Dr Arnau Rios Huguet

Senior Lecturer in Theoretical Nuclear Physics

Academic and research departments

Department of Physics.


University roles and responsibilities

  • Year 2 Coordinator, Department of Physics


    Research interests

    My teaching

    Courses I teach on


    My publications


    Rios Huguet A, Polls A, Vidana I, Ramos A, Dickhoff WH, Muther H (2010) Nucleon correlations and the equation of state of nuclear matter,AIP Conference Proceedings132299pp. 99-107 American Institute of Physics
    The self-consistent Green's function method within the ladder approximation provides a microscopic description of correlated nuclear systems which properly treats the nucleon-nucleon correlations induced by the short-range and tensor components present in any realistic interaction. These correlations produce a sizable depletion of low momenta below the Fermi surface as well as the occupation of high momenta in the nuclear ground state. A few representative results for nuclear matter are presented to illustrate the present progress in the application of this method to nuclear systems
    Rios A, Polls A, Ramos A, Vidaña I (2005) Bulk and single-particle properties of hyperonic matter at finite temperature, Phys.Rev. C72
    Bulk and single-particle properties of hot hyperonic matter are studied 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.
    Rios Huguet A, Carbone A, Polls A (2012) High-momentum components in the nuclear symmetry energy,Europhysics Letters: a letters journal exploring the frontiers of physics9722001 EDP Sciences
    The short-range and tensor correlations associated to realistic nucleon-nucleon interactions induce a population of high-momentum components in the many-body nuclear wave function. We study the impact of such high-momentum components on bulk observables associated to isospin asymmetric matter. The kinetic part of the symmetry energy is strongly reduced by correlations when compared to the non-interacting case. The origin of this behavior is elucidated using realistic interactions with different short-range and tensor structures.
    Rios Huguet A, Dickhoff WH, Polls A (2011) Role of short-range and tensor correlations in nuclei,Journal of Physics, Conference Series312(2)022007 Institute of Physics
    The present theoretical understanding of the role of short-range correlations in nuclei near stability is reviewed. Two effects are identified in particular: first, the depletion of mean-field single-particle strength that is no longer available to participate in low-lying excitations. Second, the admixture of high-momentum nucleons in the ground state that is implied by the vanishing relative wave functions of pairs in the medium. The role of the tensor force will be further clarified by discussing isospin-polarized matter. It is demonstrated that the depletion of the proton and neutron Fermi seas depends strongly on the nuclear tensor force and appears to be determined by nucleon-nucleon scattering data. The increased role of short-range and tensor correlations for the minority species makes the case for further experimental scrutiny of nuclei with large neutron excess. Appropriate data of single- and two-nucleon knockout experiments are employed to illustrate the role of short-range and tensor correlations.
    Rios A, Roca-Maza X (2014) Covariance analysis of finite temperature density functional theory: symmetric nuclear matter,
    We study symmetric nuclear matter at finite temperature, with particular 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.
    Feroci M, Den Herder JW, Bozzo E, Barret D, Brandt S, Hernanz M, Van Der Klis M, Pohl M, Santangelo A, Stella L, Watts A, Wilms J, Zane S, Ahangarianabhari M, Albertus C, Alford M, Alpar A, Altamirano D, Alvarez L, Amati L, Amoros C, Andersson N, Antonelli A, Argan A, Artigue R, Artigues B, Atteia JL, Azzarello P, Bakala P, Baldazzi G, Balman S, Barbera M, Van Baren C, Bhattacharyya S, Baykal A, Belloni T, Bernardini F, Bertuccio G, Bianchi S, Bianchini A, Binko P, Blay P, Bocchino F, Bodin P, Bombaci I, Bonnet Bidaud JM, Boutloukos S, Bradley L, Braga J, Brown E, Bucciantini N, Burderi L, Burgay M, Bursa M, Budtz-Jørgensen C, Cackett E, Cadoux FR, Caïs P, Caliandro GA, Campana R, Campana S, Capitanio F, Casares J, Casella P, Castro-Tirado AJ, Cavazzuti E, Cerda-Duran P, Chakrabarty D, Château F, Chenevez J, Coker J, Cole R, Collura A, Cornelisse R, Courvoisier T, Cros A, Cumming A, Cusumano G, D'ai A, D'elia V, Del Monte E, De Luca A, De Martino D, Dercksen JPC, De Pasquale M, De Rosa A, Del Santo M, Di Cosimo S, Diebold S, Di Salvo T, Donnarumma I, Drago A, Durant M, Emmanoulopoulos D, Erkut MH, Esposito P, Evangelista Y, Fabian A, Falanga M, Favre Y (2014) The large observatory for x-ray timing, Proceedings of SPIE - The International Society for Optical Engineering9144
