151 research outputs found
HERA data and collinearly-improved BK dynamics
Within the framework of the dipole factorisation, we use a recent collinearly-improved version of the Balitsky-Kovchegov equation to fit the HERA data for inclusive deep inelastic scattering at small Bjorken . The equation includes an all-order resummation of double and single transverse logarithms and running coupling corrections. Compared to similar equations previously proposed in the literature, this work makes a direct use of Bjorken as the rapidity scale for the evolution variable. We obtain excellent fits for reasonable values for the four fit parameters. We find that the fit quality improves when including resummation effects and a physically-motivated initial condition. In particular, the resummation of the DGLAP-like single transverse logarithms has a sizeable impact and allows one to extend the fit up to relatively large photon virtuality
Jet reconstruction and jet background classification with the ALICE experiment in PbPb collisions at the LHC
For a quantitative interpretation of reconstructed jet properties in
heavy-ion collisions it is paramount to characterize the contribution from the
underlying event and the influence of background fluctuations on the jet
signal. In addition to the pure number fluctuations, region-to-region
correlated background within one event can enhance or deplete locally the level
of background and modify the jet energy. We show a first detailed assessment of
background effects using different probes embedded into heavy-ion data and
quantify their influence on the reconstructed jet spectrum.Comment: 4 pages, 2 figures, Proceedings for the XXII International Conference
on Ultra-Relativistic Nucleus-Nucleus Collisions, Quark Matter 2011, Annec
cross section from the dipole model in momentum space
We reproduce the DIS measurements of the proton structure function at high
energy from the dipole model in momentum space. To model the dipole-proton
forward scattering amplitude, we use the knowledge of asymptotic solutions of
the Balitsky-Kovchegov equation, describing high-energy QCD in the presence of
saturation effects. We compare our results with the previous analysis in
coordinate space and discuss possible extensions of our approach.Comment: 9 pages, 3 figure
On the use of a running coupling in the calculation of forward hadron production at next-to-leading order
We study a puzzle raised recently regarding the running coupling prescription used in the calculation of forward particle production in proton-nucleus collisions at next-to-leading order: using a coordinate space prescription which is consistent with the one used in the high energy evolution of the target leads to results which can be two orders of magnitude larger than the ones obtained with a momentum space prescription. We show that this is an artefact of the Fourier transform involved when passing between coordinate and momentum space and propose a new coordinate space prescription which avoids this problem.Peer reviewe
QCD at small x and nucleus-nucleus collisions
At large collision energy sqrt(s) and relatively low momentum transfer Q, one
expects a new regime of Quantum Chromo-Dynamics (QCD) known as "saturation".
This kinematical range is characterized by a very large occupation number for
gluons inside hadrons and nuclei; this is the region where higher twist
contributions are as large as the leading twist contributions incorporated in
collinear factorization. In this talk, I discuss the onset of and dynamics in
the saturation regime, some of its experimental signatures, and its
implications for the early stages of Heavy Ion Collisions.Comment: Plenary talk given at QM2006, Shanghai, November 2006. 8 pages, 8
figure
Exploring the QCD landscape with high-energy nuclear collisions
Quantum chromodynamics (QCD) phase diagram is usually plotted as temperature
(T) versus the chemical potential associated with the conserved baryon number
(\mu_{B}). Two fundamental properties of QCD, related to confinement and chiral
symmetry, allows for two corresponding phase transitions when T and \mu_{B} are
varied. Theoretically the phase diagram is explored through non-perturbative
QCD calculations on lattice. The energy scale for the phase diagram
(\Lambda_{QCD} ~ 200 MeV) is such that it can be explored experimentally by
colliding nuclei at varying beam energies in the laboratory. In this paper we
review some aspects of the QCD phase structure as explored through the
experimental studies using high energy nuclear collisions. Specifically, we
discuss three observations related to the formation of a strongly coupled
plasma of quarks and gluons in the collisions, experimental search for the QCD
critical point on the phase diagram and freeze-out properties of the hadronic
phase.Comment: Submitted to the New Journal of Physics focus issue "Strongly
Correlated Quantum Fluids: From Ultracold Quantum Gases to QCD Plasmas
Use of a running coupling in the NLO calculation of forward hadron production
We address and solve a puzzle raised by a recent calculation [1] of the cross section for particle production in proton-nucleus collisions to next-to-leading order: the numerical results show an unreasonably large dependence upon the choice of a prescription for the QCD running coupling, which spoils the predictive power of the calculation. Specifically, the results obtained with a prescription formulated in the transverse coordinate space differ by 1 to 2 orders of magnitude from those obtained with a prescription in momentum space. We show that this discrepancy is an artifact of the interplay between the asymptotic freedom of QCD and the Fourier transform from coordinate space to momentum space. When used in coordinate space, the running coupling can act as a fictitious potential which mimics hard scattering and thus introduces a spurious contribution to the cross section. We identify a new coordinate-space prescription, which avoids this problem, and leads to results consistent with those obtained with the momentum-space prescription.Peer reviewe
Model for SU(3) vacuum degeneracy using light-cone coordinates
Working in light-cone coordinates, we study the zero-modes and the vacuum in
a 2+1 dimensional SU(3) gauge model. Considering the fields as independent of
the tranverse variables, we dimensionally reduce this model to 1+1 dimensions.
After introducing an appropriate su(3) basis and gauge conditions, we extract
an adjoint field from the model. Quantization of this adjoint field and field
equations lead to two constrained and two dynamical zero-modes. We link the
dynamical zero-modes to the vacuum by writing down a Schrodinger equation and
prove the non-degeneracy of the SU(3) vacuum provided that we neglect the
contribution of constrained zero-modes.Comment: 22 pages, 5 figure
Les Houches 2013: Physics at TeV Colliders: Standard Model Working Group Report
This Report summarizes the proceedings of the 2013 Les Houches workshop on
Physics at TeV Colliders. Session 1 dealt primarily with (1) the techniques for
calculating standard model multi-leg NLO and NNLO QCD and NLO EW cross sections
and (2) the comparison of those cross sections with LHC data from Run 1, and
projections for future measurements in Run 2.Comment: Proceedings of the Standard Model Working Group of the 2013 Les
Houches Workshop, Physics at TeV Colliders, Les houches 3-21 June 2013. 200
page
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