Niels Bohr International Academy

Copenhagen, Denmark

Niels Bohr International Academy

Copenhagen, Denmark

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News Article | November 16, 2016
Site: www.eurekalert.org

One of the most highly endowed German research awards funded by the Alexander von Humboldt Foundation will be used to establish a junior research group to study dark matter at Mainz University Together with six other award winners, particle physicist Dr. William Shepherd received the highly endowed Sofja Kovalevskaja Award of the Alexander von Humboldt Foundation in Berlin yesterday. The award was presented by Georg Schütte, the State Secretary at the German Federal Ministry of Education and Research (BMBF), and Enno Aufderheide, the Secretary General of the Alexander von Humboldt Foundation. With the prize money of almost EUR 1.5 million Shepherd will be able to set up a junior research group at the Institute of Physics at Johannes Gutenberg University of Mainz (JGU). The US-American physicist works on theoretical research into dark matter and was formerly based at the Niels Bohr International Academy at the University of Copenhagen in Denmark. Dark matter is one of the greatest unsolved mysteries of our universe. Its nature has still to be established although we know it exists because of the indirect evidence provided by the structural forms assumed in the early universe. Its presence can be experimentally demonstrated in part by the way it distorts the light that reaches us from distant galaxies. "The search for dark matter is so exciting because it is one of the few research fields that provide tangible evidence that the Standard Model of particle physics does not adequately describe our universe," explained William Shepherd. "So our endeavor to understand the fundamental nature of this new particle should also help us to better understand the universe." The Sofja Kovalevskaja Award is granted to young exceptionally promising researchers and enables them to embark on academic careers by establishing their own junior research groups at research institutions in Germany. The award funds are placed at the award winner's disposal for a period of five years to carry out the approved research project of his or her own choice in Germany. Dr. William Shepherd joined the THEP -Theoretical High Energy Physics group headed by Professor Matthias Neubert at Mainz University in September 2016. "In Mainz there is an outstanding group of scientists working in areas closely related with my own particular field of research. There are many theoretical particle physicists here whose expertise I can call on to complement my own work. This is in addition to the large group of experimental physicists who are also working on this research problem, ranging from the ATLAS Group to the CASPER Group. It is really helpful that I can put questions arising from my theoretical work to the researchers who are closely involved in the experiments," said Shepherd of his new host institute. With the funds from the Sofja Kovalevskaja Prize, the physicist can set up his own junior research group consisting of two postdoctoral researchers and up to three doctoral students. "Dr. Shepherd's work enhances our profile in the area of Dark Matter exploration significantly," said Professor Matthias Neubert. "His research is based on a new innovative approach and is highly regarded internationally. His presence in Mainz will make it possible for students and postdocs in particular to contribute to the solution of one of the most profound puzzles of modern physics." The Sofja Kovalevskaja Award is the most highly endowed research prize in Germany for early-career scientists. It is funded by the German Federal Ministry of Education and Research (BMBF). The work of the Sofja Kovalevskaja Award winner Dr. William Shepherd also forms part of the Mainz-based Cluster of Excellence "Precision Physics, Fundamental Interactions and Structure of Matter" (PRISMA). This program is funded to the tune of EUR 35 million through the current Excellence Initiative of the German federal and state governments.


Zeuner J.M.,Friedrich - Schiller University of Jena | Rechtsman M.C.,Pennsylvania State University | Plotnik Y.,Technion - Israel Institute of Technology | Lumer Y.,Technion - Israel Institute of Technology | And 4 more authors.
Physical Review Letters | Year: 2015

We present the first experimental observation of a topological transition in a non-Hermitian system. In contrast to standard methods for examining topological properties, which involve probing edge (or surface) states, we monitor the topological transition by employing bulk dynamics only. The system is composed of a lattice of evanescently coupled optical waveguides, and non-Hermitian behavior is engineered by inducing bending loss by spatially "wiggling" every second waveguide. © 2015 American Physical Society.


