arXiv subjects

P. Rossi

Publications and source records attributed to P. Rossi.

At least 19 recordsLinked to original sources

arXiv2026

White Paper on a Novel FFA-Based CEBAF Upgrade to 22 GeV

We present a conceptual design for a cost-effective upgrade of the Continuous Electron Beam Accelerator Facility (CEBAF) to 22 GeV using non-scaling fixed-field alternating-gradient (FFA) arcs built from Halbach-style permanent magnets. Building on the eight-pass energy-recovery demonstration at the Cornell--BNL CBETA test accelerator, the design replaces the highest-energy recirculation arcs with a pair of FFA arcs that simultaneously transport six passes spanning a factor-of-two momentum range. We describe the machine layout; including the injector, recirculating linacs, spreaders and recombiners, FFA arcs, splitters, transition and extraction regions; together with beam-dynamics validation studies covering emittance growth, synchrotron-radiation-driven depolarization, and orbit correction, and we summarize permanent-magnet design, prototyping, and radiation-resiliency results. This white paper documents the accelerator physics underpinning a staged path to 22 GeV that preserves CEBAF's multi-hall, high-luminosity, polarized-beam capabilities.

physics.acc-ph↗

arXiv2026

Non-thermal emission in jets and winds: Expected emission and spectral index distributions

The origin of synchrotron emission in compact radio sources associated with active galactic nuclei (AGN) remains poorly understood. In a series of papers, we have examined diagnostic tools to disentangle the dominant underlying processes. In this study, we investigate the in situ evolution of cosmic-ray electrons (CREs) in compact AGN jets and winds, and examine how their evolution shapes the resulting observable radio properties. In jets, CREs experience multiple shock interactions as they propagate along the spine toward the hotspot and flow into the cocoon via backflows. In winds, CREs are predominantly accelerated at the Mach disc, with occasional re-acceleration within turbulent cocoon backflows. The continuous mixing of different CRE populations within the cocoon produces observational signatures that cannot be inferred from instantaneous conditions alone. In all jet simulations, spectral indices are flattest near the hotspot and steepen progressively away from the hotspots. In winds, spectra steepen with increasing distance from the Mach disc, with this trend becoming more pronounced at high radio frequencies due to radiative losses. We find the Mach disc to be a significantly more efficient CRE acceleration site than the forward shock in winds, which weakens as the wind expands to large scales. Since morphology, especially at low resolution, can be ambiguous for compact sources, spatially resolved spectral indices, particularly when combined with emission and polarization signatures, can provide a powerful diagnostic.

astro-ph.HE↗

arXiv2026

Frascati 22 GeV Workshop Summary

This document summarizes the outcomes of the "Science at the Luminosity Frontier: Jefferson Lab at 22 GeV" workshop, held at the INFN Laboratori Nazionali di Frascati in December 2024. The primary goal of the workshop was to critically assess and refine the scientific case for a proposed energy upgrade of the Continuous Electron Beam Accelerator Facility (CEBAF) to 22 GeV. This document intends to capture the progress on developing the scientific case since the publication of a lengthy "White Paper" in summer 2024 signed by about 450 authors.

nucl-ex↗

arXiv2025

Three-dimensional imaging of hadrons with hard exclusive reactions: advances in experiment, theory, phenomenology, and lattice QCD

Generalized Parton Distributions (GPDs) have emerged as a powerful framework for exploring the internal structure of hadrons in terms of their partonic constituents. Over the past three decades, the field has witnessed significant theoretical and experimental advancements. The interpretation of GPDs in impact parameter space offers a vivid three-dimensional visualization of hadron structure, correlating longitudinal momentum and transverse spatial distributions, thereby enabling tomographic imaging of hadrons. Furthermore, the link between GPDs and the matrix elements of the QCD energy-momentum tensor provides access to fundamental properties of hadrons, including spin decomposition and internal pressure distributions. Notably, recent analyses of Deeply Virtual Compton Scattering (DVCS) data have enabled the empirical extraction of the quark pressure profile inside the proton. Motivated by the rapidly evolving experimental landscape, this white paper provides a timely and focused overview of recent developments in GPD theory, phenomenology, and lattice QCD studies. Its scope is shaped by the needs and opportunities of forthcoming experimental programs, and it highlights advances that are particularly relevant for the next generation of dedicated measurements, including the extended Jefferson Lab 12 GeV program and its potential 22 GeV upgrade, J-PARC, COMPASS/AMBER, LHC ultra-peripheral collisions, and the future electron-ion colliders EIC and EicC.

