Past seminars

Recent development in multi-messenger astronomy leading to a wealth of new observational results through gravitational waves (LIGO/VIRGO) and X-ray spectra (NICER) have opened new possibilities to understand hadronic matter at densities at and beyond our terrestrial reach. They provide new constraints to the theories of nuclear physics, where an absolute energy density functional from ab-initio modelling is still not available.

General relativity guarantees that there is a unique one-to-one correspondence between static observables of neutron stars such as mass-radius relation or tidal deformability and equation of state (EoS) of beta equilibrated matter. However, these static properties are not enough to predict the composition of the interiors of neutron stars. In this talk, I will present EoS meta-modelling technique which was developed in our group, particularly to emphasize that even within pure nucleonic assumption, there is a big room for ambiguity in the composition. I will also demonstrate how recent observational data put bounds on the high density nucleonic matter using this (nucleonic) meta-modelling approach.

The interacting conditional wavefunction approach is a recently introduced method for performing quantum dynamics simulations that is multiconfigurational by construction, and hence that is able to capture quantitative accuracy for situations where mean-field theory fails. The technique is highly parallelizable and reformulates the traditional "curse of dimensionality" by using a stochastic wavefunction ansatz that is based on an interacting set of single-particle conditional wave functions. Despite the successes of the method to describe non-equilibrium quantum processes in model systems, the efficiency of the method for describing the dynamics of real systems is yet to be proven. A particular drawback of the method is the lack of a consistent solution of the time-independent Schrödinger equation, which makes the applicability of the method limited to systems for which, e.g., the ground state can be exactly calculated. We have recently filled this gap by putting forth an imaginary-time variation of the method. I will provide a throughout revision of the real- and imaginary-time versions of the method and show the ability of the method for capturing static and dynamic properties in model systems made of interacting electrons, nuclei and photons.

We report on our calculation of the hypertriton partial decay rate Γ(Hyp --> 3He + π^-). Our calculation uses full three-body hypertriton and 3He wave functions, computed with the no-core shell model (NCSM) and taking as input the NNLOsim NN+NNN and LO YN effective field theory Hamiltonians.

The hypertriton wave function includes Σ-Λ admixtures, and we consider both weak vertices in the matrix element, Λ→N + π and Σ→N+π. The small Sigma probability in the hypertriton, P<0.5%, enhances the decay rate by about 10%. This effect is more than countered when the Coulomb distortion of the outgoing pion wave is considered, which decreases the rate by ~15%.

Considering both effects, the decay rate evaluates to Γ=1.265 GHz. Γ is found to be strongly correlated with the Lambda separation energy B(Λ), not precisely determined by the state of the art nuclear Hamiltonians. This can be tracked down to the main contribution to the matrix element, coupling the deuteron-baryon components of hypertriton and ^3He amplitudes. Using the world average ratio R_3=Γ(Hyp --> 3Ηe+ π^-)/Γ_{π^-}(Hyp)=0.35 ± 0.04 and the isospin ΔI=1/2 rule, we derive a value for the hypertriton lifetime and show how it correlates with B(Λ).

Over the past decade, ab initio methods for finite nuclei have expanded their reach towards heavier nuclei and gained both in precision and complexity. With further progress and developments to be expected, the formal work is bound to take increasingly more time and errors to happen. This can hopefully be circumvented by developing safe and fast automated tools to generate the necessary expressions for the final numerical codes.

In this seminar, after introducing recent progress in ab initio methods [1,2], I will present the automated diagram generator ADG [3,4], focusing on its use for the recently-proposed Bogoliubov In-Medium Similarity Renormalization Group (BIMSRG) approach [5]. By generating automatically the diagrams and associated expressions at arbitrary orders in a given formalism in a safe way, such a tool can help fast-track the development of new formalisms or the implementation of older ones at higher precision, reducing the time to high-precision physical results.

