J/ Y azimuthal anisotropy relative to the reaction plane in Pb-Pb collisions at 158 AGeV/c

1. J/Y azimuthal anisotropy relative to the reaction plane in Pb-Pb collisions at 158 AGeV/c Francesco Prino INFN – Sezione di Torino for the NA50 collaboration Hard Probes 2008, Illa da Toxa, June 12th 2008

2. Physics motivation

3. OUT OF PLANE IN PLANE Possible sources of J/Y azimuthal anisotropy • Flow (= collective motion superimposed on top of the thermal motion) of c quarks • Conditions: • c quarks are early-thermalized • J/Y formed by c-c recombination at freeze-out • Not likely to occur at SPS energies • Anisotropic (=  dependent) J/Y absorption • cc break-up by QGP hard gluons • Wang, Yuan, PLB540 (2002) 62 • Zhu, Zhuang, Xu, PLB607 (2005) 207 • cc break-up by co-moving particles • Heiselberg, Mattiello, PRC60 (1999) 44902 - - -

4. Observable: v2 • Anisotropy in the observed particle azimuthal distribution due to correlations between the azimuthal angle of the outgoing particles and the direction of the impact parameter Elliptic flow coefficient

5. J/Y anisotropy : models • Charmonium break-up on hard gluons present in the deconfined medium • Gluon density azimuthally asymmetric in non central collisions due to the almond shaped overlap region • Sets in when the critical conditions for deconfinement are attained • Measuring J/Y v2 may help to constrain models aimed at explaining the anomalous suppression observed in Pb-Pb • Wang Yuan, Phys. Lett. B540 (2002) 62 • Zhu, Zhuang, Xu, Phys. Lett. B607 (2005) 207

6. central peripheral J/ azimuthal anisotropy (In-In, NA60) R. Arnaldi, QM2008 • Limited statistics(<30000 J/ events)prevents a fine binning in centrality/pT • Define2 broad centrality classes v2 consistent with zero for central events, v2 > 0 (2.3) for peripheral

7. Experimental setup

8. Pb beam Ebeam = 158 GeV/nucleon Beam detectors Pb target in vacuum thickness = 4 mm Centrality detectors EM calorimeter (1.1<hlab<2.3) Multiplicity Detector (1.1<hlab<4.2) Zero Degree Calorimeter (hlab>6.3) Muon spectrometer (2.7<hlab<3.9) toroidal magnet+MWPC+hodoscopes NA50 setup – year 2000 Study of muon pair production in Pb-Pb collisions

9. Reaction plane estimation • Electromagnetic calorimeter • Measures neutral (g and p0 ) ET • Made of Pb and scintillating fibers • Distance from target: 20 cm • Pseudorapidity coverage: 1.1 < lab < 2.3 • Azimuthal segmentation: 6 (60° wide) sextants • Radial segmentation: 4 rings • Event plane angle Yn= estimator of the reaction plane angle YRP • Exploit anisotropy of produced ET to estimate the reaction plane • n = Fourier harmonic, φi = central azimuth of sextant i, ETi = ET in sextant i

10. Event plane resolution - method 1 • Monte Carlo simulation of the calorimeter response • Needed input: • Measured v2 of particles in the calorimeter acceptance • Sextant-to-sextant fluctuations (via an effective MC parameter tuned on real data) • Result: • NOTE: event plane resolution in flow analyses usually expressed as: RCF =<cos[2(Y2-YRP)]> called resolution correction factor • RCF = 1 in case of perfect determination of YRP

11. Event plane resolution - method 2 • Sub-event method • Based on the correlation between the event planes calculated from two halves of the detector (Poskanzer, Voloshin, PRC58 (1998) 62) • Sub-events defined exploiting the radial segmentation of the calorimeter (rings) • Same energy collected in the 2 sub-events • Rapidity gap to limit non-flow correlation between the sub-events • Energy of the excluded ring taken into account when extrapolating to the full calorimeter resolution

12. Analysis techniques

13. Analysis strategy • Dimuons under the J/Y mass peak should be properly accounted • Other dimuon sources in J/Y mass range ≈ 5-15% (depending on centrality) • Two kinds of analysis • Azimuthal anisotropy from number of J/Y in bins of azimuthal angle • 2 different methods for subtraction of dimuons under the J/Y peak • Azimuthal anisotropy from Fourier coefficient v2 • 2 different methods to calculate v2

14. Number of J/Y in azimuthal bins Method 1 = “Fitting” • Build mass spectra of dimuons in bins of: • centrality (ET) • azimuthal angle relative to the event plane (DF2=Fdimu-Y2) • Fit to dimuon mass spectra • 5 components • J/Y, Y’, DY, open charm, combinatorial background • functional forms from Monte Carlo simulations with detailed description of the NA50 setup • 6 free parameters • 4 normalizations + J/Y mass and width • Limited by DY statistics.

