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The role of resonances production in heavy ion collisions and search for Lambda(1520) in ALICE 

The role of resonances production in heavy ion collisions and search for Lambda(1520) in ALICE . Paraskevi Ganoti University of Athens. Introduction Physics Motivation Rescattering, Regeneration and Theoretical Models Results from the STAR experiment (p-p and Au-Au collisions)

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The role of resonances production in heavy ion collisions and search for Lambda(1520) in ALICE 

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  1. The role of resonances production in heavy ion collisions and search for Lambda(1520) in ALICE  Paraskevi Ganoti University of Athens P.Ganoti

  2. Introduction Physics Motivation Rescattering, Regeneration and Theoretical Models Results from the STAR experiment (p-p and Au-Au collisions) Basic kinematical variables distributions from 100 Pb-Pb central ALICE simulated events Invariant mass distributions in the reconstruction level from 65 Pb-Pb events Conclusion and future plans Outline P.Ganoti

  3. Introduction • Resonances are particles with higher mass than the corresponding ground state particle with the same quark content. • Resonances decay strongly and therefore they have a short lifetime τ (few fm/c) and a broad Γ=ћ/τ . • Due to short lifetime they can only be measured by reconstruction of their decay products. • Some known Hadron Resonances are: • K(892), Δ(1232),Σ(1385),φ(1020), Λ(1520) P.Ganoti

  4. Physics Motivation • Resonances with their short lifetime and strong coupling to the dense and hot medium are suggested as a signature of the early stage of the fireball created in a heavy ion collision. The measurement of Λ(1520) together with other resonances may provide information about the early state of the expanding source in terms of the influence of the medium on the resonance. • Their study allows us to place limits on the chemical freeze-out temperature and the time between chemical and thermal freeze-out P.Ganoti

  5. Rescattering and Regeneration Life-time [fm/c] : ρ= 1.3 ++= 1.7 K(892)= 4.0 Σ(1385)= 5.7 Λ(1520)= 13  (1020)= 45 time space Hadronization: Hadrons are formed from quarks Rescattering π K- Signal lost Λ(1520) Pb-Pb p Κ- K- Λ(1520) p p Regeneration Signal measured Chemical freeze-out: end of inelastic interactions (particle yields) Thermal freeze-out: end of elastic interactions (particle spectra) P.Ganoti

  6. 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 Regenaration 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] P.Ganoti

  7. STAR results from p-p collisions Christina Markert, Hirschegg, Jan 16-22, 2005 Invariant mass — original invariant mass histogram from K- and p combinations in same event. — normalized mixed event histogram from K- and p combinations from different events. (rotating and like-sign background) Extracting signal: After Subtraction of mixed event background from original event and fitting signal (Breit-Wigner). P.Ganoti

  8. STAR results from Au-Au collisions Christina Markert, Hirschegg, Jan 16-22, 2005 STAR Preliminary Resonance over non-resonance particle ratios of heavy ion collisions compared to pp interactions may indicate signal yield changes due to rescattering and regeneration processes. 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. P.Ganoti

  9. Study of ALICE Pb-Pb simulated events • In the new versions of AliRoot (the ALICE Simulation and Analysis Framework) it is possible to generate Λ(1520) resonance. • 35 Λ(1520)s/event are generated with transverse momentum 0-12GeV/c and forced to decay (the number 35 has been extracted from model predictions of the Λ(1520)/Λ ≈ 0.03) • 100 Pb-Pb events have been generated(cocktail) involving the • central HIJING generator (0-5fm). • the generator of Λ(1520)s. • A fast generation was produced in order to study the properties of the generated Λ(1520)’s and their decay products (K, p). • The distributions of the basic kinematical variables of the individual particles (η, pT) as well as those concerning the pairs K-p are presented in the following P.Ganoti

  10. η of the generated Λ’s and their decay products Full generation in -8 < η < 8 η η η η η Restricted to -0.9 < η < 0.9 η P.Ganoti

  11. Geometrical Acceptance of Λ(1520) decay products η The Λs are considered to have been generated with -0.9 < η < 0.9 and the decay products are required to have been entered the same region. P.Ganoti

  12. pT of the generated Λ’s and their decay products Full generation in -8 < η < 8 pT(GeV/c) pT(GeV/c) pT(GeV/c) pT(GeV/c) pT(GeV/c) Restricted to -0.9 < η < 0.9 pT(GeV/c) P.Ganoti

  13. Variables characterizing the K-p pairs • In order to select the K-p combinations which become from the Λ(1520) decay in the reconstruction level, we study first the: • ratio proton momentum/kaon momentum • the decay angle between the two decay products in the generation level. This study gives an idea of the limits of the above variables that have to be imposed in the reconstruction level and therefore in the real experiment. P.Ganoti

  14. proton-kaon momentum ratio Full generation in -8 < η < 8 pproton(GeV/c) pproton(GeV/c) pproton/pkaon Pkaon(GeV/c) Restricted to -0.9 < η < 0.9 pproton/pkaon pkaon(Gev/c) P.Ganoti

  15. Opening angle between the two decay products Full generation in -8 < η < 8 cos(opening angle between the two decay products ) Restricted to -0.9 < η < 0.9 cos(opening angle between the two decay products ) P.Ganoti

  16. Study at the Reconstruction Level I 65 events (with the same characteristics as the previous 100 events) generated by the Alice Offline Team last September have been used in the reconstruction level in order to study the invariant mass distribution of the K-p pairs. In the case of Perfect PID(we know exactly which K, p become from Λ(1520) decays): P.Ganoti

  17. Study at the Reconstruction Level II In the case that we have the combined PID from the central detectors: Three steps have been followed using these “identified” K, p in each event: • Calculation of the invariant mass of all the K-p pairs in each event • Calculation of the invariant mass distribution considering that the particles come from different events (mixed events method) • Application of the appropriate cuts in the resulting two distributions and normalization at high invariant masses ( >1.7GeV/c2 ) where no signal is expected. P.Ganoti

  18. Before the event mixing All pairs from the same event m(GeV/c2 ) Pairs from Λ(1520) decays(perfect PID) m(GeV/c2 ) P.Ganoti

  19. Study of K-p pairs from the same event: By applying sequentially the cuts below the S/B improves by an order of magnitude: 1.6*10-5if no cuts are applied and 1.2*10-4 after the application of the last cut P.Ganoti

  20. Invariant mass distributions after the various cuts “Identified” K,p(combined PID) from the same event with p>0.2GeV/c All pairs -0.9 < η < 0.9 pproton / pkaon > 0.75 Opening angle > 0.7 m(GeV/c2 ) P.Ganoti

  21. “Identified” K,p from the same event and pair mixing Black line : all pairs after cuts Red line: Mixed event background m(GeV/c2 ) Is there the Λ(1520) signal? m(GeV/c2 ) P.Ganoti

  22. Conclusions and Future plans • In the 65 events used for the analysis the signal is very low and can be fairlydistinguished from the background Estimated sources of the problem are • low statistics • not satisfactory combined PID in the events used (reconstruction algorithms and detectors response functions have been improved since the production of these events) • The goal of the presented analysis was to prepare all the “machinery” needed in order to be ready when the next official simulated data-set of the experiment will be available (end of April) • Investigation of other cuts that might improve the S/B ratio • Comparison of the results with those from p-p events. The project is co-funded by the European Social Fund and National Resources P.Ganoti

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