211 research outputs found
Testing the Gaussian Approximation to the JIMWLK Equation
In processes involving small-x partons, like in deep inelastic scattering and
in hadronic collisions at high energy, the final state can be expressed in
terms of correlators of Wilson lines. We study such high-point correlators
evolving according to the JIMWLK equation and we confirm the results of
previous numerical and analytic work, by using an independent method, that the
solution to the JIMWLK equation can be very well approximated by an appropriate
Gaussian wavefunction. We explore both fixed and running coupling evolution,
where in the latter the scale is set according to various prescriptions. As a
byproduct, we also numerically confirm to high accuracy the validity of the law
governing the behavior of the S-matrix close to the unitarity limit, the
Levin-Tuchin formula. We furthermore outline how to calculate correlators with
open color indices.Comment: 25 pages, 11 figures. v2: minor corrections, one equation added,
updated to match published versio
Resumming large higher-order corrections in non-linear QCD evolution
Linear and non-linear QCD evolutions at high energy suffer from severe issues
related to convergence, due to higher order corrections enhanced by large
double and single transverse logarithms. We resum double logarithms to all
orders by taking into account successive soft gluon emissions strongly ordered
in lifetime. We further resum single logarithms generated by the first
non-singular part of the splitting functions and by the one-loop running of the
coupling. The resulting collinearly improved BK equation admits stable
solutions, which are used to successfully fit the HERA data at small-x for
physically acceptable initial conditions and reasonable values of the fit
parameters.Comment: 4 pages, 4 figures, based on talk given at Hard Probes 2015, 29 June
- 3 July 2015, Montreal, Canad
Resummation of Large Logarithms in the Rapidity Evolution of Color Dipoles
Perturbative corrections beyond leading-log accuracy to BFKL and BK
equations, describing the rapidity evolution of QCD scattering amplitudes at
high energy, exhibit strong convergence problems due to radiative corrections
enhanced by large single and double transverse logs. We identify explicitly the
physical origin of double transverse logs and resum them directly in coordinate
space as appropriate for BK equation, in terms of an improved local-in-rapidity
evolution kernel. Numerical results show the crucial role of double-logarithmic
resummation for BK evolution, which is stabilized and slowed down by roughly a
factor of two.Comment: 6 pages, 4 figures; Proceedings of the XXIII International Workshop
on Deep-Inelastic Scattering (27 April-May 1 2015, Dallas (USA)
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
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
Hadron-nucleus scattering in the local reggeon model with pomeron loops for realistic nuclei
Contribution of simplest loops for hadron-nucleus scattering cross-sections
is studied in the Local Reggeon Field Theory with a supercritical pomeron. It
is shown that inside the nucleus the supercritical pomeron transforms into a
subcritical one, so that perturbative treatment becomes possible. The pomeron
intercept becomes complex, which leads to oscillations in the cross-sections.Comment: 13 pages, 6 figure
Optimal jet radius in kinematic dijet reconstruction
Obtaining a good momentum reconstruction of a jet is a compromise between
taking it large enough to catch the perturbative final-state radiation and
small enough to avoid too much contamination from the underlying event and
initial-state radiation. In this paper, we compute analytically the optimal jet
radius for dijet reconstructions and study its scale dependence. We also
compare our results with previous Monte-Carlo studies.Comment: 30 pages, 11 figures; minor corrections; published in JHE
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
FastJet user manual
FastJet is a C++ package that provides a broad range of jet finding and
analysis tools. It includes efficient native implementations of all widely used
2-to-1 sequential recombination jet algorithms for pp and e+e- collisions, as
well as access to 3rd party jet algorithms through a plugin mechanism,
including all currently used cone algorithms. FastJet also provides means to
facilitate the manipulation of jet substructure, including some common boosted
heavy-object taggers, as well as tools for estimation of pileup and
underlying-event noise levels, determination of jet areas and subtraction or
suppression of noise in jets.Comment: 69 pages. FastJet 3 is available from http://fastjet.fr
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