    © 2014 SPIE.The Large Observatory For x-ray Timing (LOFT) was studied within ESA M3 Cosmic Vision framework and participated in the final downselection for a launch slot in 2022-2024. Thanks to the unprecedented combination of effective area and spectral resolution of its main instrument, LOFT will study the behaviour of matter under extreme conditions, such as the strong gravitational field in the innermost regions of accretion flows close to black holes and neutron stars, and the supranuclear densities in the interior of neutron stars. The science payload is based on a Large Area Detector (LAD, 10 m2 effective area, 2-30 keV, 240 eV spectral resolution, 1° collimated field of view) and a Wide Field Monitor (WFM, 2-50 keV, 4 steradian field of view, 1 arcmin source location accuracy, 300 eV spectral resolution). The WFM is equipped with an on-board system for bright events (e.g. GRB) localization. The trigger time and position of these events are broadcast to the ground within 30 s from discovery. In this paper we present the status of the mission at the end of its Phase A study.
    Rios A, Soma V (2013) Self-consistent Green's functions calculation of the nucleon mean-free path,PROGRESS IN NONEQUILIBRIUM GREEN'S FUNCTIONS V (PNGF V)427 IOP PUBLISHING LTD
    Rios Huguet A, Vidana I, Providencia C, Polls A (2011) Symmetry Energy, Neutron Star Crust and Neutron Skin Thickness,Few Body Physics50(1-4)pp. 327-329 Springer
    Rios Huguet A, Baldo M, Polls A, Schulze H-J, Vidana I (2012) Comparative study of neutron and nuclear matter with simplifi ed Argonne nucleon-nucleon potentials,Physical Review C: Nuclear Physics86(6)064001 American Physical Society
    We present calculations of the energy per particle of pure neutron and symmetric nuclear matter with simplified Argonne nucleon-nucleon potentials for different many-body theories. We compare critically the Brueckner-Hartree-Fock results to other formalisms, such as the Brueckner-Bethe-Goldstone expansion up to third order, self-consistent Green's functions, auxiliary field diffusion Monte Carlo, and Fermi hypernetted chain. We evaluate the importance of spin-orbit and tensor correlations in the equation of state and find these to be important in a wide range of densities.
    Rios A, Polls A, Vidaña I (2005) Ferromagnetic instabilities in neutron matter at finite temperature with the Skyrme interaction, Phys.Rev. C71
    The properties of spin polarized neutron matter are studied both at zero and 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.
    Rios A, Polls A, Margueron J (2006) Neutrino propagation in dense hadronic matter, Acta Phys.Polon. B37pp. 2403-2410
    Neutrino propagation in protoneutron stars requires the knowledge of the 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.
    Rios Huguet A, Polls A, Dickhoff WH (2014) Density and isospin asymmetry dependence of high-momentum components,Physical Review C: Nuclear Physics89
    We study the one-body momentum distribution at different densities in nuclear matter, with special emphasis on its components at high momentum. Explicit calculations for finite neutron-proton asymmetry, based on the ladder self-consistent Green's function approach, allow us to access the isospin dependence of momentum distributions and elucidate their role in neutron-rich systems. Comparisons with the deuteron momentum distribution indicate that a substantial proportion of high-momentum components are dominated by tensor correlations. We identify the density dependence of these tensor correlations in the momentum distributions. Further, we find that high-momentum components are determined by the density of each sub-species and we provide a new isospin asymmetry scaling of these components. We use different realistic nucleon-nucleon interactions to quantify the model dependence of our results.
    Rios Huguet A, Polls A, Carbone A, Vidana I (2012) Liquid-gas phase transition in nuclear matter: Mean-field and beyond,EPJ Web of Conferences3200003 EDP Sciences - Web of Conferences
    Carbone A, Polls A, Rios A, Vidana I (2011) Latent heat of nuclear matter,Physical Review C (Nuclear Physics)83(2)024308 American Physical Society
