1,664 research outputs found
On correlations and discreteness in non-linear QCD evolution
We consider modifications of the standard non-linear QCD evolution in an
attempt to account for some of the missing ingredients discussed recently, such
as correlations, discreteness in gluon emission and Pomeron loops. The
evolution is numerically performed using the Balitsky-Kovchegov equation on
individual configurations defined by a given initial value of the saturation
scale, for reduced rapidities . We consider the
effects of averaging over configurations as a way to implement correlations,
using three types of Gaussian averaging around a mean saturation scale.
Further, we heuristically mimic discreteness in gluon emission by considering a
modified evolution in which the tails of the gluon distributions are cut-off.
The approach to scaling and the behavior of the saturation scale with rapidity
in these modified evolutions are studied and compared with the standard
mean-field results. For the large but finite values of rapidity explored, no
strong quantitative difference in scaling for transverse momenta around the
saturation scale is observed. At larger transverse momenta, the influence of
the modifications in the evolution seems most noticeable in the first steps of
the evolution. No influence on the rapidity behavior of the saturation scale
due to the averaging procedure is found. In the cut-off evolution the rapidity
evolution of the saturation scale is slowed down and strongly depends on the
value of the cut-off. Our results stress the need to go beyond simple
modifications of evolution by developing proper theoretical tools that
implement such recently discussed ingredients.Comment: 20 pages, LaTeX, 18 eps figures included using epsfig; changes in v3:
new kind of averaging added, 5 new plots, discussions and conclusions
extended, final versio
Study of the derivative expansions for the nuclear structure functions
We study the convergence of the series expansions sometimes used in the
analysis of the nuclear effects in Deep Inelastic Scattering (DIS) proccesses
induced by leptons. The recent advances in statistics and quality of the data,
in particular for neutrinos calls for a good control of the theoretical
uncertainties of the models used in the analysis. Using realistic nuclear
spectral functions which include nucleon correlations, we find that the
convergence of the derivative expansions to the full results is poor except at
very low values of
Nuclear Structure Functions at Small x from Inelastic Shadowing and Diffraction
Nuclear structure functions at small x and small or moderate are
studied using the relation with diffraction on nucleons which arises from
Gribov's Reggeon Calculus. A reasonable description of experimental data is
obtained with no fitted parameters. A comparison with other models and
predictions for future lepton-ion colliders are provided. Consequences for the
reduction of multiplicities in nucleus-nucleus collisions at energies of RHIC
and LHC are examined.Comment: LaTeX, 32 pages, 14 eps figures included using epsfig; 1 reference
update
On improving robot image-based visual servoing based on dual-rate reference filtering control strategy
It is well known that the use of multi-rate control techniques have improved the performance of many systems in general, and robotic systems, in particular. The main contribution of this paper is the generalization of the Reference Filtering control strategy from a dual-rate point of view, improving its inherent properties by overcoming the problem of sensor latency. In the paper, we discuss and analyze the improvements introduced by the novel dual-rate reference filtering control strategy in terms of convergence time, reachability and robustness. More specifically, we discuss the capability to solve positioning tasks, when hardware limitations are present with large sampling rates. In addition, a comparison is made between the single-rate and the proposed dual-rate control strategies to prove the advantages of the latter approach. A complete set-up has been prepared for validation, including a six degree of freedom (DOF) industrial manipulator, a smart camera and embedded hardware used as a high level controller.This work was supported by VALi+d Program (Generalitat Valenciana), DIVISAMOS Project (Spanish Ministry, DPI-2009-14744-C03-01), PROMETEO Program (Conselleria d'Educacio, Generalitat Valenciana) and SAFEBUS: Ministry of Economy and Competitivity, IPT-2011-1165-370000).Solanes Galbis, JE.; Muñoz Benavent, P.; Girbés, V.; Armesto Ángel, L.; Tornero Montserrat, J. (2015). On improving robot image-based visual servoing based on dual-rate reference filtering control strategy. Robotica. 1-18. https://doi.org/10.1017/S0263574715000454S11
Nuclear size and rapidity dependence of the saturation scale from QCD evolution and experimental data
The solutions of the Balitsky-Kovchegov evolution equations are studied
numerically and compared with known analytical estimations. The rapidity and
nuclear size dependences of the saturation scale are obtained for the cases of
fixed and running coupling constant. These same dependences are studied in
experimental data, on lepton-nucleus, deuteron-nucleus and nucleus-nucleus
collisions, through geometric scaling and compared with the theoretical
calculations.Comment: 8 pages, 8 figures. Contribution based on talks given by J. G.
Milhano and C. A. Salgado to the proceedings of ``Hard Probes 2004'',
Ericeira (Portugal), November 4-10, 200
On Hierarchy and Equivalence of Relativistic Equations for Massive Fields
A non-canonical correspondence of the complete sets of solutions to the Dirac
and Klein-Gordon free equations in Minkowski space-time is established. This
allows for a novel viewpoint on the relationship of relativistic equations for
different spins and on the origin of spinor transformations. In particular,
starting from a solution to the Dirac equation, one obtains a chain of other
solutions to both Dirac and Klein-Gordon equations. A comparison with the
massless case is performed, and examples of non-trivial singular solutions are
presented. A generalization to Riemannian space-time and inclusion of
interactions are briefly discussed.Comment: 7 pages, twocolumn. Problem of construction the hierarchy of
solutions in the massive case reformulate
Nuclear Shadowing and Diffraction
The relation between diffraction in lepton-proton collisions and shadowing of
nuclear structure functions which arises from Gribov inelastic shadowing, is
described. A model realizing such relation, which produces a parameter-free
description of experimental data on nuclear structure functions at small ,
is presented. The application to the description of multiplicities in nuclear
collisions is discussed and related to other approaches.Comment: LaTeX, 6 pages, 5 eps figures, uses enclosed ws-ijmpa.cls; invited
talk given by N. Armesto at the Eighth Workshop on Non-Perturbative Quantum
Chromodynamics, Paris, France, June 7th-11th 200
Relating high-energy lepton-hadron, proton-nucleus and nucleus-nucleus collisions through geometric scaling
A characteristic feature of small-x lepton-proton data from HERA is geometric
scaling -- the fact that over a wide range of x and Q^2 all data can be
described by a single variable , with all x-dependence
encoded in the so-called saturation momentum . Here, we observe
that the same scaling ansatz accounts for nuclear photoabsorption cross
sections and favors the nuclear dependence , . We then make the empirical finding
that the same A-dependence accounts for the centrality evolution of the
multiplicities measured in Au+Au collisions at RHIC. It also allows to
parametrize the high-p_t particle suppression in d+Au collisions at forward
rapidities. If these geometric scaling properties have a common dynamical
origin, then this A-dependence of should emerge as a consequence
of the underlying dynamical model.Comment: 4 pages, 3 postscript figure
- …