Estimation of Systematic Errors in Relative Luminosity Measurements for p0 Analysis in PHENIX

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 ++ 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 144.35 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 calculationBlue:+ - + - + - + -....Yellow:+ + - - + + - -.... We have all spin combinations in each fill • GL1p scaler counts both BBCLL1 and ZDCLL1for 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 possibleeffect of ALL in BBC and ZDC • For i-th crossing, calculater(i) = NZDCLL1(i)/NBBCLL1(i)corrected for vertex width • Fit r(i) byr(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)2ssyst = 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)2ssyst = 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 10-3(without vertex width correction, dALL is ~2 larger) • This corresponds to relative luminosity accuracy ofdR = 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 10-3- 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: • 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

22. N0statistical 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

23. ALL measurements ++ same helicity + opposite helicity • Procedure • 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 • 1-2 GeV/c • ALL= -2.8%±1.2% • 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” beam All are zeros within 1.5

26. AL check for “blue” beam All are zeros within 1.5, except

27. 0ALL result 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 tocrossing by crossing variation of Z-vertex width and acceptance difference correction applied • Relative luminosity accuracydALL = 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.