170,265 research outputs found
THE ANOMALOUS CHIRAL SCHWINGER MODEL AND ITS QUANTIZATION - EIGENFUNCTIONALS AND ENERGY-SPECTRUM
No full text4-FERMION OPERATORS WITH IMPROVED NEAREST-NEIGHBOR ACTION
No full textWe discuss the improvement of local four-fermion operators and we present the one-loop perturbative calculation of their renormalization constants and mixing coefficients in the theory defined by a nearest-neighbour improved lattice action. This calculation is a necessary ingredient for improved Monte Carlo computations of hadronic matrix elements relevant for weak interactions
The running QCD coupling in the pre-asymptotic region
No full textWe study deviations from the perturbative asymptotic behaviour in the running QCD coupling by analysing non-perturbative measurements of alpha_s (p) at low momenta as obtained from the lattice three-gluon vertex. Our exploratory study provides some evidence for power corrections to the perturbative running proportional to 1/p^2
Pittori ferraresi nelle raccolte di città
No full textAttraverso l'analisi di inventari e testimonianze d'archivio il contributo prende in esame le opere dei pittori ferraresi nelle raccolte private a Ferrara nel Seicento
Neutrino emission via proton-proton interaction and magnetic field screening in GRBs
No full textGamma-Ray Bursts (GRBs) are one of the most energetic astrophysical events of our Universe and a
precise study of all the physical mechanisms occuring in these systems involves different branches of
physics (from particles physics to General Relativity). The main subjects of this thesis concern the
particles physics and the plasma physics fields. I study two different physical processes, operated by
elementary particles as protons, electron/positron paris, photons and neutrinos, occuring in GRBs.
In Ch. (1) I give a general introduction to GRBs, with some of their structural physical generalities
and properties (as their different emission phases, spectral and temporal properties etc.). I introduce also
the fireshell model, which has been developed during the years by Prof. Ruffini, R. and his group, in
order to study and understand the several mechanisms behind the GRB emission. I will also highlight the
principal differences between this model and the fireball model (which was the first model adopted in
order to study the GRBs emission).
The structure of the fireshell model considers a Reissner-Nordoström Black Hole, with a strong electric
field that converts part of the BH total energy in e + e − plasma by the vacuum polarization process. These
particles are accelerated and emit photons, and this leads to the formation of a relativistic optically thin
fireshell of e + e − γ plasma (the “PEM–pulse”). This shell interacts with baryons, deposited in the ambient
near the BH due to the collapse event, forming a new accelerated optically thick plasma of e + e − γ-baryons
(PEMB–pulse). The transparency of this shell brings to the formation of the proper GRB emission
(P-GRB emission).
In Ch. (2) I introduce a classification of the GRBs in classes and subclasses. They differ from each
other principally for their different progenitors, formation process, their isotropic energy E iso , their rest-
frame spectral peak energy E p,i and local observed rate.
In the Thesis, I have focused my attention principally on a particular type of long GRB class: the type
I Binary-driven HyperNova (BdHN). The physical scenario and process, that leads to the formation of
the BdHN class, is the Induced Gravitational Collapse (IGC), with the hypercritical accretion process
paradigm. The two original studies of the Thesis, developed in Chs. (3) and (4), are based on the physical
scenario of BdHN. In this chapter, I also show the connections between the several observations of a GRB
event and the basic processes of the BdHN model.
In Ch. (3), the first topic of the Thesis is presented, namely the neutrinos and photons production by
proton-proton interaction, between accelerated protons and protons at rest. Keeping in mind the above
discussed scenario for the dynamics of the e + e − γ-baryons plasma, recent numerical simulations have
shown that the SN ejecta becomes highly asymmetric. Therefore, the electron-positron (e ± ) plasma
created in the BH formation, during its isotropic and self-accelerating expansion, engulfs different
amounts of ejecta baryons along different directions, leading to a direction-dependent Lorentz factor. In
this configuration, I have studied the pp interaction occurring in two regions: an high density region and a
low density region. In the conclusion of this chapter I also try to give an estimate of a possible, direct or
indirect, detection of the neutrinos and photons created throught the above mechanism. From this analysis
it came out that a possible detection of these neutrinos with currently operating detectors is plausible only
for sources several order of magnitude more energetic than the ones considered in this work, and very-high
energy ineracting protons (this subject is treated in App. (D) and will be better developed in future works).
It also came out that an indirect detection of these neutrinos by means of the related photons emission is
possible.
The second subject of the Thesis is presented in Ch. (4) and concerns the study of the screening
process of an electromagnetic field near a BH operated by electron-positron pairs. It has been shown
that a rotating BH immersed in a test background magnetic field, of initial strength B 0 and aligned
parallel to the BH rotation axis, generates an induced electric field, whose strength is proportional to the
background magnetic field, E = 1/2 ΥB (where Υ is the BH spin parameter). In this analysis, I consider
the configuration of crossed fields: B = Bẑ and E = E ŷ. In this system, an huge number of e + e − pairs can be emitted by vacuum polarization process, start to be accelerated to high energies by the induced electric
field and emit synchrotron photons. These photons interact with the magnetic field via the magnetic pair
production process (MPP): γ + B → e + + e − . The motion of all these particles around the magnetic field
lines generates also an induced magnetic field oriented in the opposite direction to the background one,
which implies a reduction of the background magnetic field. The principal results are that the combination
of the processes described above can reduce the magnetic field in a small time scale, even if the production
of pairs is not so efficient due to the low energy of the emitted photons, for the selected initial conditions
of the field strengths and particles densities
New improved operators: a convenient redefinition
No full textIn this paper we consider a new definition of improved fermion operators which is very convenient for numerical QCD simulations with the nearest-neighbour improved action, and we present the one-loop values of the corresponding renormalization constants. This redefinition includes terms that are formally of order a2 or higher and do not affect the conclusions about order a improvement
Results from a non-perturbative renormalization of lattice operators
No full textWe propose a general renormalization method, which avoids completely the use of lattice perturbation theory. We present the results from its numerical applications to two-fermion operators on a 16^3 \times 32 lattice, at \beta=6.0.We propose a general renormalization method, which avoids completely the use of lattice perturbation theory. We present the results from its numerical applications to two-fermion operators on a lattice, at
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Renormalization of lattice two-fermion operators with improved nearest-neighbour action
No full textWe present a one-loop calculation of the renormalization constants of two-fermion local operators with the nearest-neighbour improved action. This calculation is a necessary ingredient in lattice computations of continuum hadronic matrix elements relevant for the predictions of many interesting physical quantities, including fπ,ρ, electromagnetic form factors, semi-leptonic D-decays, etc
- …