[1] H. Hergert, Front. Phys. 8, 379 (2020), https://doi.org/10.3389/fphy.2020.00379

[2] P. Arthuis, C. Barbieri, M. Vorabbi and P. Finelli, Phys. Rev. Lett. 125 18, 182501 (2020), https://doi.org/10.1103/PhysRevLett.125.182501

[3] P. Arthuis, T. Duguet, A. Tichai, R.-D. Lasseri and J.-P. Ebran, Comput. Phys. Commun. 240, 202-227 (2019), https://doi.org/10.1016/j.cpc.2018.11.023

[4] P. Arthuis, A. Tichai, J. Ripoche and T. Duguet, Comput. Phys. Commun. 261, 107677 (2021), https://doi.org/10.1016/j.cpc.2020.107677

[5] A. Tichai, P. Arthuis, H. Hergert and T. Duguet, arXiv:2102.10889 [nucl-th] (2021), https://arxiv.org/abs/2102.10889

In this seminar, I will discuss the recent progress in connecting the symmetry-projected Generator Coordinate Method (PGCM) [1,2], which has been used for decades in the context of energy density functional (EDF) calculations, to the ab initio project. First, I will present the new numerical suite TAURUS [3] that is under development at the Universidad Autónoma de Madrid and that can perform very general variational calculations exploring the space of symmetry-projected Bogoliubov quasiparticle states and using state-of-the-art chiral interactions. Then, I will present the newly proposed In-Medium Generator Coordinate Method (IMGCM) that merges the PGCM with the In-Medium Similarity Renormalization Group (IMSRG) approach into a single consistent many-body technique. Finally, I will show first results concerning the calculation of the nuclear matrix elements for the neutrinoless double beta decay from first principles [4].

[1] J. J. Griffin and J. A. Wheeler, Phys. Rev. 108, 311 (1957). https://doi.org/10.1103/PhysRev.108.311

[2] B. Bally and M. Bender, Phys. Rev. C 103, 024315 (2021). https://doi.org/10.1103/PhysRevC.103.024315

[3] B. Bally, A. Sánchez-Fernández, and T. R. Rodríguez, Eur. Phys. J. A 57, 69 (2021). https://doi.org/10.1140/epja/s10050-021-00369-z

[4] J. M. Yao, B. Bally, J. Engel, R. Wirth, T. R. Rodríguez, and H. Hergert, Phys. Rev. Lett. 124, 232501 (2020). https://doi.org/10.1103/PhysRevLett.124.232501

Neutron stars unite many extremes of physics which cannot be recreated on Earth, making them excellent cosmic laboratories to study dense matter. One exciting characteristic is the presence of superfluid and superconducting components in mature neutron stars. Albeit created under very different circumstances, such macroscopic quantum behaviour exhibits many similarities with terrestrial condensates such as superfluid phases in helium, ultra-cold atomic gases, heavy-ion collisions or superconducting transitions in metals. In this talk, I will focus on the last relationship and discuss how we can describe the interiors of neutron stars by means of a two-component Ginzburg-Landau model, a framework well-known from the study of laboratory superconductors. By adapting this description to the neutron star interior and connecting it with realistic superfluid parameters and equations of state (specifically the Skyrme functional), we are able to determine the equilibrium properties of the superconducting component throughout the entire neutron star core. I will present the phase diagrams resulting from these analyses and discuss how our new approach can provide insights into the microphysical magnetic flux distribution in neutron star interiors.

The difference in the level of understanding between baryonic systems containing strangeness and the usual nuclear matter is so large due to the scarcity of experimental data. This limits the constraints that can be put on the low-energy coefficients appearing in the effective field theory (EFT) Lagrangian describing the interaction between two non-relativistic octet baryons. Trying to bridge this gap is lattice QCD, a demanding numerical approach to solve the complex dynamics of strongly-interacting systems. In this talk, I will present the results obtained by the NPLQCD collaboration for two octet-baryon systems, with strangeness ranging from 0 to -4, at two sets of quark masses that are heavier than those in nature [1,2]. In particular, I will present their scattering parameters and binding energies, as well as the constraints on the relevant EFT coefficients. The findings point to interesting symmetries observed in hypernuclear forces as predicted in the limit of QCD with a large number of colors.

[1] M. L. Wagman et al. (NPLQCD) Baryon-Baryon Interactions and Spin-Flavor Symmetry from Lattice Quantum Chromodynamics, Phys. Rev. D 96, 114510 (2017); arXiv:1706.06550 [hep-lat]

[2] M. Illa et al. (NPLQCD) Low-energy Scattering and Effective Interactions of Two Baryons at mπ ∼ 450 MeV from Lattice Quantum Chromodynamics; arXiv:2009.12357 [hep-lat]

The present knowledge of particle physics is based on the Standard Model (SM), which is an acknowledged theory of fundamental interactions and all known elementary particles. However, recent solar-neutrino experiments have proved that neutrinos have a non-zero mass, which conflicts with the SM as we know it. This signifies that the SM's perception of neutrinos is not accurate, making the search of new physics Beyond the Standard Model (BSM) most intriguing. Currently, the most practical way to access the yet-to-be-determined properties of neutrinos is observing neutrinoless double-beta (0νββ) decay.