    We study the latent heat of the liquid-gas phase transition in symmetric nuclear matter using self-consistent mean-field calculations with a few Skyrme forces. The temperature dependence of the latent heat is rather independent of the mean-field parametrization and it can be characterized by a few parameters. At low temperatures, the latent heat tends to the saturation energy. Near the critical point, the latent heat goes to zero with a well-determined mean-field critical exponent. A maximum value of the latent heat in the range l similar to 25-30 MeV is found at intermediate temperatures, which might have experimental relevance. All these features can be explained from very basic principles.
    Stevenson PD, Rios A, Goddard PM (2015) Shapes and Dynamics from the Time-Dependent Mean Field, Bulgarian Journal of Physics42(4)pp. 354-361
    Explaining observed properties in terms of underlying shape degrees 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.
    Goddard PM, Stevenson PD, Rios A (2014) Cause of the charge radius isotope shift at the N =126 shell gap,EPJ Web of Conferences66
    We discuss the mechanism causing the 'kink' in the charge radius isotope shift at the N = 126 shell closure. The occupation of the 1i11/2 neutron orbital is the decisive factor for reproducing the experimentally observed kink. We investigate whether this orbital is occupied or not by different Skyrme effective interactions as neutrons are added above the shell closure. Our results demonstrate that several factors can cause an appreciable occupation of the 1i11/2 neutron orbital, including the magnitude of the spin-orbit field, and the isoscalar effective mass of the Skyrme interaction. The symmetry energy of the effective interaction has little influence upon its ability to reproduce the kink. © Owned by the authors, published by EDP Sciences, 2014.
    Rios Huguet A, Vidana I, Polls A, Providencia C (2012) Symmetry energy within the BHF approach,Journal of Physics: Conference Series342012012 Institute of Physics
    We analyze the correlations of the slope and curvature parameters of the symmetry energy with the neutron skin thickness of neutron-rich isotopes, and the crust-core transition density in neutron stars. Microscopic Brueckner?Hartree?Fock results are compared with those obtained with several Skyrme and relativistic mean field models. Our results confirm that there is an inverse correlation between the neutron skin thickness and the transition density.
    Rios A, Polls A, Ramos A, Müther H (2006) The entropy of a correlated system of nucleons, Phys.Rev. C74
    Realistic nucleon-nucleon interaction induce correlations to the nuclear 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.
    Rios A, Polls A, Dickhoff WH (2009) Depletion of the nuclear Fermi sea, Phys.Rev.C79
    The short-range and tensor components of the bare nucleon-nucleon interaction 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.
    Rios Huguet A, Dickhoff WH, Ding D, Witte SJ, Dussan H, Polls A Pairing in bulk nuclear matter beyond BCS,
    Rios A, Danielewicz P (2008) Time-dependent Green's functions approach to nuclear reactions, AIPConf.Proc.995pp. 98-103
    Nonequilibrium Green's functions represent underutilized means of studying 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.
    Carbone A, Polls A, Providência C, Rios A, Vidaña I (2013) Tensor force effects and high-momentum components in the nuclear symmetry energy,
    We analyze microscopic many-body calculations of the nuclear 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.
    Rios A, Barker B, Buchler M, Danielewicz P (2011) Towards a nonequilibrium Green's function description of nuclear reactions: One-dimensional mean-field dynamics,Annals of Physics326(5)pp. 1274-1319 Elsevier
    Nonequilibrium Green?s function methods allow for an intrinsically consistent description of the evolution of quantal many-body body systems, with inclusion of different types of correlations. In this paper, we focus on the practical developments needed to build a Green?s function methodology for nuclear reactions. We start out by considering symmetric collisions of slabs in one dimension within the mean-field approximation. We concentrate on two issues of importance for actual reaction simulations. First, the preparation of the initial state within the same methodology as for the reaction dynamics is demonstrated by an adiabatic switching on of the mean-field interaction, which leads to the mean-field ground state. Second, the importance of the Green?s function matrix-elements far away from the spatial diagonal is analyzed by a suitable suppression process that does not significantly affect the evolution of the elements close to the diagonal. The relative lack of importance of the far-away elements is tied to system expansion. We also examine the evolution of the Wigner function and verify quantitatively that erasing of the off-diagonal elements corresponds to averaging out of the momentum?space details in the Wigner function.