hep-ph↗

arXiv2025

Recoil Polarization in $K^+Y$ Electroproduction in the Nucleon Resonance Region with CLAS12

Hyperon recoil polarization measurements for the exclusive electroproduction of $K^+\Lambda$ and $K^+\Sigma^0$ final states from an unpolarized proton target have been carried out using the CLAS12 spectrometer at Jefferson Laboratory. The measurements at beam energies of 6.535~GeV and 7.546~GeV span the range of four-momentum transfer $Q^2$ from 0.3 to 4.5~GeV$^2$ and invariant mass $W$ from 1.6 to 2.4~GeV, while covering the full center-of-mass angular range of the $K^+$. These new $\Lambda$ polarization observables extend the existing data in a similar kinematic range but from a significantly larger dataset. However, they represent the first electroproduction measurements of this observable for the $\Sigma^0$. These data will allow for better exploration of the reaction mechanism in strangeness production, for further understanding of the spectrum and structure of excited nucleon states that couple to $KY$, and for improved insight into the strong interaction in the non-perturbative domain.

nucl-ex↗

arXiv2025

VLBA observations of a sample of low-power compact symmetric objects

Compact symmetric objects (CSOs) are intrinsically compact extragalactic radio sources that are thought to be the progenitors of classical radio galaxies. To date, evolutionary models have mainly focused on the formation and growth of high-power radio sources, leaving unanswered many questions related to low-power objects, whose relativistic jets are likely more prone to instabilities. We present a new sample of candidate low-power CSOs selected from the Faint Images of the Radio Sky at Twenty-cm (FIRST) survey. The main selection criteria are (i) a parsec-scale double radio morphology from archival Very Long Baseline Array (VLBA) images and (ii) a VLBA total flux density consistent with that from the FIRST survey, which rules out the presence of significant radio emission extending beyond the parsec scale. The final sample consists of 60 sources with radio luminosities between 10$^{24}$ and 10$^{27}$ W Hz$^{-1}$ at 1.4 GHz and projected linear sizes between 45 and 430 pc, which fill a region in the radio power-size plane that is currently underpopulated. We carried out VLBA observations at 4.98 GHz of a sub-sample of 20 sources among the brightest candidate CSOs with the aim of confirming their classification. We classify 12 sources as CSOs on the basis of their radio structure and spectral index distribution. In two out of the four CSOs with core identification, the asymmetry in the flux density of the outer components is in agreement with light travel time effects, and there is no evidence of jet-cloud interaction. If we assume a simplistic parametric model, most of the sources in the total sample have a jet power of $\sim 10^{44} - 10^{45}$ erg s$^{-1}$, making their evolutionary paths sensitive to the individual conditions of the jet and its environment.

astro-ph.GA↗

arXiv2025

How do recollimation-induced instabilities shape the propagation of hydrodynamic relativistic jets?

Recollimation is a phenomenon of particular importance in the dynamic evolution of jets and in the emission of high-energy radiation. Additionally, the full comprehension of this phenomenon provides insights into fundamental properties of jets in the vicinity of the Active Galactic Nucleus (AGN). Three-dimensional (magneto-)hydrodynamic simulations revealed that the jet conditions at recollimation favor the growth of strong instabilities, challenging the traditional view-supported from two-dimensional simulations-of confined jets undergoing a series of recollimation and reflection shocks. To investigate the stability of relativistic jets in AGNs at recollimation sites, we perform a set of long duration three-dimensional relativistic hydrodynamic simulations with the state-of-the-art PLUTO code, to focus on the development of hydrodynamical instabilities. We explore the non-linear growth of the instabilities and their effects on the physical jet properties as a function of the initial jet parameters: jet Lorentz factor, temperature, opening angle and jet-environment density-contrast. The parameter space is designed to describe low-power, weakly magnetized jets at small distances from the core (around the parsec scale). All collimating jets we simulated develop instabilities. Recollimation instabilities decelerate the jet, heat it, entrain external material, and move the recollimation point to shorter distances from the core. This is true for both conical and cylindrical jets. The instabilities, that are first triggered by the centrifugal instability, appear to be less disruptive in the case of narrower, denser, more relativistic, and warmer jets. These results provide valuable insights into the complex processes governing AGN jets and could be used to model the properties of low-power, weakly magnetized jetted AGNs.

