study of Λ (1520) production in pp interactions @14 tev with the alice detector

24/6/200724/6/2007 P.Ganoti, [email protected], SQM2007@Levoca-Slovakia 11

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

Paraskevi Ganoti

University of Athens

for the ALICE Collaboration


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


Introduction-Physics MotivationThe 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. 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.

Resonance Life-time [fm/c] 1.3++ 1.7 f0(980) 2.6 K*(892) 4.0 (1520) 13 ω(783) 23(1020) 45

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


Signal measured

Pb-Pb

Hadronization: Hadrons are formed from quarks

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

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

Λ(1520)p

πK-

Rescattering

Κ-

p

Λ(1520) K-

pRegeneration

Signal lost

space

time

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


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 pT > 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

Event Sample for Λ(1520) study


Λ(1520):Kinematic Properties

Generated Λ(1520)

241265 generated Λ(1520) with <pT> = 0.82GeV/c

Λ(1520)

<pT> = 0.82GeV/c

<pT> = 0.57GeV/c

<pT> = 0.37GeV/c


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

18200 generated Λ(1520) with <pT> = 1.28GeV/c

<pT> = 1.28GeV/c

<pT> = 0.87GeV/c

<pT> = 0.53GeV/c


Geometrical Acceptance

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

# Λ(1520) generated in |η| < 0.9 with pT > 0.2GeV/c

Mean Geometrical Acceptance : ~73%


K, p interactions with the detector

material

Reconstruction Efficiency I

p

K- (all)

K- interactions

K- decays

ITS TPC TRD TOF


Reconstruction Efficiency II

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

pK+ (all)

K+ interactions

K+ decays

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.


Reconstruction Efficiency III

Reconstructed Λ(1520) in |η| < 0.9 with pT, K,p > 0.2GeV/c assuming perfect pid

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

Mean Reconstruction Efficiency : 57%


Perfect PID

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

B.W. fit results

χ2 /ndf =15.17/9

MΛ(1520) = (1.520.00)GeV/c2

Γ = (0.016440.00039)

True Signal


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


B.W.+polynomial fit results

χ2 /ndf =199.2/6

MΛ(1520) = (1.520.00)GeV/c2

Γ = (0.019680.00051)

Extracted Signal

4533162 reconstructed Λ(1520) inside 2σ (in comparison with the 5441).

S/B = 0.42 and S/(S+B) = 36.5


Comparison of True - Extracted signals


B.W. fit results

χ2 /ndf =15.25/9

MΛ(1520) = (1.520.00)GeV/c2

Γ = (0.016610.00065)

True Signal

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

Detectors PID (TPC + TOF)


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


Extracted Signal

B.W.+polynomial fit results

χ2 /ndf =130.8/5

MΛ(1520) = (1.5230.000)GeV/c2

Γ = (0.03920.0012)

1951162 reconstructed Λ(1520) inside 2σ.

S/B = 0.16 and S/(S+B) = 16.4


Comparison of True - Extracted signals


Generated TotalGenerated Total 241265241265

Generated in |Generated in |η|<0.9 η|<0.9 and pand pTT>0.2GeV/c>0.2GeV/c 18200(100%)18200(100%)

Both decay products Both decay products are geometrically are geometrically

acceptedaccepted

13248(72.8%13248(72.8%))

No K/p interaction No K/p interaction nor K decaynor K decay 9625(52.9%)9625(52.9%)

Reconstructed Reconstructed ΛΛ** (true pairs) (true pairs) Perfect Perfect

PID casePID case5441(29.9%)5441(29.9%)

ReconstructedReconstructed Λ Λ** (true pairs) (true pairs) Real PID Real PID

casecase2026(11.1%)2026(11.1%)

Summarizing the numbers…

Geometrical Acceptance

Reconstruction Efficiency


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)


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)


Backup


Particle Particle ratioratio

pp STARpp STAR Au-Au STARAu-Au STAR ThermaThermal modell model

Δ(1232)/Δ(1232)/pp 0.720.72 ± ± 0.1080.108 0.96 0.96 ± ± 0.1480.148 0.680.68

K(892)/KK(892)/K 0.389 0.389 ± ± 00..029029 0.228 0.228 ± ± 0.0440.044 0.320.32

Λ(1520)/ΛΛ(1520)/Λ 0.104 0.104 ± ± 0.030.03 0.033 0.033 ± ± 0.0170.017 0.0710.071

Φ(1020)/ΚΦ(1020)/Κ 0.124 0.124 ± ± 0.0250.025 0.1556 0.1556 ± ± 0.03110.0311 0.130.13

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.

Results from STAR experiment


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]

study of Λ (1520) production in pp interactions @14 tev with the alice detector

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