# Dr Arnau Rios Huguet

### Biography

Please have a look at my website here for further details.

I started my research career with a PhD on theoretical nuclear physics at the University of Barcelona (2007). I was hired as a postdoctoral research fellow (2007-2009) at the National Superconducting Cyclotron Laboratory (Michigan State University, USA) to work alongside Prof Pawel Danielewicz. I moved to Surrey in 2009 with an EU Marie Curie Fellowship. In 2011, I was awarded a 5-year STFC Advanced Fellowship to continue advancing new methods for nuclear theory. I became a Lecturer at Surrey in 2015.

I lead developments in many-body Green’s functions techniques for over a decade, in collaboration with groups at Barcelona & St Louis. I am also engaged in work at the nuclear structure and reactions levels, using time-dependent density functional techniques to study fission and fusion. My predicitions for neutron-star pairing gaps and equation of state have been used in astrophysical publications.

### University roles and responsibilities

- Year 2 Coordinator, Department of Physics

### Research

### Research interests

My research is at the forefront of nuclear theory and attempts to understand the emergent properties of atomic nuclei from its basic constituents – neutrons and protons. The methods I have developed over the 15 years of my research career have implications for experimental research at facilities worldwide, for astronomical observations (e.g. GW170817, the recent multimessenger binary neutron star inspiral) and for practical applications (like fission processes in nuclear reactors).

I specialise in self-consistent Green’s functions (SCGF) techniques, but I also have demonstrated expertise in density functional techniques. During my PhD thesis, I reviewed a series of problems in dense nuclear matter, ranging from ferromagnetism to hyperonic matter. As a postdoctoral researcher at Michigan State University, I developed a time-dependent Green’s functions program in (ongoing). I obtained a Marie Curie Fellowship at Surrey to develop predictions for correlated nuclear systems to be tested in experiments. My 5-year STFC Advanced Fellowship allowed me to establish a research program tying up correlated time-dependent and equilibrium nuclear properties.

### My teaching

#### 2013-Present *The Universe (PHY1037) *

More information on the Module Descriptor and SurreyLearn.

#### 2016-Present *Explosive Stellar Phenomena (PHYM052)*

More information on the Module Descriptor and SurreyLearn.

#### 2016-Present *Electromagnetism, Scalar and Vector Fields (PHY2064)*

More information on the Module Descriptor and SurreyLearn.

### Courses I teach on

## Undergraduate

### My publications

### Publications

temperature, Phys.Rev. C 72

within the Brueckner-Hartree-Fock approximation extended to finite temperature.

The bare interaction in the nucleon sector is the Argonne V18 potential

supplemented with an effective three-body force to reproduce the saturating

properties of nuclear matter. The modern Nijmegen NSC97e potential is employed

for the hyperon-nucleon and hyperon-hyperon interactions. The effect of the

temperature on the in-medium effective interaction is found to be, in general,

very small and the single-particle potentials differ by at most 25% for

temperatures in the range from 0 to 60 MeV. The bulk properties of infinite

matter of baryons, either nuclear isospin symmetric or a beta-stable

composition which includes a non-zero fraction of hyperons, are obtained. It is

found that the presence of hyperons can modify the thermodynamical properties

of the system in a non-negligible way.

symmetric nuclear matter,

emphasis on the liquid-gas phase transition. We use a standard covariance

analysis to propagate statistical uncertainties from the density functional to

the thermodynamic properties. We use four functionals with known covariance

matrices to obtain as wide a set of results as possible. Our findings suggest

that thermodynamical properties are very well constrained by fitting data at

zero temperature. The propagated statistical errors in the liquid-gas phase

transition parameters are relatively small.

the Skyrme interaction, Phys.Rev. C 71

finite temperature using Skyrme-type interactions. It is shown that the

critical density at which ferromagnetism takes place decreases with

temperature. This unexpected behaviour is associated to an anomalous behaviour

of the entropy which becomes larger for the polarized phase than for the

unpolarized one above a certain critical density. This fact is a consequence of

the dependence of the entropy on the effective mass of the neutrons with

different third spin component and a new constraint on the parameters of the

effective Skyrme force is derived in order to avoid such a behaviour.

composition as well as the dynamical response function of dense hadronic

matter. Matter at very high densities is probably composed of other particles

than nucleons and little is known on the Fermi liquid properties of hadronic

multicomponent systems. We will discuss the effects that the presence of

$\Lambda$ hyperons might have on the response and, in particular, on its

influence on the thermodynamical stability of the system and the mean free path

of neutrinos in dense matter.

