Program Archive of the Ortvay Seminar Series 2016 Spring

2016. január 21., csütörtök, 15:00-kor

Imre Bartos (Columbia University, USA)

"IceCube: the High-energy Universe and Multimessenger Astrophysics with Neutrinos"

Kivonatos ismertetés:

Astrophysical processes that produce the observed energetic cosmic particles (up to 10^20 eV) and high-energy gamma radiation involve extreme non-thermal acceleration, strongly constraining the list of possible sources. Nevertheless, the origin of the most energetic cosmic rays, and the electromagnetic emission mechanism in extreme sources such as gamma-ray bursts, are currently unknown. Neutrinos may well be the silver bullet to unravel these processes. They can reveal the hadronic nature of the emission, and due to their weak interaction they lead right back to the source. The IceCube neutrino observatory at the South Pole has recently discovered a cosmic flux of TeV-PeV neutrinos, making the first step in lifting the curtain on cosmic particle accelerators. I will discuss recent multimessenger observational developments, and source candidates in the high-energy universe. I will describe plans and capabilities for the next-generation neutrino detector IceCube-Gen2.


2016. február 18., csütörtök, 15:00-kor

Frei Zsolt (ELTE, Atomfizikai Tanszék)

„A gravitációs hullámok felfedezése a LIGO-val"

Kivonatos ismertetés:

Először történt, hogy tudósok a téridő „gravitációs hullámoknak" nevezett fodrozódásait megfigyelték, miután azok a távoli univerzum egy kataklizmikus eseményében keletkezve elérték a Földet. Az észlelés igazolja Albert Einstein 1915-ben közölt általános relativitáselméletének egyik legfőbb előrejelzését, és eddig példa nélküli, új ablakot nyit a világegyetemre. A gravitációs hullámokat 2015. szeptember 14-én, magyar idő szerint délelőtt 10:51 perckor (9:51 UTC) észlelte a Laser Interferometer Gravitational-wave Observatory (LIGO) mindkét detektora, amelyek az amerikai egyesült államokbeli Livingstonban (Louisana állam) és Hanfordban (Washington állam) találhatók.

Előadásomban elmondom a LIGO történetét, bemutatom a felfedezést, és ismertetem a magyar hozzájárulást is a közös munkához.

Az Eötvös Gravity Research Group (EGRG), amely a budapesti Eötvös Loránd Tudományegyetem és a debreceni MTA Atommagkutató Intézet összefogásában működik, 2007 óta az LSC tagja. A csoport az LSC valamennyi tevékenységi köréhez nyújt hozzájárulást: műszerépítéssel segítettük a LIGO detektorok zajszintcsökkentését; a csoporttagok műszak- és riasztási felügyeletet látnak el a LIGO detektorok adatgyűjtési időszakai alatt mind a detektorállomásokon, mind a távolból; forrásmodellező munkánkkal és jelkeresőprogram fejlesztésével a gravitációshullám-jelek észlelési és kiértékelési hatékonyságát maximalizáljuk. Az EGRG készítette azt a galaxiskatalógust, amelyet az LSC az észlelt jelek (köztük a szeptember 14-én észlelt jel) forrásgalaxisainak azonosításához használ. Az EGRG tagjai segítik a jövőben építeni tervezett detektorok (köztük a tervek szerint Indiában épülő LIGO detektor) optimális elhelyezésének megtalálását. A LSC magyar nyelvű honlapját az EGRG csoport készítette és fejleszti.


18th February 2016., Thursday, 3pm

Frei Zsolt (ELTE, Atomfizikai Tanszék)

"The Detection of Gravitational Waves by LIGO"

Abstract:

From the original press release: "For the first time, scientists have observed ripples in the fabric of spacetime called gravitational waves, arriving at the earth from a cataclysmic event in the distant universe. This confirms a major prediction of Albert Einstein's 1915 general theory of relativity and opens an unprecedented new window onto the cosmos. The gravitational waves were detected on September 14, 2015 at 5:51 a.m. Eastern Daylight Time (09:51 UTC) by both of the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, located in Livingston, Louisiana, and Hanford, Washington, USA.

In my seminar talk I will describe the history of LIGO, introduce the discovery, and list contributions from Hungarian participants of the project.

The Eotvos Gravity Research Group (EGRG) is a joint effort by Eotvos University, Budapest, and the Institute for Nuclear Research (ATOMKI), in Debrecen. EGRG joined LIGO in 2007 and participates in all aspects of the work of the LIGO Scientific Collaboration (LSC): We developed hardware for the Physical Environmental Monitoring (PEM) subsystem, participants of EGRG served as science monitors during data taking runs, both in the US and remotely from the offices of Eotvos University, and helped to maximise the efficiency of LIGO in finding gravitational wave signals by modelling possible sources and developing algorithms and software for data analysis. EGRG has constructed the galaxy catalog that was used to identify possible host galaxies for sources, including the event on the 14th of September, 2015. EGRG members help in finding the optimal location for the future LIGO India, and also created the Hungarian version of the LIGO web pages. 