astro-ph.HE↗

arXiv2024

Evolution of current and pressure driven instabilities in relativistic jets

Instabilities in relativistic magnetized jets are thought to be deeply connected to their energy dissipation properties and to the consequent acceleration of the non-thermal emitting relativistic particles. Instabilities lead to the development of small scale dissipative structures, in which magnetic energy is converted in other forms. In this paper we present three-dimensional numerical simulations of the instability evolution in highly magnetized plasma columns, considering different kinds of equilibria. In fact, the hoop stresses related to the azimuthal component of magnetic field can be balanced either by the magnetic pressure gradient (force-free equilibria, FF) or by the thermal pressure gradient (pressure-balanced equilibria, PB) or by a combination of the two. FF equilibria are prone to current-driven instabilities (CDI), while PB equilibria are prone to pressure-driven instabilities (PDI). We perform a global linear stability analysis, from which we derive the different instability properties in the two regimes, showing that PDI have larger growth rates and are also unstable for high wavenumbers. The numerical simulations of the non-linear instability evolution show similar phases of evolution in which the formation of strong current sheets is followed by a turbulent quasi steady-state. PDI are however characterized by a faster evolution, by the formation of smaller scale dissipative structures and larger magnetic energy dissipation.

astro-ph.HE↗

arXiv2023

FR0 jets and recollimation-induced instabilities

The recently discovered population of faint FR0 radiogalaxies has been interpreted as the extension to low power of the classical FRI sources. Their radio emission appears to be concentrated in very compact (pc-scale) cores, any extended emission is very weak or absent and VLBI observations show that jets are already mildly or sub-relativistic at pc scales. Based on these observational properties we propose here that the jets of FR0s are strongly decelerated and disturbed at pc scale by hydrodynamical instabilities.}{With the above scenario in mind, we study the dynamics of a low-power relativistic jet propagating into a confining external medium, focusing on the effects of entrainment and mixing promoted by the instabilities developing at the jet-environment interface downstream of a recollimation shock. We perform a 3D relativistic hydrodynamical simulation of a recollimated jet by means of the state-of-the-art code PLUTO. The jet is initially conical, relativistic (with initial Lorentz Factor $\Gamma$=5), cold and light with respect to the confining medium, whose pressure is assumed to slowly decline with distance. The magnetic field is assumed to be dynamically unimportant. The 3D simulation shows that, after the first recollimation/reflection shock system, a rapidlygrowing instability develops, as a result of the interplay between recollimation-induced instabilities and Richtmyer-Meshkov modes. In turn, the instabilities promote strong mixing and entrainment that rapidly lead to the deceleration of the jet and spread its momentum to slowly moving, highly turbulent external gas. We argue that this mechanism could account for the peculiarities of the low-power FR0 jets. For outflows with higher power, Lorentz factor or magnetic field, we expect that the destabilizing effects are less effective, allowing the survival of the jet up to the kpc scale, as observed in FRIs.

astro-ph.HE↗

arXiv2022

Physics with CEBAF at 12 GeV and Future Opportunities

We summarize the ongoing scientific program of the 12 GeV Continuous Electron Beam Accelerator Facility (CEBAF) and give an outlook into future scientific opportunities. The program addresses important topics in nuclear, hadronic, and electroweak physics including nuclear femtography, meson and baryon spectroscopy, quarks and gluons in nuclei, precision tests of the standard model, and dark sector searches. Potential upgrades of CEBAF are considered, such as higher luminosity, polarized and unpolarized positron beams, and doubling the beam energy.

nucl-ex↗

arXiv2021

Making Fanaroff-Riley I radio sources III. The effects of the magnetic field on relativistic jets' propagation and source morphologies

