High Pt hadron-hadron correlations

We propose the formulation of a dihadron fragmentation function in terms of parton matrix elements. Under the collinear factorization approximation and facilitated by the cut-vertex technique, the two hadron inclusive cross section at leading order (LO) in e+ e- annihilation is shown to factorize into a short distance parton cross section and the long distance dihadron fragmentation function. We also derive the DGLAP evolution equation of this function at leading log. The evolution equation for the non-singlet and singlet quark fragmentation function and the gluon fragmentation function are solved numerically with the initial condition taken from event generators. Modifications to the dihadron fragmentation function from higher twist corrections in DIS off nuclei are computed. Results are presented for cases of physical interest.Comment: 7 pages, 8 figures, Latex, Proceedings of Hot Quarks 2004, July 18-24, Taos, New Mexic

Dihadron fragmentation functions and high Pt hadron-hadron correlations

We propose the formulation of a dihadron fragmentation function in terms of parton matrix elements. Under the collinear factorization approximation and facilitated by the cut-vertex technique, the two hadron inclusive cross section at leading order (LO) in e+ e- annihilation is shown to factorize into a short distance parton cross section and the long distance dihadron fragmentation function. We also derive the DGLAP evolution equation of this function at leading log. The evolution equation for the non-singlet quark fragmentation function is solved numerically with a simple ansatz for the initial condition and results are presented for cases of physical interest.Comment: Latex, 4 pages, 4 figures, talk given at Quark Matter 2004, To appear in J. Phys.

A comparative study of Jet-quenching Schemes

The four major approximation schemes devised to study the modification of jets in dense matter are outlined. The comparisons are restricted to basic assumptions and approximations made in each case and the calculation methodology used. Emergent underlying similarities between apparently disparate methods brought about by the approximation schemes are exposed. Parameterizations of the medium in each scheme are discussed in terms of the transport coefficient $\hat{q}$. Discrepancies between the estimates obtained from the four schemes are discussed. Recent developments in the basic theory and phenomenology of energy loss are highlighted.Comment: 12 pages, 10 figures, latex, plenary presentation at the 19th International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions (QM2006), Shanghai, China Nov. 14-20, 200

Evolution of Parton Fragmentation Functions at Finite Temperature

The first order correction to the parton fragmentation functions in a thermal medium is derived in the leading logarithmic approximation in the framework of thermal field theory. The medium-modified evolution equations of the parton fragmentation functions are also derived. It is shown that all infrared divergences, both linear and logarithmic, in the real processes are canceled among themselves and by corresponding virtual corrections. The evolution of the quark number and the energy loss (or gain) induced by the thermal medium are investigated.Comment: 21 pages in RevTex, 10 figure

Nuclear Attenuation of high energy two-hadron system in the string model

Nuclear attenuation of the two-hadron system is considered in the string model. The two-scale model and its improved version with two different choices of constituent formation time and sets of parameters obtained earlier for the single hadron attenuation, are used to describe available experimental data for the $z$-dependence of subleading hadron, whereas satisfactory agreement with the experimental data has been observed. A model prediction for $\nu$-dependence of the nuclear attenuation of the two-hadron system is also presented.Comment: 8 page

Particle Production at Large Transverse Momentum with ALICE

We present transverse momentum distributions of inclusive charged particles and identified hadrons in $pp$ and Pb--Pb collisions at \rs= 2.76 TeV, measured by ALICE at the LHC. The Pb--Pb data are presented in intervals of collision centrality and cover transverse momenta up to 50 GeV/$c$. Nuclear medium effects are studied in terms of the nuclear modification factor \raa. The results indicate a strong suppression of high-$p_T$ particles in Pb--Pb collisions, consistent with a large energy loss of hard-scattered partons in the hot, dense and long-lived medium created at the LHC. We compare the preliminary results for inclusive charged particles to previous results from RHIC and calculations from energy loss models. Furthermore, we compare the nuclear modification factors of inclusive charged particles to those of identified $\pi^0$, $\pi^{\pm}$, K$^0_s$, and $\Lambda$.Comment: Talk given at Quark Matter 2011 conferenc

Jet tomography

I summarize the recent advances in jet tomographic studies of cold and hot nuclear matter based on perturbative QCD calculations of medium-induced gluon bremsstrahlung. Quantitative applications to ultrarelativistic heavy ion reactions at RHIC indicate the creation of a deconfined state of QCD with initial energy density on the order of 100 times cold nuclear matter density.Comment: Plenary talk given at the seventeenth international conference on Ultra-Relativistic Nucleus-Nucleus Collisions (Quark Matter 2004). 8 pages, 12 figures. Updated references, updated Table

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

Multiparticle angular correlations: a probe for the sQGP at RHIC

A novel decomposition technique is used to extract the centrality dependence of di-jet properties and yields from azimuthal correlation functions obtained in Au+Au collisions at $\sqrt{s_{_{\rm NN}}}$=200 GeV. The width of the near-side jet shows very little dependence on centrality. In contrast, the away-side jet indicates substantial broadening as well as hints for for a local minimum at $\Delta \phi=\pi$ for central and mid-central events. The yield of jet-pairs (per trigger particle) slowly increases with centrality for both the near- and away-side jets. These observed features are compatible with several recent theoretical predictions of possible modifications of di-jet fragmentation by a strongly interacting medium. Several new experimental approaches, including the study of flavor permutation and higher order multi-particle correlations, that might help to distinguish between different theoretical scenarios are discussed.Comment: Proceedings of the MIT workshop on correlations and fluctation