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

Inside same event, correlations between

K+ and π- candidates

K- and π+ candidates

Evaluate invariant mass spectrum

Combinatorial background:

Signal extraction

Mixed-event technique

Like-sign technique

Example: K*(892) Kπ (~100%)

)()(2)( mNmNmNKKSignLike

Studied dependence on event selection criteria

●Charged multiplicity

●z-vertex location

Study of the combinatorial background by the mixed-event technique

Multiplicity

z-vertex

Comparison of the event mixing background to the “true” combinatorial backgroung

‘True’ background = (Signal) – (True pairs)

Only events with Δm<5 and Δzv < 3 cm mixed

No event selection

Only events with Δm<5 and Δzv < 3 cm mixed

Effect of event selection on mixing procedure

(True Background)

(Mixed events background)

Like-sign technique also explored

K* K*

Comparison of the like-

sign background to the “true” combinatorial background

Unlike-sign

Like-sign

Perfect PID Realistic PID

PID influence on K*(892)

reconstruction

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

Maxprob > 0.7 (K)No PID (π)

Maxprob > 0.7 (K)Maxprob > 0.7 (π)

No thresh on maxprob

True

Found

Perfect PID

Realistic PID

Selection of primary and identified tracks:

AliRsnReaderRL

Analysis, Cuts,histograms,…

AliRsnAnalysis

From generation and reconstruction

ESD: Event Summary Data

Kinematics

…

AOD:Analysis Object Data:

AliRsnEvent, contains arrays ofAliRsnDaughter objects

How the code works

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

Results @ 900 GeV

K*(892) results with realistic PID

True K* (within 2σ) = 4728

Found K* (within 2σ) = 4274

S/B = 0.105

S/√B = 20.12

(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

*

Yields and particle ratios uncertainties

Particle ratios Statistical

K*/K- 1.6 %

Λ*/Λ 9 %

Φ/K* 8 %

Φ/Λ* 12 %

K*/Λ* 9 %

Resonance Discrepancy w.r.t. true pairs

K* 4 %Φ 10 %Λ* 12 %

K*(892)± KS0 + π± B.R. ~ 33%

π+ + π- B.R. ~ 69%

Total B.R. ~ 23 %

K*(892)± analysis

Need to identify the associated V0

They are reconstructed by association of two opposite charge tracks, and then applying some topological cuts

cosθp > 0.994dca < 3.46 cmr > 0.34 cmb+/- > 0.0115 cm

0.48 < m < 0.51 GeV3042 KS

0 reconstructed (2978 true)

Efficiency = 52.0 %

Purity = 97.9 %

PPR cuts

Results on charged K*(892)

Results on charged K*(892)

7 inputs, 1 hidden layer

bj = tanh (∑ wij ai – Θj)

output = ∑ wk bk

w’s = synaptic weights

Θ = neuron thresholds

ai = input

bi = hidden layerVarious possibilities for the training algorithm exploited, BFGS chosen

Neural network approach to V0 finding

Aim: ● Improve V0 selection (efficiency & purity) by ANN● Combine V0 with pion tracks

True and found K*(892)±

Results on charged K*(892)using ANN approach

V0 selection strategy True K* Found K* NotesPPR cuts 944 1042 Overestimated yield/widthANN 1122 1173 Reason. agreement, best present condition

Yield improvement by 15-20% with respect to PPR

Reasonable agreement between true and found K*

Results @ 14 TeV

(1020) and *(1520) with realistic PID

(1020) (1520)

True =4893

Found =4967

True *=3879

Found *=3649

K*(892) with realistic PID

Mass resolution ~ 3MeV/c2

Found K*(±2)=89182

True K*(±2)=85360

K*(892)0 pT-analysis with realistic PID

pT= 0 - 0.5 pT= 1.5 - 2

pT= 3.5 - 4

Correction matrix

(1020): signal vs. event mixing background

5.00 Tp 15.0 Tp 5.11 Tp

25.1 Tp 5.22 Tp 35.2 Tp

5.33 Tp 45.3 Tp

Λ(1520): comparison between signal and event mixing

Looking for f0(980)

100k pp events @14 TeV

• 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.

Summary

Backup

Neural network approach to charged K*(892) reconstruction

Huge background