Extragalactic radio sources appear under different morphologies, the most frequent ones are classified as Fanaroff-Riley type I (FR I), typically with lower luminosities, and Fanaroff-Riley type II, (FR II), typically more luminous. This simple classification, however, has many exceptions that we intend to investigate. Following previous analyses in the three-dimensional Hydrodynamic and Magneto-Hydrodynamic limits, we extend the numerical investigation to the Relativistic Magneto-Hydrodynamic regime, to include sources whose jet kinetic power sets in the range that separates FR Is from FR IIs. We consider weakly and mildly relativistic, underdense, supersonic jets that propagate in a stratified medium. In the model, the ambient temperature increases with distance from the jet origin maintaining constant pressure. We present three cases with low, high and intermediate kinetic luminosity that evolve into different morphologies. We find that the resulting morphology can be highly time dependent and that, apart from the jet power, the jet-to-ambient density ratio and the magnetization parameter play a crucial role in the jet evolution as well.

astro-ph.HE↗

arXiv2021

Current driven kink instabilities in relativistic jets: dissipation properties

We analyze the evolution of current driven kink instabilities of a highly magnetized relativistic plasma column, focusing in particular on its dissipation properties. The instability evolution leads to the formation of thin current sheets where the magnetic energy is dissipated. We find that the total amount of dissipated magnetic energy is independent of the dissipation properties. Dissipation occurs in two stages: a peak when the instability saturates, which is characterized by the formation of a helicoidal current sheet at the boundary of the deformed plasma column, followed by a weaker almost flat phase, in which turbulence develops. The detailed properties of these two phases depend on the equilibrium configuration and other parameters, in particular on the steepness of the pitch radial profile, on the presence of an external axial magnetic field and on the amount of magnetization. These results are relevant for high energy astrophysical sources, since current sheets can be the sites of magnetic reconnection where particles can be accelerated to relativistic energies and give rise to the observed radiation.

astro-ph.HE↗

arXiv2020

Diquark Correlations in Hadron Physics: Origin, Impact and Evidence

The last decade has seen a marked shift in how the internal structure of hadrons is understood. Modern experimental facilities, new theoretical techniques for the continuum bound-state problem and progress with lattice-regularised QCD have provided strong indications that soft quark+quark (diquark) correlations play a crucial role in hadron physics. For example, theory indicates that the appearance of such correlations is a necessary consequence of dynamical chiral symmetry breaking, viz. a corollary of emergent hadronic mass that is responsible for almost all visible mass in the universe; experiment has uncovered signals for such correlations in the flavour-separation of the proton's electromagnetic form factors; and phenomenology suggests that diquark correlations might be critical to the formation of exotic tetra- and penta-quark hadrons. A broad spectrum of such information is evaluated herein, with a view to consolidating the facts and therefrom moving toward a coherent, unified picture of hadron structure and the role that diquark correlations might play.

hep-ph↗

arXiv2020

The different flavors of extragalactic jets: the role of relativistic flow deceleration

We perform three-dimensional numerical simulations of relativistic (with a Lorentz factor of 10), non magnetized jets propagating in a uniform density environment, in order to study the effect of the entrainment and the consequent deceleration. Our simulations investigate the jet propagation inside the galaxy core, where, most likely, the deceleration occurs more efficiently. We compare cases with different density and pressure ratios with respect to the ambient medium finding that low density jets are efficiently decelerated and reach a quasi steady state in which, over a length of 600 jet radii, slow down from highly to sub-relativistic velocities. At the opposite, denser jets keep highly relativistic velocity over the same length. We discuss these results in relation to the Faranoff Riley (FR) radio-sources classification. We infer that lower density jets can give rise to FR 0 and FR I radio-sources, while higher density jets may be connected to FR II radio-sources.

astro-ph.HE↗

arXiv2019

Dark matter search in a Beam-Dump eXperiment (BDX) at Jefferson Lab -- 2018 update to PR12-16-001

