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Oana Catu, Yale University

System size dependence of dihadron correlations and fragmentation functions in Cu+Cu and Au+Au at √s=200 GeV. Oana Catu, Yale University for the STAR Collaboration. Quark Matter 2008, February 4-10, Jaipur, India.

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Oana Catu, Yale University

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  1. System size dependence of dihadron correlations and fragmentation functions in Cu+Cu and Au+Auat √s=200 GeV Oana Catu, Yale University for the STAR Collaboration Quark Matter 2008, February 4-10, Jaipur, India

  2. H.Zhang, J.F. Owens, E. Wang, X.N. Wang - Phys. Rev. Lett. 98: 212301 (2007) MOTIVATION • Study medium-induced modification of fragmentation function due to energy loss • Without full jet reconstruction, jet/parton energy not measurable → z not measured (z=phadron/pparton ) • Di-hadron fragmentation function =ratio of di-hadron jet-like correlated yield to single hadron yield (for experimental data d-Au data used as substitute of p-p) zT=pTassoc/pTtrig Oana Catu Quark Matter 2008, February 4-10,Jaipur, India

  3. (IAA) H. Zhang, J.F. Owens, E. Wang, X.N. Wang – Phys. Rev. Lett. 98: 212301 (2007) 2.8 ± 0.3 GeV2/fm MOTIVATION (II) • Back-to-back di-hadron spectra - more sensitive to initial gluon density than single spectra • smaller surface bias Distribution of points of origin of di-hadron pairs and single hadrons, respectively y (fm) y (fm) x (fm) x (fm) T. Renk and Eskola- Phys. Rev. C75: 054910 (2007) • Comparing STAR di-hadron and single hadron spectra with model calculations - extract the transport coefficient • Test model prediction in different size system – Cu-Cu → constrain path length dependence Oana Catu Quark Matter 2008, February 4-10,Jaipur, India

  4. pTtrig=3-6 GeV/c, 1.5 GeV/c <pTassoc< pTtrig jet ridge Dh Df Au+Au central @ 200 GeV STAR ANALYSIS TECHNIQUE – NEAR SIDE JET • -  histograms of particles correlated with trigger particle • mixed events technique to correct for detector effects • non-uniform efficiency in  • limited acceptance in → pair acceptance in  (“triangle”) • In Au-Au near-side (  0 ) correlations of particles with large  separation observed – “ridge” Oana Catu Quark Matter 2008, February 4-10,Jaipur, India

  5. Jet+Ridge +Flow Ridge+ Flow 2  1  2  1  2 Ridge + flow jet Ridge + flow ANALYSIS TECHNIQUE – NEAR SIDE JET (II) • 2 methods: projecting onto ,  1) PROJECTION ON  • subtract ridge - construct  correlations for particles with || < 0.7 and subtract correlations with 0.7< || < 1.7. 2) PROJECTION ON  • Project jet  range (|  |<0.78) • assuming ridge flat in  - subtract flat background Oana Catu Quark Matter 2008, February 4-10,Jaipur, India

  6. Background subtracted (black minus red) NEAR-SIDE JET – ΔΦ METHOD CuCu 0-10% 4< pT trig < 6 GeV , pT assoc > 3 GeV •|| < 0.7 •0.7<|| < 1.7 • normalize large Δη correlations to match the small Δη correlations in the away-side region • method subtracts uncorrelated background pairs, ridge and elliptic flow – assumed constant in Δη • add content of bins in ||<0.78 range of background-subtracted correlation plots → jet yield Oana Catu Quark Matter 2008, February 4-10,Jaipur, India

  7. NEAR-SIDE JET - η METHOD CuCu 0-10% 4< pT trig < 6 GeV , pT assoc > 3 GeV • Used projection in Δη to extract yield ( also a consistency check) • Subtract flat background (uncorrelated background correlations, “ridge” ) • Get jet yield by bin-counting in |Δη| < 0.7 Oana Catu Quark Matter 2008, February 4-10,Jaipur, India

  8. NEAR-SIDE JET YIELD DEPENDENCE ON Npart 4< pT trig < 6 GeV 6< pT trig < 10 GeV pT assoc >3 GeV • jet yield consistent in the two methods • Near-side jet yield consistent in Cu-Cu and Au-Au for same Npart • Jet yield independent of Npart - unmodified vacuum fragmentation? Oana Catu Quark Matter 2008, February 4-10,Jaipur, India

  9. NEAR SIDE DI-HADRON FRAGMENTATION FUNCTIONS 4< pT trig < 6 GeV 6< pT trig < 10 GeV • Near-side di-hadron fragmentation functions in Au-Au and Cu-Cu – similar to d-Au → consistent with parton fragmenting in vacuum (it still allows energy loss prior to fragmentation) Oana Catu Quark Matter 2008, February 4-10,Jaipur, India

