1. 1 Nuclear modification and elliptic flow measurements for  mesons at  s NN = 200 GeV d+Au and Au+Au collisions by PHENIX Dipali Pal for the PHENIX collaboration Vanderbilt University

2. 2 Outline Motivation  meson spectra at different centralities Nuclear modification factors for the  mesons Elliptic flow of the  mesons Summary and outlook

3. 3 Baryon/meson puzzle  Scaling properties of yields  Different suppression  Elliptic flow Quark number scaling of the elliptic flow parameter v 2 “Mass effect” or “baryon/meson effect”? Au + Au @ √s NN = 200 GeV PHENIX Motivation: Baryon/meson anomaly  meson has a mass similar to a proton.  Appropriate probe to address the baryon/meson puzzle.

4. 4   K + K - measurement in PHENIX Number of events analyzed: Spectra: 409 M for TOF 170M for PbSc Elliptic flow: 800 M  separation in TOF  0.3 < p (GeV/c) < 2.5  separation in EMCal: 0.3 < p(GeV/c) < 1.0 Four independent K + K - pairing TOF – TOF (9% of the total  ’s) TOF – PbSc (East) (27% of the total  ’s) PbSc(East) – PbSc(East) (9% of the total  ’s) PbSc(West) – PbSc(West) (55% of the total  ’s)  Allows a self-consistent measurement on . K+K- pairing topology

5. 5  meson reconstruction technique Combinatorial background (CB) is estimated by event mixing. Normalized to 2  (N ++.N -- ), N ++ and N – are like sign measured yields. Signal = Same event - CB Experimental mass resolution ~ 1 MeV/c 2  Better than  mass width. Subtracted spectrum is fitted with Relativistic Breit Wigner convolved with Gaussian  mass resolution. Centroid and width are consistent with PDG. Number of  mesons analyzed Spectra: ~ 44K Elliptic flow :~ 180K = 1.01891 ± 0.00003 (stat) ± 0.00085 (syst) GeV/c 2  = 4.22 ± 0.09 (stat) ± 0.506 (syst) MeV/c 2 Subtracted spectrum

6. 6 Raw yield extraction  N  (rec) (m T ) = Same event (m T ) – CB(m T ) Yield is extracted by integrating the subtracted mass spectrum over a fixed mass window of ± 5 MeV with respect to the centroid.  Optimized signal and S/B ratio. Corrections –Acceptance: K + K - pair MC through PHENIX simulation chain. –Efficiency: time (experimental run) dependent variations. –Occupancy dependent corrections: Embedding simulated   K + K - pairs into the real data. Spectra: raw yields to absolutely normalized spectra

7. 7 Minimum-bias spectra Excellent agreement between the subsystems Understanding of the systematics. Run4 result is consistent with Run2.

8. 8  meson spectra at different centralities Talk by A. Kozlov [6(b)] Poster by D. Pal (158) Centrality dN/dy T (MeV) MB 1.08 ± 0.04 ± 0.20 388 ± 5 ± 27 0-10% 3.80 ± 0.30 ± 0.72 372 ± 11 ± 26 10-20% 2.32 ± 0.16 ± 0.44 394 ± 10 ± 27 20 – 30% 1.62 ± 0.11 ± 0.31 397 ± 10 ± 28 30 – 40% 0.95 ± 0.07 ± 0.18 401 ± 10 ± 28 40 – 50% 0.75 ± 0.04 ± 0.13 377 ± 8 ± 26 50 – 60% 0.35 ± 0.03 ± 0.06 392 ± 12 ± 27 60 – 90% 0.11 ± 0.01 ± 0.02 348 ± 11 ± 24 0.0074 ± 0.0007 ± 0.0020 391 ± 25 ± 50 pp Au + Au @ 200 GeV

9. 9 Scaling of protons and  spectra N coll scaled  spectra vs pT compared to protons  Radial Flow at low-p T  At intermediate p T, (anti)protons scale with N coll  No N coll scaling for    Baryon/meson effect? Or mass effect? Quantify nuclear effects by Central-to-peripheral ratios (Rcp) and ratio of Au-Au central to pp yields (R AA ).

