# Past activities

« previous | page 3 of 7 | next »#### CRC - The Dark Universe

The evaluation of the TRR33/3 application for the third funding period 7/2014 - 6/2018 was scheduled for December 12/13, 2013 at Physikalisches Institut, Nussallee 12, Universität Bonn.

(A schedule can be found here.)

#### Bethe Colloquium by Prof. Holger Bech Nielsen

December's Bethe Colloquium took place on December 5th (3:15 pm) in Hörsaal I:

- Holger Bech Nielsen (Niels-Bohr-Institut, Copenhagen)
- Why just the Standard Model Group, 4 dimensions and the Higgs representation?
- Hörsaal I, Physikalisches Institut

**Abstract:** Don Bennett and I looked for relatively simple quantity/function depending on a Lie group being so cleverly found that it takes its maximum value just for the
Standard Model group. The group dependent function/quantity we found is an averaging in the logarithm weighted with the dimension of the Lie algebras in of essentially the inverse
Dynkin index with some special rule added for the Abelian components of the Lie group. A slightly adjusted application of this function for the Poincare (and/or Lorentz) group singles
out just the experimental number of dimensions of space time. In addition, converting our quantity to a measure for a 'size' of a representation leads to single out the representation
to which the Standard Model Higgs field belongs as being the 'smallest' one possible. Also the Fermion representations of the Standard Model are the 'smallest' mass protected and
anomaly free representations. So we have roughly invented a (sports)game among various quantum gauge-theories leading to the Standard Model winning the 'gold medal'.

#### Bethe Colloquium by Prof. Roderich Moessner

November's Bethe Colloquium took place on November 7th (3:15 pm) in Hörsaal I:

- Roderich Moessner (Max-Planck-Institute for physics of complex systems, Dresden)
- Magnetic monopoles in spin ice
- Hörsaal I, Physikalisches Institut

**Abstract:** Magnetic monopoles were first proposed to exist by Dirac many decades ago as the natural counterparts of electrically charged particles such as the electron. Despite
much searching, no elementary monopoles have ever been observed, even though many theories of high-energy physics suggest that they should be present. Here, we present an alternative
route for the observation of monopoles, as a low- rather than a high-energy phenomenon. It involves a process known as fractionalisation, which is a striking emergent phenomenon, in
which an 'elementary' particle breaks up into two independent entities. A celebrated example of this is spin-charge separation, in which an electron's magnetic (spin) and electric
(charge) properties appear to become independent degrees of freedom. The spin ice materials -- Dy_2Ti_2O_7 and Ho_2Ti_2O_7 -- provide a rare instance of fractionalisation in three
dimensions: their atomic magnetic dipole moments fractionalise, resulting in elementary excitations which can be thought of as magnetic monopoles.

This colloquium presents a self-contained introduction to theoretical concepts and experimental phenomena in the physics of spin ice. It focuses on the unique signatures of the
peculiar nature of its ground state and its excitations. These include unusual neutron scattering structure factors, rich non-equilibrium physics, as well as a response to external
magnetic fields that likens spin ice to a magnetic analogue of an electrolyte. Finally, this talk addresses open questions and future perspectives for detecting individual monopoles,
among them a (thought-)experiment inspired by high energy physics.

#### Bethe Colloquium by Prof. Stefan Theisen

October's Bethe Colloquium took place on October 24th (3:15 pm) in Hörsaal I:

- Stefan Theisen (Max-Planck-Institute for gravitational physics, Potsdam-Golm)
- Conformal Field Theory, old and new
- Hörsaal I, Physikalisches Institut

**Abstract:** Conformal field theories are both interesting and important. Recently there has been a renewed interest in these theories in four dimensions. After reviewing some old
results about conformal field theories, I will turn to new developments such as the a-theorem and the relation between conformal and scale invariance.

