Reconstruction of short-lived resonances in pp collisions

1. Reconstruction of short-lived resonances in pp collisions F.Blanco – INFN e Università di Catania - Italy • Content • Identification strategy • Code development • Results on resonances (K*(892),(1520),(1020),..) from simulated p-p events @ 900 GeV and @14 TeV • Summary and Outlook Convegno Nazionale Fisica di Alice, Frascati, 12-14 Novembre 2007

2. Signal extraction Inside same event, correlations between K+ and π- candidates K- and π+ candidates Evaluate invariant mass spectrum Example: K*(892)  Kπ (~100%) Mixed-event technique Combinatorial background: Like-sign technique

3. Study of the combinatorial background by the mixed-event technique Studied dependence on event selection criteria ●Charged multiplicity ●z-vertex location Only events with Δm<5 and Δzv < 3 cm mixed Multiplicity Comparison of the event mixing background to the “true” combinatorial backgroung ‘True’ background = (Signal) – (True pairs) z-vertex

4. Effect of event selection on mixing procedure (True Background) (Mixed events background) Only events with Δm<5 and Δzv < 3 cm mixed No event selection

5. K* K* Like-sign technique also explored Unlike-sign Like-sign Comparison of the like-sign background to the “true” combinatorial background

6. Perfect PID Realistic PID True PID influence on K*(892) reconstruction Found No thresh on maxprob Maxprob > 0.7 (K) Maxprob > 0.7 (π) Maxprob > 0.7 (K) No PID (π) Perfect PID Realistic PID True K* = 7599 Found K* = 7488 S/B = 0.138 S/√B = 30.68 True K* = 4306 Found K* = 4139 S/B = 0.11 S/√B = 20.28

7. How the code works Selection of primary and identified tracks: AliRsnReaderRL From generation and reconstruction AOD: Analysis Object Data: AliRsnEvent, contains arrays of AliRsnDaughter objects ESD: Event Summary Data Kinematics Analysis, Cuts,histograms,… AliRsnAnalysis …

8. Results from p-p events @ 900 GeV and 14 TeV • Events of PYTHIA were generated and fully reconstructed using • Realistic simulation of the detector response for the whole ALICE assembly • Realistic clusters and tracks reconstruction 2 Data Sets 2 x 105 minimum bias p-p PHYTIA events @ 900 GeV. Running scenario at LHC startup 1.5 x 106 minimum bias p-p PHYTIA events @ 14 TeV (about 0.2% 1-year data taking) PDC06 data, distributed GRID analysis

9. Results @ 900 GeV

10. K*(892) results with realistic PID True K* (within 2σ) = 4728 Found K* (within 2σ) = 4274 S/B = 0.105 S/√B = 20.12

11. (1020) and (1520) with realistic PID  True  (± 2σ) = 186 Found  (±2σ) = 168 S/B = 2.87 S/√B = 13.86 True Λ (±2σ) = 146 Found Λ (±2σ) = 128 S/B = 1.23 S/√B = 19.5 *

12. Yields and particle ratios uncertainties Resonance Discrepancy w.r.t. true pairs K* 4 % Φ 10 % Λ* 12 % Particle ratios Statistical K*/K- 1.6 % Λ*/Λ 9 % Φ/K* 8 % Φ/Λ* 12 % K*/Λ* 9 %

13. K*(892)± analysis K*(892)± KS0 + π± B.R. ~33% Total B.R. ~ 23 % π+ + π- B.R. ~ 69% Need to identify the associated V0 They are reconstructed by association of two opposite charge tracks, and then applying some topological cuts

14. Results on charged K*(892) PPR cuts cosθp > 0.994 dca < 3.46 cm r > 0.34 cm b+/- > 0.0115 cm Efficiency = 52.0 % Purity = 97.9 % 0.48 < m < 0.51 GeV 3042 KS0 reconstructed (2978 true)

15. Results on charged K*(892)

16. Neural network approach to V0 finding Aim: ● Improve V0 selection (efficiency & purity) by ANN ● Combine V0 with pion tracks bj = tanh (∑ wij ai – Θj) output = ∑ wk bk w’s = synaptic weights Θ = neuron thresholds ai = input bi = hidden layer Various possibilities for the training algorithm exploited, BFGS chosen 7 inputs, 1 hidden layer

17. Results on charged K*(892) using ANN approach True and found K*(892)± V0 selection strategy True K* Found K* Notes PPR cuts 944 1042 Overestimated yield/width ANN 1122 1173 Reason. agreement, best present condition Yield improvement by 15-20% with respect to PPR Reasonable agreement between true and found K*

18. Results @ 14 TeV

19. (1020) and *(1520) with realistic PID (1020) (1520) True =4893 Found =4967 True *=3879 Found *=3649

20. K*(892) with realistic PID Found K*(±2)=89182 True K*(±2)=85360 Mass resolution ~ 3MeV/c2

21. pT= 0 - 0.5 pT= 1.5 - 2 pT= 3.5 - 4 K*(892)0 pT-analysis with realistic PID

22. Correction matrix

23. (1020): signal vs. event mixing background

24. Λ(1520): comparison between signal and event mixing

25. Looking for f0(980) 100k pp events @14 TeV

26. Summary • Short-lived resonances in pp collisions @ LHC energies could be studied in ALICE from the very beginning • With a small sample of events [O(105)] @ 900 GeV and realistic PID: • ● Extraction of yields at least for K*(892), Φ(1020), Λ*(1520) • ● Rough pT - distribution for K*(892) up to 1.5 GeV/c • ● Particle ratios Φ/K*, Λ*/K*, Φ/Λ* measurable • Analysis of O(106) pp events at 14 TeV fully reconstructed on the GRID • ● Resonance yields with large statistics • ● pT-analysis of K*(892)0 • ● Correction matrix (y,pT) • Extension to other resonances (Σ(1385), Δ, f0(980)) is in progress.

27. Backup

28. Neural network approach to charged K*(892) reconstruction Huge background