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

2. Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 2 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=p hadron /p parton ) Di-hadron fragmentation function = ratio of di-hadron jet-like correlated yield to single hadron yield z T =p Tassoc /p Ttrig (for experimental data d-Au data used as substitute of p-p) H.Zhang, J.F. Owens, E. Wang, X.N. Wang - Phys. Rev. Lett. 98: 212301 (2007)

3. Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 3 MOTIVATION (II) Back-to-back di-hadron spectra - more sensitive to initial gluon density than single spectra smaller surface bias Test model prediction in different size system – Cu-Cu → constrain path length dependence Comparing STAR di-hadron and single hadron spectra with model calculations - extract the transport coefficient y (fm) x (fm) T. Renk and Eskola- Phys. Rev. C75: 054910 (2007) H. Zhang, J.F. Owens, E. Wang, X.N. Wang – Phys. Rev. Lett. 98: 212301 (2007) 2.8 ± 0.3 GeV 2 /fm (I AA ) y (fm) x (fm) Distribution of points of origin of di-hadron pairs and single hadrons, respectively

4. Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 4 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”   p T trig =3-6 GeV/c, 1.5 GeV/c <p T assoc < p T trig Au+Au central @ 200 GeV jet ridge STAR

5. Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 5 Ridge + flow   Ridge + flow jet     Jet+Ridge +Flow Ridge+ Flow 1 2 2 1 2 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 ANALYSIS TECHNIQUE – NEAR SIDE JET (II)

6. Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 6 NEAR-SIDE JET – ΔΦ METHOD CuCu 0-10% ● normalize large Δη correlations to match the small Δη correlations in the away- side region 4 3 GeV Background subtracted (black minus red) |  | < 0.7 0.7<|  | < 1.7 ● 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

7. Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 7 ● 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 CuCu 0-10% 4 3 GeV NEAR-SIDE JET -  η METHOD

8. Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 8 NEAR-SIDE JET YIELD DEPENDENCE ON N part 4< p T trig < 6 GeV6< p T trig < 10 GeV p T assoc >3 GeV ● jet yield consistent in the two methods ● Near-side jet yield consistent in Cu-Cu and Au-Au for same N part ● Jet yield independent of N part - unmodified vacuum fragmentation?

9. Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 9 NEAR SIDE DI-HADRON FRAGMENTATION FUNCTIONS 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) 4< p T trig < 6 GeV6< p T trig < 10 GeV

10. Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 10 ANALYSIS TECHNIQUE – AWAY SIDE 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 v 2 ) get syst errors - subtract area under green and blue curves (higher and lower limit on v 2 ) AuAu 40-80%,4 3 GeV

11. Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 11 AWAY-SIDE SUPPRESSION - DEPENDENCE ON N part 4< p T trig < 6 GeVp T assoc > 3 GeV PQM: C. Loizides, Eur. Phys. J. C 49, 339-345 (2007) Modified frag: nucl-th/0701045 - H.Zhang, J.F. Owens, E. Wang, X.N. Wang 6 < p T trig < 10 GeV values are given for 0-10% AuAu and CuCu 2.8 ± 0.3 GeV 2 /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 jet yield consistent in Cu-Cu and Au-Au for the same N part ● models fail to describe magnitude of suppression for the lower p T trig 6< p T trig < 10 GeV p T assoc >3 GeV

12. Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 12 AWAY SIDE DI-HADRON FRAGMENTATION FUNCTIONS 6< p T trig < 10 GeV di-hadron frag functions suppressed but shape not modified in the considered z T 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 – N part dependence? COMPARE TO MODIFIED FRAGMENTATION MODEL 2.8 ± 0.3 GeV 2 /fm Theory - X.N.Wang private communication H.Zhang, J.F. Owens, E. Wang, X.N. Wang - Phys. Rev. Lett. 98: 212301 (2007)

13. Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 13 AWAY SIDE DIHADRON FRAGMENTATION FUNCTIONS (II) Investigate N part dependence of di-hadron fragmentation function consistent in Au-Au and Cu-Cu for similar N part 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 N part path length dependence of surviving partons - surface bias – different ? - model dependent N part ≈ 100 N part ≈ 20 N part ≈ 100 N part ≈ 20 6< p T trig < 10 GeV Loizides – private comunication

14. 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 N part Away side suppression increases with N part same behavior in Cu-Cu and Au-Au for same N part –not expected in PQM –modified frag funct model – describes relationship but misses shape Di-hadron frag funct shape not modified for considered z T range – consistent with vacuum fragmentation after energy loss –agreement with theory for higher p T triggers Available data would allow to quantitatively constrain the models

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

16. 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

17. 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

18. 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