RECONSTRUCTION OF LOW MOMENTUM PARTICLES

1. RECONSTRUCTION OF LOW MOMENTUM PARTICLES ‘Stopping’ particles  particles that end its life (stop, decay or interact) in a given layer of the Spectrometer R. Hołyński, P. Sawicki, A. Trzupek, B. Wosiek, K. Woźniak Reconstruction procedure • Calibration • Analysis of MC events • Analysis of single MC particles with higher momenta • Positive versus negative particles • Analysis of real data • Plans Barbara Wosiek, Dec. 2000, PHOBOS Collaboration Meeting

2. RECONSTRUCTION PROCEDURE • Define a layer where particle stops, decays or interacts Now: Layer E of SpecP ----- +, K+, p • Energy Cut: Select hits with large energy deposits (after merging*) in layers A – E Dei> Deicut i = A – D DeE > DeEcut for the last layer * currently only ‘horizontal’ merging, for MC tracks used for calibration we require that the merged hit should contain a hit from the primary particle • Create track candidates from selected hits in layers A – E all possible combinations of 5 hits per track • Accept only these candidates which pass the following cuts:  FirstAngularCut  Veto Hit in Layer F  Mass Cut  Second Angular Cut  Last Cut

3. First Angular Cut | i+1 - i | < i+1,icut | i+1 - i | < i+1,icut For each pair of layers- AB, BC,CD,DE i+1,icut , i+1,icut : obtained from+ tracks used to calibrate the procedure (i, i angles from vertex to hiti) Veto Hit in Layer F no hit with DeF > DeEcut and | FE | < EDcut | FE | < EDcut

4. Mass Cut •  Calculate mass parameter for • layer i, i = A – D: • (massp)i = (De/dx)i * • (1/2) (m2) • Average over layers A – D: < mass_p>xxxxx •  Calculate total energy loss: • Eloss = •  Use <massp> vs. Eloss/nhits • scatter plot to accept the track • candidate and get it’s ID Second Angular Cut | i+1,i | < i+1,icut | i+1.i | < i+1.icut Cuts for +, K+ and p are used depending on the track ID Last Cut Select among candidates which share at least one hit.  Track is reconstructed!

5. CALIBRATION Single +, K+ and p tracks • all secondary processes turned on • Eth = 0.1 MeV •  range 45o  25o (flat in ) •  range 5o (flat) • momentum range (flat): • 0.035 0.065 GeV/c for + • 0.110  0.150 GeV/c for K+ • 0.180  0.260 GeV/c for p • For calibration use only particles: •  which give hits in layers A – E (the last hit in layer E ) •  p < pmax (0.060 - +, 0.148 - K+, 0.235 - p) •  their passage through the detector is terminated at layer E • 2.5 % of + and 5 % of K+ and p survived the above requirements

6. CALIBRATION + K+ p Ntracks23000 46000  49000 p(GeV/c) 0.050 – 0.060 0.124 – 0.148 0.196 – 0.235 Stopped 0 0 99.8% Decayed 99.991% 99.4% - Interacted 0.009% 0.6% 0.2% • Energy Cut: • Layers A-D: De > 0.65 MeV+ at layer A • Layer E : De > 0.36 MeV p at layer E • First Angular Cut( for + rejects 0.5% of largest deviations ) : •  < [1.9o, 4.1o, 2.2o, 6.7o] •  < [2.5o, 3.9o, 3.1o, 7.1o] • AB BC CD DE

7. CALIBRATION • Mass Cut: • massp vs. Eloss/5 contours for +, K+, p • (contours contain 90% of tracks) • Second Angular Cut: •  cut, cut for +, K+, p Initial ID reconstructed as : + K+ p random hits +69.56% 0.02% 0.00% 2.93% K+ 0.83% 80.94% 0.08% 1.23% p 0.00% 2.53% 82.90% 0.00%

8. Analysis of MC Events • Events with |Zvtx| < 2.5 cm and Nch (prim.) >3000 • simulated with all secondary processes and Eth = 1 MeV. • 89 events: •  + (p=0.049 - 0.060 GeV/c) • K+ (p=0.123 - 0.148 GeV/c) • p (p=0.195 – 0.235 GeV/c) • 3 + in the  acceptance of the SpecP0in the  acceptance • 9 K+ in the  acceptance of the SpecP0in the  acceptance • 12 p in the  acceptance of the SpecP0in the  acceptance • Use these events as a background for reconstructing low momentum particles.

