# Past activities

« previous | page 2 of 7 | next »#### Bethe Colloquium by Prof. Alain Connes

A second Bethe Colloquium was scheduled for November. It took place on November 27th (3:15 pm) in Hörsaal I:

- Alain Connes (College de France, Paris)
- Quanta of Geometry
- Hörsaal I, Physikalisches Institut

**Abstract:** In the construction of spectral manifolds in noncommutative geometry, a higher degree Heisenberg commutation relation involving the Dirac operator and the Feynman
slash of real scalar fields naturally appears and implies, by equality with the index formula, the quantization of the volume. We first show that this condition implies that the
manifold decomposes into disconnected spheres which will represent quanta of geometry. We then refine the condition by involving the real structure and two types of geometric quanta,
and show that connected manifolds with large quantized volume are then obtained as solutions. When this condition is adopted in the gravitational action it leads to the quantization
of the four volume with the cosmological constant obtained as an integration constant. Restricting the condition to a three dimensional hypersurface implies quantization of the three
volume and the possible appearance of mimetic dark matter. When restricting to a two dimensional hypersurface, under appropriate boundary conditions, this results in the quantization
of area and has many interesting applications to black hole physics.

#### Bethe Colloquium by Prof. Carsten Urbach

November's Bethe Colloquium took place on November 13th (3:15 pm) in Hörsaal I:

- Carsten Urbach (HISKP, Bonn)
- Axial U(1) anomaly in QCD and the Witten-Veneziano formula
- Hörsaal I, Physikalisches Institut

**Abstract:** In the 70ties of the last century the mass spectrum of the light pseudoscalar mesons posed a puzzle: the eta' meson is way too heavy for being a (pseudo) Golstone
boson. This was explained eventually with the anomalously broken axial U(1) symmetry together with topological effects in QCD. While this solution is mainly based on perturbative
arguments, we will discuss how to determine the light pseudoscalar meson spectrum non-perturbatively using lattice QCD. Moreover, we will relate the so determined masses to the U(1)
anomaly using the famous Witten-Veneziano formula.

#### Bethe Colloquium by Prof. Ettore Remiddi

October's Bethe Colloquium took place on October 16th (3:15 pm) in Hörsaal I:

- Ettore Remiddi (INFN, Bologna)
- The (state of the) art of Feynman graph evaluation - a personal recollection
- Hörsaal I, Physikalisches Institut

**Abstract:** After a short presentation of the current situation of the (g-2) of the electron, I will try to describe some of the new tools appeared in the years in the technology
of Feynman graph evaluation. The analytical evaluation of the electron (g-2) required new integration techniques (mostly the differentiation of a suitable integral representation
before its actual integration), whose repeated use emphasized the relevance of the continuous dimensional regularisation and of the Integration-by-Parts identities for expressing all
the terms to be evaluated to the Master Integrals (MI's) of the problem. When applied to the M.I.s, the differentiation-integration technique naturally evolved into a system of first
order differential equations, whose investigation brought into the game the good old Euler's variation of constants, repeated integrations, Euler's dilogarithm, Nielsen's
polylogarithms - obviously generalised into harmonic polylogarithms HPL's, two-dimensional HPL's, etc. Large (and still growing) sets of amplitudes are nowdays evaluated in terms of
HPL's by solving at once the corresponding huge sets of differential equations by iterating, almost algebraically, clever (but rather simple) integrations procedures, which exploit
homogeneity properties and/or the Magnus formalism. Despite all that progress, it is not yet clear whether (or how) the approach can be systematically extended to amplitudes with non
vanishing internal masses; to illustrate that point, the case of the two loop (sunrise) self-mass graph will be concisely recalled.