D'Onofrio M.,Helsinki Institute of Physics | Rummukainen K.,Helsinki Institute of Physics | Tranberg A.,Niels Bohr International Academy
Journal of High Energy Physics | Year: 2012

Using lattice simulations, we measure the sphaleron rate in the Standard Model as a function of temperature through the electroweak cross-over, for the Higgs masses mH = 115 and mH = 160GeV. We pay special attention to the shutting off of the baryon rate as the temperature is lowered. This quantity enters computations of Baryogenesis via Leptogenesis, where non-zero lepton number is converted into non-zero baryon number by equilibrium sphaleron transitions. Combining existing numerical methods applicable in the symmetric and broken electroweak phases, we find the temperature dependence of the sphaleron rate at very high temperature, through the electroweak cross-over transition, and deep into the broken phase. © 2012 SISSA.


Kastoryano M.J.,Niels Bohr Institute | Kastoryano M.J.,Niels Bohr International Academy | Reiter F.,Niels Bohr Institute | Sorensen A.S.,Niels Bohr Institute
Physical Review Letters | Year: 2011

We propose a novel scheme for the preparation of a maximally entangled state of two atoms in an optical cavity. Starting from an arbitrary initial state, a singlet state is prepared as the unique fixed point of a dissipative quantum dynamical process. In our scheme, cavity decay is no longer undesirable, but plays an integral part in the dynamics. As a result, we get a qualitative improvement in the scaling of the fidelity with the cavity parameters. Our analysis indicates that dissipative state preparation is more than just a new conceptual approach, but can allow for significant improvement as compared to preparation protocols based on coherent unitary dynamics. © 2011 American Physical Society.


Greensite J.,Niels Bohr International Academy | Langfeld K.,University of Plymouth
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

We apply the relative weights method [J. Greensite, Phys. Rev. D 86, 114507 (2012)] to determine the effective Polyakov line action for SU(2) lattice gauge theory in the confined phase, at lattice coupling β=2.2 and N t=4 lattice spacings in the time direction. The effective action turns out to be bilinear in the fundamental representation Polyakov line variables, with a rather simple expression for the finite range kernel. The validity of this action is tested by computing Polyakov line correlators, via Monte Carlo simulation, in both the effective action and the underlying lattice theory. It is found that the correlators in each theory are in very close agreement. © 2013 American Physical Society.


Rudner M.S.,Niels Bohr International Academy | Rudner M.S.,Ohio State University | Rudner M.S.,Austrian Academy of Sciences | Levitov L.S.,Massachusetts Institute of Technology
Physical Review Letters | Year: 2013

Early experiments on spin-blockaded double quantum dots revealed robust, large-amplitude current oscillations in the presence of a static (dc) source-drain bias. Despite experimental evidence implicating dynamical nuclear polarization, the mechanism has remained a mystery. Here we introduce a minimal albeit realistic model of coupled electron and nuclear spin dynamics which supports self-sustained oscillations. Our mechanism relies on a nuclear spin analog of the tunneling magnetoresistance phenomenon (spin-dependent tunneling rates in the presence of an inhomogeneous Overhauser field) and nuclear spin diffusion, which governs dynamics of the spatial profile of nuclear polarization. The proposed framework naturally explains the differences in phenomenology between vertical and lateral quantum dot structures as well as the extremely long oscillation periods. © 2013 American Physical Society.


Hood A.W.,University of St. Andrews | Archontis V.,University of St. Andrews | MacTaggart D.,Niels Bohr International Academy
Solar Physics | Year: 2012

This paper reviews some of the many 3D numerical experiments of the emergence of magnetic fields from the solar interior and the subsequent interaction with the pre-existing coronal magnetic field. The models described here are idealised, in the sense that the internal energy equation only involves the adiabatic, Ohmic and viscous shock heating terms. However, provided the main aim is to investigate the dynamical evolution, this is adequate. Many interesting observational phenomena are explained by these models in a self-consistent manner. © 2011 Springer Science+Business Media B.V.


Greensite J.,Niels Bohr International Academy | Greensite J.,San Francisco State University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

I adapt a numerical method, previously applied to investigate the Yang-Mills vacuum wave functional, to the problem of extracting the effective Polyakov line action from SU(N) lattice gauge theories, with or without matter fields. The method can be used to find the variation of the effective Polyakov line action along any trajectory in field configuration space; this information is sufficient to determine the potential term in the action and strongly constrains the possible form of the kinetic term. The technique is illustrated for both pure and gauge Higgs SU(2) lattice gauge theory at finite temperature. A surprise, in the pure gauge theory, is that the potential of the corresponding Polyakov line action contains a nonanalytic (yet center-symmetric) term proportional to |P|3, where P is the trace of the Polyakov line at a given point, in addition to the expected analytic terms proportional to even powers of P. © 2012 American Physical Society.


