24/6/2007

Study of Λ(1520) production in pp interactions @14 TeV with the ALICE detector

Paraskevi Ganoti University of Athens for the ALICE Collaboration

P.Ganoti, SQM2007@Levoca-Slovakia 1

Outline



Introduction and Physics Motivation



The kinematic properties of the produced Λ(1520) by Simulation



The Geometrical Acceptance and the Reconstruction Efficiency of Λ(1520) in pp interactions inside the ALICE detector



Invariant mass spectra of K-p pairs



Conclusion and future plans

24/6/2007 P.Ganoti, SQM2007@Levoca-Slovakia 2

Introduction-Physics Motivation

The production of Resonances is a method to study the dynamics of QGP hadronization. The idea is to use unstable resonances with varying lifetime (typical lifetimes comparable to that of the hot and dense matter created in heavy ion collisions) and to determine the effect of rescattering and regeneration on the Resonances yields.

f Resonance Life-time [fm/c]

r 

++ 0 1.3

1.7 (980) 2.6 K*(892) 4.0

S(1385) 5.7

L

ω(783) 23



(1520) 13 (1020) 45

After hadronization of the QGP, but before the interactions of the hadrons cease, the physical properties of resonances, such as their masses and widths, might be modified by the density of the surrounding medium. In addition, the yield might change. The overall net effect of rescattering and regeneration on the total observed yields depends on the time interval between

chemical and thermal freeze-

out, the lifetime of the resonances and the magnitudes

of the interaction cross sections of the decay particles

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Resonances may probe the timescale between chemical and kinetic freeze-out time

Hadronization: Hadrons are formed from quarks

Pb-Pb

Λ(1520) Rescattering π K p Κ Λ(1520) K -

Chemical freeze-out: end of inelastic interactions (particle yields) Thermal freeze-out: end of elastic interactions (particle spectra)

24/6/2007 P.Ganoti, SQM2007@Levoca-Slovakia p Regeneration p

Signal lost Signal measured

4

Event Sample for Λ(1520) study

Λ(1520)

is a resonance with: m = 1519.5 MeV/c Width = 15.6MeV/c, τ = 13fm/c, BR(NK)=45% In this analysis

1.08M

min bias pp simulated events @14TeV have been used.

In these events, 18200 Λ(1520) are generated in |η| < 0.9 and with

p T

> 0.2GeV/c -> 0.017/event.

Glossary

Perfect PID : we know exactly the kind of a reconstructed particle from comparison with generation Detectors PID : the PID that the combination of the central ALICE detectors give. In this analysis only the PID information from TPC and TOF are used.

Λ(1520) : Both Λ * + Λ * . Similarly, K: Both K + + K , p: Both p + p 24/6/2007 P.Ganoti, SQM2007@Levoca-Slovakia 5

Λ(1520):Kinematic Properties

= 0.82GeV/c Λ(1520) Generated Λ(1520)

= 0.57GeV/c

= 0.37GeV/c

24/6/2007 241265 generated Λ(1520) with

= 0.82GeV/c P.Ganoti, SQM2007@Levoca-Slovakia 6

Generated in |η| < 0.9 with p T > 0.2GeV/c

18200 generated Λ(1520) with

= 1.28GeV/c

= 1.28GeV/c

= 0.87GeV/c

= 0.53GeV/c

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Geometrical Acceptance

# Λ(1520) in |η| < 0.9 with p T Λ * ,K,p > 0.2GeV/c # Λ(1520) generated in |η| < 0.9 with p T > 0.2GeV/c

Mean Geometrical Acceptance : ~73%

8 24/6/2007 P.Ganoti, SQM2007@Levoca-Slovakia

Reconstruction Efficiency I K, p interactions with the detector material

K (all) p ITS TPC

24/6/2007

K interactions TRD TOF

P.Ganoti, SQM2007@Levoca-Slovakia

K decays

9

Reconstruction Efficiency II

K + (all) p K + interactions K + decays

3623 Λ(1520) (out of 13248 that are geometrically accepted) are lost due to interactions/decay of K/p in a radius R < 100cm.

The numbers in the above plots reflect the different cross sections of K + , K , p and p. σ(K ) ~ 2σ(K + ) and σ( p ) ~ 6σ(p) around 1GeV.

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Reconstruction Efficiency III

Reconstructed Λ(1520) in |η| < 0.9 with p T, K,p > 0.2GeV/c assuming perfect pid # Λ(1520) generated in |η| < 0.9 with p T, K,p interactions/decay > 0.2GeV/c after accounting for K, p

Mean Reconstruction Efficiency : 57%

11 24/6/2007 P.Ganoti, SQM2007@Levoca-Slovakia

Perfect PID True Signal

Count 5441 reconstructed Λ(1520) if only the K-p pairs becoming from a true generated Λ(1520) are accepted.

