1. p0 ALL analysis in PHENIX
K. Tanida (RIKEN/RBRC) RSC meeting 11/18/03
Outline
Relative luminosity - estimation of systematic error - ALL in ZDC/BBC
LocalPol
Counting number of p0’s
Result

2. ALL (P) Polarization (L) Relative Luminosity (N) Number of pi0s
++ same helicity
+ opposite helicity
(P) Polarization
(L) Relative Luminosity
(N) Number of pi0s

3. How to get relative luminosity?
Principle: R=N++/N+- -- from any counter (trigger)
BBCLL1 trigger - coincidence of two Beam-Beam Counters - small background (~ 10-4) - high statistics (detects ~ 1/2 of all collisions) - ALLBBC: expected to be small, need confirmation
BBC-South
BBC-North
⊿η = 3.0 ~ 3.9
⊿φ = 2π cm

4. Method of accuracy estimation
Cross-check by ZDCLL1 trigger (Zero Degree Calorimeter) - low background (~ 10-4) - statistics: moderate (~ 3% of BBCLL1)
N(ZDCLL1)/N(BBCLL1) must be constant
Crossing-by-crossing, fill-by-fill ratio calculation Blue: Yellow:  We have all spin combinations in each fill
GL1p scaler counts both BBCLL1 and ZDCLL1 for each crossing
Required accuracy: constant to 10-3 level for each fill.

5. crossing-by-crossing ratio
c2/dof = 346/45
Significant deviation from constant. Why?

6. Possible explanation – detector acceptance
run 87693
Extreme cases - Flat acceptance  N: area - Narrow acceptance  N: height
crossing 74
counts
crossing 90
BBC Z vertex (cm)
ZDCLL1: Z-vertex resolution ~ 20 cm (wider acceptance)
BBCLL1: ~ 5 cm (narrower) (both have Z-vertex cut at ±30 cm)

7. Correlation with Z-vertex width
c2/dof = 55.1/45
[cm]
Z-vertex width [cm]
We understand the reason now  correction

8. After correction
c2/dof = 54.8/45
Relative luminosity accuracy < 0.09% (stat. limited)

9. Alternative correction
Actual vertex width is not yet available in most runs (will come soon)
Alternative width estimation  use ZDCout/ZDCin ratio ZDCin: ZDCLL1 counts with Z-vertex cut of 30 cm ZDCout: ZDCLL1 outside of ZDCin (both are counted crossing-by-crossing)  wider vertex distribution  larger ratio

10. Actual accuracy estimation -- Bunch fitting
Crossing-by-crossing analysis, taking care of possible effect of ALL in BBC and ZDC
For i-th crossing, calculate r(i) = NZDCLL1(i)/NBBCLL1(i) corrected for vertex width
Fit r(i) by r(i) = C[1+ALLBBC/ZDCPB(i)PY(i)] fit parameter: C, ALLBBC/ZDC
You will get C, ALLBBC/ZDC, their errors, and c2 - accuracy is obtained as dALLBBC/ZDC (x PBPY) - c2 gives a good check for systematic errors.

11. c2 vs stat. error – before correction
1 point/fill
w/o vertex width correction
correlating
 ssyst ~ 0.002
c2/dof = 1+(ssyst/sstat)2 ssyst = 0.002
c2/dof
statistical error (deLL, sstat)

12. c2 vs stat. error – after correction
fill 3735
correlation
is weak, but
still some systematic effect remains
bad fill
c2/dof = 1+(ssyst/sstat)2 ssyst = 0.001
c2/dof
statistical error (dALL, sstat)

13. Result & BBC/ZDC ALL Systematic error seen  c2 correction required
If c2/dof is larger than 1, then enlarge statistical error by sqrt(c2/dof) – conservative correction
Averaged result: ALLBBC/ZDC = (1.8±1.8) x (without vertex width correction, dALL is ~2 larger)
This corresponds to relative luminosity accuracy of dR = 2.5 x 10-4  Achieved the goal
ZDC-BBC ALL is 0 consistent  we can use both for reference of ALL = 0

14. Some notes
We reached dALLBBC/ZDC=1.8 x this is dominated by statistics of ZDC, so it’s rather conservative estimation - actual relative luminosity is given by BBC
In p0 analysis, vertex position dependence of acceptance is the same as BBCLL1 because - BBC hits are required in the analysis - PHENIX central arm acceptance is almost flat for p0 within the region we are using (±30 cm)

15. Remaining tasks for rel. luminosity
Apply offline Z-vertex cut - better resolution: ~ 5 cm  ~ 2 cm (effect is found to be small)
Full accuracy estimation using real vertex distribution. - correction for other than vertex width?
Study effect of multiple collisions - negligible in Run3, significant for higher luminosity.
Z-vertex dependence of acceptance (efficiency) for BBCLL1, p0 and other reaction channels. - reaction channel specific relative luminosity. - BBCLL1 itself is almost perfect for p0 case, but may be not for other channels.