In this seminar, I will introduce my dissertation research in which I studied probing 0νββ decay by exploiting available data on charge-exchange reactions and ordinary muon capture. I studied the strength distributions of these processes in the isobaric triplets corresponding to ββ-decay of the key 0νββ-decay candidates in the pnQRPA framework. By studying these strength distributions we can not only probe the intermediate states 0νββ decay (see the figure), but also eventually shed light on the unknown effective values of the couplings in wide excitation-energy and momentum-exchange regions relevant for 0νββ decay.

I will also cover some future directions for my postdoctoral research period at the University of Barcelona. The main aim is to extend the recent large-scale nuclear shell-model calculations of coherent elastic neutrino-nucleus scattering to inelastic scattering, heading up to high energies utilising pnQRPA.

[The dissertation study has been published in the University of Jyväskylä's dissertation series JYU Dissertations, number 288, Jyväskylä 2020. ISBN 978-951-39-8304-8 (PDF), URN: ISBN: 978-951-39-8304-8, ISSN 2489-9003. The publication is available in the JYX publication archive at: https://jyx.jyu.fi/handle/123456789/71822]

Beside quark-antiquark meson and three-quark baryons, quantum chromodynamics predicts that other types of quark and gluon configurations could bind into hadron resonances. During the last decade, a lot of theoretical and experimental efforts have been devoted to the  search of tetraquark, pentaquark and hybrid mesons. Several candidates have been observed by experimental collaborations but the interpretation of these signals requires a joint collaboration between phenomenologists and experimentalists. In this talk, I'll review the challenges of discovering and interpreting exotic hadrons, with a particular attention to the lightest hybrid meson.

Density Functional Theory (DFT) and the so called ab initio methods constitute two different and complementary approaches to the nuclear many-body problem. While the latter encounter computational limitations, the former is currently the only available method that can be applied to the whole nuclear chart. DFT allows the study of both ground state properties and nuclear excitations, and finds successful applications in nuclear structure and nuclear astrophysics.

I will briefly introduce the strong synergy between “heaven and earth” which is instrumental in the study of the nuclear Equation of State (EoS). That is, the relevance of ground experiments as well as accurate astronomical observations on neutron stars and gravitational waves to shed light into one of the most challenging problems of our times: how does subatomic matter organize itself.

On the basis of DFT, I will present our recent proposed solution to the apparent inconsistency between our current knowledge of the EoS, the energy of the isobaric analog state (IAS) in a heavy nucleus such as ²⁰⁸Pb (see a simple example in the figure for ⁹⁰Zr), and the isospin symmetry breaking forces in the nuclear medium [1]. This is achieved by performing state-of-the-art IAS calculations that include all isospin symmetry breaking contributions. To this aim, we propose a new energy density functional that is successful in reproducing the IAS excitation energy without compromising other properties of finite nuclei. Applications to the mass radius relation of a cold non-accreting neutron star will be briefly discussed.

Future perspectives on the efforts of building more accurate nuclear energy density functionals will be also given.

[1] X. Roca-Maza, G. Colò, and H. Sagawa, Phys. Rev. Lett. 120, 202501 (2018); arXiv:1803.09120

Neutrinoless double-beta (0νββ) decay is a proposed nuclear decay which turns out to be the most promising process to observe lepton number violation in the laboratory, and to establish whether neutrinos are its own antiparticle. Due to this unique potential, several international collaborations are actively searching for this rare decay, among them the NEXT experiment running at the Canfranc Underground Laboratory, in the Spanish Pyrenees. In order to plan these searches, estimations for the decay lifetimes, which are known to exceed 10^26 years, are crucial. However, the decay rate depends quadratically on nuclear matrix elements (NMEs) which are not well known, as state-of-the-art nuclear structure methods predict NMEs disagreeing by factors 2-3. An alternative avenue to learn about 0νββ NMEs is to find other observables correlated with 0νββ decay that may be easier to access experimentally. In this seminar, I will discuss results for double dipole magnetic (M1M1) transitions, which show a good correlation with 0νββ decay. This could be used to constrain 0νββ NMEs from measurements of nuclear 2γ M1M1 decays.