    Carbone A, Polls A, Rios Huguet A (2013) Symmetric nuclear matter with chiral three-nucleon forces in the self-consistent Green's functions approach, Physical Review C: Nuclear Physics88(4)044302pp. 044302-1-044302-11 American Physical Society
    Rios Huguet A, Carbone A, Polls A, Vidana I (2011) Liquid-gas phase transition in nuclear matter in the mean-field approximation,Journal of Physics, Conference Series321(1)012058 Institute of Physics
    The liquid gas phase transition in nuclear systems is a unique phenomenon, at the frontier of nuclear, many-body and statistical physics. We use self-consistent mean-field calculations to quantify the properties of the transition in symmetric nuclear matter. We explore the available parameter space of critical properties by analyzing the mean-field dependence of the phase transition. The latent heat of the transition is computed and we find that it exhibits a model independent temperature dependence due to basic physical principles.
    Rios Huguet A, Polls A, Ramos A, Muther H (2008) Liquid-gas phase transition in nuclear matter from realistic many-body approaches,Physical Review C (Nuclear Physics)78044314 American Physical Society
    The existence of a liquid-gas phase transition for hot nuclear systems at subsaturation densities is a well-established prediction of finite-temperature nuclear many-body theory. In this paper, we discuss for the first time the properties of such a phase transition for homogeneous nuclear matter within the self-consistent Green's function approach. We find a substantial decrease of the critical temperature with respect to the Brueckner-Hartree-Fock approximation. Even within the same approximation, the use of two different realistic nucleon-nucleon interactions gives rise to large differences in the properties of the critical point.
    Rios A, Polls A, Müther H (2005) Sum rules of single-particle spectral functions in hot asymmetric nuclear matter,
    The neutron and proton single-particle spectral functions in asymmetric 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.
    Goddard P, Stevenson P, Rios A (2016) Fission dynamics within time-dependent Hartree-Fock. II. Boost-induced fission,PHYSICAL REVIEW C93(1)ARTN 014620 AMER PHYSICAL SOC
    Goddard P, Stevenson P, Rios A (2015) Fission dynamics within time-dependent Hartree-Fock: Deformation-induced fission,PHYSICAL REVIEW C92(5)ARTN 054610 AMER PHYSICAL SOC
    Goddard PM, Cooper N, Werner V, Rusev G, Stevenson PD, Rios A, Bernards C, Chakraborty A, Crider BP, Glorius J, Ilieva RS, Kelley JH, Kwan E, Peters EE, Pietralla N, Raut R, Romig C, Savran D, Schnorrenberger L, Smith MK, Sonnabend K, Tonchev AP, Tornow W, Yates SW (2013) Dipole response of $^76$Se above 4 MeV,Phys. Rev. C88(6)pp. 064308-064308 American Physical Society
    The dipole response of 76 34 Se in the energy range from 4 to 9 MeV has been analyzed using a (³ × ,³ 2 ) polarized photon scattering technique, performed at the High Intensity ³ -Ray Source facility at Triangle Universities Nuclear Laboratory, to complement previous work performed using unpolarized photons. The results of this work offer both an enhanced sensitivity scan of the dipole response and an unambiguous determination of the parities of the observed J=1 states. The dipole response is found to be dominated by E1 excitations, and can reasonably be attributed to a pygmy dipole resonance. Evidence is presented to suggest that a significant amount of directly unobserved excitation strength is present in the region, due to unobserved branching transitions in the decays of resonantly excited states. The dipole response of the region is underestimated when considering only ground state decay branches. We investigate the electric dipole response theoretically, performing calculations in a three-dimensional (3D) Cartesian-basis time-dependent Skyrme-Hartree-Fock framework.
    Isaule F, Arellano-Garcia H, Rios Huguet A (2016) Di?neutrons in neutron matter within Brueckner?Hartree?Fock approach,Physical Review C: Nuclear Physics94034004 American Physical Society