of freedom is a well?established prism with which to understand atomic nuclei. Self?consistent mean?field models provide one tool to understand nuclear shapes, and their link to other nuclear properties and observables. We present examples of how the time?dependent extension of the mean?field approach can be used in particular to shed light on nuclear shape properties, particularly looking at the giant resonances built on deformed nuclear ground states, and at dynamics in highly-deformed fission isomers. Example calculations are shown of 28Si in the first case, and 240Pu in the latter case.

many-body system which lead to a fragmentation of the single-particle strength

over a wide range of energies and momenta. We address the question of how this

fragmentation affects the thermodynamical properties of nuclear matter. In

particular, we show that the entropy can be computed with the help of a

spectral function which can be evaluated in terms of the self-energy obtained

in the Self-Consistent Green's Function approach. Results for the density and

temperature dependences of the entropy per particle for symmetric nuclear

matter are presented and compared to the results of lowest order finite

temperature Brueckner--Hartree--Fock calculations. The effects of correlations

on the calculated entropy are small, if the appropriate quasi-particle

approximation is used. The results demonstrate the thermodynamical consistency

of the self-consistent T-matrix approximation for the evaluation of the Green's

functions.

induce a sizeable depletion of low momenta in the ground state of a nuclear

many-body system. The self-consistent Green's function method within the ladder

approximation provides an \textit{ab-initio} description of correlated nuclear

systems that accounts properly for these effects. The momentum distribution

predicted by this approach is analyzed in detail, with emphasis on the

depletion of the lowest momentum state. The temperature, density, and nucleon

asymmetry (isospin) dependence of the depletion of the Fermi sea is clarified.

A connection is established between the momentum distribution and the

time-ordered components of the self-energy, which allows for an improved

interpretation of the results. The dependence on the underlying nucleon-nucleon

interaction provides quantitative estimates of the importance of short-range

and tensor correlations in nuclear systems.

the time evolution of quantum many-body systems. In view of a rising computer

power, an effort is underway to apply the Green's functions formalism to the

dynamics of central nuclear reactions. As the first step, mean-field evolution

for the density matrix for colliding slabs is studied in one dimension. The

strategy to extend the dynamics to correlations is described.

symmetry energy,

and its density dependence. The calculations are performed in the framework of

the Brueckner-Hartree-Fock and the Self-Consistent Green's Functions methods.

Within Brueckner-Hartree-Fock, the Hellmann-Feynman theorem gives access to the

kinetic energy contribution as well as the contributions of the different

components of the nucleon-nucleon interaction. The tensor component gives the

largest contribution to the symmetry energy. The decomposition of the symmetry

energy in a kinetic part and a potential energy part provides physical insight

on the correlated nature of the system, indicating that neutron matter is less

correlated than symmetric nuclear matter. Within the Self-Consistent Green's

Function approach, we compute the momentum distributions and we identify the

effects of the high momentum components in the symmetry energy. The results are

obtained for the realistic interaction Argonne V18 potential, supplemented by

the Urbana IX three-body force in the Brueckner-Hartree-Fock calculations.

nuclear matter,

nuclear matter fulfill energy weighted sum rules. The validity of these sum

rules within the self-consistent Green's function approach is investigated. The

various contributions to these sum rules and their convergence as a function of

energy provide information about correlations induced by the realistic

interaction between the nucleons. These features are studied as a function of

the asymmetry of nuclear matter.

dressed quasiparticle properties from a microscopic perspective. Using self-consistent ladder selfenergies,

we find both spectra and lifetimes of such quasiparticles in nuclear matter. With a consistent

choice of the group velocity, the nucleon mean-free path can be computed. Our results indicate that, for

energies above 50 MeV at densities close to saturation, a nucleon has a mean-free path of 4 to 5 fm.

relevant for astrophysical phenomena and provides a starting point for the discussion

of pairing properties in nuclear structure. Short-range correlations can significantly

deplete the available single-particle strength around the Fermi surface and thus provide

a reduction mechanism of the pairing gap. Here, we study this effect in the singlet and

triplet channels of both neutron matter and symmetric nuclear matter. Our calculations

use phase-shift equivalent interactions and chiral two-body and three-body interactions

as a starting point. We find an unambiguous reduction of the gap in all channels with

very small dependence on the NN force in the singlet neutron matter and the triplet

nuclear matter channel. In the latter channel, SRC alone provide a 50% reduction of

the pairing gap.

understanding of the fragmentation of nuclear states due to short-range and tensor correlations.

Purpose: The aim of this paper is to compare on a quantitative basis the single-particle spectral function

generated by different nuclear hamiltonians in symmetric nuclear matter using the first three energy-weighted

moments.