16. Vertex distribution (BBCLL1)
reject
reject
counts
accept
offline Zvertex [cm]
changes rel. lumi. by < 0.04% per fill (negligible)

17. PHENIX Local Polarimeter
Forward neutron transverse asymmetry (AN) measurements
SMD (position) + ZDC (energy)
f distribution
SMD
Vertical  f ~ ±p/2
Radial  f ~ 0
Longitudinal  no asymmetry
ZDC

18. Spin Longitudinal Component
ST is measured with PHENIX Local Polarimeter
Left-Right asymmetry
Up-Down asymmetry

19. p0 counting Data collected with high pT photon trigger
Based on EMCal; Threshold ~1.4 GeV/c
Rejection factor ~110
Analyzed data sample: 42.7M events (~0.215 pb-1)
sqrt(<PbPy>)~26%
Minimum Bias data
To obtain “unbiased” 0 cross section at low pT
For high pT photon trigger efficiency study

20. p0 reconstruction for ALL
1-2 GeV/c
Bckgr=45%
2-3 GeV/c
Bckgr=17%
Results obtained for four pt bins from 1 to 5 GeV/c
Pi0 peak width varies from 12 to 9.5 MeV/c2 from lowest to highest pt bins
Background contribution under pi0 peak for 25 MeV/c2 mass cut varies from 45% to 5% from lowest to highest pt bins
3-4 GeV/c
Bckgr=7%
4-5 GeV/c
Bckgr=5%

21. p0 counting for ALL N0: Nbck1: Nbck2:
25 MeV/c2 around 0 peak (and also 15 and 35 MeV/c2 for cross checks)
Nbck1:
Two 50 MeV/c2 wide areas adjacent to 0 peak
Nbck2:
250 MeV/c2 wide area between 0 and  peaks
N0 and Nbck accumulated statistics pt
GeV/c
N0
15 MeV/c2
25 MeV/c2
35 MeV/c2
Nbck1
Nbck2
1-2
1278k
1777k
2129k
1470k
3478k
2-3
874k
1059k
1146k
335k
989k
3-4
176k
201k
208k
27k
83k
4-5
34k
38k
39k
3.9k
12k

22. N0 statistical error estimation
dk/dNev
dk/dNev
N0
Nbck2
In multi-0 events:
Comparison to naïve approach:
Fluctuation enhancement (squared) due to multi-particle events pt
GeV/c
N0
15 MeV/c2
25 MeV/c2
35 MeV/c2
Nbck1
Nbck2
1-2
1.134
1.220
1.302
1.360
1.708
2-3
1.047
1.081
1.114
1.178
1.406
3-4
1.015
1.026
1.037
1.101
1.222
4-5
1.008
1.021
1.059
1.162

23. ALL measurements Procedure ++ same helicity + opposite helicity
Collect N and L for ++ and + configurations (sum over all crossings) and calculate ALL for each fill
Average ALL over fills; use 2/NDF to control fit quality; use “bunch shuffling” to check syst. errors

24. ALL Calculations ALL averaged over fills 1-2 GeV/c 2-3 GeV/c 3-4 GeV/c
2/ndf = 64/48
2-3 GeV/c
ALL= -2.2%±1.5%
2/ndf = 34/48
3-4 GeV/c
ALL= -0.2%±3.3%
2/ndf = 49/48
4-5 GeV/c
ALL= -2.3%±7.4%
2/ndf = 39/48
4-5 GeV/c
3-4 GeV/c

25. AL check for “yellow” beampt
GeV/c
15 MeV/c2
25 MeV/c2
35 MeV/c2
1-2
0.0010.004
0.0000.003
0.0020.004
2-3
0.0000.004
0.0020.007
0.0020.005
3-4
0.0070.009
0.0110.009
0.0080.009
-0.0330.025
-0.0100.015
4-5
-0.0010.021
0.0040.020
0.0080.020
0.0200.064
0.0500.039
All are zeros within 1.5

26. AL check for “blue” beampt
GeV/c
15 MeV/c2
25 MeV/c2
35 MeV/c2
1-2
-0.0010.004
0.0010.003
0.0000.003
-0.0020.004
-0.0000.003
2-3
0.0010.004
0.0000.004
0.0020.004
0.0090.007
0.0000.005
3-4
0.0040.009
0.0060.009
0.0060.008
-0.0040.024
-0.0360.015
4-5
-0.0240.021
-0.0160.020
-0.0160.019
-0.0110.062
0.0130.038
All are zeros within 1.5, except

27. (Background subtracted)
0 ALL result pT
GeV/c
(rbck)
(Background subtracted)
1-2
-0.0280.012 (45%)
-0.0060.014
-0.0460.025
2-3
-0.0220.015 (17%)
-0.0350.027
-0.0190.019
3-4
-0.0020.033 (7%)
0.0940.092
-0.0090.036
4-5
-0.0230.074 (5%)
0.380.24
-0.0450.079
Polarization scaling error P ~30%: is not included
Enters the ALL quadratically
Analyzing power AN(100 GeV) ~ AN(22GeV) is assumed
P~30%: combined stat. and syst. error for AN(22GeV) (AGS E950)
pT smearing correction is not included

28. Summary
Relative luminosity – systematics is mostly due to crossing by crossing variation of Z-vertex width and acceptance difference  correction applied
Relative luminosity accuracy dALL = 1.8 x 10-3 achieved
Relative luminosity is reaction channel dependent - For p0 analysis, BBCLL1 is good enough.
Preliminary result for p0 ALL obtained
Various null tests – all OK statistical and systematic errors are well studied.