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Gobinda C Mishra Georgia State University Atlanta, GA 30303, USA (For the PHENIX Collaboration)

Study of J/ y Polarization in pp Collisions at = 200 GeV with the PHENIX Experiment at RHIC. Gobinda C Mishra Georgia State University Atlanta, GA 30303, USA (For the PHENIX Collaboration) Quark Matter 2004 Oakland, CA January, 2004. Introduction.

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Gobinda C Mishra Georgia State University Atlanta, GA 30303, USA (For the PHENIX Collaboration)

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  1. Study of J/y Polarization in pp Collisions at = 200 GeV with the PHENIX Experiment at RHIC Gobinda C Mishra Georgia State University Atlanta, GA 30303, USA (For the PHENIX Collaboration) Quark Matter 2004 Oakland, CA January, 2004

  2. Introduction In this poster we present the status of our first attempt to extract the J/y polarization for p+p collisions at GeV at RHIC using PHENIX muon arm data. The limited statistics we have collected so far, does not allow to precisely extract the polarization parameter l. So here we will describe the procedure to extract l and estimate how much minimum J/y statistics needed to obtain statistically significant result.

  3. Motivation Why do we need to study polarization  This study is a crucial test for various quarkonium production mechanism as different model predicts different polarization. Various theoretical models for quarkonium production: color singlet model: Only pair in color singlet state can bind to form physical quarkonium. color octet model:at color octet state can bind to make physical quarkonium by emitting a soft gluon. color evaporation model:This model assumes any produced with small relative momentum can form a physical quarkonium by emission of soft gluons, irrespective of its color or angular momentum configuration

  4. NRQCD factorization approach including the color octet mechanism predicts that the directly produced J/ywill be increasingly transversely polarized at higher pt. This is because at high pt region production of J/yis dominated by gluon fragmentation and gluon transverse polarization is preserved as the pair turns to a bound state J/y meson. The analysis of J/y and y’polarization at large pt is one of the most decisive test of NRQCD factorization approach. hep-ph/0106120 • Color evaporation model predicts that J/y is unpolarized, because it assumes unsuppressed gluon emission from pair during hadronization, which randomizes spin and color. • CDF result doesn’t agree with the NRQCD prediction at higher pt >12 GeV/c neither for J/y nor y’ ( pp at 1.8TeV ) PRL 85 (2000) 2888

  5. One of the Fermilab experiments with p + N at 252 GeV observed that J/yis unpolarized in most of the kinematics regions, but tends to be longitudinally polarized as xF1 PRL 58 (1987)2523 • It is claimed that the change of spin alignment of J/y at large xF involves higher twist effect (gluon exchange interactions between annihilating quarks and spectator quarks is responsible for this), assuming annihilation mechanism dominate near xF = 1 where as gluon-gluon fusion dominate at low and moderate xF.

  6. NRQCD Prediction for Cross-Section NRQCD formalism with color octet model appears to explain differential cross-section result reasonably well. hep-ph/0106120

  7. NRQCD Prediction for Polarization There is large discrepancy in polarization between the data and model prediction. Data are from CDF.hep-ph/0106120

  8. Angular Distributions • Experimentally one studies the polarization through angular distribution of decay muons. • Most general form of angular distribution is given as • The q dependence of cross-section gives only l and is comparatively easier to measure than f dependence. • q is the polar angle between one of the muons (m+) in J/y rest frame. There are various ways one can choose axes for the J/y rest frame. We have chosen the Collins-Soper (CS) frame where the z-axix is along the bisector direction of colliding hadrons.

  9. J/y Rest Frame (Collins-Sopper Frame) Formula to calculate cosq in Collins-Soper frame from measured kinematics in lab frame

  10. = 0.5 = -0.5 = 1 = 0 = -1 = 1 (Transverse Polarization) = -1 (Longitudinal Polarization) = 0 (No polarization) cos q Distribution for Different Cases

  11. PHENIX Muon Arm North Muon Arm Acceptance: 1.2 <h<2.4 South Muon Arm Acceptance: -2.2 <h<-1.2

  12. Data Selection We have used ~4 M triggered events in north arm and ~1.1 M triggered events in south arm for this analysis. These correspond to 184 nb-1 and 208 nb-1 integrated luminosity for north and south arm respectively. The number of J/y obtained from these data sample are 248 and 104 after background subtraction and with pT > 0.5 GeV/c in north and south arm respectively. The data were divided into two pT bins to study the pT dependence of J/y polarization.

  13. Mass Distribution in two pt bins North Arm These mass distributions are after a pT cut of 0.5 GeV/c South Arm

  14. Analysis Procedure To get the true angular distribution (cosq distribution) we have to do acceptance correction. The cosq acceptance heavily depends other kinematics variables, viz. pT and xF. Monte Carlo (MC) events with same pT, xF as in data were used to correct for the acceptance for cosq distribution from data. We employ two step acceptance correction technique to get correct cosq acceptance. In first step, we generate a set of Monte Carlo events with flat pT, xF, f and cosq. Use these events to correct pT and xF distribution obtained from real data. The resultant pT and xF distributions were then used in second set of Monte Carlo events to get true cosq acceptance.

  15. Analysis Procedure (Contd.) The data have been divided into 2 pt bins: 0.5-2 GeV/c and 2-4 GeV/c to study the pT dependence. The pT<0.5 GeV/c was excluded from analysis because of pT resolution. The number of J/y at two pT bins from north arm are 197 and 71 respectively, and from south arm 77 and 27 respectively after 0.5 GeV/c pT threshold. The cosq acceptance for each pT bin and each arms are shown in following figures. Because of very low statistics, we have not taken the ratio for now. Rather, we have estimated the statistical accuracy of extracted polarization parameter by using a pure J/y events in a perfect detector.

  16. Uncorrected cosq Dist. North Arm South Arm

  17. Uncorrected background subtracted cosq Distribution North Arm Bin by bin Background Subtraction South Arm

  18. cosq Acceptance Distribution. North Arm South Arm

  19. Statistical Error Estimation Here different number of simulated J/y data events were generated in PHENIX muon arm acceptance, and were corrected for acceptance using very high statistics Monte Carlo events. The acceptance corrected cosq distributions were fitted by 1+lcos2q. The resulting ls with errors are plotted vs number of simulated data events. The two red points were obtained using the procedure described above, but the blue points were obtained for the cases where the simulated data and Monte Carlo events have exactly same input pT and xF distribution.

  20. Conclusions and outlook • The current J/y polarization study from 200 GeV p+p collisions with PHENIX muon arm data suffers from low J/y statistics. • A pure statistical study indicates that to get reliable polarization measurement we should have ~5000 or more J/y, which we hope to get in future RHIC runs. See the poster on J/y polarization study for d+Au Collisions at RHIc by X. R. Wang (Flavor 8)

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