08-03-2021 (in modalita' telematica )
(DOTTORATO IN FISICA)
|Tidal defirmability of exotic compact objects
Introduction to xAct|
1) The equation of state of neutron star matter leaves a clean imprint on the gravitational-wave signal emitted by inspiraling binary neutron stars. This imprint depends on the tidal deformability parameter of the neutron stars involved: measurements of such parameter can therefore directly constrain the neutron star equation of state. This theoretical framework of may be applied by extension to exotic objects with density and gravitational behaviour similar to neutron stars. More specifically, two kind of exotic Neutron Stars: one in which we allow them to present anisotropies in the internal pressure, and a second one in which we allow them to contain mirror dark matter. We will obtain the equilibrium sequences for both classes of exotic neutron stars, thus determining the maximum mass and the compactness as a functionof the mass.
We compare our results to the maximum mass constraint imposed by PSR J0348+0432 and to the GW170817 constraint on the tidal deformability. We bare able to find exotic neutron star configurations that can satisfy both constraints, despite using an equation of state that violates one of the constraints if one considers canonical neutron star models. We also quantify the deviations of our exotic neutron star models from the universal behaviour found in that links compactness and tidal deformability. We show that deviations from this universal behaviour can be significant, particularly in the presence of mirror dark matter. This highlights that when one interprets observations by using universal relations, one must be well aware of their realm of applicability.
XAct is a powerful tools in Mathematica that can perform abstract tensor calcolous and can be useful in perturbation theory (xPert) or in cosmology (xPand). We will present some basic functions and show a minimal example in wich we will find the Einstein Fiel Equation of a perturbed spherical metric.
(DOTTORATO IN FISICA)
|Magneto-thermal evolution and X-ray emission of neutron stars: a 3D approach |
1)Neutron stars (NSs) harbor extremely strong magnetic fields within their solid crust, they are born very hot and cool down as they age. Their thermal evolution is inherently coupled to that of their magnetic field. Knowledge of the secular magneto-thermal evolution can discriminate between different cooling scenarios when compared to observations, thus constraining the equation of state of ultra-dense matter.
The aim of this study is to model the evolution of the temperature and the magnetic field in the NS crust by means of numerical simulations performed with PARODY, a 3D, pseudo-spectral code which solves the coupled induction and temperature equations in the crust of an highly-magnetised isolated NS. A fully 3D approach is required in order to treat non-axysimmetric magnetic configurations and to explore small-scale structures which naturally arise as a consequence of the Hall term.
Moreover, in a NS the thermal map is shaped by the magnetic field topology, since heat flows in the crust mostly along the magnetic field lines. Self-consistent surface thermal maps can hence be produced by simulating the coupled magnetic and thermal evolution of the star to model the X-ray emission from the NS surface, deriving the spectrum and the pulse profile as seen by an observer at infinity, accounting for general-relativistic effects.
24-03-2021 (in modalita' telematica )
(DOTTORATO IN FISICA)
|Towards a new approach for the Pulse Shape Discrimination in the GERDA experiment.|
Neutrinoless double beta (0νββ) decay is a powerful tool to investigate lepton
number violation and the only practical way to assess the neutrinos nature (Dirac or
Majorana particles). It can therefore provide unique information about the physics
beyond the Standard Model (SM). Thanks to observing the neutrino oscillation
phenomenon, we know that neutrinos have mass, but the nature and the size of their
mass are still open issues in the neutrino sector beyond the SM. At present, the
only feasible experiments that have the potential to define the absolute scale of
the neutrino mass and to establish their nature are those searching for 0νββ decay.
GERDA experiment is considered among the leading experiments in the field, obtaining
the to-date best limit on the half-life of the 0νββ decay. This achievement results
from the careful selection of highly radio-pure materials and the efficient
background suppression techniques, such as pulse shape discrimination (PSD).
The initial phase of my master degree’s work concerned the investigation of the PSD
method applied to the data collected with germanium detectors in the GERDA
experiment. A new and innovative approach was subsequently implemented to improve
the PSD capabilities through a new digital filter that effectively treats the
experimental noise. The long-term goal of this thesis’s work is to apply the newly
developed technique to the forthcoming LEGEND-200 data, which will continue the
search of 0νββ decay, within the upgraded GERDA infrastructure.
