唐泽波 中国科学技术大学近代物理系

1. Resonance K* reconstruction and its rolein heavy-ion collisions 相对论重离子碰撞中 矢量介子K*的重建及其应用 唐泽波 中国科学技术大学近代物理系 • Why do we need resonance • How to reconstruct it • What can it tell us Zebo Tang, Lecture for Zhangbu's class

2. Kinetic Freeze-Out Chemical Freeze-Out Colliding QGP evolution What we can measure • How to study the property of the hot dense medium and its evolution? • What kind of probe we can use? A time scale compatible to the lifetime of the medium is needed Zebo Tang, Lecture for Zhangbu's class

3. Property of K* t=4 fm/c Zebo Tang, Lecture for Zhangbu's class

4. Re-Scattering vs. Re-Generation K K* K  K* Lost  Measured K K K*   K K K*   t Colliding hadronization begins Chemical freeze-out Thermal freeze-out Resonances produced at chemical freeze-out stage Daughter particle’s re-scattering effect destroys part of resonance signal Re-generation effect compensates on resonance yield Zebo Tang, Lecture for Zhangbu's class

5. 130 GeV nucl-ex/0206011 hadrons q q hadrons leading particle suppressed hadrons leading particle q q hadrons leading particle suppressed The high pT suppression behavior is different for KS0 and  Is this a mass effect or depending on particle species (meson vs. baryon) ? Hydrodynamic Model Quark Recombination Model K* and : mesons but their masses Close to  K* at high pT ( > 2 GeV/c ) are the same as individual hadrons K* RAA can distinguish this difference p+p Jet Quenching Au+Au 9 Zebo Tang, Lecture for Zhangbu's class

6. p K K p K* K* A typical Au+Au event at 200 GeV Primary vertex ~1000 tracks/particles produced within STAR acceptance in a central Au+Au event K* dN/dy ~ 10 in central Au+Au collisions at 200 GeV Zebo Tang, Lecture for Zhangbu's class

7. K* reconstruction • What we can detect is final stable particles • How to identify K*? • What properties does K* have? Zebo Tang, Lecture for Zhangbu's class

8. Pion and Kaon Identification |Nsp|<2 and |NsK|<2 Zebo Tang, Lecture for Zhangbu's class

9. Nothing! Why? Kaon Pion paring How to find the K-p pair from the same parent? No way! What should we do? Pair every pion with every kaon in the same event Zebo Tang, Lecture for Zhangbu's class

10. Random Combinatorial BG Reconstruction • Like-sign technique • Mixed-event technique • Rotate technique Zebo Tang, Lecture for Zhangbu's class

11. K+ p- K* Like-sign p+ uncorrelated K- uncorrelated Zebo Tang, Lecture for Zhangbu's class

12. K+ p- K* Rotate p-’ Remove correlation Zebo Tang, Lecture for Zhangbu's class

13. K+ p- K* Mixed-event uncorrelated p-’’ Zebo Tang, Lecture for Zhangbu's class

14. Invariant Mass Distribution Zebo Tang, Lecture for Zhangbu's class

15. Combinatorial BG subtracted Zebo Tang, Lecture for Zhangbu's class

16. BG reconstruction method comparison Zebo Tang, Lecture for Zhangbu's class

17. Y   K K* Y K X K K* K* K* X Source of residual background • Elliptic flow • Particle mis-identification • D0Kp etc. Zebo Tang, Lecture for Zhangbu's class

18. K* signal Zebo Tang, Lecture for Zhangbu's class J.Adams et al., PRC71(2005)064902

19. K* raw pT spectra Branching ratio corrected Zebo Tang, Lecture for Zhangbu's class

20. Efficiency×acceptance determination • Generate K*, flat y, f, pT distribution • Decay in GEANT, simulate STAR sub-detectors’ responses • Embed into a real event • Reconstruct embedded event • Associate MC tracks with reconstructed tracks • Run through embedded events, applied all of the cuts and get the counts of reconstructed K*, compare to MC K* counts Zebo Tang, Lecture for Zhangbu's class

21. Efficiency×acceptance vs. pT • Increase as increasing pT • Decrease as increasing multiplicity Zebo Tang, Lecture for Zhangbu's class

22. K* pT spectra Extract integrated yield dN/dy and inverse slope T Extrapolate to unmeasured pT range Exponential function: Zebo Tang, Lecture for Zhangbu's class

23. dN/dy • Increase with increasing energy • No system size dependence Zebo Tang, Lecture for Zhangbu's class

