A Constrained-Path Quantum Monte-Carlo Approach for the Nuclear Shell Model

A new Quantum Monte-Carlo (QMC) approach is proposed to investigate low-lying states of nuclei within the shell model. The formalism relies on a variational symmetry-restored wave-function to guide the underlying Brownian motion. Sign/phase problems that usually plague QMC fermionic simulations are controlled by constraining stochastic paths through a fixed-node like approximation. Exploratory results in the sd and pf valence spaces with realistic effective interactions are presented. They prove the ability of the scheme to yield nearly exact yrast spectroscopies for both even- and odd-mass nuclei.Comment: 5 pages, 2 figure

Boundary conditions for star matter and other periodic fermionic systems

Bulk fermionic matter, as it can be notably found in supernova matter and neutrons stars, is subject to correlations of infinite range due to the antisymmetrisation of the N-body wave function, which cannot be explicitly accounted for in a practical simulation. This problem is usually addressed in condensed matter physics by means of the so-called Twist Averaged Boundary Condition method. A different ansatz based on the localized Wannier representation has been proposed in the context of antisymmetrized molecular dynamics. In this paper we work out the formal relation between the two approaches. We show that, while the two coincide when working with exact eigenstates of the N-body Hamiltonian, differences appear in the case of variational approaches, which are currently used for the description of stellar matter. Some model applications with Fermionic Molecular Dynamics are shown

New Lower Bound on Fermion Binding Energies

We derive a new lower bound for the ground state energy $E^{\rm F}(N,S)$ of N fermions with total spin S in terms of binding energies $E^{\rm F}(N-1,S \pm 1/2)$ of (N-1) fermions. Numerical examples are provided for some simple short-range or confining potentials.Comment: 4 pages, 1 eps figur

Planck pre-launch status : The Planck mission

Peer reviewe

Impact of COVID-19 on cardiovascular testing in the United States versus the rest of the world

Objectives: This study sought to quantify and compare the decline in volumes of cardiovascular procedures between the United States and non-US institutions during the early phase of the coronavirus disease-2019 (COVID-19) pandemic. Background: The COVID-19 pandemic has disrupted the care of many non-COVID-19 illnesses. Reductions in diagnostic cardiovascular testing around the world have led to concerns over the implications of reduced testing for cardiovascular disease (CVD) morbidity and mortality. Methods: Data were submitted to the INCAPS-COVID (International Atomic Energy Agency Non-Invasive Cardiology Protocols Study of COVID-19), a multinational registry comprising 909 institutions in 108 countries (including 155 facilities in 40 U.S. states), assessing the impact of the COVID-19 pandemic on volumes of diagnostic cardiovascular procedures. Data were obtained for April 2020 and compared with volumes of baseline procedures from March 2019. We compared laboratory characteristics, practices, and procedure volumes between U.S. and non-U.S. facilities and between U.S. geographic regions and identified factors associated with volume reduction in the United States. Results: Reductions in the volumes of procedures in the United States were similar to those in non-U.S. facilities (68% vs. 63%, respectively; p = 0.237), although U.S. facilities reported greater reductions in invasive coronary angiography (69% vs. 53%, respectively; p < 0.001). Significantly more U.S. facilities reported increased use of telehealth and patient screening measures than non-U.S. facilities, such as temperature checks, symptom screenings, and COVID-19 testing. Reductions in volumes of procedures differed between U.S. regions, with larger declines observed in the Northeast (76%) and Midwest (74%) than in the South (62%) and West (44%). Prevalence of COVID-19, staff redeployments, outpatient centers, and urban centers were associated with greater reductions in volume in U.S. facilities in a multivariable analysis. Conclusions: We observed marked reductions in U.S. cardiovascular testing in the early phase of the pandemic and significant variability between U.S. regions. The association between reductions of volumes and COVID-19 prevalence in the United States highlighted the need for proactive efforts to maintain access to cardiovascular testing in areas most affected by outbreaks of COVID-19 infection

Approches Monte-Carlo quantiques à chemins contraints pour le modèle en couches nucléaire

Le modèle en couches constitue aujourd'hui un cadre théorique de référence pour appréhender les propriétés des noyaux. Son applicabilité reste toutefois limitée par une croissance rédhibitoire de la dimension de l espace des états avec les nombres de couches de valence et de nucléons. Les méthodes Monte-Carlo quantiques (QMC) permettent a priori de contourner une telle difficulté en offrant une alternative à la diagonalisation du hamiltonien. Elles reposent sur une reformulation stochastique de l équation de Schrödinger qui ramène le problème à N-corps à un ensemble de problèmes à un corps dans des champs extérieurs fluctuants. L originalité de l échantillonnage proposé dans ce mémoire réside dans l utilisation d une approche avec restauration des symétries avant variation pour guider le mouvement brownien, et pour le contraindre afin de contrôler le problème de phase inhérent aux schémas QMC pour des fermions. Dans les couches sd et fp et avec les interactions résiduelles réalistes, nous avons ainsi obtenu une spectroscopie yrast en excellent accord avec les résultats issus de la diagonalisation du hamiltonien. En outre, une ouverture vers les systèmes d électrons fortement corrélés est présentée au travers de schémas QMC récemment suggérés pour le modèle de Hubbard bidimensionnel. Contrairement aux échantillonnages traditionnels, ils garantissent des trajectoires à poids positifs quel que soit le régime considéré. Nous avons prouvé que ces méthodes sont en réalité reliées à l'approche QMC mise en œuvre pour le modèle en couches. L'origine des erreurs systématiques qu'exhibent ces schémas, pourtant exempts de problème de signe dans ce cas, est par ailleurs élucidée.The shell model is a powerful theoretical framework for studying the nuclear structure. Unfortunately, the exponential scaling of the many-body space with the number of nucleons or the number of valence levels strongly restricts its applicability. The Quantum Monte-Carlo (QMC) methods may then be considered as a possible alternative to the direct diagonalization of the Hamiltonian. They are based on a stochastic reformulation of the Schrödinger equation that reduce the many-body problem to a set of one-body problems, numerically tractable, describing independent particles evolving in fluctuating external fields. The originality of the QMC scheme proposed in the present thesis is the use of a variational approach, with symmetry restoration before variation, to guide the Brownian motion and to constrain it in order to control the sign/phase problem that generally occurs in the QMC samplings for fermions. The yrast spectroscopy we obtain for sd- and fp-shell nuclei with realistic residual interactions agree remarkably well with the results from an exact diagonalization of the Hamiltonian. Moreover, an openness towards strongly correlated electronic systems is presented through new QMC schemes recently developed for the two-dimensional Hubbard model. In contrast with the traditional QMC samplings, they guarantee positive-weighted trajectories regardless the interaction strength or the doping of the lattice. We demonstrate that these schemes are in fact related to the stochastic approach applied to the nuclear shell model. The origin of the systematic errors that emerge in these methods, although free from sign/phase problem with the Hubbard Hamiltonian, is also discussed.CAEN-BU Sciences et STAPS (141182103) / SudocSudocFranceF