The GERDA experimental setup, the pulse shape discrimination technique and the new
filtering technique with the obtained results will be summarized.
(DOTTORATO IN FISICA)
|Theory of jet energy loss in the quark-gluon plasma|
The quark-gluon plasma, the deconfined phase of Quantum ChromoDynamics (the theory
of the strong interaction) can be probed in the laboratory through ultrarelativistic
heavy-ion collision experiments, such as those at the Large Hadron Collider (LHC) at
CERN. The investigation of the medium produced in these experiments proceeds through
two complementary approaches: bulk properties (collective behavior of the many
lower-energy produced particles) and hard probes (the fewer higher-energy
Jets (collimated sprays of hadrons) are one of the key hard probes: understanding
the difference between jet development in a hot plasma, compared to development in
the vacuum or near-vacuum environment of a lower-multiplicity collision (e.g
proton-proton) is of the utmost importance for quantifying the properties of the
quark-gluon plasma. Also, this study is used in the light of the challenge posed by
the recent discovery of collective behavior in “small systems”, i.e. proton-nucleus
and high-multiplicity proton-proton collisions, unaccompanied by the modification of
jet spectra (“jet quenching”) observed in heavy-ion collisions.
Motivated by these current challenges and intense activity, the presentation will
investigate jet propagation in a hot quark-gluon plasma, concentrating on the theory
approaches to energy loss of the energetic partons composing the jet as they
traverse the medium.
14-04-2021 (modalita' telematica )
||Chloe Helene Martella|
(DOTTORATO IN FISICA)
|Reanalysis of the Active Seismic Experiments performed on the Moon during Apollo 14 and 16 Missions|
Robotic and human Moon exploration is currently living a new renaissance, as it is considered a benchmark for future human expansion in the Solar System. Many space agencies and private companies are developing strategies to bring the man back on the Moon by the end of this decade. This new international race will be driven by scientific, technological, and economic interests, and will require an in-depth understanding of the Moon subsurface environment. Several new geophysical instruments have been proposed to explore the Moon subsoil at different spatial scales. Amongst these, active seismic methods (reflection and refraction) are two of the most reliable techniques to investigate the lunar shallow stratigraphy. Indeed, refraction seismic technique was already tested during Apollo14, Apollo16 and Apollo 17 missions, providing some information on the thickness and the mechanical properties of the lunar regolith at the three landing sites.
In this work we present a reanalysis of the Active Seismic Experiments (ASE) conducted on the Moon during Apollo 14 and 16 missions. We analyzed all data collected along the seismic lines using the thumper source and we applied modern seismic attributes approach to better perform the first arrival picking. Conversely to previous works that used the LM ascent impacts and grenades, we were able to detect for both sites the direct and the head waves only using the thumper shots and compute the compressional wave velocities for the first and second layer. Finally, we compared our results with those published in previous works.
(DOTTORATO IN FISICA)
|Polarization properties of the X-ray emission of the accretion disk around black holes|
The radiation emitted by accretion disks around stellar mass black holes peaks in the soft X-rays when the source is in the thermal state. For symmetry reasons, the emission is expected to be polarized in a direction parallel or perpendicular to the disk symmetry axis projected onto the plane of the sky as an effect of the electron scattering that photons may undergo before leaving the disk. However, absorption effects in the disk material can influence as well the polarization state of emerging photons. Here we present, for the first time, a study of the polarization properties of the radiation emitted from a typical stellar mass black hole accretion disk, accounting for both scattering and absorption effects, that we have included computing the ionization structure of the disk material. We explore the two cases of a Schwarzschild and an extreme Kerr black hole, focusing our study on a BH with mass M = 10M? and for different values of mass accretion rate and scattering optical depth of the disk. Our results show that absorption plays a crucial role in determining the polarization properties of the X-ray emission. In fact, the polarization degree of radiation tends to be higher in the colder and more absorbed regions of the disk. The polarization angle is found to be perpendicular to the disk symmetry axis. For the time being, in the calculations we did not include any strong gravity effect, so we studied the spectra and the polarization observables as seen by an observer located on the disk surface. An immediate application of this model will be to include all the relevant GR effects to specialize these calculations to the case of stellar mass black holes in binary systems in our Galaxy.