2016. február 25., csütörtök, 15:00-kor
25th February 2016., Thursday, 3pm

Peter Berczik (Astronomisches Rechen-Institut (ARI) am Zentrum fuerAstronomie der Universitaet Heidelberg (ZAH), Heidelberg, Germany; Main Astronomical Observatory (MAO), National Academy of Sciences of Ukraine (NASU), Kyiv, Ukraine)

"Galaxy collision simulations with central Post Newtonian Supermassive Binary Black Holes"

Abstract:

The most energetic gravitational waves (GWs) are generated by merging supermassive binary black holes (SMBBHs) in the centers of galaxies. In our high order post-Newtonian N-body simulations, we study galaxy-galaxy collisions with the maximum numerical resolution possible today. For the simulations, we use the largest astrophysical Graphical Processing Unit (GPU) accelerated clusters, including the TITAN GPU supercomputer in the USA which holds the second place in the recent top500 list. The central SMBBH is simulated as special particles with post-Newtonian force corrections (up to 1/c**7 terms) implemented for the BHs' gravitational interactions. We find that the SMBBH merger time shows a quite strong dependency on the merging galaxies' mass ratio. Using our simulations, we obtain upper limits for the final GW merger time for SMBBHs in observational galaxy pair candidates.

 


2016. március 3., csütörtök, 15:00-kor
3rd March 2016., Thursday, 3pm
Oroszlány László (ELTE, Komplex Rendszerek Fizikája Tanszék)

"Magnetism of Gadolinium: A First-Principles Perspective"

Abstract:

By calculating the spectral density of states in the ferromagnetic ground state and in the high temperature paramagnetic phase we provide the first concise study of finite temperature effects on the electronic structure of the bulk and the surface of gadolinium metal. The variation of calculated spectral properties of the Fermi surface and the density of states in the bulk and at the surface are in good agreement with recent photoemission experiments performed in both ferromagnetic and paramagnetic phases. In the paramagnetic state we find vanishing spin splitting of the conduction band, but finite local spin moments both in bulk and at the surface. We clearly demonstrate that the formation of these local spin moments in the conduction band is due to the asymmetry of the density of states in the two spin channels, suggesting a complex, non-Stoner behavior. We, therefore, suggest that the vanishing or nearly vanishing spin splitting of spectral features cannot be used as an indicator for Stoner-like magnetism. 

 


2016. március 10., csütörtök, 15:00-kor
10th March 2016., Thursday, 3pm 
Pécz Béla (MTA EK MFA)

„GaN rétegek és eszközök, a kék LED-en túl"

Kivonatos ismertetés:

GaN és rokon anyagai a széles tiltottsávú félvezetők közül az optoelektronikai alkalmazások (kék LED, majd világítás) miatt nagy figyelmet kaptak. Ugyanakkor ezekből az anyagokból nagyteljesítményű eszközök is készíthetők. A nagy teljesítmény miatt felmerül a jó hővezető anyagok alkalmazásának igénye ezekben az eszközökben. A legjobb hővezető anyagok a gyémánt és a grafén. A 2D grafén kombinációja a vegyületfélvezetőkkel (AlN és ZnO) újszerű 2D félvezető rétegeket ígér.

Az előadásom mondanivalója, hogy a nanoszerkezet ismerete vezethet a makro tulajdonságok megértéséhez. E megismerésben legfontosabb eszközünk a transzmissziós elektronmikroszkópia, amelyben az elmúlt 10 évben forradalmi fejlődésnek lehetünk tanúi. Előadásomban röviden ismertetem a kék-LED és általában a jó minőségű GaN rétegek fejlesztésének történetét. Tárgyalom a nagyteljesítményű (high electron mobility transistor) HEMT tranzisztorok működés közbeni fűtésének problémáját és a lehetséges megoldásokat gyémánt és grafén segítségével. Az utóbbi elvezet a grafén/SiC hordozón létrehozott 2D AlN és ZnO félvezetőkhöz.


