arXiv subjects

Alessandro Drago

Publications and source records attributed to Alessandro Drago.

At least 55 records · Page 3Linked to original sources

arXiv2014

Dense hadronic matter in neutron stars

The existence of stars with masses up to $2 M_{\odot}$ and the hints of the existence of stars with radii smaller than $\sim 11$ km seem to require, at the same time, a stiff and a soft hadronic equation of state at large densities. We argue that these two apparently contradicting constraints are actually an indication of the existence of two families of compact stars: hadronic stars which could be very compact and quark stars which could be very massive. In this respect, a crucial role is played, in the hadronic equation of state, by the delta isobars whose early appearance shifts to large densities the formation of hyperons. We also discuss how recent experimental information on the symmetry energy of nuclear matter at saturation indicate, indirectly, an early appearance of delta isobars in neutron star matter.

nucl-th↗

arXiv2014

EMMI Rapid Reaction Task Force Meeting on 'Quark Matter in Compact Star'

The recent measurement of two solar mass pulsars has initiated an intense discussion on its impact on our understanding of the high-density matter in the cores of neutron stars. A task force meeting was held from October 7-10, 2013 at the Frankfurt Institute for Advanced Studies to address the presence of quark matter in these massive stars. During this meeting, the recent oservational astrophysical data and heavy-ion data was reviewed. The possibility of pure quark stars, hybrid stars and the nature of the QCD phase transition were discussed and their observational signals delineated.

astro-ph.HE↗

arXiv2014

Can very compact and very massive neutron stars both exist?

The existence of neutron stars with masses of $\sim 2\,M_{\odot}$ requires a stiff equation of state at high densities. On the other hand, the necessary appearance also at high densities of new degrees of freedom, such as hyperons and $Δ$ resonances, can lead to a strong softening of the equation of state with resulting maximum masses of $\sim 1.5\, M_{\odot}$ and radii smaller than $\sim 10$ km. Hints for the existence of compact stellar objects with very small radii have been found in recent statistical analysis of quiescent low-mass X-ray binaries in globular clusters. We propose an interpretation of these two apparently contradicting measurements, large masses and small radii, in terms of two separate families of compact stars: hadronic stars, whose equation of state is soft, can be very compact, while quark stars, whose equation of state is stiff, can be very massive. In this respect an early appearance of $Δ$ resonances is crucial to guarantee the stability of the branch of hadronic stars. Our proposal could be tested by measurements of radii with an error of $\sim 1$ km, which is within reach of the planned LOFT satellite, and it would be further strengthened by the discovery of compact stars heavier than $\sim 2\,M_{\odot}$.

nucl-th↗

arXiv2012

Growth of magnetic fields in accreting millisecond pulsars

R-modes can generate strong magnetic fields in the core of accreting millisecond neutron stars (NSs). The diffusion of these fields outside the core causes the growth of the external magnetic field and thus it affects the evolution of the spin down rates $\dot{P}$ of the millisecond pulsars (MSPs). The diffusion of the internal magnetic field provides a new evolutionary path for the MSPs. This scenario could explain the large $\dot{P}$ of the pulsars J1823-3021A and J1824-2452A.

astro-ph.HE↗

arXiv2012

Generation of strong magnetic fields by r-modes in millisecond accreting neutron stars: induced deformations and gravitational wave emission

Differential rotation induced by the r-mode instability can generate very strong toroidal fields in the core of accreting, millisecond spinning neutron stars. We introduce explicitly the magnetic damping term in the evolution equations of the r-modes and solve them numerically in the Newtonian limit, to follow the development and growth of the internal magnetic field. We show that the strength of the latter can reach large values, $B \sim 10^{14} $ G, in the core of the fastest accreting neutron stars. This is strong enough to induce a significant quadrupole moment of the neutron star mass distribution, corresponding to an ellipticity $|ε_B}| \sim 10^{-8}$. If the symmetry axis of the induced magnetic field is not aligned with the spin axis, the neutron star radiates gravitational waves. We suggest that this mechanism may explain the upper limit of the spin frequencies observed in accreting neutron stars in Low Mass X-Ray Binaries. We discuss the relevance of our results for the search of gravitational waves.