The realization of efficient and controlled interactions between photons and atomic media, or other quantum emitters, is a central goal within quantum optics. Photon loss — re-scattering of a photon into unwanted directions — represents a fundamental limitation to this aim. In typical atomic ensemble models, the atoms are assumed to emit independently and in an uncorrelated fashion. This paradigm however, is expected to break down for dense and ordered atomic arrays, where interference between emitted photons from different atoms becomes relevant, and can give rise to exciting phenomena such as the well known "super-" and "sub-radiance", with a strong modification of the atomic collective photon emission.

In this talk, we will provide a comprehensive treatment of this problem, and show through several examples that it is possible to take advantage of cooperative light-scattering in dense atomic ensembles. This includes a dramatic suppression of the error in single photon storage and retrieval [1, 2], the enhancement of excitation transport between two ring-shaped nano-structures [3], or an enhanced single-photon absorption by one of these rings when a single absorptive emitter is placed at its center [4].

[1] A. Asenjo-Garcia, M. Moreno-Cardoner, A. Albrecht, H. J. Kimble, and D. E. Chang, Phys. Rev. X 7, 031024 (2017).

[2] M. T. Manzoni, M. Moreno-Cardoner, A. Asenjo-Garcia, J. V. Porto, A. V. Gorshkov, and D. E. Chang, New Journal of Physics 20, 083048 (2018).

[3] M. Moreno-Cardoner, D. Plankensteiner, L. Ostermann, D. E. Chang, and H. Ritsch, Phys. Rev. A 100, 023806 (2019).

[4] M. Moreno-Cardoner, D. Holzinger, and H. Ritsch, arXiv:2010.09771 (2020).

The modeling of nuclear structure properties of neutron-rich nuclei is a crucial ingredient for understanding the production of heavy elements during the rapid neutron capture process (or r process). In order to properly interpret future kilonova observations, sensitivity studies addressing the impact of nuclear theoretical uncertainties are required. In this talk, I will present some recent network calculations based on nuclear input obtained within the Density Functional Theory (DFT) framework. In particular, I will focus in the role of nuclear masses and fission properties in the production of translead nuclei, and the possible implications for the electromagnetic counterparts produced during neutron star mergers. In the second part of this talk, I will introduce some recent advances regarding the large-scale DFT calculation of fission fragments distributions and the estimation of theoretical uncertainties using Bayesian machine-learning techniques.

The quark-gluon plasma (QGP) is expected to be produced with heavy-ion collisions at RHIC and LHC at very high temperatures and vanishing baryon densities. Due to the large mass and relaxation time of the c quark, charmed mesons are a powerful probe of the QGP and a proper theoretical understanding of their propagation in a hot medium is required. While lattice QCD is becoming an increasingly important tool in this regime, we employ a complementary theoretical approach that allows us to reach the QGP transition from the hadronic phase. In particular, we study the modification of open-charm mesons in a hot mesonic medium below Tc within an effective field theory based on chiral and heavy-quark spin-flavor symmetries within the imaginary-time formalism [1,2]. The in-medium unitarized amplitudes of the scattering of the heavy mesons with the pseudoscalar light mesons and the ground-state self-energies are calculated self-consistently. I will show that the D(*) and D_s(*) mesons acquire a substantial width and their masses drop with increasing temperatures, and that the excited mesonic states generated dynamically in our heavy-light molecular model (D_0^*(2300), D_1^*(2430), D_{s0}^*(2317) and D_{s1}^*(2460)) are also modified in a thermal medium. Besides, from the thermal ground-state spectral functions, we have computed for the first time open-charm Euclidean correlators that can be compared with those obtained with lattice QCD [3].

[1] G. Montaña, A. Ramos, L. Tolos and J. M. Torres-Rincon, Phys. Lett. B 806 (2020), 135464 doi:10.1016/j.physletb.2020.135464 [arXiv:2001.11877].

[2] G. Montaña, A. Ramos, L. Tolos and J. M. Torres-Rincon, (Submitted to Phys.Rev.D) [arXiv:2007.12601].

[3] G. Montaña, O. Kaczmarek, L. Tolos and A. Ramos, (Submitted to Eur.Phys.J.A) [arXiv:2007.15690].

Machine learning techniques are ubiquitous in science, and nuclear physics is not an exception. Neural networks have found a variety of nuclear physics applications in the past, including capturing systematic trends in data. Recently, machine learning tools have also been used to solve directly for the properties of quantum many-body fermionic and bosonic systems. These developments open the door for a first-principles machine learning description of atomic nuclei. I will discuss the embryonic applications of these techniques to few-body nuclear systems, and lay down some of the challenges ahead.

Keeble & Rios, Physics Letters B 809 135743 (2020); arxiv:1911.13092