23 research outputs found
Feynman diagrams versus Fermi-gas Feynman emulator
Precise understanding of strongly interacting fermions, from electrons in
modern materials to nuclear matter, presents a major goal in modern physics.
However, the theoretical description of interacting Fermi systems is usually
plagued by the intricate quantum statistics at play. Here we present a
cross-validation between a new theoretical approach, Bold Diagrammatic Monte
Carlo (BDMC), and precision experiments on ultra-cold atoms. Specifically, we
compute and measure with unprecedented accuracy the normal-state equation of
state of the unitary gas, a prototypical example of a strongly correlated
fermionic system. Excellent agreement demonstrates that a series of Feynman
diagrams can be controllably resummed in a non-perturbative regime using BDMC.
This opens the door to the solution of some of the most challenging problems
across many areas of physics
Childhood exposure due to the Chernobyl accident and thyroid cancer risk in contaminated areas of Belarus and Russia
The thyroid dose due to 131I releases during the Chernobyl accident was reconstructed for children and adolescents in two cities and 2122 settlements in Belarus, and in one city and 607 settlements in the Bryansk district of the Russian Federation. In this area, which covers the two high contamination spots in the two countries following the accident, data on thyroid cancer incidence during the period 1991-1995 were analysed in the light of possible increased thyroid surveillance. Two methods of risk analysis were applied: Poisson regression with results for the single settlements and Monte Carlo (MC) calculations for results in larger areas or sub-populations. Best estimates of both methods agreed well. Poisson regression estimates of 95% confidence intervals (CIs) were considerably smaller than the MC results, which allow for extra-Poisson uncertainties due to reconstructed doses and the background thyroid cancer incidence. The excess absolute risk per unit thyroid dose (EARPD) for the birth cohort 1971-1985 by the MC analysis was 2.1 (95% CI 1.0-4.5) cases per 10(4) person-year Gy. The point estimate is lower by a factor of two than that observed in a pooled study of thyroid cancer risk after external exposures. The excess relative risk per unit thyroid dose was 23 (95% CI 8.6-82) Gy(-1). No significant differences between countries or cities and rural areas were found. In the lowest dose group of the settlements with an average thyroid dose of 0.05 Gy the risk was statistically significantly elevated. Dependencies of risks on age-at-exposure and on gender are consistent with findings after external exposures
Thyroid cell irradiation by radioiodines: a new Monte Carlo electron track-structure code
The Future of the Correlated Electron Problem
The understanding of material systems with strong electron-electron
interactions is the central problem in modern condensed matter physics. Despite
this, the essential physics of many of these materials is still not understood
and we have no overall perspective on their properties. Moreover, we have very
little ability to make predictions in this class of systems. In this manuscript
we share our personal views of what the major open problems are in correlated
electron systems and we discuss some possible routes to make progress in this
rich and fascinating field. This manuscript is the result of the vigorous
discussions and deliberations that took place at Johns Hopkins University
during a three-day workshop January 27, 28, and 29, 2020 that brought together
six senior scientists and 46 more junior scientists. Our hope, is that the
topics we have presented will provide inspiration for others working in this
field and motivation for the idea that significant progress can be made on very
hard problems if we focus our collective energies.Comment: 55 pages, 19 figure
The Unitary Gas and its Symmetry Properties
The physics of atomic quantum gases is currently taking advantage of a
powerful tool, the possibility to fully adjust the interaction strength between
atoms using a magnetically controlled Feshbach resonance. For fermions with two
internal states, formally two opposite spin states, this allows to prepare long
lived strongly interacting three-dimensional gases and to study the BEC-BCS
crossover. Of particular interest along the BEC-BCS crossover is the so-called
unitary gas, where the atomic interaction potential between the opposite spin
states has virtually an infinite scattering length and a zero range. This
unitary gas is the main subject of the present chapter: It has fascinating
symmetry properties, from a simple scaling invariance, to a more subtle
dynamical symmetry in an isotropic harmonic trap, which is linked to a
separability of the N-body problem in hyperspherical coordinates. Other
analytical results, valid over the whole BEC-BCS crossover, are presented,
establishing a connection between three recently measured quantities, the tail
of the momentum distribution, the short range part of the pair distribution
function and the mean number of closed channel molecules.Comment: 63 pages, 8 figures. Contribution to the Springer Lecture Notes in
Physics "BEC-BCS Crossover and the Unitary Fermi gas" edited by Wilhelm
Zwerger. Revised version correcting a few typo
Thyroid cancer risk in Belarus after the Chernobyl accident: Comparison with external exposures.
Thermodynamics of the unitary Fermi gas
The understanding of quantum many-body systems is one of the most daunting challenges of modern physics. Thanks to recent progress in cooling and trapping techniques, it is now possible to investigate their properties in the well controlled environment of ultra-cold gas systems. In this article, we present experimental results on the thermodynamics of strongly correlated Fermi gases and we provide a reinterpretation of the equation of state of a strongly polarized Fermi gas in terms of Fermi liquid parameters<br/