin

• Philosophy (initial design)
• High rate capability granularity
• Good mass resolution particle ID
• Sacrifice acceptance

p0/g/h Electromagnetic Calorimeter p/p- Drift
Chamber Ring Imaging Cherenkov Counter J/y Muon
Id/Muon Tracker Relative Luminosity Beam Beam
Counter (BBC) Zero Degree Calorimeter
(ZDC) Local Polarimetry - ZDC
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PHENIX Long. Spin runs
Year Ös 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
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Unpol. Cross Section in pp
pp??0 X hep-ex-0704.3599
pp?? X PRL 98, 012002
?lt0.35

• Good agreement between NLO pQCD calculations and
  data ? confirmation that pQCD can be used to
  extract spin dependent pdfs from RHIC data.
• Same comparison fails at lower energies

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Probing ?G in pol. pp collisions
pp ? hX
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Measuring ALL

• (N) Yield
• (R) Relative Luminosity
• BBC vs ZDC
• (P) Polarization
• RHIC Polarimeter (at 12 oclock)
• Local Polarimeters (SMDZDC)

• 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

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ALL ?0
PHENIX Preliminary Run6 (?s200 GeV)
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pT(GeV)
GRSV model ?G 0 ?G(Q21GeV2)0.1 ?G
std ?G(Q21GeV2)0.4
Stat. uncertainties are on level to distinguish
std and 0 scenarios?
Run3,4,5 PRL 93, 202002 PRD 73, 091102
hep-ex-0704.3599
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From soft to hard
hep-ex-0704.3599
Exponent (e-??pT) describes our pion cross
section data perfectly well at pTlt?1 GeV/c
(dominated by soft physics) ?5.56?0.02
(GeV/c)-1 ?2/NDF6.2/3 Assume that exponent
describes soft physics contribution also at
higher pTs ? soft physics contribution at pTgt2
GeV/c is lt10
exponential fit
For ?G constrain use pi0 ALL data at pTgt2 GeV/c
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From pT to xgluon

• NLO pQCD ?0 pT2?9 GeV/c ? xgluon0.02?0.3
• GRSV model ?G(xgluon0.02?0.3) 0.6??G(xgluon
  0?1 )
• 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

Log10(xgluon)
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From ALL to ?G (with GRSV)
Calc. by W.Vogelsang and M.Stratmann
?

• std scenario, ?G(Q21GeV2)0.4, is excluded by
  data on gt3 sigma level ?2(std)??2mingt9
• Only exp. stat. uncertainties are included (the
  effect of syst. uncertainties is expected to be
  small in the final results)
• Theoretical uncertainties are not included

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Extending x range is crucial!
Gehrmann-Stirling models
GSC ?G(xgluon 0?1) 1 GRSV-0
?G(xgluon 0?1) 0 GRSV-std ?G(xgluon
0?1) 0.4
Current data is sensitive to ?G for xgluon
0.02?0.3
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?G whats 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

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Improve exp. uncertainties
Need more FOMP4 ?L (stat. uncertainty ?FOM)
?0 expectations from Run-8
Higher pT measurements ? probe higher x ?
constrain ?G vs x
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Different channels
Need more FOMP4 ?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

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Different channels
Need more FOMP4 ?L (stat. uncertainty ?FOM)

• Complementary to ?0 measurements
• Need ? fragmentation functions

• Probe ?G with heavy quarks
• Open charm will come soon
• Need more theoretical input

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pp ? ? jet
PHENIX Projection

• Theoretically clean (no fragmentation at LO)
• Gluon Compton dominates ? sensitive to sign of ?G
• Requires substantial FOMP4 ?L

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Different ?s
?s62 GeV ?0 cross section described by NLO pQCD
within theoretical uncertainties
Sensitivity of Run6 ?s62 GeV data collected in
one week is comparable to Run5 ?s200 GeV data
collected in two months, for the same
xT2pT/?s ?s500 GeV will give access to lower
x starts in 2009
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Flavor decomposition
Measured through longitudinal single spin
asymmetry AL in W? production at ?s500 GeV
First data expected in 2009-2010
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Other measurements
Helicity correlated kT from PHENIX May be
sensitive to orbital angular momentum
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PHENIX Upgrades
See talk by I.Nakagawa
Silicon Tracking VTX (barrel) by 2009 FVTX
(forward) by 2011 Electromagnetic Calorimetry NCC
by 2011 MPC, already installed! Muon trigger
upgrade By 2009 Momentum selectivity in the LVL-1
trigger
rapidity
?G from heavy flavor, photon-tagged jets Expanded
reach in x Flavor separation of spin
asymmetries W physics at 500GeV Transverse Spin
Physics (see talk by M.Liu)
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Summary

• RHIC is the worlds 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 (0.02?0.3)
• ?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

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Polarimetry
Utilizes small angle elastic scattering in the
coulomb-nuclear interference (CNI) region
p or C12
p

• 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 measurements

?
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Backup Rel. Lum. In PHENIX
Year GeV dR dALL
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
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Backup ?s62 vs 200 GeV
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Partonic Orbital Angular Momentum
?? Beam momenta
Like helicities

• 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 ltkT2gt

Net pT kick
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Partonic Orbital Angular Momentum
?? Beam momenta
Unlike helicities

• 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 ltkT2gt

Net pT kick
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Backup W

• W production
• Produced in parity violating V-A process
• Chirality / helicity of quarks defined
• Couples to weak charge
• Flavor almost fixed

xagtgtxb AL(W) ? ?u/u(x)
- - xbgtgtxa AL(W) ? ?d/d(x)
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Backup SIDIS for ?G
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Backup ????GFrom M. Stratmann
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Backup from ?G to ALL
By MarcoWerner
GRSV ?G(Q21GeV2) ?1.76 ? 1.89