####
6th Bethe Center Workshop

Topological Strings and Applications

The workshop took place from September 29th to October 3rd. There were pedagogical introductory lectures each day followed by research talks by the participants. The introductory talks were

- Andrea Brini: "Introduction to matrix models"
- Hans Jockers: "Topological Strings and Effective Interactions"
- Albrecht Klemm: "Introduction into Refined Topological String Theory"
- Johannes Walcher: "Introduction to Matrix Factorizations and D-branes"
- Alessandro Tomasiello: "Geometric aspects of supersymmetric field theories on curved manifolds"

#### 2nd Alumni meeting of the network Teilchenwelt

Between September 19th and September 21st high school students visited the bctp for a meeting of the network Teilchenwelt. More information can be found here.

#### Workshop on CMOS Active Pixel Sensors for Particle Tracking

Focused workshop dedicated to CMOS pixel technologies and developments in the field of tracking detectors. The aim of the workshop was to share detailed technical insight and
experience with CMOS technologies suitable for active pixel sensors for particle and radiation detection. In order to achieve the aforementioned goals, presentations and attendance
were solicited by invitation only.

Topical interests (tentative):

- Design aspects
- CMOS technologies
- Signal collection
- Radiation hardness
- Simulations - Lessons learned

#### Mini String Forum

- Mauricio Romo (IPMU Japan)
- Gauged Linear Sigma Models disk partition function and nonabelian matrix factorizations
- Time: 10-12, Place: bctp 1 (2.019)

**Abstract:** I will explain how the supersymmetric disk partition function Z_{D^2} of gauged linear sigma models relate to the central charge of objects in the category of
B-branes of a Calabi-Yau (CY). The advantage of this approach is that Z_{D^2} provides an expression at every point in the quantum corrected moduli space of the CY. The B-branes in
these models are realized naturally as matrix factorizations, equivariant under the gauge group. I will explain how to relate them to more familiar objects such as coherent sheaves on
the CY and show applications of this result to different type of CYs, including more exotic non-complete intersections.

- Eric Sharpe (Virginia Tech)
- Recent developments in two-dimensional (0,2) theories
- Time: 14-16, Place: bctp 1 (2.019)

**Abstract:** In this talk I will survey recent progress in heterotic string compactifications and two-dimensional theories with (0,2) supersymmetry. For the first half of the
talk, I will give an overview of `quantum sheaf cohomology,' a heterotic analogue or generalization of quantum cohomology. For the second half of the talk, I will discuss various new
and old dualities in two dimensions. Specifically, I will first review progress towards (0,2) mirror symmetry, a generalization of ordinary mirror symmetry. I'll then turn to gauge
dualities in two dimensions, and how easy bits of math can be used to derive such dualities. Finally, I'll discuss decomposition for two-dimensional nonabelian gauge theories with
center-invariant matter, which is the observation that such theories can decompose into a disjoint union of closely related theories.

#### Bethe Colloquium by Prof. Bert Schellekens

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

- Bert Schellekens (NIKHEF, Amsterdam)
- Particle Physics in the Multiverse
- Hörsaal I, Physikalisches Institut

**Abstract:** Is our universe a single entity or part of a multiverse? Can the Standard Model of Particle Physics be derived uniquely from a fundamental theory, or is it just one
of many possibilities? Insights from inflation and string theory in the past three decades suggest an answer that has raised a lot of controversy, but that is slowly gaining ground.
It would have important implications for our attempts at understanding the Standard Model and the expectations for “new physics”. It also implies that we cannot ignore an important
bias in our observations: only universes with observers can be observed. This leads inevitably to the most controversial issue: the infamous "anthropic principle”.

#### Visit of Korean students

#### Bethe Forum on Non-geometry, asymmetric orbifolds and model building

Recently the field of non-geometrical string compactifications has revived. One motivation for this has been the search for constructions that provide a build-in mechanism for moduli stabilization. Another reason is that it has been realized that maximal (N=8) supergravities in four dimensions admit many gaugings, but only a small subset of those can be associated with compactifications of 10D supergravity. Some of the other gauge supergravities can be obtained by applying T-dualities to the geometrical compactifications. Hence, one expects that there must be some sort of lift of these 4D gauged supergravities to 10D string theory. They go under the name of non-geometrical flux backgrounds.