15. Elliptic anisotropy from fitting • Slightly more J/Y’s observed “in plane” • Anisotropy quantified as:

16. Number of J/Y in azimuthal bins Method 2 = “Counting” • Build ET spectra of Opposite Sign dimuons with 2.9 < M < 3.3 GeV/c2 in bins of azimuthal angle relative to the event plane and subtract: • Combinatorial bckfrom Like Sign dimuons in 2.9 < M < 3.3 GeV/c2 • DYfrom Opposite Sign dimuons in M > 4.2 GeV/c2 • Open Charm from Opposite Sign dimuons in 2.2 < M < 2.6 GeV/c2

17. Elliptic anisotropy: counting vs. fitting • Good agreement between the 2 analysis methods • Counting analysis (not limited by low DY statistics) allows for more centrality bins

18. Estimate J/Y v2Method 1 = “Cosine spectra” • Build cos[2(Fdimu-Y2)] spectra of dimuons with 2.9<M<3.3 GeV/c2 in ET (pT) bins • Subtract combinatorial bck, DY and open charm cos[2(Fdimu-Yn)] spectra • Calculate v ’2 • Same formula as v2 but with the measured event plane Y2 instead of the unknown reaction plane YRP • Correct for event plane resolution: • |v2| >= |v’2|

19. J/Y v2 : from cosine spectra • Positive J/Y v2 • more J/Y’s exiting in plane • Errors: • Error bar = statistical error from J/Y v2’ • (Upper) band = systematic error from event plane resolution

20. Estimate J/Y vnMethod 2 = “Fit to NJ/Y in azimuthal bins” • Count dimuons in 2.9<M<3.3 GeV/c2 in bins of ET and DF2=Fdimu-Y2 • Subtract combinatorial bck, DY and open charm • Fit with: • dN/dDF2 =A[ 1 + 2 v’2 cos (2·DF2)] (2 free parameters) • Apply resolution correction factor to go from v’2 to v2 Bad quality fit

21. J/Y v2 : cosine spectra vs. fits • Errors: • Error bar = statistical error from J/Y v2’ • (Upper) band = systematic error from event plane resolution • Good agreement between the two analysis methods

22. Elliptic anisotropy and v2 vs. pT • No centrality selection • NOTES: • (NIN – NOUT) / (NIN + NOUT) = (p/4) · v2’ • Event plane resolution not accounted for in (NIN – NOUT) / (NIN + NOUT) • Elliptic anisotropy seems to increase with J/Y pT

23. Conclusions • J/Y anisotropy relative to the reaction plane measured in Pb-Pb collisions at 158 A GeV/c from NA50 experiment • Event plane from azimuthal distribution of neutral transverse energy • Event plane resolution extracted in 2 independent ways: • from Monte Carlo simulations • by adapting the sub-event method to the geometrical segmentation of the NA50 electromagnetic calorimeter • J/Y reconstructed from m+m- decay in the dimuon spectrometer • Elliptic anisotropy results: • Observed a positive (in-plane) J/Y v2 = more J/Y’s “in plane” than “out-of plane” • Observed anisotropy v2 slightly increasing with increasing J/Y pT

24. Backup

25. Flow from the NA50 calorimeter • Original method specifically developed for the sextant geometry of the NA50 calorimeter • Fourier expansion of ET azimuthal distribution: • limited to 6 terms because there are 6 sextants • an and bn coefficients calculated with Discrete Fourier Transform: • v2 calculated from a2 and b2 • b3 used to account for statistical fluctuations

26. Event plane flattening • Event plane azimuthal distribution should be flat (isotropic) • no preferential direction for the impact parameter • Acceptance correction • Flattening by weighting each sextant with the inverse of <ET> measured by that sextant (Poskanzer, Voloshin, PRC58 (1998) 62) • very small correction (practically uniform calorimeter azimuthal acceptance)

27. Y1 pions Proj. spect. nucleons x z pions Target spect. nucleons y Y2 x Event plane direction • Electromagnetic calorimeter in the backward rapidity region • v1 of pions in the backward region is positive (NA49, WA98) • pions flow in the opposite direction with respect to spectator nucleons • Event plane Y1 directed: • opposite to the direction of spectator nucleons in the backward hemisphere • along the direction of spectator nucleons in the forward hemisphere • v2 of pions in the backward region is positive (NA49, CERES) • event plane Y2 directed in plane

28. Directed anisotropy v1 • Unexpectedly large directed anisotropy • Excess of J/Y observed opposite to the event plane direction (i.e. negative v1) • Very large systematic error bar coming from the estimation of the resolution of the first order event plane • (Part of) this anti-correlation between J/Y and Y1 directions may be due to momentum conservation effect • Event plane Y1 not corrected for the momentum taken by the J/Y

29. Cross-checks: shifted ET bins • < cos[2(Fdimu-Y2)] > analysis repeated by shifting the ET bins by half a bin (open circles)

30. Cross-checks: EZDC centrality bins • EMCALO used both for event plane and for centrality determination • EZDC independent centrality estimator • Similar centrality dependence as from ET analysis • Exclude a bias from centrality selection From <cosDF> peripheral events ( high EZDC ) central events ( low EZDC )

31. Cross-checks: event-plane flattening • Different choices of flattening the event plane do not change significantly vn’ results From <cosDF> From <cosDF>

32. J/Y azimuthal distribution • J/Y azimuthal angle from reconstructed muon momenta • azimuthal distribution not flat due to acceptance effects