2016. március 17., csütörtök, 15:00-kor
17th March 2016., Thursday, 3pm

Christian Hoelbling (Universität Wuppertal, Németország)

"The proton-neutron mass difference and the stability of ordinary matter"

 

Abstract:

Most of the mass of the visible universe is a consequence of the dynamics of the strong nuclear force. Through a steady progress in the understanding of this nonperturbative phenomenon over the last decade, we have gained increasing insight into the details of this mechanism. The numerical solution of the theories of the strong nuclear force, quantum chromodynamics (QCD), and the electromagnetic force (QED) have recently provided us with a quantitative understanding of even the permil level difference of the masses of protons and neutrons, which is fundamental for the existence and stability of the universe as we know it. In this talk, I will present these calculations and discuss their implications on the question of how finely tuned our universe really is.


2016. március 31., csütörtök, 15:00-kor

Yoshiharu Omura (Research Institute for Sustainable Humanosphere, Kyoto University)

"Dynamic variation of radiation belts due to nonlinear
wave-particle interactions during space weather events" 

 

 

Kivonatos ismertetés:

During space weather events, energetic particles are injected from the magnetotail to the inner magnetosphere, and various kinds of wave-particle interactions take place. Whistler-mode chorus emissions are one of the most important waves for the dynamics of relativistic electrons forming the outer radiation belt. Chorus emissions are excited via interaction with 10 – 100 keV electrons outside the plasmasphere [1,2], and they can accelerate a fraction of resonant electrons to MeV energy through nonlinear wave trapping mechanisms called relativistic turning acceleration (RTA) [3] and ultra-relativistic acceleration (URA) [4]. The time scale of acceleration is much shorter than that predicted by the quasi-linear theory [5]. Another kind of waves important for the radiation belt dynamics is EMIC triggered emissions with rising-tone frequencies excited by nonlinear interaction with 10 – 100 keV protons both inside and outside the plasmasphere [6]. The EMIC emissions can interact with relativistic electrons (> 0.3 MeV) and scatter them to lower pitch angles efficiently by nonlinear wave trapping, resulting in significant precipitation of radiation belt electrons [7,8] as well as energetic protons [9]. We review recent development of nonlinear theory and simulations that can describe dynamic nature of the radiation belts under intense space weather events.

References:

1. Y. Omura, et al., J. Geophys. Res., 114, A07217, 2009.

2. Y. Omura and D. Nunn, J. Geophys. Res., 116, A05205, 2011.

3. Y. Omura N. Furuya, D. Summers, J. Geophys. Res., Vol. 112, A06236, 2007.

4. D. Summers, and Y. Omura, Geophys. Res. Lett., 34, L24205, 2007.

5. Y. Omura et al., J. Geophys. Res., 120, 9545–9562, 2015.

6. S. Nakamura, et al., J. Geophys. Res., 120, 7318–7330, 2015.

7. Y. Omura and Q. Zhao, J. Geophys. Res., 118, 5008, 2013.

8. Y. Kubota et al., J. Geophys. Res., 120, 4384-4399, 2015.

9. M. Shoji and Y. Omura, J. Geophys. Res., 118, 5553, 2013.


2016. április 7., csütörtök, 15:00-kor
7th April 2016., Thursday, 3pm

Legeza Örs (MTA Wigner FK Szilárdtestfizikai és Optikai Intézet)

"Tensor product factorization and entanglement in nuclei,
through molecules to ultra cold atom and condensed matter systems"

Abstract:

Hierarchical Tucker tensor (HT) format and Tensor Trains (TT) have been introduced recently for low rank tensor product approximation. TT representation, also known as Matrix Product States (MPS), and HT representation, apparent in tensor network states (TNS), have been used in quantum physics for several years. Hierarchical tensor decompo- sitions are based on subspace approximation by extending Tucker de- composition into a multilevel framework. Therefore, they inherit the favorable properties of Tucker tensors, i.e., they offer a stable and ro- bust approximation, but still enabling low order scaling with respect to the dimensions. For many high dimensional problems, hard to treat so far, this approach may offer a novel strategy to circumvent the curse of dimensionality. In this contribution, we will present recent results on entanglement, which is in fact the key ingredient of such methods, in var- ious strongly correlated quantum many body systems with long range interactions.


2016. április 14., csütörtök, 15:00-kor
14th April 2016., Thursday, 3pm

Szunyogh László (BME, ElméletiFizika Tanszék)

"Chiral twist of magnetic domain walls in ultrathin films"

Abstract:

Breaking of inversion symmetry at surfaces or interfaces of magnetic solids and the presence of spin-orbit coupling can generate remarkably large Dzyaloshinskii-Moriya (DM) interaction that leads to numerous intriguing phenomena in nanomagnetism. In this talk, first I give a short overview about effects induced by DMI, like the stabilization of exotic spin textures (spin-spirals, magnetic skyrmions), asymmetry of the spin-wave dispersion, homochirality of magnetic domain walls, or exchange bias due to DMI.

