A NLO analysis on fragility of dihadron tomography in high energy AA collisions

The dihadron spectra in high energy AA collisions are studied within the NLO pQCD parton model with jet quenching taken into account. The high pT dihadron spectra are found to be contributed not only by jet pairs close and tangential to the surface of the dense matter but also by punching-through jets survived at the center while the single hadron high pT spectra are only dominated by surface emission. Consequently, the suppression factor of such high-pT hadron pairs is found to be more sensitive to the initial gluon density than the single hadron suppression factor. One of the most exciting phenomena observed[1] at the Relativistic Heavy ion Collider (RHIC) is jet quenching[2]—a hard probe of a strongly-interacting quark gluon plasma in high energy heavy ion collisions. The observed suppression of large pT hadron spectrum is caused by the total parton energy loss which is related to the average gluon density along the jet propagation path and the total propagation length[3]. Therefore, measurements of large pT hadron suppression can be directly related to the averaged gluon density. Here we will employ a NLO pQCD parton model[4] to study the suppression of both single and dihadron spectra due to jet quenching. Different from the previous LO study[3], because the number ratio of gluon/quark jets is larger in NLO than in LO calculation and the energy loss of a gluon jet is assumed to be 9/4 larger than that of a quark jet, NLO contribution will behave with stronger quenching effect than LO contribution (see Fig. 4, R AA < R LO AA, I NLO AA < I LO AA). In particular, we will check the robustness of back-to-back dihadron spectra as a probe of the initial gluon density when single hadron spectra supression become fragile[5]. Within a NLO pQCD parton model [4], large pT particle production cross section in N +N collisions can be expressed as a convolution of NLO parton-parton scattering cross sections, parton distributions inside the collided nucleons and parton fragmentation functions (FF). In order to study large pT particle production in A + A collisions, one can extrapolate N + N cross section to A + A collisions. The effect of jet quenching in A + A collisions is incorporated via the modified jet fragmentation functions due to radiative parton energy loss in dense medium [3, 6]. The modified jet fragmentation A NLO analysis on fragility of dihadron tomography in high energy AA collisions 2 4 6 8 10 12 14 16 18 10 10 10 10 10 10 PHENIX Pre. 200GeV p+p (scaled) AuAu 0 0-10% pT(GeV) pp NLO =0.9, 1.2, 1.5 pT pp LO =1.2pT AuAu NLO 0-10% =1.2pT 0=1.08, 1.28, 1.48, 1.68, 1.88, 2.08, 2.28 1/ 2 p T d 2 N /d p T dy | y= 0 ( G eV -2 ) 4 6 8 10 12 14 16 18 0.0 0.2 0.4 0.6 0.8 1.0 R A A PHENIX Preliminary pQCD NLO =1.2pT 0=1.08, 1.28, 1.48, 1.68, 1.88, 2.08, 2.28 200GeV AuAu 0-10% pT (GeV) Figure 1. The single π spectra (left) in p+ p and in central Au+Au collisions, and the nuclear modification factors (right). The data are from Ref.[7]. 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 10 10 10 STAR 200GeV d+Au x2 AuAu 20-40% AuAu 0-5%