Abstract: Risto Paatelainen

Multiplicities, p_T spectra and v_2 in A+A collisions at LHC and RHIC from NLO-improved pQCD + saturation + hydrodynamics model

We report the results from the recent studies [1,2], where we have brought the EKRT framework [3] to next-to-leading order (NLO) in pQCD, and shown the viability of the model in describing the produced initial QGP energy densities at the LHC and RHIC. Our updated framework [1] combines a rigorous NLO pQCD computation of the minijet transverse energy production with the saturation of gluons and fluid dynamics. Latest knowledge of NLO nuclear parton distributions (nPDFs) is utilized. Identifying the key parameters and charting the uncertainties of the model, we obtain an encouragingly good agreement with the charged-particle multiplicities and identified bulk hadron p_T spectra measured in 5% most central Au+Au collisions at RHIC and Pb+Pb at the LHC [1]. To obtain the initial QGP energy-density profiles dynamically in non-central collisions [2], we supplement the calculation by a local saturation condition and impact-parameter dependent nPDFs (EPS09s [4]). We address the main uncertainties associated with the obtained initial states. Using viscous fluid dynamics, we show that a good simultaneous description of the centrality dependence of multiplicity, p_T spectra and elliptic flow is obtained both at the LHC and RHIC.


Ref. [1] R. Paatelainen, K.J. Eskola, H. Holopainen and K. Tuominen, Multiplicities and $p_T$ spectra in ultrarelativistic heavy ion collisions from a next-to-leading order improved perturbative QCD + saturation + hydrodynamics model. Phys.Rev. C 87 (2012) 044904.

Ref. [2] R. Paatelainen, K.J. Eskola, and H. Niemi, work in progress.

Ref. [3] K.J. Eskola, K. Kajantie, P.V. Ruuskanen, K. Tuominen Scaling of transverse energies and multiplicities with atomic number and energy in ultrarelativistic nuclear collisions. Nuc Phys B 570 (2000) 379.

Ref. [4] I. Helenius, K.J. Eskola, H. Honkanen and C.A. Salgado, Impact-Parameter Dependent Nuclear Parton Distribution Functions_ EPS09s and EKS98s and Their Applications in Nuclear Hard Processes. JHEP 1207 (2012) 073.