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1Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Recent . Results for ALL and Current Constraints
on GKieran Boyle
(RIKEN BNL Research Center) Outline:
• Quick Physics overview
• RHIC, PHENIX, and ALL
• Results from Run5 and Run6
• Constraints on G
• Current and Future Issues
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2Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Accessing G in pp scattering
• From Run5 and Run6, we consider an array of probes:– large statistics, specified trigger– large statistics, low trigger efficiency,
PID only for pT<2.8 and pT>4.7 GeV/c
– Direct photon low statistics, no G2 very clean signal– low statistics – multiparticle “cone”– J/
G2 Gq q2
Hard Scattering Process
2P2 2x P
1P
1 1x P
0
with ~25%, G not well constrained, L?
ALL~ agg * G2 + bgq * G q + cqq q2
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3Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Why ALL?
• If f = q, then we have this from pDIS• So roughly, we have
+- =
+++
+=
From ep (&pp)(HERA mostly)
pQCD NLO From e+e-
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4Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Measuring ALL
Helicity Dependent Particle Yields , , , , , etc
(Local) Polarimetry Relative Luminosity (R=L++/L+-) ALL
+ - = Opposite helicity =
++ = Same helicity
+
+=
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5Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
RHIC
BRAHMS & PP2PP (p)
STAR (p)PHENIX (p)
AGS
LINAC BOOSTER
Pol. Proton Source
Spin RotatorsPartial Siberian Snake
Siberian Snakes
200 MeV Polarimeter AGS Internal PolarimeterRf Dipoles
RHIC CNI (pC) PolarimetersAbsolute Polarimeter (H jet)
Year [GeV] Luminosity [pb-1] (recorded) Polarization [%] Figure of Merit (P4L)
2003 * 200 0.35 27 0.0019
2004 * 200 0.12 40 0.0031
2005 * 200 3.4 49 0.20
2006 * 200 7.5 62 (57) 1.1 (7.9)
2006 * 62.4 0.08 48** 0.0042*** Longitudinal** Online
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6Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
PHENIX Detector
BBC
ZDCZDC
EMCal detection
• Electromagnetic Calorimeter (PbSc/PbGl):• High pT photon trigger to collect trigger
to collect 0's, ’s, ’s • Acceptance: ||x • High granularity (~10*10mrad2)
• Drift Chamber (DC) for Charged Tracks• Ring Imaging Cherenkov Detector (RICH)
• High pT charged pions (pT>4.7 GeV).Relative Luminosity• Beam Beam Counter (BBC)
• Acceptance: 3.0< 3.9• Zero Degree Calorimeter (ZDC)
• Acceptance: ±2 mradLocal Polarimetry• ZDC• Shower Maximum Detector (SMD)
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7Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Local Polarimetry at PHENIX• Use ZDC and SMD to measure a
L-R and U-D asymmetry in forward neutrons (Acceptance: ±2 mrad).
• When transversely polarized, we see clear asymmetry.
• When longitudinally polarized, there should be no asymmetry.
BLUE YELLOW
Raw
as
ymm
etry
Raw
as
ymm
etry
BLUE YELLOW
Raw
as
ymm
etry
Raw
as
ymm
etry
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8Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Measured Asymmetry During Longitudinal Running (2005)
<PT/P>=10±2(%)
<PL/P> =99.48±0.12±0.02(%)
LR 2/NDF = 88.1/97p0 = -0.00323±0.00059
LR
UD 2/NDF = 82.5/97p0 = 0.00423±0.00057
XF>0 XF>0
XF<0 XF<0
2/NDF = 119.3/97p0 = 0.00056±0.00063
UD 2/NDF = 81.7/97p0 = -0.00026±0.00056
Fill NumberFill Number
<PT/P>=14±2(%)
<PL/P>=98.94±0.21±0.04(%)
• Measurement of remaining transverse component spin pattern is correct
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9Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Relative Luminosity
• R = L++/L+- ; L = luminosity.
• Luminosity is given by the number of BBC triggered events (NBBC).
• For estimate of uncertainty, measure ALL in the ratio of our two luminosity detectors
Year [GeV] ALL
2005 * 200 2.3e-4
2006 * 200 5.4e-4*** Longitudinal** Online
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10Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Strategy:1.Measure Cross Section to
confirm that pQCD is applicable to data
2.Measure ALL to extract G
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11Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
pQCD works
arXiv:0704.3599 [hep-ex]
0 @ 200 GeV Direct @ 200 GeV
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12Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Calculating 0 ALL
1. Calculate ALL(+BG) and ALL(BG) separately.
2. Get background ratio (wBG) from fit of all data.
3. Subtract ALL(BG) from ALL(+BG):
ALL(+BG) = w· ALL() + wBG · ALL(BG)
+BG region :±25 MeV around
peakBG region :
two 50 MeV regionsaround peak
Two Photon Mass Spectrum
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13Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Final Run5 0 ALL
• Data is compared to GRSV model with several input values of G.