26-04-2021 (modalita' telematica )
||Paolo La Francesca|
(DOTTORATO IN FISICA)
|Phase Ordering Dynamics in 2D Ferromagnetic Systems|
In the last few years there has been a considerable interest in the theory of phase
ordering dynamics in ferromagnetic systems and in models based on the introduction
of auxiliary fields with Gaussian statistics. The aim of this work was to study the
dynamics of the Landau-Ginzburg model and to numerically test the properties
predicted by the theory on 2D discrete lattices of sizes never considered before.
The occurence of the formation of competing domains of opposite magnetisation with
characteristic linear length obeying to the Lifshitz-Allen-Cahn Law l²(t) ∼ t was
shown. The behaviour of the correlation function G(r,t) at large t was also analysed
and it appeared to be compatible with the scaling hypothesis G(r,t)=F(r/l(t)) except
for the initial region x → 0 with x=r/l(t).
The extent of this non-universality region decreases over time both because the size
of the domains increases, and because the size of the interface regions among the
domains decreases. Furthermore, in the variable x, its linear size must become
negligible in the long term due to the growth of l(t) but it has not been possible,
for now, to provide an estimate of the necessary time for this to happen.
Therefore, excluding the regions in which the correlation function deviates from the
scaling regime, the scaling function was compared with the approximations obtained
by introducing auxiliary fields: the OJK and Mazenko approximations. Specifically,
neither of the two theories has been able to provide an accurate description of the
The rest of the work focused on comparing the behaviour of the Landau-Ginzburg model
and that of the Monte Carlo simulations for the Ising model. With the exception of
the region r → 0 considered above (for which the Ising model continues to have a
universal behaviour), the correlation functions were found to be compatible between
the two models, giving reason to assume that the scaling phenomena they describe
belong to the same universality class. Consistently, even in this case the
approximations for the scaling function did not work.
(DOTTORATO IN FISICA)
|Coherent Raman Imaging of Amyloid-? Plaques in Alzheimer’s Diseased Brain|
Alzheimer’s disease (AD) is a progressive neurological disease determining both the
reduction of cognitive function and the loss of memory and memories. The actual
therapeutic treatments are not able to heal the AD and to recover cognitive and
memory functions, therefore it is fundamental to get an early and accurate diagnosis
to slow down and contain as much as possible the disease progress. In the studies of
the diseases it has been experienced that the use of biological markers (biomarkers)
allows speeding up the diagnosis with a positive impact on the prognosis too.
This thesis aims to study the Amyloid-β plaques that have been considered as
Alzheimer’s disease biomarker.
During this work the triple-transgenic mouse model of AD (3xTg-AD) was used; the
model exhibit Amyloid-β plaques that are characteristic of the human form of
Alzheimer’s disease. An advanced stage of the disease has been studied: the
late-symptomatic stage present in 72-week-old mice. The study also included the
analysis of a healthy tissue of non-Tg mice. This control tissue has the same age,
equal to 72 weeks, as the AD tissues.
Initially, Fluorescence Micro-Spectroscopy was used to characterize, with two
specific fluorophores, the brain tissue under examination. Trying to minimize any
interferences (ex. samples preparation, labelling) and any physiochemical changes
that can occur inadvertently during the test executions, the complex interactions
between lipids and Amyloid-β plaque were studied employing a label-free vibrational
spectral imaging: Coherent Anti-Stokes Raman Scattering (CARS) is a non-invasive
label-free ultrafast method based on intrinsic vibrational contrast and is
characterized by high spectral resolution, 3D-sectioning capability and high
sensitivity. Vibrational Microscopy, compared to a Fluorescence Microscopy, has the
additional advantage that it can also be used for in-vivo medical applications: the
use of staining or labelling with fluorophores on humans is not preferable in terms
of risk of toxicity. Thanks to this vibrational spectral imaging, the region of the
typical lipid CH groups was probed.
This study clearly demonstrates that the chemical composition of the plaque is
different from the chemical composition of the brain tissue that surrounds it as
well as from the chemical composition of a non-AD brain.