798 research outputs found
Anomalous diffusion originated by two Markovian hopping-trap mechanisms
We show through intensive simulations that the paradigmatic
features of anomalous diffusion are indeed the features of
a (continuous-time) random walk driven by two different Markovian hopping-trap mechanisms.
If and are the probabilities
of occurrence of each Markovian mechanism,
then the anomalousness parameter results to be
.
Ensemble and single-particle observables of this model have been studied
and they match the main characteristics of anomalous diffusion
as they are typically measured in living systems.
In particular, the celebrated transition of the walker's distribution
from exponential to stretched-exponential and finally
to Gaussian distribution is displayed by including also
the Brownian yet non-Gaussian interval.BERC 2018â2021
BERC 2022â2025
MOSAIC project DIT.AD004.14
Fractional Diffusion and Medium Heterogeneity: The Case of the Continuos Time Random Walk
In this contribution we show that fractional diffusion emerges from a simple Markovian Gaussian random walk when the medium displays a power-law heterogeneity. Within the framework of the continuous time random walk, the heterogeneity of the medium is represented by the selection, at any jump, of a different time-scale for an exponential survival probability. The resulting process is a non-Markovian non-Gaussian random walk. In particular, for a power-law distribution of the time-scales, the resulting random walk corresponds to a time-fractional diffusion process. We relates the power-law of the medium heterogeneity to the fractional order of the diffusion. This relation provides an interpretation and an estimation of the fractional order of derivation in terms of environment heterogeneity. The results are supported by simulations
Higgs-Mediated tau --> mu and tau --> e transitions in II Higgs doublet Model and Supersymmetry
We study the phenomenology of the mu-tau and e-tau lepton flavour violation
(LFV) in a general two Higgs Doublet Model (2HDM) including the supersymmetric
case. We consider several LFV decay modes of the charged fermion tau, namely
tau-> l_jgamma, tau->l_j l_k l_k and tau-> l_jeta. The predictions and the
correlations among the rates of the above processes are computed. In
particular, it is shown that tau->l_jgamma processes are the most sensitive
channels to Higgs-mediated LFV specially if the splitting among the neutral
Higgs bosons masses is not below the 10% level.Comment: v2=published version: 13 pages, 4 figures, text improved and
reference added. Two loop effects (relevant for tau->l_jgamma) added.
Conclusions unchange
Search for Tau Flavour Violation at the LHC
We explore the prospects for searches at the LHC for sparticle decays that
violate lepton number, in the light of neutrino oscillation data and the
seesaw model for neutrino masses and mixing. We analyse the theoretical and
phenomenological conditions required for tau flavour violation to be observable
in \chi_2 \to \chi + \tau^\pm \mu^\mp decays, for cosmologically interesting
values of the relic neutralino LSP density. We study the relevant
supersymmetric parameter space in the context of the Constrained Minimal
Supersymmetric Extension of the Standard Model (CMSSM) and in SU(5) extensions
of the theory. We pay particular attention to the possible signals from
hadronic tau decays, that we analyse using PYTHIA event simulation. We find
that a signal for \tau flavour-violating \chi_2 decays may be observable if the
branching ratio exceeds about 10%. This may be compatible with the existing
upper limit on \tau \to \mu \gamma decays if there is mixing between
right-handed sleptons, as could be induced in non-minimal SU(5) GUTs.Comment: 24 pages, 10 fig
Langevin equation in complex media and anomalous diffusion
The problem of biological motion is a very intriguing and topical issue.
Many efforts are being focused on the development of novel modelling
approaches for the description of anomalous diffusion in biological systems,
such as the very complex and heterogeneous cell environment. Nevertheless,
many questions are still open, such as the joint manifestation of statistical
features in agreement with different models that can also be somewhat alternative to each other, e.g. continuous time random walk and fractional Brownian
motion. To overcome these limitations, we propose a stochastic diffusion
model with additive noise and linear friction force (linear Langevin equation),
thus involving the explicit modelling of velocity dynamics. The complexity of
the medium is parametrized via a population of intensity parameters (relaxation time and diffusivity of velocity), thus introducing an additional
randomness, in addition to white noise, in the particleâs dynamics. We
prove that, for proper distributions of these parameters, we can get both
Gaussian anomalous diffusion, fractional diffusion and its generalizations.V.S. acknowledges BCAM Internship Program, Bilbao, for the financial support to her internship research period during which she developed her masterâs thesis research
useful for her masterâs degree in Physics at University of Bologna.