9. Analysis of MC Events with embedded low momentum tracks One low momentum +, or K+ or p added per event. 2225 events analyzed for each particle ID. + Single tracks Full events Reconstructed as: +70.4 % 54.6 % K+ 0.04% 0.13% p 0.0 0.0 Rejected: Energy Cut 2.4% 0.4% First Angular Cut 5.5% 5.7% Veto Hit in Layer F 13.7% 32.0% Mass Cut 7.6% 6.7% Second Angular Cut 0.5% 0.5%

10. K+ Single tracks Full events Reconstructed as: + 0.0 0.58% K+82.40% 63.90% p 0.22% 0.18% Rejected: Energy Cut 0.36% 0.09% First Angular Cut 1.12% 1.21% Veto Hit in Layer F 1.71% 21.03% Mass Cut 10.52% 10.02% Second Angular Cut 3.64% 2.97%

11. p Single tracks Full events Reconstructed as: + 0.0 0.49% K+ 0.45% 0.81% p 86.43% 67.28% Rejected: Energy Cut 0.99% 0.04% First Angular Cut 0.72% 0.90% Veto Hit in Layer F 0.04% 19.19% Mass Cut 6.20% 6.61% Second Angular Cut 5.17% 4.67% • Loss of 15-16% in efficiency in an event environment! (Veto Hit in Layer F)

12. Analysis of single MC particles with the extended momentum ranges Estimation of the fraction of miss-identified particles Extended momentum ranges: 0.035 0.200 GeV/c for + 0.110  0.300 GeV/c for K+ 0.180  0.400 GeV/c for p Initial ID reconstructed as + K+ p +100.00% 0.00% 0.00% K+22.45% 77.55% 0.00% p8.75% 6.21% 85.04% Corrections for miss-identification should be applied. (model dependent-dN/dp for , K and p)

13. Positive  Negative Particles Using the calibration based on positive particles we reconstruct: Initial ID reconstructed as: rejected by Mass Cut - K- p- - 28.36% 0.00% 0.00% 55.19% K- 1.62% 17.93% 4.86% 48.49% p- 0.09% 0.40% 27.28% 50.34%  + - K+ K- p p- Fraction of stopped 0 0 0 0 .998 0 Fraction of decayed .9999 .002 .994 .006 0 0 Fraction of interacted .0001 .998 .006 .994 .002 1.000 Large energy losses in layer E for negatively charged particles.

14. New Calibration • Using Mass Cut based on the mass parameter and • energy loss calculated only for layers A-D. • first attempt: • i) if DeE > 7.5 MeV no Veto Hit in layer F • ii) massp is calculated for Layers A, B, C, D • but the two lowest massp are excluded • Modification of the energy loss in layer E • (under study)

15. Reconstruction of single low-momentum MC particles with the new mass contours. Initial ID reconstructed as  K p +71.8% 0.0% 0.0% - 83.8% 0.1% 0.0% K+ 0.0% 87.7% 1.3% K- 0.1% 71.8% 1.0% p0.0% 1.8% 88.1% p-0.0% 2.3% 67.9%  encouraging, optimization still possible !

16. Plans & Schedule • Optimized calibration Dec.30,2000 • Improved merging (‘vertical’ merging) Dec.30,2000 • Calibration for SpecN Jan. 5, 2001 • Analysis of data (B0,B+,B-) Jan.30,2001 • Hot/dead channels, De systematics Feb.15,2001 • Zvtx dependence Jan.30,2001 • Corrections : Feb.28,2001 • acceptance, efficiensy, • miss-identification, rec. momentum • Extension to other layers (C,D,..) Feb.28,2001 • Charge separation Feb.28,2001

17. Plans • Optimized calibration • Improved merging (‘vertical’ merging) • Calibration for SpecN • Analysis of data (B0,B+,B-) • Hot/dead channels, De systematics • Zvtx dependence • Corrections : • acceptance, efficiency, • miss-identification, rec. momentum • Extension to other layers (C,D,..) • Charge separation

18. Analysis of the data 6200 events B=0 |Zvtx| < 15 cm selected from 50k events R5544 Old calibration (A-E): reconstructed:  K p 22 2 0 SpecP 16 particles SpecN 8 particles New calibration (A-D): reconstructed:  K p 25 2 1 SpecP 19 particles SpecN 9 particles  Needed: Large data statistics Separate calibration for SpecN

19. Analysis of the data 82.4%of events rejected byEnergy Cut(empty HitList) 13.4%of events rejected byFirst Angular Cut(empty list of candidates) 3.5%of events rejected byVeto Hit in Layer F(empty list of candidates) 0.4%of events rejected byMass Cut(empty list of candidates) 0.03%of events rejected bySecond Angular Cut(empty list of candidates) 0.23% of events have a reconstructed low momentum particle!