  10. ANALYSIS TECHNIQUE – AWAY SIDE AuAu 40-80%,4< pT trig < 6 GeV , pT ass > 3 GeV • Project -  histograms on  summing  range ||<1.7 - For consistency with previous studies - no  (triangle) correction • determine background level at ||~1 (ZYA1) • Construct flow modulated background • jet yield – in  region 1.7-4.4 - add bin content and subtract area under red curve (best estimate v2 ) • get syst errors - subtract area under green and blue curves (higher and lower limit on v2) Oana Catu Quark Matter 2008, February 4-10,Jaipur, India

  11. 6< pT trig < 10 GeV pT assoc >3 GeV values are given for 0-10% AuAu and CuCu 2.8 ± 0.3 GeV2/fm • modified fragmentation model – describes Cu-Cu vs Au-Au but slightly misses shape • PQM – does not describe relationship between Cu-Cu and Au-Au yields with the chosen values; misses shape AWAY-SIDE SUPPRESSION - DEPENDENCE ON Npart 4< pT trig < 6 GeV pT assoc > 3 GeV 6 < pT trig < 10 GeV Modified frag: nucl-th/0701045 - H.Zhang, J.F. Owens, E. Wang, X.N. Wang PQM: C. Loizides, Eur. Phys. J. C 49, 339-345 (2007) • Away side jet yield consistent in Cu-Cu and Au-Au for the same Npart • models fail to describe magnitude of suppression for the lower pT trig Oana Catu Quark Matter 2008, February 4-10,Jaipur, India

  12. Theory - X.N.Wang private communication H.Zhang, J.F. Owens, E. Wang, X.N. Wang - Phys. Rev. Lett. 98: 212301 (2007) 2.8 ± 0.3 GeV2/fm AWAY SIDE DI-HADRON FRAGMENTATION FUNCTIONS 6< pT trig < 10 GeV COMPARE TO MODIFIED FRAGMENTATION MODEL • di-hadron frag functions suppressed but shape not modified in the considered zT range– consistent with vacuum fragmentation after energy loss • comparison to modified fragmentation model - use parameters fitted to previous • Au-Au data to predict Cu-Cu – match data • less suppressed in Cu-Cu than central Au-Au – Npart dependence? Oana Catu Quark Matter 2008, February 4-10,Jaipur, India

  13. Npart ≈ 100 Npart ≈ 100 Npart ≈ 20 Npart ≈ 20 Loizides – private comunication AWAY SIDE DIHADRON FRAGMENTATION FUNCTIONS (II) 6< pT trig < 10 GeV • Investigate Npart dependence of di-hadron fragmentation function • consistent in Au-Au and Cu-Cu for similar Npart • trends observed in the away-side yields – verified in di-hadron fragmentation functions • Average path length of all partons (geometric path length) – quite similar in the two systems for same Npart • path length dependence of surviving partons • - surface bias – different ? • - model dependent Loizides – private comunication Oana Catu Quark Matter 2008, February 4-10,Jaipur, India

  14. CONCLUSIONS • Systematic study of jet-like di-hadron correlations in 3 systems • Near-side • jet yield same in heavy ion collisions as in d-Au – consistent with vacuum frag in the near side • same behavior in Cu-Cu and Au-Au for same Npart • Away side suppression • increases with Npart • same behavior in Cu-Cu and Au-Au for same Npart • not expected in PQM • modified frag funct model – describes relationship but misses shape • Di-hadron frag funct shape not modified for considered zT range – consistent with vacuum fragmentation after energy loss • agreement with theory for higher pT triggers Available data would allow to quantitatively constrain the models Oana Catu Quark Matter 2008, February 4-10,Jaipur, India

  15. Oana Catu Quark Matter 2008, February 4-10,Jaipur, India

  16. PQM • qhat = product of parton-medium cross-section * color-charge density • BDMPS quenching weights – only coherent radiative energy loss via gluon bremsstrahlung • Realistic geometry • Static • Avg qhat -> time-dependent density = scheme dependent\ • No initial state multiple scattering • No modified nuclear parton distribution functions Oana Catu Quark Matter 2008, February 4-10,Jaipur, India

  17. MODIFIED FRAG FUNCTION • Longitudinally expanding medium • Hard spheres • PDFs factorized into nucleon ones and HIJING parameterization of the impact parameter dependent nuclear modification factor • radiative gluon energy loss – incorporated in effective medium modified FF Oana Catu Quark Matter 2008, February 4-10,Jaipur, India

  18. GLV • Realistic geometry • Bjorken expanding medium • Radiative energy loss – gluon brehmsstralung to all orders in opacity • Calculate a priori (w/o en loss) the average path length to the edge – use it to calculate partonic energy loss Oana Catu Quark Matter 2008, February 4-10,Jaipur, India

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