24. <pT> Higher in central collisions Re-scattering effect Close to proton, higher than Pion and Kaon Mass instead of particle type Zebo Tang, Lecture for Zhangbu's class

25. K*/K- Ratio • K* and K- have same quark content • Ratio in Au+Au collisions smaller than that in p+p collisions (res-cattering dominant over regeneration) • Decrease as increasing system size (increasing fireball lifetime) Zebo Tang, Lecture for Zhangbu's class

26. f/K* Ratio • K* and f have similar mass and same spin, different strangeness • K* re-scattering or strangeness enhancement or both Zebo Tang, Lecture for Zhangbu's class

27. Nuclear Modification Factor K* RAA @ pT<1.5 GeV/c  Lower than other particles  Rescattering Effect K* RAA @ pT>1.5 GeV/c  Close to KS0, different from   No strong mass dependence Zebo Tang, Lecture for Zhangbu's class

28. y Non-central Au+Au Collisions coordinate space x z x y Momentum space py px Elliptic Flow Reaction plane r Fourier expansion for particle azimuthal distribution in momentum space: Elliptic flow carries information at initial stage Multiple interactions and pressure gradient lead to the final observable elliptic flow Elliptic Flow v2: second Fourier coefficient 10 Zebo Tang, Lecture for Zhangbu's class

29. LOW INTERMEDIATE HIGH STAR PHENIX v2 vs. pT Low pT : affected by hydrodynamic flow Intermediate pT : may be related to quark matter anisotropy High pT : space emission Zebo Tang, Lecture for Zhangbu's class

30. Y   K K* pY K X K K* K* K* pX Momentum phase space Coordinate phase space K* v2 scaling Low pT: Resonance daughter particles’ re-scattering and re-generation effects depend on the fire ball shape in the coordinate space Resonance v2 compared to hadron v2 at low pT can tell the fireball shape in the coordinate space at late stage Intermediate pT: For directly produced K*, n =2 For regenerated K*, n=4 Zebo Tang, Lecture for Zhangbu's class

31. Event Plane Reconstruction Zebo Tang, Lecture for Zhangbu's class

32. Event Plane Distribution Zebo Tang, Lecture for Zhangbu's class

33. Extract v2 • In certain pT bin, get the same-event and mixed-event invariant mass distribution in several (f-y2) bins • Fit background subtracted invariant mass distribution, get K* yield N(f-y2,pT) • Fit N(f-y2,pT) with A*(1+2v2cos(2(f-y2))), get v2(pT) Zebo Tang, Lecture for Zhangbu's class

34. v2 results n=2.0±0.4, c2/ndf = 2/6 Zebo Tang, Lecture for Zhangbu's class

35. Spin alignment Zebo Tang, Lecture for Zhangbu's class

36. 1. Z.T.Liang and X.N.Wang, PRL 94 102301 (2005), PLB 629 (2005) 20-26 2. S.A.Voloshin, nuch-th/0410089 Global polarization in non-central A+A collisions In non-central A+A collisions • large initial orbital angular momentum of partonic system • quarks and anti-quarks will be polarized opposite to reaction plane • For vector mesons, spin density matrix elementρ00 should be 1/3 in unpolarized case. The deviation of ρ00 from 1/3 manifests the global polarization of vector mesons. Zebo Tang, Lecture for Zhangbu's class

37. vs. hadronization mechanism Two kinds of quarks and anti-quarks: 1) quarks and anti-quarks in QGP ---- Polarized; 2) created in accompanying process ----- higher pT and unpolarized. Three different hadronization scenario : 1) recombine quark and anti-quark in QGP; 2) recombine in QGP with those from accompanying process; 3) fragmentation of a fast quark/anti-quark from the QGP. Zebo Tang, Lecture for Zhangbu's class

38. K* is relatively short-lived (~4fm/c)vector meson. In the rest frame of K*, K/π distribute as: Event plane measurement • good Particle IDentification capability • large acceptance • uniform at both azimuth and polar directions Zebo Tang, Lecture for Zhangbu's class

39. Extract r00 Zebo Tang, Lecture for Zhangbu's class

40. r00 vs pT Consistent with 1/3, provide no evidence of global spin alignment Zebo Tang, Lecture for Zhangbu's class

41. Summary • Resonance is a unique/important tool to probe the property of the hot dense medium and its evolution Zebo Tang, Lecture for Zhangbu's class

42. Homework • 已知K*0Kp的分支比约100%，K*0K+p- 的分支比是多少？ • 从RAA和v2的测量中我们可以看出在STAR中KS和L的测量要比K*要好得多，为什么？ Zebo Tang, Lecture for Zhangbu's class