    We investigate the appearance of di?neutron bound states in pure neutron matter within the Brueckner?Hartree?Fock approach at zero temperature. We consider the Argonne v18 and Paris bare interactions as well as chiral two? and three?nucleon forces. Self?consistent single?particle potentials are calculated by controlling explicitly singularities in the g matrix associated with bound states. Di?neutrons are loosely bound, with binding energies below 1 MeV, but are unambiguously present for Fermi momenta below 1 fm?1 for all interactions. Within the same framework we are able to calculate and characterize di?neutron bound states, obtaining mean radii as high as <110 fm. Implications of these findings are presented and discussed.
    Rios Huguet A, Polls A, Vidana I (2009) Hot neutron matter from a self-consistent Green's-functions approach,Physical Review C (Nuclear Physics)79025802 American Physical Society
    A systematic study of the microscopic and thermodynamical properties of pure neutron matter at finite temperature within the self-consistent Green's-function approach is performed. The model dependence of these results is analyzed by both comparing the results obtained with two different microscopic interactions, the CD Bonn and the Argonne V18 potentials, and by analyzing the results obtained with other approaches, such as the Brueckner-Hartree-Fock approximation, the variational approach, and the virial expansion.
    Rios Huguet A, Soma V (2012) Self-consistent Green's functions calculation of the nucleon mean free path,Physical Review Letters108012501 American Physical Society
    The extension of Green?s functions techniques to the complex energy plane provides access to fully 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.
    Vidana I, Providencia C, Polls A, Rios A (2009) Density dependence of the nuclear symmetry energy: A microscopic perspective,Physical Review C80(4)045806 American Physical Society
    We perform a systematic analysis of the density dependence of nuclear symmetry energy within the microscopic Brueckner-Hartree-Fock (BHF) approach using the realistic Argonne V18 nucleon-nucleon potential plus a phenomenological three-body force of Urbana type. Our results are compared thoroughly with those arising from several Skyrme and relativistic effective models. The values of the parameters characterizing the BHF equation of state of isospin asymmetric nuclear matter fall within the trends predicted by those models and are compatible with recent constraints coming from heavy ion collisions, giant monopole resonances, or isobaric analog states. In particular we find a value of the slope parameter L=66.5 MeV, compatible with recent experimental constraints from isospin diffusion, L=88±25 MeV. The correlation between the neutron skin thickness of neutron-rich isotopes and the slope L and curvature Ksym parameters of the symmetry energy is studied. Our BHF results are in very good agreement with the correlations already predicted by other authors using nonrelativistic and relativistic effective models. The correlations of these two parameters and the neutron skin thickness with the transition density from nonuniform to ²-stable matter in neutron stars are also analyzed. Our results confirm that there is an inverse correlation between the neutron skin thickness and the transition density.
    Rios A, Barker B, Danielewicz P (2013) Towards a nonequilibrium Green's function description of nuclear reactions,PROGRESS IN NONEQUILIBRIUM GREEN'S FUNCTIONS V (PNGF V)427 IOP PUBLISHING LTD
    Goddard PM, Stevenson PD, Rios A (2013) Charge radius isotope shift across the N=126 shell gap, Physical Review Letters110(3)
    We revisit the problem of the kink in the charge radius shift of neutron-rich even isotopes near the N=126 shell closure. We show that the ability of a Skyrme force to reproduce the isotope shift is determined by the occupation of the neutron 1i11/2 orbital beyond N=126 and the corresponding change it causes to deeply-bound protons orbitals with a principal quantum number of 1. Given the observed position of the single-particle energies, one must either ensure occupation is allowed through correlations, or not demand that the single-particle energies agree with experimental values at the mean-field level. © 2013 American Physical Society.
    Rios Huguet A, Polls A, Dickhoff W (2017) Pairing and Short-Range Correlations in Nuclear Systems,Journal of Low Temperature Physics189(5-6)pp. 234-249 Springer Verlag
    The structure and density dependence of the pairing gap in infinite matter is 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.