• To interpret data, we use pQCD
Question: At what pT does soft physics contribution become small enough to allow comparison to pQCD models
9.4% scale uncertainty not included
GRSV: M. Gluck, E. Reya, M. Stratmann, and W. Vogelsang, PRD 63 (2001) 094005.
Adare et al., PRD76 (2007)051106
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14Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Soft physics• By comparing 0 data with
charged pion data, which has very good statistics at low pT, can estimate soft physics contribution
• Fitting an exponential to the low pT charged pion data (pT<1 GeV/c) gives an estimate on the soft physics contribution.
• Fit result: = 5.56±0.02 (GeV/c)−1
2/NDF = 6.2/3• From this, we see that for
pT>2 GeV, the soft physics component is down by more than a factor of 10.
exponential fit
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15Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Final Run5 0 ALL
• Measure below 1 GeV (~100% soft physics). ALL = 0.002±0.002.
• Above pT=2 GeV/c, soft physics contribution estimated to be <10%
• Contamination from soft physics asymmetry is small
• Note that DSSV used data with pT>1 GeV
Theory 2/NDF C.L. (%)
GRSV-std 10.9/8 20
GRSV G=0 12.7/8 12
GRSV G=G 256/8 0.00
GRSV G=-G 58.4/8 0.00
• Extend this approach
Adare et al., PRD76 (2007) 051106
9.4% scale uncertainty not included
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16Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Run6 0 ALL
• Run6 scaling error based on online polarization values. Final scaling error expected to be ~10%
• Grey band is systematic uncertainty due to Relative Luminosity, and is pT independent.
• Run6 Data favor “GRSV G=0” over GRSV-std
Theory 2/NDF CL(%)
GRSV-std 23.8/8* 0.25
GRSV G=0 7.9/8* 44
*Theoretical uncertainties not included
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17Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Sensitivity of 0 ALL to G
Scaling Errors not included
• Stat errors only• What about:
– exp. syst.?– theory
uncertainties?
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18Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Exp. Systematics• Polarization uncertainty:
– scales both the data and the error.
– Very small effect on interpretation in terms of G
– Final results for Run6 will be similarly small with final pol. values
• Issues:– Already below required limit of
5% per beam– Note that this is fully correlated
between all data points measured at RHIC (STAR and PHENIX) for a given year and energy
Adare et al., PRD76 (2007) 051106
P-PP+P
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19Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Exp. Systematics• Relative Luminosity uncertainty:
– Effectively a common shift of all data.– As the results for ALL are about zero, has
more significant effect than polarization uncertainty, but still small
• Relative Luminosity– In Run6, most significant experimental
uncertainty in extracting G– Larger in Run6 than Run5.– Expected to become worse in the future:
• higher luminositymore multiple collisions• squeezing longitudinal bunch profile may
increase problem
– Note that this is entirely correlated between all measurements at PHENIX, and so must be handled correctly
– To reduce effect, will REQUIRE spin flipper
ALL+ALL|RALL-ALL|R
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20Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Sensitivity of 0 ALL to G
Scaling Errors not included
present x-range
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21Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
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
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22Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Sign Ambiguity
• Dominance of two gluon interaction at low pT present 0 ALL data cannot determine sign of G.
• Solution:– Higher pT higher FOM (P4L)
– Look to other probes:• Direct Photon, in which G2 term
is not allowed at lowest order.• Charged pions
G2 Gq q2
Hard Scattering Process
2P2 2x P
1P
1 1x P
0
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23Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Sign Ambiguity II
Fraction of pion production
• With PHENIX RICH, charged pions above 4.7 GeV can be identified.
• At higher pT, qg interactions become dominant and so qg term in ALL becomes significant allowing access to the sign of G
• Expect if G>0, then: ALL(+)>ALL(0)>ALL(-)
• Additional data will help.
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24Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Direct photon
• First step, direct photon cross section.
• ALL results from Run5 with very small statistics
• Will need significant data set for this measurement (more than Run6)
• Background ALL estimates increase errors by ~30% at low pT
– Currently studying other methods to estimate background
gq
Hard Scattering Process
2P 2 2x P
1P
1 1x P
Gluon-photon compton dominant
RUN5
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25Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
p+p + X
g2 gq q2
Hard Scattering Process
2P 2 2x P
1P
1 1x P
• Similar analysis technique as with 0, using two photon decay channel.
• Independent measure for gluon polarization.
• Due to different fragmentation functions, expectations are slightly different.
• Statistically limited currently
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26Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Extending the x range:0 ALL at s = 62.4 GeVfor high x measurement
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27Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
0 ALL @ s=62.4 GeV
GRSV: M. Gluck, E. Reya, M. Stratmann, and W. Vogelsang, Phys. Rev. D 53 (1996) 4775.
