Systematics of parton-medium interaction from RHIC to LHC

Despite a wealth of experimental data for high-P_T processes in heavy-ion collisions, discriminating between different models of hard parton-medium interactions has been difficult. A key reason is that the pQCD parton spectrum at RHIC is falling so steeply that distinguishing even a moderate shift in parton energy from complete parton absorption is essentially impossible. In essence, energy loss models are effectively only probed in the vicinity of zero energy loss and, as a result, at RHIC energies only the pathlength dependence of energy loss offers some discriminating power. At LHC however, this is no longer the case: Due to the much flatter shape of the parton p_T spectra originating from 2.76 AGeV collisions, the available data probe much deeper into the model dynamics. A simultaneous fit of the nuclear suppression at both RHIC and LHC energies thus has great potential for discriminating between various models that yield equally good descriptions of RHIC data alone.Comment: Talk given at Quark Matter 2011, 22-28 May 2011, Annecy, Franc

Thermal photons from fluctuating initial conditions

Event-by-event fluctuations of initial QCD-matter density produced in heavy-ion collisions at RHIC enhance the production of thermal photons significantly in the region $2 \le p_T \le 4$ GeV/$c$ compared to a smooth initial-state averaged profile in the ideal hydrodynamic calculation. This enhancement is a an early time effect due to the presence of hotspots or over-dense regions in the fluctuating initial state. The effect of fluctuations is found to be stronger in peripheral than in central collisions.Comment: 4 pages, 3 figures. Talk given at Quark Matter 2011, 22-28 May 2011, Annecy, Franc

Angular hadron correlations probing the early medium evolution

Hard processes are a well calibrated probe to study heavy-ion collisions. However, the information to be gained from the nuclear suppression factor R_AA is limited, hene one has to study more differential observables to do medium tomography. The angular correlations of hadrons associated with a hard trigger appear suitable as they show a rich pattern when going from low p_T to high p_T. Of prime interest is the fate of away side partons with an in-medium pathlength O(several fm). At high p_T the correlations become dominated by the punchtrough of the away side parton with subsequent fragmentation. We discuss what information about the medium density can be gained from the data.Comment: Talk given at the 19th International Conference on Ultrarelativistic Nucleus-Nucleus Collisions: Quark Matter 2006 (QM 2006), Shanghai, China, 14-20 Nov 200

Hard and soft probe - medium interactions in a 3D hydro+micro approach at RHIC

We utilize a 3D hybrid hydro+micro model for a comprehensive and consistent description of soft and hard particle production in ultra-relativistic heavy-ion collisions at RHIC. In the soft sector we focus on the dynamics of (multi-)strange baryons, where a clear strangeness dependence of their collision rates and freeze-out is observed. In the hard sector we study the radiative energy loss of hard partons in a soft medium in the multiple soft scattering approximation. While the nuclear suppression factor $R_{AA}$ does not reflect the high quality of the medium description (except in a reduced systematic uncertainty in extracting the quenching power of the medium), the hydrodynamical model also allows to study different centralities and in particular the angular variation of $R_{AA}$ with respect to the reaction plane, allowing for a controlled variation of the in-medium path-length.Comment: 5 pages, 4 figures, Quark Matter 2006 proceedings, to appear in Journal of Physics

Hadron Correlations in Pb-Pb collisions at sqrt(s_NN) = 2.76 TeV with ALICE

Untriggered di-hadron correlations studies are shown which provide a map of the bulk correlation structures in Pb-Pb collisions. Long-range correlations are further studied by triggered correlations which address the dependence on trigger and associated pT. Measured correlation functions are decomposed with a multi-parameter fit and into Fourier coefficients. The jet-yield modification factor I_AA is presented.Comment: Proceedings of plenary talk at the XXII International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions, Quark Matter 2011, Annec

System size dependence of nuclear modification and azimuthal anisotropy of jet quenching

We investigate the system size dependence of jet-quenching by analyzing transverse momentum spectra of neutral pions in Au+Au and Cu+Cu collisions at $\sqrt{s_{\textrm{NN}}}$ =200 GeV for different centralities. The fast partons are assumed to lose energy by radiating gluons as they traverse the plasma and undergo multiple collisions. The energy loss per collision, $\epsilon$, is taken as proportional to $E$(where $E$ is the energy of the parton), proportional to $\sqrt{E}$, or a constant depending on whether the formation time of the gluon is less than the mean path, greater than the mean free path but less than the path length, or greater than the path length of the partons, respectively. NLO pQCD is used to evaluate pion production by modifying the fragmentation function to account for the energy loss. We reproduce the nuclear modification factor $R_\textrm{AA}$ by treating $\epsilon$ as the only free parameter, depending on the centrality and the mechanism of energy loss. These values are seen to explain the nuclear modification of prompt photons, caused by the energy lost by final state quarks before they fragment into photons. These also reproduce the azimuthal asymmetry of transverse momentum distribution for pions within a factor of two and for prompt photons in a fair agreement with experimental data.Comment: 26 pages, 17 figures. One more figure added. Discussion expanded. Typographical corrections done, several references added. To appear in Journal of Physics

Nuclear modification factors from the CMS experiment

We report the measurements of nuclear modification factors of the Z bosons, isolated photons and charged particles in $\sqrt{s_{NN}}$ = 2.76 TeV PbPb collisions with the CMS detector. The nuclear modification factors are constructed by dividing the PbPb $p_T$ spectra, normalized to the number of binary collisions, by the pp references. No modifications are observed in isolated photon and Z boson production with respected to the pp references while large suppression is observed in the charged particles.Comment: Presented at QM2011: Quark Matter 201

Charged Particle Production at Large Transverse Momentum in Pb−-Pb Collisions at sNN=2.76\sqrt{s_{NN}}=2.76 TeV Measured with ALICE at the LHC

Transverse momentum ($p_{T}$) spectra of charged particles are measured as a function of event centrality in Pb$-$Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV with ALICE at the LHC. The spectra are compared to those measured in pp collisions at the same collision energy in terms of the nuclear modification factor $R_{AA}$. The high-$p_{T}$ charge particle production in central Pb$-$Pb collisions ($0-5$) is strongly suppressed by a factor $\approx6$ at transverse momenta $p_{T}=6-7$ GeV/c as compared to expectation from independent superposition of nucleon-nucleon collisions. Above $p_{T}=7$ GeV/c there is a significant rise in the nuclear modification factor, which reaches $R_{AA} \approx 0.4$ at $p_{T}=50$ GeV/c. The measured suppression of high-$p_{T}$ particles is stronger than that measured at RHIC.Comment: 4 pages, 6 figures, Quark Matter 2011 Proceeding

Transport Theoretical Description of Collisional Energy Loss in Infinite Quark-Gluon Matter

We study the time evolution of a high-momentum gluon or quark propagating through an infinite, thermalized, partonic medium utilizing a Boltzmann equation approach. We calculate the collisional energy loss of the parton, study its temperature and flavor dependence as well as the the momentum broadening incurred through multiple interactions. Our transport calculations agree well with analytic calculations of collisional energy-loss where available, but offer the unique opportunity to address the medium response as well in a consistent fashion.Comment: 12 pages, updated with additional references and typos correcte