    Rios Huguet Arnau, Ding D, Dussan H, Dickhoff WH, Witte SJ, Carbone A, Polls A (2016) Pairing in high-density neutron matter including short- and long-range correlations,Physical Review C: Nuclear Physics94025802 APS
    Pairing gaps in neutron matter need to be computed in a wide range of densities to address open questions in neutron-star phenomenology. Traditionally, the Bardeen-Cooper-Schrieffer approach has been used to compute gaps from bare nucleon-nucleon interactions. Here we incorporate the influence of short- and long-range correlations in the pairing gaps. Short-range correlations are treated, including the appropriate fragmentation of single-particle states, and substantially suppress the gaps. Long-range correlations dress the pairing interaction via density and spin modes and provide a relatively small correction. We use different interactions, some with three-body forces, as a starting point to control for any systematic effects. Results are relevant for neutron-star cooling scenarios, in particular in view of the recent observational data on Cassiopeia A.
    Rios Huguet A, Sellahewa R (2014) Isovector properties of the Gogny interaction,PHYSICAL REVIEW C90(5) AMER PHYSICAL SOC
    We analyse the properties of the Gogny interaction in homogeneous matter, with special emphasis on the isovector sector. We provide analytical expressions for both the single-particle and the bulk properties of symmetric and asymmetric nuclear matter. We perform an extensive analysis of these properties using 11 parametrizations extracted from the literature. We find that most Gogny interactions have low values for the slope of the symmetry energy, outside the range of empirically extracted values. As a test of extreme isospin dependence, we also study the mass-radius relations implied by the different Gogny equations of state. Our results call for a more careful fitting procedure of the isovector properties of Gogny functionals.
    Rios Huguet A, Carbone A, Polls A (2017) Comparison of nuclear hamiltonians using spectral function sum rules,Physical Review C96(1)014003 American Physical Society
    Background: The energy weighted sum rules of the single-particle spectral functions provide a quantitative 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.
    Carbone A, Cipollone A, Barbieri C, Rios Huguet A, Polls A (2013) Self-consistent Green's functions formalism with three-body interactions,Physical Review C: Nuclear Physics88(5) American Physical Society
    We extend the self-consistent Green's functions formalism to take into account three-body interactions. We analyze the perturbative expansion in terms of Feynman diagrams and define effective one- and two-body interactions, which allows for a substantial reduction of the number of diagrams. The procedure can be taken as a generalization of the normal ordering of the Hamiltonian to fully correlated density matrices. We give examples up to third order in perturbation theory. To define nonperturbative approximations, we extend the equation of motion method in the presence of three-body interactions. We propose schemes that can provide nonperturbative resummation of three-body interactions. We also discuss two different extensions of the Koltun sum rule to compute the ground state of a many-body system.
    Arellano-Garcia H, Isaule F, Rios Huguet A (2017) Di-nucleon structures in homogeneous nuclear matter based on two- and three-nucleon interactions,EUROPEAN PHYSICAL JOURNAL A52(299) SPRINGER
    We investigate homogeneous nuclear matter within the Brueckner-Hartree-Fock (BHF) approach in the limits of isospin-symmetric nuclear matter (SNM) as well as pure neutron matter at zero temperature. The study is based on realistic representations of the internucleon interaction as given by Argonne v18, Paris, Nijmegen I and II potentials, in addition to chiral N3LO interactions, including three-nucleon forces up to N2LO. Particular attention is paid to the presence of di-nucleon bound states structures in 1S0 and 3SD1 channels, whose explicit account becomes crucial for the stability of self-consistent solutions at low densities. A characterization of these solutions and associated bound states is discussed. We confirm that coexisting BHF single-particle solutions in SNM, at Fermi momenta in the range 0.13 ? 0.3 fm?1 , is a robust feature under the choice of realistic internucleon potentials.
    Gonzalez-Boquera C, Centelles M, Vinas X, Rios Huguet Arnau (2017) Higher-order symmetry energy and neutron star core-crust transition in Gogny forces,Physical Review C96(6)065806 American Physical Society

