Hiroshi Masui / Univ. of Tsukuba for the Collaboration JPS, 62 nd Annual Meeting

1. Measurement of Centrality dependence of identified hadron Elliptic Flow in sNN = 200 GeV Au+Au collisions at RHIC - PHENIX Hiroshi Masui / Univ. of Tsukuba for the Collaboration JPS, 62nd Annual Meeting JPS, Sep/24/2007

2. Why Elliptic Flow ? Z Y • Sensitive probe in the early stage of heavy ion collisions • Initial geometry overlap (eccentricity) • Initial density + EOS • System size (number of collisions) • Final momentum anisotropy (v2) X Reaction plane Py Pz Px JPS, Sep/24/2007

3. Universal Scaling of v2 ? • Universal scaling of v2 suggest • v2 for a variety of particle species at RHIC are determined through (, KET/nq) • Its magnitude is established in partonic level before hadronization QM2006 A. Taranenko At mid-rapidity JPS, Sep/24/2007

4. Motivation • Universal scaling of v2 has been observed by assuming •   v2 of non-identified charged hadrons • v2 F(KET/nq) • Questions • Are these assumptions really correct ? • The scaling of v2 is tested from central to mid-central. How about peripheral collisions ? • Study the validity of the scaling of v2 in a wide range of centrality for identified hadrons JPS, Sep/24/2007

5. PHENIX Experiment • Vertex, Centrality • BBC, ZDC • Event plane • BBC (full azimuth, || = 3.0 - 3.9)  Large rapidity interval • Tracking • DC, PC • PID • TOF (||</4, ||<0.35), t ~ 120 ps • , p : pT < 4 GeV/c, K : pT < 3 GeV/c JPS, Sep/24/2007

6. Centrality dependence of v2(pT) • v2 increase with increasing centrality • Similar mass ordering for all centrality JPS, Sep/24/2007

7. KET + NCQ scaling of v2 • Scaling holds for all centrality ? • Ratio of data to fit (next slide) JPS, Sep/24/2007

8. v2(Data)/v2(Fit) • Scaling works within systematic errors, except for low KET • Radial flow ? JPS, Sep/24/2007

9. Eccentricity • Eccentricity () is estimated by Glauber MC simulation •  value’s are not changed so much in b = 5 - 9 fm (10 - 50 % centrality). / ~ 10 % • Quite different for different definitions of  at central and peripheral Systematic error on  ~ 10 - 20 % JPS, Sep/24/2007

10. Eccentricity scaling • Scaled v2 with participant eccentricity • Start to break even in mid-central at high KET • Scaling works at low KET within systematic errors • Clear difference between central and peripheral • The difference of v2 is also observed for different eccentricity * Only statistical errors are shown * Systematic error on  ~ 10 - 20 % JPS, Sep/24/2007

11. Summary • Study the scaling of v2(pT) for identified hadrons in a wide range of centrality • KET + quark number scaling • Scaling holds within systematic errors, except for low KET • New RXNP detector could help us to • reduce systematic errors • add more statistics at peripheral events • Eccentricity scaling • Scaling breaks even in mid-central at high KET • works for low KET within systematic errors • Clear difference between central and peripheral • Suggest that v2    v2    f(Npart) JPS, Sep/24/2007

12. Back up JPS, Sep/24/2007

13. Elliptic Flow at RHIC • Substantial elliptic flow signals are observed for a variety of particle species at RHIC JPS, Sep/24/2007

14. Event plane resolution • Event plane resolution is determined by multiplicity and v2 • Maximum at mid-central • High multiplicity, small v2 at central • Low multiplicity, large v2 at peripheral JPS, Sep/24/2007

15. Terminology • Definition of eccentricity • std :Standard eccentricity • var (participant eccentricity part) • 2 (event plane eccentricity) : var, subtract auto-correlation event-by-event (an idea from ShinIchi) • Weighting • part (coll) • Calculated by weighting with Npart (Ncoll) distribution • Averaging • {2}  2 • Averaging of 2 over all events, then take square root • More natural definition like measured v2 • Total : 3  2  2 = 12 definitions JPS, Sep/24/2007

16. Eccentricity • Estimated by Glauber MC simulation • coll > part due to steeper Ncoll distributions compared to Npart JPS, Sep/24/2007