• Short run with longitudinal polarized protonsALL
• Grey band is systematic uncertainty due to Relative Luminosity
pQCD works measure ALL
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28Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Comparison with 200 GeV
• At fixed xT, 0 cross section is 2 orders of magnitude higher at 62.4 GeV than at 200 GeV
• Converting to xT, we can get a better impression of the significance of the s=62.4 GeV data set, when compared with the Run5 final data set.
Run5 200GeV final 2.7pb-1 (49%)Run6 62.4GeV prelm. 0.04 pb-1 (48%)
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29Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
A new theoretical fit• De Florian, Sassot, Stratmann
and Vogelsang (DSSV) have recently included 0 data at 200 and 62 GeV from PHENIX.– First use of any pp data.
• Only Experimental stat and syst (summed in quadrature) uncertainties – NO THEORETICAL
UNCERTAINTIES were considered.
• Results for G were driven by RHIC data in the range x=[0.05-0.2]
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30Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Conclusions• Cross sections have been measured at s=200 GeV and s=62 GeV and NLO pQCD describes our data well.
• Run5 and Run6 0 ALL are a significant constraint on the gluon spin contribution to the proton in the x range [0.02,0.3].
• Experimental systematic uncertainties are small, but are beginning to be significant.– Spin Flipper at RHIC will be needed to reduce Rel. Lumi.
uncertainty
• More luminosity at s=200 GeV will enable us to address the G sign ambiguity with charged pions as well as direct photons.
• Extending the measured x range will be needed, and has begun with measurements at s=62 GeV.
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31Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Backups
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32Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Scale
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33Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Charged pion pt spectrum
• All high quality ERT trigger tracks with an associated RICH hit with matching, dc acceptance, EM prob, E/p and momentum dependent energy cuts applied.
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34Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Polarimetry: Direction• Require longitudinally polarized beams at
PHENIX to measure ALL, and so use spin rotators.
• How can we know direction? Consider a single spin asymmetry (SSA).
SSA found in forward neutron production at RHIC in 2002 [Bazilevsky et al. PLB 650:325-330,2007]
from M. Togawa
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35Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
ATT
• If large, could lead to false ALL.• Theory expects it to be small• Experimentally found to be consistent with
zero.
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36Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
What about polarized?• Recall the unpolarized
data.• In the polarized case,
much less data exists, covering much smaller x and Q2 range.
• Even with this small range of data, polarized quark distribution is reasonably well constrained.
~25% of proton spin
most of proton spin in either gluon spin, or OAM.
• But from fixed target DIS, gluon is very poorly constrained.
G
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37Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Polarimetry results• HJet results for target
asymmetry are consistent between Run5 and Run6.
• Results from beam asymmetry differ, indicating change in polarization.
Run5
Run6
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38Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
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
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39Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Extend x Range
present x-ranges = 200 GeV
Extend to lower x at s = 500 GeV
Extend to higher x at s = 62.4 GeV
• To measure G, need as wide an x range as possible.• Planned Upgrades such as the VTX, FVTX and Nosecone will help • By measuring at different center of mass energies, we can reach
different x ranges.• We can extend our x coverage towards lower x at s =
500 GeV. Expected to start in 2009.• We can extend our x coverage towards higher x at s =
62.4 GeV. Run6
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40Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Direct Production• Gluon Compton
scattering dominates– At LO no fragmentation
function– Small contribution from
annihilation
hep-ex/0609031
Run-3
2P 2 2x P
1P
1 1x P
g
qqgq
)(ˆ)(
)(
)(
)(
2
2
1
1 qgqaxq
xq
xg
xgA LLLL
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41Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
SMC Fit to DIS
Polarized gluon distribution poorly constrained
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42Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Backup: SIDIS for G
HERMES preliminaryHERMES preliminary
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43Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Backup: GFrom M. Stratmann
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44Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Proton Spin
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45Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Recent ALL Measurements
arXiv:0704.3599 [hep-ex] 0
Many independent probes to understand gluon spin
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46Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Results From DIS• Basic Idea (Cartoon):
– How many quarks have spin in direction of proton spin polarized PDF
• Look at DIS
• So measure Asymmetry of proton polarized along photon vs. against photon
lq =
= X
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47Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Nucleon Spin Structure
Naïve parton model:
1989 EMC (CERN):=0.120.090.14
Spin Crisis sdusdu
vv du 2
1
2
1
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 2
1
2
1
For complete descriptioninclude parton orbital angular momentum LZ:
ZLGqq 2
1
2
1
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48Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
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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49Kieran Boyle—Gluon Polarization in the Nucleon—June 16, 2008
Learning about structure• Primary method is Deeply Inelastic Scattering (DIS).• Scatter off quark with momentum fraction x, and
understand probability to find a given quark at a given x, known as the Parton Distribution Function (PDF).
Q2 = -q2