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"Nuclear force"
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From Few to Many: Observing the Formation of a Fermi Sea One Atom at a Time
Knowing when a physical system has reached sufficient size for its macroscopic properties to be well described by many-body theory is difficult. We investigated the crossover from few-to many-body physics by studying quasi-one-dimensional systems of ultracold atoms consisting of a single impurity interacting with an increasing number of identical fermions. We measured the interaction energy of such a system as a function of the number of majority atoms for different strengths of the interparticle interaction. As we increased the number of majority atoms one by one, we observed fast convergence of the normalized interaction energy toward a many-body limit calculated for a single impurity immersed in a Fermi sea of majority particles.
Journal Article
Lookout America! the secret Hollywood studio at the heart of the Cold War
\"The story of the Cold War era Lookout Mountain Laboratory, or the 1352nd Photographic Group of the United States Air Force, which employed hundreds of Hollywood studio veterans. Engages with issues of the Cold War state and visual culture\"-- Provided by publisher.
Book
Unraveling the Gray Area Problem
In Unraveling the Gray Area
Problem , Luke Griffith examines the US role
in why the Intermediate-Range Nuclear Forces (INF) Treaty took
almost a decade to negotiate and then failed in just thirty
years. The INF Treaty enhanced Western security by
prohibiting US and Russian ground-based missiles with maximum
ranges of 500 to 5,500 kilometers. Significantly, it eliminated
hundreds of Soviet SS-20 missiles, which could annihilate targets
throughout Eurasia in minutes. Through close scrutiny of US theater
nuclear policy from 1977 to 1987, Griffith describes the Carter
administration's masterminding of the dual-track decision of
December 1979, the North Atlantic Treaty Organization (NATO)
initiative that led to the INF Treaty. The Reagan administration,
in turn, overcame bureaucratic infighting, Soviet intransigence,
and political obstacles at home and abroad to achieve a
satisfactory outcome in the INF negotiations.
Disagreements between the US and Russia undermined the INF
Treaty and led to its dissolution in 2019. Meanwhile, the US is
developing a new generation of ground-based, INF-type missiles that
will have an operational value on the battlefield. Griffith urges
policymakers to consider the utility of INF-type missiles in new
arms control negotiations. Understanding the scope and consistency
of US arms control policy across the Carter and Reagan
administrations offers important lessons for policymakers in the
twenty-first century.
eBook
How the thymus designs antigen-specific and self-tolerant T cell receptor sequences
T lymphocytes (T cells) orchestrate adaptive immune responses that clear pathogens from infected hosts. T cells recognize short peptides (p) derived from antigenic proteins bound to protein products of the MHC genes. Recognition occurs when T cell receptor (TCR) proteins expressed on T cells bind sufficiently strongly to antigen-derived pMHC complexes on the surface of antigen-presenting cells. A diverse repertoire of self-pMHC-tolerant TCR sequences is shaped during development of T cells in the thymus by processes called positive and negative selection. Combining computational models and analysis of experimental data, we parsed the contributions of positive and negative selection to the design of TCR sequences that recognize antigenic peptides with specificity, yet also exhibit cross-reactivity. A dominant role for negative selection in mediating antigen specificity of mature T cells and a molecular mechanism for TCR recognition of antigen are described.
Journal Article
Evidence for quark-matter cores in massive neutron stars
The theory governing the strong nuclear force—quantum chromodynamics—predicts that at sufficiently high energy densities, hadronic nuclear matter undergoes a deconfinement transition to a new phase of quarks and gluons
1
. Although this has been observed in ultrarelativistic heavy-ion collisions
2
,
3
, it is currently an open question whether quark matter exists inside neutron stars
4
. By combining astrophysical observations and theoretical ab initio calculations in a model-independent way, we find that the inferred properties of matter in the cores of neutron stars with mass corresponding to 1.4 solar masses (
M
⊙
) are compatible with nuclear model calculations. However, the matter in the interior of maximally massive stable neutron stars exhibits characteristics of the deconfined phase, which we interpret as evidence for the presence of quark-matter cores. For the heaviest reliably observed neutron stars
5
,
6
with mass
M
≈ 2
M
⊙
, the presence of quark matter is found to be linked to the behaviour of the speed of sound
c
s
in strongly interacting matter. If the conformal bound
c
s
2
≤
1
/
3
(ref.
7
) is not strongly violated, massive neutron stars are predicted to have sizable quark-matter cores. This finding has important implications for the phenomenology of neutron stars and affects the dynamics of neutron star mergers with at least one sufficiently massive participant.
The cores of neutron stars could be made of hadronic matter or quark matter. By combining first-principles calculations with observational data, evidence for the presence of quark matter in neutron star cores is found.
Journal Article
Bias and Overconfidence in Parametric Models of Interactive Processes
We assess the ability of logit, probit and numerous other parametric models to test a hypothesis that two variables interact in influencing the probability that some event will occur [Pr(Y)] in what we believe is a very common situation: when one's theory is insufficiently strong to dictate a specific functional form for the data generating process. Using Monte Carlo analysis, we find that many models yield overconfident inferences by generating 95% confidence intervals for estimates of the strength of interaction that are far too narrow, but that some logit and probit models produce approximately accurate intervals. Yet all models we study generate point estimates for the strength of interaction with large enough average error to often distort substantive conclusions. We propose an approach to make the most effective use of logit and probit in the situation of specification uncertainty, but argue that nonparametric models may ultimately prove to be superior.
Journal Article
Quantum Gases
Ultracold quantum gases are proving to be a powerful model system for strongly interacting electronic many-body systems. This Perspective explores how such atomic ensembles can help to unravel some of the outstanding open questions in the field.
Journal Article