67 research outputs found
Following Gluonic World Lines to Find the QCD Coupling in the Infrared
Using a parametrization of the Wilson loop with the minimal-area law, we
calculate the polarization operator of a valence gluon, which propagates in the
confining background. This enables us to obtain the infrared freezing (i.e.
finiteness) of the running strong coupling in the confinement phase, as well as
in the deconfinement phase up to the temperature of dimensional reduction. The
momentum scale defining the onset of freezing is found both analytically and
numerically. The nonperturbative contribution to the thrust variable,
originating from the freezing, makes the value of this variable closer to the
experimental one.Comment: 25 pages, 5 figure
On the covariant quantization of tensionless bosonic strings in AdS spacetime
The covariant quantization of the tensionless free bosonic (open and closed)
strings in AdS spaces is obtained. This is done by representing the AdS space
as an hyperboloid in a flat auxiliary space and by studying the resulting
string constrained hamiltonian system in the tensionless limit. It turns out
that the constraint algebra simplifies in the tensionless case in such a way
that the closed BRST quantization can be formulated and the theory admits then
an explicit covariant quantization scheme. This holds for any value of the
dimension of the AdS space.Comment: 1+16 pages; v4 two clarifications adde
Generalized Integrability and two-dimensional Gravitation
We review the construction of generalized integrable hierarchies of partial
differential equations, associated to affine Kac-Moody algebras, that include
those considered by Drinfel'd and Sokolov. These hierarchies can be used to
construct new models of 2D quantum or topological gravity, as well as new -algebras.Comment: 24 pages, fixed broken tex sourc
Solving loop equations by Hitchin systems via holography in large-N QCD_4
For (planar) closed self-avoiding loops we construct a "holographic" map from
the loop equations of large-N QCD_4 to an effective action defined over
infinite rank Hitchin bundles. The effective action is constructed densely
embedding Hitchin systems into the functional integral of a partially quenched
or twisted Eguchi-Kawai model, by means of the resolution of identity into the
gauge orbits of the microcanonical ensemble and by changing variables from the
moduli fields of Hitchin systems to the moduli of the corresponding holomorphic
de Rham local systems. The key point is that the contour integral that occurs
in the loop equations for the de Rham local systems can be reduced to the
computation of a residue in a certain regularization. The outcome is that, for
self-avoiding loops, the original loop equations are implied by the critical
equation of an effective action computed in terms of the localisation
determinant and of the Jacobian of the change of variables to the de Rham local
systems. We check, at lowest order in powers of the moduli fields, that the
localisation determinant reproduces exactly the first coefficient of the beta
function.Comment: 65 pages, late
Hunting down the X17 boson at the CERN SPS
Indexación ScopusRecently, the ATOMKI experiment has reported new evidence for the excess of e+e- events with a mass ∼ 17 MeV in the nuclear transitions of 4He, that they previously observed in measurements with 8Be. These observations could be explained by the existence of a new vector X17 boson. So far, the search for the decay X17 → e+e- with the NA64 experiment at the CERN SPS gave negative results. Here, we present a new technique that could be implemented in NA64 aiming to improve the sensitivity and to cover the remaining X17 parameter space. If a signal-like event is detected, an unambiguous observation is achieved by reconstructing the invariant mass of the X17 decay with the proposed method. To reach this goal an optimization of the X17 production target, as well as an efficient and accurate reconstruction of two close decay tracks, is required. A dedicated analysis of the available experimental data making use of the trackers information is presented. This method provides independent confirmation of the NA64 published results [1], validating the tracking procedure. The detailed Monte Carlo study of the proposed setup and the background estimate show that the goal of the proposed search is feasible. © 2020, The Author(s).https://link-springer-com.recursosbiblioteca.unab.cl/article/10.1140%2Fepjc%2Fs10052-020-08725-
Hydrogen and nitrogen plasma treatment effects on surface properties of GaSb and InGaAsSb
P-i-n diodes of GaSb InGaAsSb were treated in atomic hydrogen and atomic nitrogen flows in a microwave plasma crossed beams machine and in direct H2 and N2 plasma. The first type of treatment led to degradation of reverse currents of the diodes. Direct plasma treatments in H2 alone also led to a deterioration of the surface properties. However, combined treatment at 450°C in direct H2 and N2 plasmas improved the reverse currents and photoluminescence (PL) intensity by more than an order of magnitude. Formation of a passivating GaN layer at the surface of GaSb depleted of Sb by preliminary intense H2 treatment is thought to be responsible for the effect. © 1995
Passivation of GaAs by atomic hydrogen flow produced by the crossed beams method
A new method of hydrogen passivation is described in which a flow of molecular hydrogen is passed through dense microwave plasma across a magnetic field. This magnetic field serves to protect the sample surface from being bombarded by charged particles. Hence the crossed beam (CB) method makes it possible to avoid plasma etching and heavy ion bombardment that are so detrimental for many semiconductor devices. The positive effect of CB treatment on n-GaAs Schottky diodes is described. The effect consists of shallow donor passivation and improvement of I-V characteristics
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