Method: The spectral functions are calculated in the framework of the self-consistent Green's function approach

at finite temperature within a ladder resummation scheme. We analyze the first three moments of the spectral

function and connect these to the correlations induced by the interactions between the nucleons in symmetric

nuclear matter. In particular, the variance of the spectral function is directly linked to the dispersive contribution

of the self-energy. The discussion is centered around two- and three-body chiral nuclear interactions, with and

without renormalization, but we also provide results obtained with the traditional phase-shift-equivalent CD-Bonn

and Av18 potentials.

Results: The variance of the spectral function is particularly sensitive to the short-range structure of the force,

with hard-core interactions providing large variances. Chiral forces yield variances which are an order of magnitude

smaller and, when tamed using the similarity renormalization group, the variance reduces significantly and in

proportion to the renormalization scale. The presence of three-body forces does not substantially affect the

results.

Conclusions: The first three moments of the spectral function are useful tools in analysing the importance

of correlations in nuclear ground states. In particular, the second-order moment provides a direct insight into

dispersive contributions to the self-energy and its value is indicative of the fragmentation of single-particle states.

**Background:**

An accurate determination of the core-crust transition is necessary in the modelling of neutron

stars for astrophysical purposes. The transition is intimately related to the isospin dependence of the nuclear

force at low baryon densities

**Purpose:**

To study the symmetry energy and the core-crust transition in neutron stars using the finite-range

Gogny nuclear interaction and to examine the deduced crustal thickness and crustal moment of inertia.

Methods: The second-, fourth- and sixth-order coefficients of the Taylor expansion of the energy per particle in

powers of the isospin asymmetry are analyzed for Gogny forces. These coefficients provide information about the

departure of the symmetry energy from the widely used parabolic law. The neutron star core-crust transition is

evaluated by looking at the onset of thermodynamical instability of the liquid core. The calculation is performed

with the exact Gogny EoS (i.e., the Gogny EoS with the full isospin dependence) for the ²-equilibrated matter

of the core, and also with the Taylor expansion of the Gogny EoS in order to assess the influence of isospin

expansions on locating the inner edge of neutron star crusts.

**Results:**

The properties of the core-crust transition derived from the exact EoS differ from the predictions of

the Taylor expansion even when the expansion is carried through sixth order in the isospin asymmetry. Gogny

forces, using the exact EoS, predict the ranges 0.094 fm?3 . Át . 0.118 fm?3

for the transition density and

0.339 MeV fm?3 . Pt . 0.665 MeV fm?3

for the transition pressure. The transition densities show an anticorrelation

with the slope parameter L of the symmetry energy. The transition pressures are not found to correlate

with L. Neutron stars obtained with Gogny forces have maximum masses below 1.74Mý and relatively small

moments of inertia. The crustal mass and moment of inertia are evaluated and comparisons are made with the

constraints from observed glitches in pulsars.

**Conclusions:**

The finite-range exchange contribution of the nuclear force, and its associated non-trivial isospin

dependence, is key in determining the core-crust transition properties. Finite-order isospin expansions do not

reproduce the core-crust transition results of the exact EoS. The predictions of the Gogny D1M force for the

stellar crust are overall in broad agreement with those obtained using the Skyrme-Lyon EoS.

^{16}O+

^{16}O

^{32}S are extracted based on both the time-dependent Hartree-Fock and the time-dependent density matrix methods. The latter goes beyond the mean-field approximation by taking into account the effect of two-body correlations, but in practical simulations of fusion reactions we find that the total energy is not conserved. We analyze this problem and propose a solution that allows for a clear quantification of dissipative effects in the dynamics. Compared to mean-field simulations, friction coefficients in the density-matrix approach are enhanced by about 20 %. An energy-dependence of the dissipative mechanism is also demonstrated, indicating that two-body collisions are more efficient at generating friction at low incident energies.

matter employing both two- and three-nucleon chiral interactions. Our discussion focuses on the

sources of systematic errors in microscopic quantum many body predictions. On the one hand, we

test uncertainties of our results arising from changes in the construction of chiral Hamiltonians. We

use five different chiral forces with consistently derived three-nucleon interactions. On the other

hand, we compare the ladder resummation in the Self-Consistent Green?s Functions approach to

finite temperature Brueckner?Hartree?Fock calculations. We find that systematics due to Hamiltonians

dominate over many-body uncertainties. Based on this wide pool of calculations, we estimate

that the critical temperature is T

_{c}= 16±2 MeV, in reasonable agreement with experimental results.

We also find that there is a strong correlation between the critical temperature and the saturation

energy in microscopic many-body simulations.

^{100}Sn.