A vast number of recent experimental and theoretical works focused on manipulating the DMI by varying the geometry and chemical composition of magnetic multilayers. Motivated by this challenge, I present a study of domain walls in Co/Irn/Pt(111) (n = 0-6) magnetic thin films by a combined approach of first-principles calculations and spin-dynamics simulations. We determine the tensorial exchange interactions and the magnetic anisotropies for the Co overlayer, depending on the thickness of Ir buffer layers. We find strong nearest neighbor ferromagnetic isotropic exchange interactions between the Co atoms and an out-of-plane magnetic anisotropy for the majority of the studied thin films. We show that the Néel type of magnetic domain walls (DWs) change their rotational sense (chirality) upon the insertion of an Ir buffer layer as compared to Co/Pt(111).

Quite unexpectedly, our spin-dynamics simulations indicate a twist of the DW profile off the plane normal to the surface. This feature can uniquely be attributed to the out-of-plane components of the DM vectors. Using symmetry arguments, we discuss the appearance of this twist for DWs propagating along different directions on a hexagonal lattice. In terms of an appropriate micromagnetic model we argue that the system gains energy through twisting of the DW profile. We show that the twisting amplitude proportionally depends on the size of the out-of-plane component of the effective DM vector, Δζ, while the direction of the twisting is determined by the sign of Δζ. [1]

 

[1] Gy. J. Vida, E. Simon, K. Palotás, and L. Szunyogh, preprint just before submission (2016).

 


 

2016. április 28., csütörtök, 15:00-kor
28th April 2016., Thursday, 3pm

György Szabó (Institute of Technical Physics and Materials Science, Centre for Energy Research)

"Interactions and phenomena in evolutionary games"

Abstract:

Evolutionary games provide a general and plastic mathematical background to study the macroscopic behaviors in living and non-living systems. In these models the players can represent biological molecules, cells, multi-cellular organisms including humans and their associations. If the players have a finite number of options then the interactions can be described by payoff matrices (introduced in the traditional game theory) that significantly extend the types of interactions studied in physical systems. We compare the particle-particle interactions with those occurring when the players are distinguishable. Accordingly the interactions can be classified into four types: games with self- or cross-dependent payoffs, coordination between the strategy pairs, and games with cyclic dominance. The anti-symmetric parts of the payoff matrices describe cyclic and hierarchical dominances among the strategies and cause biodiversity or social dilemmas in the living systems.


2016. május 26., csütörtök, 15:00-kor

26th May 2016., Thursday, 3pm

Walter Winter (Deutsches Elektronen-Synchrotron, Hamburg, DE)

"Perspectives for neutrino tomography of Earth"

/Marx Memorial Lecture/

Abstract:

Neutrinos are among the most abundant particles of the universe, which are however difficult to catch. We sketch the theory of neutrino oscillations, and we review the results which have led to the Nobel prize 2015 awarded to Takaaki Kajita and Arthur McDonald. We furthermore discuss approaches to use neutrinos for the tomography of Earth, from possible ideas to realistic perspectives - including the recent findings in neutrino oscillations.


2016. június 2., csütörtök, 15:00-kor

2nd June 2016., Thursday, 3pm

Tania Robens (Technische Universität Dresden, Dresden, DE)

"Particle Physics after the Higgs Discovery: Questions and Challenges in the LHC era"

Abstract:

With the start of data taking at the LHC in 2009 and its more than successful physics program since then, particle physics has entered an exciting era. After the discovery of a Higgs boson, a major task within the high-energy community is now the exact determination of its properties, which in turn calls for both accurate experimental measurements as well as precise theoretical predictions. While current measurements are well in agreement with predictions from the Standard Model (SM) alone, both theoretical as well as experimental uncertainties still allow for interpretation in models which extend the particle sector of the SM.

In my talk, I will focus on two areas of research of high relevance to the LHC physics program. The first concerns the inclusion of higher-order corrections into Monte Carlo event generators, which are a necessary ingredient for the direct comparison between theoretical predictions and experimental data. I will specifically discuss the treatment of infrared singularities. Numerical implementations of these divergent terms need to be treated with great care to avoid instabilities. A common tool for this are subtraction schemes. I have developed a new scheme for such subtractions and will discuss its properties as well as advantages over alternative schemes. I will also discuss the calculation of higher-order corrections to the process WW+jet using analytic methods.

A second part of my research deals with models that extend the particle content of the SM. I will discuss possible extensions of the SM Higgs sector, with a special focus on the singlet extension, where an additional spin-0 particle is added to the Higgs potential. This model, which is subject to both theoretical as well as experimental bounds, currently serves as an important benchmark scenario for LHC experimental searches. I will discuss the currently available bounds within this model as well as the discovery prospects for the additional particle at the LHC experiments.