La Rivista del Nuovo Cimento最新文献

TeV haloes are a recently discovered class of very high energy gamma-ray emitters. These sources consist of extended regions of multi-TeV emission, originally observed around the two well-known and nearby pulsars, Geminga and PSR B0656+14 (Monogem), and possibly, with different degrees of confidence, around few more objects with similar age. Since their discovery, TeV haloes have raised much interest in a large part of the scientific community, for the implications their presence can have on a broad range of topics spanning from pulsar physics to cosmic ray physics and dark matter indirect searches. In this article, we review the reasons of interest for TeV haloes and the current status of observations. We discuss the proposed theoretical models and their implications, and conclude with an overlook on the prospects for better understanding this phenomenon.

Silicon Photomultipliers (SiPMs) have emerged as leading photon detectors in experimental physics since their introduction in the late 1990s. With performance characteristics superior to those of traditional photodetectors, SiPMs exhibit up to 60% photon detection efficiency, rapid signal rise times, and resistance to magnetic fields. Their solid-state construction enables mass production, compactness, and high spatial resolution, facilitating their integration into a wide range of experimental setups. Although susceptible to radiation damage, mitigation strategies are being studied to allow their reliable operation even in environments with elevated radiation levels. SiPMs excel in detecting low levels of light, making them well suited for applications involving scintillation and Cherenkov light. Their ability to operate effectively at cryogenic temperatures allows the construction of a new class of multi-tons rare event search experiments such as Darkside-20k. Insensitivity to the magnetic field and mitigation of the radiation damages are making SiPMs well-suited to be used in accelerator driven physics such as Cherenkov light detectors for Particle IDentification (PID) in the future Electron Ion Collider (EIC).

It is often said that “granular matter is ubiquitous”. Many natural components and human products look and behave like grains: stones, debris, soils, on the one hand; food, pharmaceuticals, building materials, etc., on the other. However, the physics involved is still poorly understood due to its inherent difficulties. In fact, granular materials are an example of frictional, dissipative, nonlinear, out-of-equilibrium systems. One consequence is that they exhibit, under various circumstances, large and irregular fluctuations, finite size effects, and poor reproducibility (as everyone knows from trying to slowly pour sugar or coffee powder). This article summarizes some experimental results on the response of horizontal grain beds subjected to low rate shear stress. In this case, the response is often intermittent and irregular, the so-called stick–slip regime, and can only be described statistically. Small-scale experiments are the best way to collect the necessary large amount of data and, despite the difference in scale, can provide the basis for a better understanding of larger scale phenomena such as avalanches, landslides and earthquakes.

Many cosmological observables derive from primordial vacuum fluctuations evolved to late times. These observables represent statistical draws from some underlying quantum or statistical field theoretic framework where infinities arise and require regularization. After subtraction, renormalization conditions must be imposed by measurements at some scale, mindful of scheme and background dependence. We review this process on backgrounds that transition from finite duration inflation to radiation domination, and show how in spite of the ubiquity of scaleless integrals, ultraviolet (UV) divergences can still be meaningfully extracted from quantities that nominally vanish when dimensionally regularized. In this way, one can contextualize calculations with hard cutoffs, distinguishing between UV and infrared (IR) scales corresponding to the beginning and end of inflation from UV and IR scales corresponding to the unknown completion of the theory and its observables. This distinction has significance as observable quantities cannot depend on the latter, although they will certainly depend on the former. One can also explicitly show the scheme independence of the coefficients of UV divergent logarithms. Furthermore, certain IR divergences are shown to be an artifact of the de Sitter limit and are cured for finite duration inflation. For gravitational wave observables, we stress the need to regularize stress tensors that do not presume a prior scale separation in their definition (as with the standard Isaacson form), deriving an improved stress tensor fit to purpose. We conclude by highlighting the inextricable connection between inferring (N_textrm{eff}) bounds from vacuum tensor perturbations and the process of background renormalization.