Since an underlying idea is that various configurations of fluxes are related by T-dualities, it would be useful to have a formulation of the low-energy theory of string theory that is T-duality covariant. Here, double field theory enters the scene: it is a construction in which the number of coordinates are doubled to make T-duality manifest.

Double field theory is one attempt to have a definite stringy description of non-geometry. Another approach is to use asymmetric orbifolds. Even though these orbifolds do not have a simple geometrical interpretation, they provide exactly solvable string solutions. The connection between them and the non-geometric fluxes has recently been investigated. In addition some first attempts have been made to do model building on such backgrounds. Furthermore, a natural description of asymmetric Z2xZ2 orbifolds are free-fermionic constructions. Also, quite recently, there has been a full classification of all symmetric orbifold geometries compatible with heterotic N=1 or more supersymmetry in four dimensions using the language of cristallography. It would therefore be very interesting to obtain a similar classification of asymmetric orbifolds. In addition the techniques to determine the nature of the non-geometrical fluxes might also be applicable to more involved non-geometrical string constructions like Gepner models.

In this workshop we wanted to bring together experts on the various aspects of non-geometry and exact string constructions to share their recent results and discuss how some of the open questions mentioned above can be addressed.

The homepage of the program is available here.

#### Bethe Colloquium by Prof. Graham Ross

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

- Graham Ross (Rudolf Peierls Centre, Oxford)
- Beyond the Standard Model (or not) after LHC-8TeV
- Hörsaal I, Physikalisches Institut

**Abstract:** To date, the LHC has not observed indications of structure beyond the Standard Model of particle physics. Moreover the measured properties of the Brout-Englert-Higgs
boson hint at an underlying simplicity of the model without such new physics. This has led to a reevaluation of the hierarchy problem on which much of our faith in Beyond the Standard
Model physics rests. In this talk I will review these issues and compare “Just the Standard Model”, including gravity, with models that incorporate a more complete unification of the
fundamental forces.

#### Bethe Forum on Discrete symmetries and their stringy origin

There is a second possibility to protect discrete symmetries against gravitational effects by relating their origin to the relevant energy scale: the scale where quantum effects of gravity become important, i.e. the planck scale. Hence, this approach needs a consistent theory of quantum gravity and string theory is a prime candidate. Within string theory discrete symmetries (Abelian and non-Abelian) can naturally originate from both, the traditional approach described above and, furthermore, from the compactification from ten to four dimensions. In the later case one can partially understand the origin of the discrete symmetry intuitionally from properties of the six-dimensional compactification space. But in addition to this intuition there can be vanishing couplings and relations between coupling strengths that only show up in the explicit computation of string amplitudes. Some of them can be interpreted as an enlargement of the intuitive symmetry, but some might not. Intersecting D-branes and heterotic orbifold compactifications provide consistent frameworks that allow to study the origin of discrete symmetries from string theory directly. Recently, discrete R-symmetries have been re-examined in this context. But a full study of couplings and their interpretation as discrete symmetries remains as an open problem.

The aim of this workshop was to bring together experts on discrete symmetry phenomenology with experts on their possible string theory origin.

The homepage of the program is available here.

#### Bethe Colloquium by Prof. Georgy Shlyapnikov

May's Bethe Colloquium will take place on May 22nd (3:15 pm) in Hörsaal I:

- Georgy Shlyapnikov (LPTMS, Orsay)
- Many-body physics with quantum gases in disorder
- Hörsaal I, Physikalisches Institut

**Abstract:** I will give a brief introduction to the physics of ultracold atoms in random potentials and show that ultracold quantum gases open a wide avenue to study how the
interaction between particles influences their localization and transport properties. This subtle question was crucial for electrons in solids and now it appears in a new light in the
systems of disordered ultracold neutral atoms.