astro-ph.HE↗

arXiv2012

Mitigation and control of instabilities in DAFNE positron ring

The positron beam in the DAFNE e+/e- collider has always been suffering from strong e-cloud instabilities. In order to cope with them, several approaches have been adopted along the years: flexible and powerful bunch-by-bunch feedback systems, solenoids around the straight sections of the vacuum chamber and, in the last runs, e-cloud clearing electrodes inside the bending and wiggler magnets. Of course classic diagnostics tools have been used to evaluate the effectiveness of the adopted measures and the correct setup of the devices, in order to acquire total beam and bunch-by-bunch currents, to plot in real time synchrotron and betatron instabilities, to verify the vertical beam size enlargement in collision and out of collision. Besides, to evaluate the efficacy of the solenoids and of the clearing electrodes versus the instability speed, the more powerful tools have been the special diagnostics routines making use of the bunch-by-bunch feedback systems to quickly compute the growth rate instabilities and the bunch-by-bunch tune spread in different beam conditions.

physics.acc-ph↗

arXiv2012

A chiral quark-soliton model with broken scale invariance for nuclear matter

We present a model for describing nuclear matter at finite density based on quarks interacting with chiral fields, σand πand with vector mesons introduced as massive gauge fields. The chiral Lagrangian includes a logarithmic potential, associated with the breaking of scale invariance. We provide results for the soliton in vacuum and at finite density, using the Wigner-Seitz approximation. We show that the model can reach higher densities respect to the linear-σmodel and that the introduction of vector mesons allows to obtain saturation. This result was never obtained before in similar approaches.

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arXiv2012

The baryon number two system in the Chiral Soliton Model

We study the interaction between two B = 1 states in a Chiral Soliton Model where baryons are described as non-topological solitons. By using the hedgehog solution for the B = 1 states we construct three possible B = 2 configurations to analyze the role of the relative orientation of the hedgehog quills in the dynamics. The strong dependence of the intersoliton interaction on these relative orientations reveals that studies of dense hadronic matter using this model should take into account their implications.

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arXiv2011

The Hypothesis of Superluminal Neutrinos: comparing OPERA with other Data

The OPERA Collaboration reported evidence for muonic neutrinos traveling slightly faster than light in vacuum. While waiting further checks from the experimental community, here we aim at exploring some theoretical consequences of the hypothesis that muonic neutrinos are superluminal, considering in particular the tachyonic and the Coleman-Glashow cases. We show that a tachyonic interpretation is not only hardly reconciled with OPERA data on energy dependence, but that it clashes with neutrino production from pion and with neutrino oscillations. A Coleman-Glashow superluminal neutrino beam would also have problems with pion decay kinematics for the OPERA setup; it could be easily reconciled with SN1987a data, but then it would be very problematic to account for neutrino oscillations.

hep-ph↗

arXiv2011

A hybrid-chiral soliton model with broken scale invariance for nuclear matter

We present a model for describing nuclear matter at finite density based on quarks interacting with chiral fields, sigma and pion. The chiral Lagrangian also includes a logarithmic potential, associated with the breaking of scale invariance. We provide results for the soliton in vacuum and at finite density, using the Wigner-Seitz approximation. We show that the model can reach higher densities respect to the Linear-sigma model, up to approximately 3 rho_0 for m_sigma=1200 MeV.

nucl-th↗

arXiv2010

Masses and radii of neutron and quark stars

We discuss new limits on masses and radii of compact stars and we conclude that they can be interpreted as an indication of the existence of two classes of stars: "normal" compact stars and "ultra-compact" stars. We estimate the critical mass at which the first configuration collapses into the second.

astro-ph.SR↗

arXiv2009

Magnetic fields generated by r-modes in accreting millisecond pulsars

In rotating neutron stars the existence of the Coriolis force allows the presence of the so-called Rossby oscillations (r-modes) which are know to be unstable to emission of gravitational waves. Here, for the first time, we introduce the magnetic damping rate in the evolution equations of r-modes. We show that r-modes can generate very strong toroidal fields in the core of accreting millisecond pulsars by inducing differential rotation. We shortly discuss the instabilities of the generated magnetic field and its long time-scale evolution in order to clarify how the generated magnetic field can stabilize the star.