This document complements and completes what was submitted last year to PAC45 as an update to the proposal PR12-16-001 "Dark matter search in a Beam-Dump eXperiment (BDX)" at Jefferson Lab submitted to JLab-PAC44 in 2016. Following the suggestions contained in the PAC45 report, in coordination with the lab, we ran a test to assess the beam-related backgrounds and validate the simulation framework used to design the BDX experiment. Using a common Monte Carlo framework for the test and the proposed experiment, we optimized the selection cuts to maximize the reach considering simultaneously the signal, cosmic-ray background (assessed in Catania test with BDX-Proto) and beam-related backgrounds (irreducible NC and CC neutrino interactions as determined by simulation). Our results confirmed what was presented in the original proposal: with 285 days of a parasitic run at 65 $μ$A (corresponding to $10^{22}$ EOT) the BDX experiment will lower the exclusion limits in the case of no signal by one to two orders of magnitude in the parameter space of dark-matter coupling versus mass.

physics.ins-det↗

arXiv2019

Making Faranoff-Riley I radio sources II. The effects of jet magnetization

Radio sources of low power are the most common in the universe. Their jets typically move at nonrelativistic velocity and show plume-like morphologies that in many instances appear distorted and bent. We investigate the role of magnetic field on the propagation and evolution of low-power jets and the connection between the field intensity and the resulting morphology. The problem is addressed by means of three-dimensional magnetohydrodynamic (MHD) simulations. We consider supersonic jets that propagate in a stratified medium. The ambient temperature increases with distance from the jet origin maintaining constant pressure. Jets with low magnetization show an enhanced collimation at small distances with respect to hydrodynamic (HD) cases studied in a previous paper. These jets eventually evolve in a way similar to the HD cases. Jets with higher magnetization are affected by strong nonaxisymmetric modes that lead to the sudden jet energy release. From there on, distorted plumes of jet material move at subsonic velocities. This transition is associated with the formation of structures reminiscent of the `warm spots' observed in wide-angle-tail (WAT) sources.

astro-ph.HE↗

arXiv2018

Tau-structure for the Double Ramification Hierarchies

In this paper we continue the study of the double ramification hierarchy of [Bur15]. After showing that the DR hierarchy satisfies tau-symmetry we define its partition function as the (logarithm of the) tau-function of the string solution and show that it satisfies various properties (string, dilaton and divisor equations plus some important degree constraints). We then formulate a stronger version of the conjecture from [Bur15]: for any semisimple cohomological field theory, the Dubrovin-Zhang and double ramification hierarchies are related by a normal (i.e. preserving the tau-structure [DLYZ14]) Miura transformation which we completely identify in terms of the partition function of the CohFT. In fact, using only the partition functions, the conjecture can be formulated even in the non-semisimple case (where the Dubrovin-Zhang hierarchy is not defined). We then prove this conjecture for various CohFTs (trivial CohFT, Hodge class, Gromov-Witten theory of $\mathbb{CP}^1$, $3$-, $4$- and $5$-spin classes) and in genus $1$ for any semisimple CohFT. Finally we prove that the higher genus part of the DR hierarchy is basically trivial for the Gromov-Witten theory of smooth varieties with non-positive first Chern class and their analogue in Fan-Jarvis-Ruan-Witten quantum singularity theory [FJRW].

math-ph↗

arXiv2018

Dark matter search in a Beam-Dump eXperiment (BDX) at Jefferson Lab: an update on PR12-16-001

This document is an update to the proposal PR12-16-001 Dark matter search in a Beam-Dump eXperiment (BDX) at Jefferson Lab submitted to JLab-PAC44 in 2016 reporting progress in addressing questions raised regarding the beam-on backgrounds. The concerns are addressed by adopting a new simulation tool, FLUKA, and planning measurements of muon fluxes from the dump with its existing shielding around the dump. First, we have implemented the detailed BDX experimental geometry into a FLUKA simulation, in consultation with experts from the JLab Radiation Control Group. The FLUKA simulation has been compared directly to our GEANT4 simulations and shown to agree in regions of validity. The FLUKA interaction package, with a tuned set of biasing weights, is naturally able to generate reliable particle distributions with very small probabilities and therefore predict rates at the detector location beyond the planned shielding around the beam dump. Second, we have developed a plan to conduct measurements of the muon ux from the Hall-A dump in its current configuration to validate our simulations.

physics.ins-det↗