#### Lectures on the physics Nobel Prize 2013

Scientists from the University of Bonn host and evening lecture discussing the Higgs particle

The Nobel Prize in Physics 2013 goes to Francois Englert and Peter Higgs for their prediction of the Higgs field and the associated Higgs particle. As a result the Physics Institute
of the University of Bonn welcomes you to an information evening on Thursday, October 17th at 7.30pm in the Wolfgang Paul Auditorium, Kreuzbergweg 28 in Bonn. There is no admission
fee.

The importance of the Higgs mechanism and its experimental confirmation will be discussed by Prof. Dr. Herbert Dreiner and Dr. Jürgen Kroseberg. This newly discovered Higgs particle
was predicted 50 years ago by both Higgs and Englert. There will be plenty of time for questions after the lecture.

Herbert Dreiner is a Professor of Theoretical Particle Physics at the Physics Institute who is known for initiating the Physics show. Dr. Jürgen Kroseberg is an experimental physicist
and member of ATLAS collaboration which is one of the two LHC experiments that announced the discovery of the new particle on July 4th 2012.

####
5th Bethe Center Workshop

LHC Run1 Aftermath: Where Theory Meets Experiment

The workshop took place from September 30 to October 3 at the Physikzentrum Bad Honnef. The four days of the workshop have been devoted to intense discussion between theory and experiment on 4 topics:

- Higgs: Understanding and Interpreting the new Boson
- SUSY+BSM: What's left for SUSY, how do we further improve the interpretation of the searches, and what other options should be explored?
- SM Precision: What needs to be calculated more precisely, and what needs to be measured more precisely, to best interpret the data?
- Flavour: How best to understand the flavour structure of the SM? How do we improve upon interpretation of `new physics' models in light of flavour and other precision measurements, negative search results, and the Higgs?

#### Bethe Colloquium by Prof. Katrin Wendland

July's Bethe Colloquium took place on July 4 (3:15 pm) in Hörsaal I:

- Katrin Wendland (Freiburg University)
- Moonshine in Mathematics and String Theory
- Hörsaal I, Physikalisches Institut

**Abstract:** The story of "monstrous moonshine" is a famous success story of mutual interactions between mathematics and physics. In mathematics, Conway/Norton's "monstrous
moonshine conjecture" relates a sporadic group, commonly known as the Monster, to "modular" functions on the upper complex half plane. In physics, the Monster occured first as the
automorphism group of a certain conformal quantum field theory (CFT), let us call it the Monster CFT. Richard Borcherds used the latter to prove the former and was awarded a Fields
Medal for his proof of "monstrous moonshine" in 1989. Recently, Eguchi, Ooguri and Tachikawa have observed a phenomenon in a certain class of CFTs which is reminiscent of monstrous
moonshine, where the role of the Monster is taken by another sporadic group, the Mathieu group M24.

While axiomatic approaches to CFT are tedious, such that a detailed description of any CFT would go beyond the scope of this talk, CFTs arise naturally in string theory, and in the
particular case of the Monster CFT much of its construction can be motivated by simple geometry. This is the approach taken in this talk, aiming to give an overview on some geometric
aspects of moonshine, including - if time permits - a view on Mathieu moonshine.

#### Bethe Colloquium by Prof. Laura Covi

June's Bethe Colloquium took place on Thursday, June 13 (3:15 pm) in Hörsaal I:

- Laura Covi (University Göttingen)
- Supergravity, Dark Matter and the LHC
- Hörsaal I, Physikalisches Institut

**Abstract:** Symmetries have been fundamental in driving progress in theoretical physics and are at the basis of many extensions of the Standard Model of particle physics. In this
talk, I will discuss in particular models of local supersymmetry, also called supergravity. Supergravity extendes the Poincare` algebra with fermionic generators and predicts the
existence of superpartners with different spin for all the SM particles including also the graviton. I will review the consequences of supergravity for cosmology and Dark Matter and
confront the models with the recent LHC searches for new particles.

#### Bethe Forum accompanying the Planck 2013 conference

#### Planck 2013

The annual conference "Planck 2013 - From the Planck Scale to the Electroweak scale" took place from May 20th to May 24th at Bonn University. It is a joint enterprise of several European research groups in the framework of the Marie Curie ITN network "UNILHC". There were two Bethe Formum programs in the week before and after the conference accompanying the program. The conference homepage can be found here.