S.V. acknowledges the University of Bologna for the financial support
through the âMarco Polo Programmeâ for her PhD research period
abroad spent at BCAM, Bilbao, useful for her PhD degree in Physics
at University of Bologna.
P.P. acknowledges financial support from Bizkaia Talent and European Commission
through COFUND scheme, 2015 Financial Aid Program for Researchers, project number
AYDâ000â252 hosted at BCAM, Bilbao
Model-independent Analysis of Lepton Flavour Violating Tau Decays
Many models for physics beyond the Standard Model predict lepton-flavour
violating decays of charged leptons at a level which may become observable very
soon. In the present paper we investigate the decays of a Tau into three
charged leptons in a generic way, based on effective-field-theory methods,
where the relevant operators are classified according to their chirality
structure. We work out the decay distributions and discuss phenomenological
implications.Comment: 14 pages, 10 figures, references and comments adde
Waiting for mu->eg from the MEG experiment
The Standard Model (SM) predictions for the lepton flavor-violating (LFV)
processes like mu->eg are well far from any realistic experimental resolution,
thus, the appearance of m->eg at the running MEG experiment would unambiguously
point towards a New Physics (NP) signal. In this article, we discuss the
phenomenological implications in case of observation/improved upper bound on
m->eg at the running MEG experiment for supersymmetric (SUSY) scenarios with a
see-saw mechanism accounting for the neutrino masses. We outline the role of
related observables to m->eg in shedding light on the nature of the SUSY LFV
sources providing useful tools i) to reconstruct some fundamental parameters of
the neutrino physics and ii) to test whether an underlying SUSY Grand Unified
Theory (GUT) is at work. The perspectives for the detection of LFV signals in
tau decays are also discussed.Comment: 15 pages, 12 figure
Centre-of-mass like superposition of Ornstein-Uhlenbeck processes: A pathway to non-autonomous stochastic differential equations and to fractional diffusion
We consider an ensemble of OrnsteinâUhlenbeck processes featuring a population of relaxation times and a population of noise amplitudes that characterize the heterogeneity of the ensemble. We show that the centre-of-mass like variable corresponding to this ensemble is statistically equivalent to a process driven by a non-autonomous stochastic differential equation with time-dependent drift and a white noise. In particular, the time scaling and the density function of such variable are driven by the population of timescales and of noise amplitudes, respectively. Moreover, we show that this variable is equivalent in distribution to a randomly-scaled Gaussian process, i.e., a process built by the product of a Gaussian process times a non-negative independent random variable. This last result establishes a connection with the so-called generalized grey Brownian motion and suggests application to model fractional anomalous diffusion in biological systems.âMarco Polo Programmeâ (University of Bologna
Some relations between Lagrangian models and synthetic random velocity fields
We propose an alternative interpretation of Markovian transport models based
on the well-mixedness condition, in terms of the properties of a random
velocity field with second order structure functions scaling linearly in the
space time increments. This interpretation allows direct association of the
drift and noise terms entering the model, with the geometry of the turbulent
fluctuations. In particular, the well known non-uniqueness problem in the
well-mixedness approach is solved in terms of the antisymmetric part of the
velocity correlations; its relation with the presence of non-zero mean helicity
and other geometrical properties of the flow is elucidated. The well-mixedness
condition appears to be a special case of the relation between conditional
velocity increments of the random field and the one-point Eulerian velocity
distribution, allowing generalization of the approach to the transport of
non-tracer quantities. Application to solid particle transport leads to a model
satisfying, in the homogeneous isotropic turbulence case, all the conditions on
the behaviour of the correlation times for the fluid velocity sampled by the
particles. In particular, correlation times in the gravity and in the inertia
dominated case, respectively, longer and shorter than in the passive tracer
case; in the gravity dominated case, correlation times longer for velocity
components along gravity, than for the perpendicular ones. The model produces,
in channel flow geometry, particle deposition rates in agreement with
experiments.Comment: 54 pages, 8 eps figures included; contains additional material on
SO(3) and on turbulent channel flows. Few typos correcte
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