    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


    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.


    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.


    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.

    My research has been focused on time-dependent aspects of nuclear physics both at the mean-field and at the beyond-mean-field level. At the mean field level, the objective of my PhD has been to understand how the introduction of the tensor part of the Skyrme interaction affects heavy ion collisions and giant magnetic resonances, in a self consistent and symmetry unrestricted manner. The introduction of the tensor force redistributes the strength of the giant magnetic resonances within the same energy range. Within the study of heavy ion collisions of 16O+16O the introduction of the tensor decreased the amount of dissipation in the system. At the beyond mean field level, the objective of my PhD was to implement a time dependent density matrix (TDDM) theory, self consistently, without symmetry restrictions using the full Skyrme force. TDDM allows an order by order truncation of the Bogoliubov-Born-ýýGreen-ýýKirkwood-ýýYvon (BBGKY) hierarchy, which relates the evolution of many body densities. If two-body correlations are assumed to dominate the dynamics of the system, the resulting equations incorporate one-particle-one-hole and two-particle-two-hole correlations. A variety of different nuclei below A=40 were chosen to study the formation of correlations for different nuclear ground states. Two body correlations were found to have a noticeable effect on the ground state properties of these nuclei. For example on average 4 - 5 % of the total energy is due to correlations. When time dependent calculations were performed with these correlated nuclei, computational limitations led to problems with conservation laws.
    Goddard PM, Stevenson PD, Rios A (2013) Charge Radius Isotope Shift Across the N=126 Shell Gap,Physical Review Letters110
    We revisit the problem of the kink in the charge radius shift of neutron-rich even isotopes near the N=126 shell closure. We show that the ability of a Skyrme force to reproduce the isotope shift is determined by the occupation of the neutron 1i11/2 orbital beyond N=126 and the corresponding change it causes to deeply-bound protons orbitals with a principal quantum number of 1. Given the observed position of the single-particle energies, one must either ensure occupation is allowed through correlations, or not demand that the single-particle energies agree with experimental values at the mean-field level.
    Wen Kai, Barton M. C., Rios Arnau, Stevenson P. D. (2018) Two-body dissipation effect in nuclear fusion reactions,Physical Review C98(1)014603pp. 014603-1-014603-8 American Physical Society
    Friction coefficients for the fusion reaction 16O+16O ’ 32S 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.
    Carbone Arianna, Polls Artur, Rios Arnau (2018) Microscopic predictions of the nuclear matter liquid-gas phase transition,Physical Review C98(2)025804 American Physical Society
    We present first-principle predictions for the liquid-gas phase transition in symmetric nuclear 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 Tc = 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.
    Rios Huguet Arnau (2019) Beta decay gets the ab initio treatment,Nature Physics15pp. pp 425-426 Nature Research
    One of the fundamental radioactive decay modes of nuclei is ² decay. Now, nuclear theorists have used first-principles simulations to explain nuclear ² decay properties across a range of light- to medium-mass isotopes, up to 100Sn.
    Wen K., Barton M.C., Rios A., Stevenson P.D. (2019) Dissipation Dynamics of Nuclear Fusion Reactions,Acta Physica Polonica B - proceedings of the 2018 Zakopane Conference on Nuclear Physics Extremes of the Nuclear Landscape50(3)pp. 567-572 Jagiellonian University
    Based on both the time-dependent Hartree?Fock (TDHF) and the time-dependent density matrix (TDDM) methods, we adopt a macroscopic reduction procedure to investigate the dissipation dynamics of nuclear fusion reactions. The TDDM method is an extension of TDHF, in the sense that it goes beyond the mean-field concept and takes into account two-body correlations explicitly. To investigate the effect of correlations on dissipation, the collective trajectories as well as the friction coefficients for the reaction ¹vO + ¹vO ’³²S are extracted. Our results suggest that two-body correlations play a relevant role in the fusion process.
    Pons Marina, Juliá-Díaz Bruno, Polls Artur, Rios Arnau, Vidaña Isaac (2020) The Hellmann?Feynman theorem at finite temperature,American Journal of Physics88(6)pp. 503-510 American Association of Physics Teachers
    We present a simple derivation of the Hellmann?Feynman theorem at finite temperature. We illustrate its validity by considering three relevant examples, which can be used in quantum mechanics lectures: the one-dimensional harmonic oscillator, the one-dimensional Ising model, and the Lipkin model. We show that the Hellmann?Feynman theorem allows one to calculate expectation values of operators that appear in the Hamiltonian. This is particularly useful when the total free energy is available, but there is no direct access to the thermal average of the operators themselves.
    Keeble J.W.T., Rios A. (2020) Machine learning the deuteron,Physics Letters, Section B809135743 Elsevier
    We use machine learning techniques to solve the nuclear two-body bound state problem, the deuteron. We use a minimal one-layer, feed-forward neural network to represent the deuteron S- and D-state wavefunction in momentum space, and solve the problem variationally using ready-made machine learning tools. We benchmark our results with exact diagonalisation solutions. We ?nd that a network with 6 hidden nodes (or 24 parameters) can provide a faithful representation of the ground state wavefunction, with a binding energy that is within 0.1% of exact results. This exploratory proof-of principle simulation may provide insight for future potential solutions of the nuclear many-body problem using variational arti?cial neural network techniques.