#### Bonn-Cologne Number Theory and Physics Meeting

The intention of the meeting was to bring together theoretical physicists and mathematicians working on the overlap of number theory and theoretical physics.

The speakers were Francis Brown (Paris), Sameer Murthy (Amsterdam), Gunther Cornelissen (Utrecht) and Tamás Hausel (Lausanne). The schedule can be found here.

#### Bethe Colloquium by Prof. Qimiao Si

May's Bethe Colloquium took place on Monday, May 6 (2:15 pm) in Seminar Room I (We 2.019, note unusual date and time):

- Qimiao Si (Rice University)
- Quantum Criticality and Strongly Correlated Fermions
- Seminar Room I (2.019), BCTP, Wegelerstr. 10

**Abstract:** Quantum criticality describes the collective fluctuations of matter undergoing a continuous phase transition at absolute zero. It is of extensive current interest to
a variety of quantum condensed matter systems, in which competing orders frequently occur. It is also being discussed in other systems, including in holographic settings.

In this talk, I will summarize the basic concepts of quantum criticality, developed within the framework of Landau theory of critical phenomena. I will then discuss the topical issues
concerning “beyond-Landau” quantum critical points, and their implications for the physics of strongly correlated fermion systems. Finally, I will consider the connections – both the
similarities and differences -- of the latter with the quantum critical behavior in holographic models of non-Fermi liquids.

#### Bethe Colloquium by Prof. Ulf-G. Meißner

April's second Bethe Colloquium took place on April 25 (3:15 pm) in Hörsaal I:

- Ulf-G. Meißner (Bonn University)
- The Hoyle state and the fate of carbon-based life
- Hörsaal I, Physikalisches Institut

**Abstract:** The Hoyle state has been an enigma to nuclear theory for half a century. I present the first ab initio calculation of this elusive excited state in the spectrum of
carbon-12 and discuss its structure. Simulated worlds with different quark mass and fine structure constants are also considered and the role of the Hoyle state in our anthropic view
of the universe is elucidated.

#### Bethe Colloquium by Prof. Fabian Essler

April's first Bethe Colloquium took place on April 18 (3:15 pm) in Hörsaal I:

- Fabian Essler (Oxford University)
- Quantum Quenches in one dimensional many-particle systems
- Hörsaal I, Physikalisches Institut

**Abstract:** I consider the non equilibrium time evolution after a "quantum quench": an isolated many-particle system is initially prepared in its ground state and at time t=0 a
parameter such as a magnetic field or an interaction strength is changed suddenly. I discuss realisations of such protocols in systems of ultra-cold trapped atoms and review general
expectations regarding the late-time behaviour of these systems. Interestingly, in the thermodynamic limit the expectation values of local observables relax to time-independent
values, even though the system is not coupled to a bath. I show that conservation laws affect the non equilibrium dynamics and the stationary behaviour very strongly and present
detailed results for the "Ising model" of quantum quenches, the transverse-field Ising chain.

#### Bethe Forum Lecture Series on Primordial Cosmology

The homepage of the program is available here.

#### Visit of Prof. Martin Savage

#### Bethe Colloquium by Prof. Subir Sachdev (Note unusual date)

January's Bethe Colloquium took place on January 14 (3:15 pm) in Hörsaal I:

- Subir Sachdev (Harvard University)
- Entanglement, holography and the quantum phases of matter
- Hörsaal I, Physikalisches Institut

**Abstract:** Electrons in many interesting materials, such as the high temperature superconductors, form states with complex varieties of quantum entanglement. I will present a
broad classification of quantum many-particle states with long-range entanglement, and describe recent theoretical progress in characterizing them. The methods of holography, drawn
from string theory, have given us a new tool to describe such states, by relating them to theories of gravitation in curved spacetimes with an extra dimension. I will discuss the
impact of such ideas on studies of quantum phase transitions, and of novel metals.