1. Kieran Boyle (Stony Brook University) for the PHENIX Collaboration
Double Longitudinal Spin Asymmetry in Neutral Pion Production in Polarized p+p Collisions at =200 GeV at PHENIX
Kieran Boyle (Stony Brook University)
for
the PHENIX Collaboration
Outline:
Quick Physics overview
RHIC and PHENIX, and ALL requirements
Soft Physics contribution at Low pT
Results and Dg constraints

2. Motivation with DS ~25% Dg not well constrained DL ?
How to measure Dg: p0
Hard Scattering Process
Dg2
DgDq
Dq2
~ agg * Dg2 + bgq * Dg + cqq

3. ALL Requirements Helicity Dependent Particle Yields
++ = +- + =
Helicity Dependent Particle Yields
(Local) Polarimetry
Relative Luminosity (R=L++/L+-)
ALL
Year
[GeV] dR
dALL
2005
200
1.0e-4
2.3e-4
2006
1.1e-4
1.5e-4

4. RHIC RHIC CNI (pC) Polarimeters Absolute Polarimeter (H jet)
BRAHMS & PP2PP (p)
PHENIX (p)
STAR (p)
Siberian Snakes
Spin Rotators
Partial Siberian Snake
LINAC
BOOSTER
Pol. Proton Source
AGS
200 MeV Polarimeter
AGS Internal Polarimeter
Rf Dipoles
Year
[GeV]
Luminosity [pb-1] (recorded)
Polarization [%]
Figure of Merit (P4L)
2005 *
200
3.4 46
0.15
2006 *
7.5 62
1.11
* Longitudinal
Run6 Figure of Merit 7.5 times that of Run5

5. PHENIX Detector p° detection Relative Luminosity Local Polarimetry
Electromagnetic Calorimeter:
High pT photon trigger to collect p0's
Acceptance: |h|<0.35, f = 2 x p/2
High granularity (~10*10mrad2)
Relative Luminosity
Beam Beam Counter (BBC)
Acceptance: 3.0< h<3.9
Minimum Bias event trigger
Zero Degree Calorimeter (ZDC)
Acceptance: ±2 mrad
Local Polarimetry
ZDC
Level 2 Filtering
Run6 p0 results
Filter events with high pT photons (>2.5 GeV).
Statistics limited for pT<5 GeV/c
BBC
ZDC

6. Local Polarimetry at PHENIX
Use Zero Degree Calorimeter (ZDC) 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.
Run5
<PT/P>= 10.25±2.05(%)
<PL/P> = ±0.12±0.02(%)
<PT/P>= 14.47±2.20(%)
<PL/P> = 98.94±0.21±0.04(%)
BLUE
YELLOW
Raw asymmetry f
BLUE
YELLOW
Raw asymmetry f
Run6
Awaits Full production
Currents for Rotator and Main magnets for both STAR and PHENIX are monitored during the run
Enough historical evidence exists for understanding spin direction and magnet current correlation.
Conclude that spin orientation is fine.
Idea: Use neutron asymmetry to study transversely polarized component.

7. Remaining Transverse ComponentATT
Here
ATT
azimuthally independent double transverse spin asymmetry.
ALL background (e<0.01).
expected to be small, but previously unmeasured.
In Run5, PHENIX took a short transverse run specifically to measure ATT.
Consistent with zero.
Possible systematic contribution to ALL <0.05dALL.

8. Run5 p0 Cross Section Consistent with previous results.
Extends previous results to pT of 20 GeV/c.
Theory is consistent with data over nine orders of magnitude.

9. Calculating p0 ALL Calculate ALL(p0+BG) and ALL(BG) separately.
Get background ratio (wBG) from fit of all data.
Subtract ALL(BG) from ALL(p°+BG):
ALL(p0+BG) = wp° · ALL(p0) + wBG · ALL(BG)
p0+BG region :
±25 MeV around
p0 peak
BG region :
two 50 MeV regions
around peak

10. Run5 Result
What is the soft physics contribution to the low pT region?
To study this region, we instead use a minimum bias trigger as our photon trigger has low trigger efficiency at low pT.
Can we understand the soft physics contribution in this pT range?
GRSV: M. Gluck, E. Reya, M. Stratmann, and W. Vogelsang, Phys. Rev. D 63 (2001)

11. Soft physics at Low pT
By comparing Run5 preliminary p0 cross section, with that of charged pions ([p++p-]/2), we see the data agree very well.
Fitting an exponential decay to the low pT charged pion data gives an estimate on the soft physics contribution.
From this, we see that for pT>2 GeV, the soft physics component is down by more than a factor of 10.
From this, we also see that our Minimum bias triggered p0 ALL data for pT<1 can give us an estimate for ALL from soft physics.
Charged Pion
Neutral Pion
Run5 Preliminary

12. Soft physics, continued
Using Run5 minimum bias data for pT<1 GeV, we see that ALL is consistent with zero.
As we see no large asymmetries with current uncertainties in this soft physics region, the soft physics should at most dilute the measured ALL in the 1-2 GeV pT bin.
New Run6 data agrees with Run5 data.