Greensite J.,Niels Bohr International Academy | Greensite J.,San Francisco State University | Splittorff K.,Copenhagen University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

The free energy of effective spin or "Polyakov line" models with a chemical potential, based on the U(N) group, does not depend on the chemical potential. In a mean field-inspired expansion, we show how the condition of unit determinant, taking U(N) to SU(N), reintroduces the chemical potential, and allows us to express the free energy, as a function of mean field variational parameters, in terms of an expansion in the baryon (rather than the quark) fugacity at each lattice site. We solve the SU(3) mean field equations numerically to determine the phase diagram and compute observables. We also calculate the first corrections to the leading order mean field results, and find that these can significantly shift the endpoint of a line of first order transitions. The problem of deriving an effective spin model from full QCD is discussed. © 2012 American Physical Society.


News Article | October 26, 2016
Site: physicsworld.com

A new statistical analysis of type 1a supernovae observations has failed to find substantial statistical evidence that the rate of expansion of the universe has been increasing over time. Instead, the calculations are consistent with a universe that is expanding at a mostly constant rate – something that could be at odds with the popular lambda-cold dark matter (ΛCDM) model of cosmology. Type 1a supernovae are exploding stars that play an important role in astronomy as "standard candles" that emit the same type and quantity of light. This means that the distance to a supernova can be worked out simply from its brightness in the sky. Prior to the late 1990s, cosmologists had assumed that the expansion of the universe should either be constant over time, or slowing down. But then a team led by Saul Perlmutter and another team led by Adam Riess and Brian Schmidt noticed that the rate of expansion of the universe has been increasing. The teams found that more than 50 distant type 1a supernovae are fainter than expected for their measured redshift. The expansion of the universe causes the light from a supernova to be shifted to longer wavelengths when observed on Earth. This redshift tells astronomers how quickly the supernova was moving away from us when the explosion occurred – which gives us the rate of the expansion of the universe at that time. The surprise discovery was evidence that the expansion of the universe has been accelerating. It earned Perlmutter, Riess and Schmidt the 2011 Nobel Prize for Physics and led physicists to speculate that this acceleration was driven by an unseen entity called dark energy. Since then, further independent evidence for the accelerating expansion has come to light in measurements of the cosmic microwave background (CMB) and observations of galaxies. Indeed, the accelerating expansion of the universe has become a pillar of the most popular theory of cosmology, ΛCDM, where Λ is the cosmological constant that describes the acceleration. Hundreds of other type 1a supernovae have been observed since the 1990s, but now some physicists are beginning to doubt whether these observations support an accelerating expansion. Subir Sarkar of the University of Oxford in the UK, Jeppe Nielsen of the Niels Bohr International Academy in Denmark and Alberto Guffanti of Italy's University of Turin have done a statistical analysis of data from 740 type 1a supernovae and concluded "that the data are still quite consistent with a constant rate of expansion". The difference between the trio's study and previous analyses is how variations in supernovae light are dealt with. While all type 1a supernovae are nearly identical, astrophysicists know that there are important differences that must be accounted for. Sarkar and colleagues argue that the statistical techniques adopted for previous studies are too simple and not appropriate for the growing set of observational data. Using a technique that Sarkar describes as "industry standard statistics," the trio took a different approach to dealing with variations in the supernovae. They concluded that the deviation from a constantly expanding universe is less than about 3σ, which is a relatively poor statistical significance. "The evidence for accelerated expansion is marginal," says Sarkar, who believes that the ΛCDM model needs rethinking. Roberto Trotta of Imperial College London does not go that far, pointing out that there is other independent and strong evidence for the accelerating expansion. However, he acknowledges that the evidence for acceleration in type 1a observations does not appear to be as robust as previously thought. Trotta – who has developed a new statistical method for analysing type 1a data that is different than Sarkar's – says that astronomers are poised to observe thousands of new type 1a supernovae and must be prepared to adopt more rigorous statistical techniques to analyse them. The analysis is described in Scientific Reports.

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