astro-ph.HE↗

arXiv2009

Transverse Polarisation of Quarks in Hadrons

We review the present state of knowledge regarding the transverse polarisation (or transversity) distributions of quarks. After some generalities on transverse polarisation, we formally define the transversity distributions within the framework of a classification of all leading-twist distribution functions. We describe the QCD evolution of transversity at leading and next-to-leading order. A comprehensive treatment of non-perturbative calculations of transversity distributions (within the framework of quark models, lattice QCD and QCD sum rules) is presented. The phenomenology of transversity (in particular, in Drell-Yan processes and semi-inclusive leptoproduction) is discussed in some detail. Finally, the prospects for future measurements are outlined.

hep-ph↗

arXiv2009

Magnetic fields generated by r-modes in accreting millisecond pulsars

In millisecond pulsars the existence of the Coriolis force allows the development of the so-called Rossby oscillations (r-modes) which are know to be unstable to emission of gravitational waves. These instabilities are mainly damped by the viscosity of the star or by the existence of a strong magnetic field. A fraction of the observed millisecond pulsars are known to be inside Low Mass X-ray Binaries (LMXBs), systems in which a neutron star (or a black hole) is accreting from a donor whose mass is smaller than 1 $M_\odot$. Here we show that the r-mode instabilities can generate strong toroidal magnetic fields by inducing differential rotation. In this way we also provide an alternative scenario for the origin of the magnetars.

astro-ph.HE↗

arXiv2009

A chiral lagrangian with Broken Scale: testing the restoration of symmetries in astrophysics and in the laboratory

We study matter at high density and temperature using a chiral lagrangian in which the breaking of scale invariance is regulated by the value of a scalar field, called dilaton \cite{Heide:1993yz,Carter:1995zi,Carter:1996rf,Carter:1997fn}. We provide a phase diagram describing the restoration of chiral and scale symmetries. We show that chiral symmetry is restored at large temperatures, but at low temperatures it remains broken at all densities. We also show that scale invariance is more easily restored at low rather than large baryon densities. The masses of vector mesons scale with the value of the dilaton and their values initially slightly decrease with the density but then they increase again for densities larger than $\sim 3 ρ_0$. The pion mass increases continuously with the density and at $ρ_0$ and T=0 its value is $\sim$ 30 MeV larger than in the vacuum. We show that the model is compatible with the bounds stemming from astrophysics, as e.g. the one associated with the maximum mass of a neutron star. The most striking feature of the model is a very significant softening at large densities, which manifests also as a strong reduction of the adiabatic index. While the softening has probably no consequence for Supernova explosion via the direct mechanism, it could modify the signal in gravitational waves associated with the merging of two neutron stars.

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arXiv2008

Formation of quark phases in compact stars and SN explosion

We describe possible scenarios of quark deconfinement in compact stars and we analyze their astrophysical implications. The quark deconfinement process can proceed rapidly, as a strong deflagration, releasing a huge amount of energy in a short time and generating an extra neutrino burst. If energy is transferred efficiently to the surface, like e.g. in the presence of convective instabilities, this burst could contribute to revitalize a partially failed SN explosion. We discuss how the neutrino observations from SN1987A would fit in this scenario. Finally, we focus on the fate of massive and rapidly rotating progenitors, discussing possible time separations between the moment of the core collapse and the moment of quark deconfinement. This mechanism can be at the basis of the interpretation of gamma ray bursts in which lines associated with heavy elements are present in the spectrum.

astro-ph↗

arXiv2008

Trends in Fast Feedback R&D

In this paper, starting from the basic description of the equation that governs the bunch motion and looking at the advances of the technology, three examples of feedback designs versus technology trend are presented and discussed. In particular the author compares three digital systems implemented or proposed for DAFNE and other e+/e- accelerators. Descriptions of some relevant features are also done. Conclusions on the digital feedback design trend are reported.

physics.acc-ph↗

arXiv2008

A compact star rotating at 1122 Hz and the r-mode instability

We show that r-mode instabilities severely constraint the composition of a compact star rotating at a submillisecond period. In particular, the only viable astrophysical scenario for such an object, present inside the Low Mass X-ray Binary associated with the x-ray transient XTE J1739-285, is that it has a strangeness content. Since previous analysis indicate that hyperonic stars or stars containing a kaon condensate are not good candidates, the only remaining possibility is that such an object is either a strange quark star or a hybrid quark-hadron star. We also discuss under which conditions sub-millisecond pulsars are rare.

astro-ph↗