13. Run6 p0 ALL (200 GeV)
Run6 Data set from times improvement on statistical uncertainties from Run5.
Variation due to LvL2 “turn on.”
Due to unreleased absolute polarizations, which act a scale factor in ALL and which contain correlated and uncorrelated part, we have not combined the two data sets.
For the confidence levels on the following slide, we assume complete correlation.
GRSV: M. Gluck, E. Reya, M. Stratmann, and W. Vogelsang, Phys. Rev. D 63 (2001)

14. What about Dg?
Confidence levels from a simple c2 test between our data and the four curves plotted.
Theoretical uncertainties are not taken into account (for now).
C.L. (%)
Theory model
Run5
Run6
Run5&Run6
GRSV-std
*GRSV-max (Dg=g)
*GRSV Dg=0
*GRSV Dg=−g
C.L. (%)
Theory model
Run5
Run6
Run5&Run6
GRSV-std
*GRSV-max (Dg=g)
*GRSV Dg=0
*GRSV Dg=−g
C.L. (%)
Theory model
Run5
Run6
Run5&Run6
GRSV-std
*GRSV-max (Dg=g)
*GRSV Dg=0
*GRSV Dg=−g
Range is from varying ALL by polarization scale uncertainty
* At input scale: Q2 = .4 GeV2
Run 6 rules out maximal gluon scenarios.
Expect clearer statement when lower pT data from Run6 is available; possibly will allow differentiation between STD and Dg=0.

15. Soft Physics Bias?
To remove possibility that soft physics is influencing the result through the pT<2 data, we calculated confidence levels excluding this point.
Theoretical uncertainties are not taken into account (for now).
C.L. (%)
Theory model
Run5
Run6
Run5&Run6
Run5&Run6 (pT>2 GeV)
GRSV-std
*GRSV-max (Dg=g)
*GRSV Dg=0
*GRSV Dg=−g
* At input scale: Q2 = .4 GeV2
No significant difference seen.
Expect clearer statement when lower pT data from Run6 is available; possibly will allow differentiation between STD and Dg=0.
More detailed analysis of Dg constraint underway.

16. Conclusions
Possible contaminating ATT shown to be consistent with zero for p0.
Soft physics contribution to ALL above 2 GeV estimated to be less than 10% from comparison of unpolarized cross sections from neutral and charged ([p++p-]/2) pion.
PHENIX Run5 data set excluded the positive maximal gluon scenario. With the addition of Run6 data at high pT (due to data filter), negative maximal gluon scenario also excluded.
For Run6, an increase of 7.5 in figure of merit will help reduce present uncertainty in Dg by factor of ~2-2.7, possibly allowing differentiation between STD and Dg=0.
More data expected at lower pT points, as seen below:

17. Backups: 200 GeV

18. Relative Luminosity
Number of BBC triggered events used to calculate Relative Luminosity.
For estimate of Uncertainty, fit
where
Limited by ZDC statistics.
Year
[GeV] dR
dALL
2005 *
200
1.0e-4
2.3e-4
2006 *
1.1e-4
1.5e-4
* Longitudinal

19. Measured Asymmetry During Longitudinal Running (2005)
c2/NDF = 88.1/97
p0 = ±
c2/NDF = 82.5/97
p0 = ± LR UD
XF>0
XF<0
c2/NDF = 119.3/97
p0 = ±
c2/NDF = 81.7/97
p0 = ±
<PT/P>=
10.25±2.05(%)
<PL/P> =
99.48±0.12±0.02(%)
14.47±2.20(%)
98.94±0.21±0.04(%)
Fill Number
Fill Number

20. Minimum Bias: Bunch Shuffle
Bunch Shuffling shows systematic difference from expected distribution at low pT
We add a systematic error band to account for this.

21. Minbias: Mass Spectrum
Mass distributions for 6 pT points.

22. ATT: Bunch Shuffle Match expected distributions 1.5-2 1-1.5 2.5-3
2-2.5
3.5-4
3-3.5
4-5
5-6
6-7

23. Run6: Mass Spectrum
PbSc
PbGl

24. Run6: Bunch Shuffle, Signal
Even
Odd