Fluorescent proteins (FPs) have transformed cell biology through their use in fluorescence microscopy, enabling precise labeling of proteins via genetic fusion. A key advancement is altering primary sequences to customize their photophysical properties for specific imaging needs. A particularly notable family of engineered mutants is constituted by Reversible Switching Fluorescent Proteins (RSFPs), i.e. variant whose optical properties can be toggled between a bright and a dark state, thereby adding a further dimension to microscopy imaging. RSFPs have strongly contributed to the super-resolution (nanoscopy) revolution of optical imaging that has occurred in the last 20 years and afforded new knowledge of cell biochemistry at the nanoscale. Beyond high-resolution applications, the flexibility of RSFPs has been exploited to apply these proteins to other non-conventional imaging schemes such as photochromic fluorescence resonance energy transfer (FRET). In this work, we explore the origins and development of photochromic behaviors in FPs and examine the intricate relationships between structure and photoswitching ability. We also discuss a simple mathematical model that accounts for the observed photoswitching kinetics. Although we review most RSFPs developed over the past two decades, our main goal is to provide a clear understanding of key switching phenotypes and their molecular bases. Indeed, comprehension of photoswitching phenotypes is crucial for selecting the right protein for specific applications, or to further engineer the existing ones. To complete this picture, we highlight in some detail the exciting applications of RSFPs, particularly in the field of super-resolution microscopy.

We review some aspects of higher spin symmetry, in (Anti-)de Sitter and flat space–times, aiming at closing the gap between the constantly curved and flat cases. On (Anti-)de Sitter space, non-Abelian higher spin algebras are at the core of the construction of interacting theories of higher spin gravity. By considering a suitable contraction of these algebras, we show that similar considerations can apply to Minkowski space–time. We identify a unique candidate to the role of higher spin symmetry in flat space that can also be built as a quotient of the universal enveloping algebra of the isometries of the vacuum, as in the (Anti-)de Sitter case. We then show how to recover the free dynamics from the gauging of the resulting algebra at the linear level. Finally, we show how to realise this gauge algebra as a subset of the global symmetries of a Carrollian conformal scalar field theory living on the null infinity of Minkowski space–time. This theory emerges as the limit of vanishing speed of light of a free massless relativistic scalar. The identification of the same higher spin algebra that rules the dynamics in the bulk of space–time within the global symmetries of this boundary theory paves the way to a flat counterpart of higher spin holography.

Positron emission tomography (PET) is a well-established imaging technique for “in-vivo” molecular imaging. In this review, after a brief history of PET, its physical principles and the technology developed for bringing PET from a bench experiment to a clinically indispensable instrument are presented. The limitations and performance of the PET tomographs are discussed, both for the hardware and software aspects. The status of the art of clinical, pre-clinical and hybrid scanners (i.e., PET/CT and PET/MR) is reported. Finally, the actual trend and the recent and future technological developments are illustrated. The current version of this paper is the second edition of the original version published in 2016 (Rivista del Nuovo Cimento, Vol 39(4) 2016, pp. 156–213). The authors decided to keep the same structure of the paper, operating corrections of some typos, and adjustments. However, we added a description of the most recent PET developments that took place in the last 10 years completed with the addition of the most relevant references. These topics are now described in detail and cover the last two chapters of the paper.

Abstract

Neutrinos are the only particles in the Standard Model that could be Majorana fermions, that is, completely neutral fermions that are their own antiparticles. The most sensitive known experimental method to verify whether neutrinos are Majorana particles is the search for neutrinoless double-beta decay. The last 2 decades have witnessed the development of a vigorous program of neutrinoless double-beta decay experiments, spanning several isotopes and developing different strategies to handle the backgrounds masking a possible signal. In addition, remarkable progress has been made in the understanding of the nuclear matrix elements of neutrinoless double-beta decay, thus reducing a substantial part of the theoretical uncertainties affecting the particle–physics interpretation of the process. On the other hand, the negative results by several experiments, combined with the hints that the neutrino mass ordering could be normal, may imply very long lifetimes for the neutrinoless double-beta decay process. In this report, we review the main aspects of such process, the recent progress on theoretical ideas and the experimental state of the art. We then consider the experimental challenges to be addressed to increase the sensitivity to detect the process in the likely case that lifetimes are much longer than currently explored, and discuss a selection of the most promising experimental efforts.