I then discuss two examples related to the interaction-induced many-body localization-delocalization transition. The first one concerns weakly interacting one-dimensional disordered
bosons, where we have a non-conventional finite temperature phase transition between two distinct states: fluid and insulator, and I describe the phase diagram. The second example is
related to interacting bosons in a quasiperiodic one-dimensional potential, which is a superposition of an initial strong lattice and superimposed shallow incommensurate lattice. This
system is intermediate between the true disorder and regular lattice and I will show that the many-body localization-delocalization transition in this system leads to a remarkable
result. Namely, in a wide range of parameters an increase in temperature favors the insulator state. Thus, one has an anomalous ''freezing with heating'' behavior.

#### Linear Collider Forum

The homepage of the program is available here.

#### Bethe Colloquium by Prof. Antonio Masiero

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

- Antonio Masiero (INFN, Padua)
- The Early Universe and LHC: 2 Accelerators for 1 New Physics
- Hörsaal I, Physikalisches Institut

**Abstract:** The discovery of the Higgs boson (LHC) and the results on the cosmic background radiation radiation (early Universe) have assessed the validity of the GSW Standard
Model of particle physics and the hot Big Bang ΛCDM Standard Model of cosmology. Yet, there are quite a few, but important, pieces of evidence that new physics beyond such Standard
Models exists: neutrino masses, dark matter, cosmic matter-antimatter asymmetry, primordial inflation, dark energy. Unfortunately, there's no clear clue on the mass scale of the new
particles/interactions associated with such new physics. Strong, though debatable, arguments (related to the fine-tuning of fundamental parameters connected to the Higgs boson mass
and the unification of fundamental interactions) point to a new physics mass scale accessible to the LHC and with relevant implications for the early Universe dynamics. I' ll review
the three-fold approach (high-energy and high-intensity machines, astroparticle physics) hunting for such (low-energy) new physics focusing, as an explicit example, on the probes of
the virtual presence of new particles in very rare phenomena with lepton flavor violation.

#### 5 years bctp

Founded in 2008, the Bethe Center for Theoretical Physics (bctp) is a joint enterprise of theoretical physicists and mathematicians at various institutes of Bonn University. In the spirit of Hans Bethe it fosters research activities over a wide range of theoretical and mathematical physics.

The bctp provides a forum for interdisciplinary research and education throughout the fields of physics, mathematics and cosmology. Activities of the bctp include a regular Bethe Colloquia series with 38 talks to date, workshops on dedicated research topics, lectures for doctoral students as well as short and long term visitor programs.

Research in the bctp has been instrumental in unlocking the secrets of the universe. We are tackling the deepest questions at the frontiers of theoretical and mathematical physics, and are committed to sharing our results with the scientists of the young generation.

Bethe Workshops and Lecture Series, eight events to date, have offered the opportunity to meet leading physicists to over 300 participants. Since 2011, five Bethe Forum programs have brought together 84 invited scientists for extensive discussion sessions and talks. Highlights in our activities have been two international conferences hosted by the bctp, “String-Math” in 2012 and “Planck 2013 – From the Planck Scale to the Electroweak Scale” in 2013, with about 200 participants each. The work of the bctp in research and education has resulted in close to 500 publications and 31 doctoral theses to date.

#### Bethe Forum on Detector Physics - Trends and Challenges

The homepage of the program is available here.

#### Bethe Forum Lecture Series on Scattering Amplitudes

The homepage of the program is available here.

#### Bethe Colloquium by Prof. Wolfgang Lück

January's Bethe Colloquium took place on January 16th (3:15 pm) in Hörsaal I:

- Wolfgang Lück (Hausdorff Research Institute for Mathematics, Bonn)
- Heat kernels and their application in geometry, topology and group theory
- Hörsaal I, Physikalisches Institut

**Abstract:** The heat kernel is a well known object in physics. We are interested in the kernel of the Laplace operator acting on p-forms of the universal covering of a closed
Riemannian manifold. One can extract from the spectrum of the Laplace operator interesting topological invariants which are independent of the Riemannian metric and capture some of
the geometry and topology. These concepts are extended to more general spaces and lead to surprizing applications to problems in geometry, topology and group theory.