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 900 GeV TeV
Summary and Outlook
Convegno Nazionale Fisica di Alice, Frascati, 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. Only events with Δm<5 and Δzv < 3 cm mixed
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. (Mixed events background)
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. Like-sign technique also explored
Unlike-sign
Like-sign
Comparison of the like-sign background to the “true” combinatorial background

6. PID influence on K*(892) reconstruction
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 From generation and reconstruction …
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 2 Data Sets
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 900 GeV. Running scenario at LHC startup
1.5 x 106 minimum bias p-p PHYTIA 14 TeV (about 0.2% 1-year data taking)
PDC06 data, distributed GRID analysis

9. 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* %
Φ %
Λ* %
Particle ratios Statistical
K*/K %
Λ*/Λ %
Φ/K* %
Φ/Λ* %
K*/Λ* %

13. K*(892)± analysis Total B.R. ~ 23 % π+ + π- B.R. ~ 69%
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. 3042 KS0 reconstructed (2978 true)
Results on charged K*(892)
PPR cuts
cosθp > 0.994
dca < 3.46 cm
r > 0.34 cm
b+/- > 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 Overestimated yield/width
ANN Reason. agreement, best present condition
Yield improvement by 15-20% with respect to PPR
Reasonable agreement between true and found K*

18. 14 TeV

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

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

21. 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 TeV

26. Summary
Short-lived resonances in pp LHC energies could be studied in ALICE from the very beginning
With a small sample of events 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