10. 10 Two extreme centrality classes show Completely different scales of suppression. Mesons (  and   ) in Au+Au 0 – 10% (most central) are suppressed to the same extent. They are least (or almost not) suppressed in 60-90% (most peripheral) Protons are not suppressed anywhere.  Baryons and mesons show a clear difference. What about other centralities? Suppression of the mesons decreases from central to peripheral. Two extreme centrality classes show completely different scales of suppression. Mesons (  and   ) in Au+Au 0 – 10% (most central) are suppressed to the same extent. They are least (or almost not) suppressed in 60- 90% (most peripheral) Suppression of mesons increases from peripheral to central. Protons are not suppressed anywhere.  Baryons and mesons show a clear difference. Nuclear modification factors, R AA

11. 11 Nuclear modification factor, R cp Au + Au collisions show suppressions for mesons(  and   ) and no suppression for the protons and  ’s. d+Au collisions (cold nuclear matter) shows no suppression for baryons or mesons.  The anomalous meson suppression is a property of the hot and dense matter. d + Au @ √s NN = 200 GeV Au + Au @ √s NN = 200 GeV Poster by D. Pal (158), D. Mukhopadhyay (154)

12. 12 Elliptic Flow of baryons and mesons At low p T hydro works remarkably well Above ~ 2 GeV/c : a split between mesons and baryons Universal behavior in flow per quark: expected from recombination Need to measure v 2 of 

13. 13 Event reaction plane is determined by beam beam counter -- BBC South and BBC north  Peripheral Central  100 – centrality(%)  Peripheral Central  Reaction plane resolution:  sqrt  cos2    BBCS    BBCN  Reaction plane Extraction of uncorrected v 2 from azimuthal distribution Reaction plane resolution correction factor  v 2 = v 2 (obs).  v 2 measurement in PHENIX

14. 14 v 2 extraction of  Azimuthal distribution of  mesons Fitted with the function: dN/d(   )=A [1+2v 2 (obs)cos2(  )] The p1 parameter in the figure is the uncorrected v 2. v 2 (obs) v 2 = R.P. Resolution correction 1.0 < p T (GeV/c) < 1.5 1.5 ≤p T (GeV/c) < 2.0 2.0 ≤p T (GeV/c) < 3.0 v 2 (obs) = 0.0250 +/- 0.0137 v  (obs) = 0.0338 +/- 0.0091 v 2 (obs) = 0.0198 +/- 0.0071 Minimum bias

15. 15 v 2 vs p T Non-zero v 2 observed for  mesons Statistically, it is consistent with other hadrons. Minimum bias With present error bar, the quark number-scaled  meson v 2 is consistent with other hadrons. Talk by H. Masui, Poster by A. Taranenko (identified hadron v 2 ) PHENIX Preliminary

16. 16 Summary PHENIX has measured  mesons in K + K - decay channel with its full central arm.   K + K - spectra at seven centrality bins have been measured within 1.2 < m T (GeV/c 2 ) < 4.4. Nuclear modification factors, R cp and R AA in Au-Au collisions exhibits dramatic suppression of  ’s like other mesons. R cp in d-Au clearly demonstrate absence of any suppression for the  mesons. v 2 of  has been measured for the first time. It is non- zero for intermediate p T. v 2 of  scaled with number of quark follows universal quark number scaling within statistical errors.

17. 17 Outlook Considerable improvements expected over the next few months - A factor of 4 increase in statistics for spectra analysis: finer p T bins and a wider range - Measurement of v 2 as a function of centrality - Fine tuning of the cuts and additional statistics for elliptic flow analysis may enable us to increase statistical significance of the v 2 signal.

18. 18 Backup slides

19. 19 v 2 analysis methods Method 1: dN/d(    = A(1+2v 2 (obs)cos2(  –  2 )) -- Fit azimuthal distribution of  with this function with v 2 as a fitting parameter. Method 2: v 2 (obs) = for the  mesons. Methods 1 and 2 are mathematically equivalent. Method 3: Mesure v 2 from the azimuthal distributions of the same event and CB and then extract uncorrected v 2 of  as: v 2 (obs) = [N S+B v 2 (S+B) – N B v 2 (B)]/N S S+B = Signal + CB v 2 = v 2 (obs)/   = Reaction plane resolution = 1/sqrt(2 ) Three methods have extracted the same v2 value for Phi mesons. Method 1 has been applied to the full statistics of the data.

20. 20   KK in pp collisions: Comparison with STAR