B.W. fit results χ 2 /ndf =15.17/9 M Λ(1520) = (1.52

 0.00)GeV/c 2 Γ = (0.01644

 0.00039) P.Ganoti, SQM2007@Levoca-Slovakia 12 24/6/2007

All pairs from the same event and the mixed-event background

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Extracted Signal

B.W.+polynomial fit results χ 2 /ndf =199.2/6 M Λ(1520) = (1.52

 0.00)GeV/c 2 Γ = (0.01968

 0.00051)

4533

 162 reconstructed Λ(1520) inside 2σ (in comparison with the 5441).

S/B = 0.42 and S/



(S+B) = 36.5

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Comparison of True - Extracted signals

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Detectors PID (TPC + TOF) True Signal

2026 reconstructed Λ(1520) if only the K-p pairs becoming from a true generated Λ(1520) are accepted.

B.W. fit results χ 2 /ndf =15.25/9 M Λ(1520) = (1.52

 0.00)GeV/c 2 Γ = (0.01661

 0.00065) P.Ganoti, SQM2007@Levoca-Slovakia 16 24/6/2007

All pairs from the same event and the mixed-event background

24/6/2007 P.Ganoti, SQM2007@Levoca-Slovakia 17

Extracted Signal

B.W.+polynomial fit results χ 2 /ndf =130.8/5 M Λ(1520) = (1.523

 0.000)GeV/c 2 Γ = (0.0392

 0.0012) 24/6/2007

1951

 162 reconstructed Λ(1520) inside 2σ.

S/B = 0.16 and S/



(S+B) = 16.4

P.Ganoti, SQM2007@Levoca-Slovakia 18

Comparison of True - Extracted signals

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Summarizing the numbers…

Generated Total

241265

Generated in |η|<0.9 and p T >0.2GeV/c Both decay products are geometrically accepted No K/p interaction nor K decay Reconstructed Λ * (true pairs) Perfect PID case Reconstructed Λ * (true pairs) Real PID case

18200(100%) 13248(72.8%) 9625(52.9%) 5441(29.9%) 2026(11.1%) 24/6/2007 P.Ganoti, SQM2007@Levoca-Slovakia

Geometrical Acceptance Reconstruction Efficiency

20

Conclusions and Future Plans

 The number of reconstructed Λ(1520) in the Detectors PID case is about 63% lower than in the Perfect PID case. This means that we need better particle identification and the corresponding groups in ALICE work towards this direction. Detectors like ITS, TRD and HMPID will be included in the next data production.

In addition, from a preliminary study we expect an increase of the Λ(1520) signal of ~15% from kaon identification through their kink decays.  Study of other observables concerning Resonances Study such as - Transverse momentum and transverse mass spectra - Particle Ratios (Resonant over no Resonant particles) 24/6/2007 21 P.Ganoti, SQM2007@Levoca-Slovakia

Especially for the last observable, particle ratios in pp collisions will be the reference point for the corresponding ones in Heavy Ion collisions where we expect a decrease (or increase depending on the resonance lifetime) due to the stronger influence of the hot dense matter to the reconstruction of resonances. The project is co-funded by the European Social Fund and National Resources (EPEAEK II, PYTHAGORAS II) 24/6/2007 P.Ganoti, SQM2007@Levoca-Slovakia 22

Backup

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Results from STAR experiment

Particle ratio Δ(1232)/p K(892)/K Λ(1520)/Λ Φ(1020)/Κ pp STAR 0.72

± 0.108

0.389 ± 0.029

0.104 ± 0.03

0.124 ± 0.025

Au-Au STAR 0.96 ± 0.148

0.228 ± 0.044

0.033 ± 0.017

0.1556 ± 0.0311

Thermal model 0.68

0.32

0.071

0.13

The K(892)/K and the Λ(1520)/Λ suggest that rescattering is larger than regeneration.

The φ(1020)/Κ ratio is consistent with the thermal model predictions The Δ(1232)/p suggests regenaration.

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Theoretical Models

Some models have been written, trying to explain the yield of the various resonances “seen” by the experiments:  Rescattering of the decay products - Regenaration of the resonance

[UrQMD: Marcus Bleicher and Jörg Aichelin Phys. Lett. B530 (2002) 81. M. Bleicher and H. Stöcker .Phys.G30 (2004) 111].

 Rescattering of the decay products no Regeneration of the resonance

[Torrieri G and Rafelski J 2001 Phys.Lett.B 509 239-45]

 Broadening of the width of the resonance and mass modification

[Shuryak E V and Brown G E 2003 Nucl.Phys.A 717 322-35]

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