• Tuesday 12 January 2021 at 10.15 with Zoom: Henrique Rubira (DESY)
    A hybrid simulation of gravitational wave production in first-order phase transitions
    The LISA telescope will provide the first opportunity to probe the scenario of a first-order phase transition happening close to the electroweak scale. By now, it is evident that the main contribution to the GW spectrum comes from the sound waves propagating through the plasma. Current estimates of the GW spectrum are based on numerical simulations of a scalar field interacting with the plasma or on analytical approximations — the so-called sound shell model. In this work we present a novel setup to calculate the GW spectra from sound waves. We use a hybrid method that uses a 1d simulation (with spherical symmetry) to evolve the velocity and enthalpy profiles of a single bubble after collision and embed it in a 3d realization of multiple bubble collisions, assuming linear superposition of the velocity and enthalpy. The main advantage of our method compared to 3d hydrodynamic simulations is that it does not require to resolve the scale of bubble wall thickness. This makes our simulations more economical and the only two relevant physical length scales that enter are the bubble size and the shell thickness (that are in turn enclosed by the box size and the grid spacing). The reduced costs allow for extensive parameter studies and we provide a parametrization of the final GW spectrum as a function of the wall velocity and the fluid kinetic energy.
    Link to video:
  • Tuesday 2 February 2021 at 10.15 with Zoom: Umut Gürsoy (Utrecht U.)
    Hydrodynamics and holography of spin currents
    Collective motion of spin has been playing a prominently interesting role both in high energy and condensed matter physics, especially after the observation of global polarization of $\Lambda$ hyperons in heavy ion collisions and generation of spin current in liquid helium. In this talk I will explain how to construct the theory of spin hydrodynamics, that is, relativistic hydrodynamics comprised of the spin current and the stress tensor. We then apply this to heavy ions and find surprisingly good agreement with data. Finally, I will introduce a holographic approach to spin currents employing the Lovelock-Chern-Simons theory in five dimensions.
    Link to video:
  • Tuesday 16 February 2021 at 10.15 with Zoom: Jukka Maalampi (Jyväskylä)
    The strange history of Runar Gåsström, a Finnish-American-Soviet Physicist
    Runar Viktor Gåsström (1914-1998) was born to a Finnish immigrant family in New York. The family moved back to Finland in 1917, to immigrate ten years later to Vancouver, Canada, and further on to Petroskoi, Soviet-Karelia. Gåsström studied physics in the Moscow State University, allegedly as a student of Pjotr Kapitsa. In 1946 Gåsström, quite surprisingly, moved to Helsinki. He worked for ten years at the Department of Physics in the nuclear physics group of Lennart Simons, being e.g. responsible of the construction of a Van de Graaff generator. Gradually, some of his actions, such as his frequent and inquisitive visits in nuclear physics research centers around the Western Europe and his possible role in the defection of Italian nuclear physicist Bruno Pontecorvo to Soviet Union, as well as his background, started to raise concerns. Gåsström left Finland in 1954 and moved to Groningen, Netherlands, and then to Vienna to work for the IAEA. In 1960 he returned to Soviet Union, where he thereafter held many distiguished positions in Novosibirsk, Irkutsk, and Kaliningrad, working in the fields of nuclear electronics and radiophysics. 
    Link to video:
  • Tuesday 9 March 2021 at 10.15 with Zoom: Eoin Ó Colgáin (CQUEST & Sogang U.)
    Comments on H0
    Precision LCDM Cosmology may be in a state of flux. I will spell out some implications of Hubble tension, a persistent disagreement between local determinations and cosmological inferences of the Hubble constant.
    Link to video:
  • Tuesday 16 March 2021 at 10.15 with Zoom: Watse Sybesma (Iceland)
    Evaporation and the price of curiosity
    The Page curve arises if black hole evaporation is a unitary process. It describes entanglement between the inside and outside of the black hole as a function of time. I will describe how to compute the Page curve for an evaporating black hole in asymptotically flat spacetime by adapting the recently developed ‘island’ formula to the setting of a semi-classical two-dimensional dilaton gravity theory. Afterwards I will switch gears and use the island formula to investigate semi-classical de Sitter in two dimensions. I will consider the question if somebody can use Gibbons-Hawking radiation to decode what lies beyond their cosmological horizon. The answer to this question seems to be positive, but with the caveat that this curiosity will come at a price.
    Link to video:
  • Tuesday 23 March 2021 at 10:15 with Zoom: Masanori Hanada (Surrey)
    Color confinement, Bose-Einstein condensation, and emergent geometry in gauge/gravity duality
    Abstract: In gauge/gravity duality, the information regarding the gravitational geometry (e.g., black hole and smooth exterior geometry) has to be encoded in gauge theory. The color degrees of freedom (matrix degrees of freedom) have to play the key role, because the duality can hold even when the gauge theory side is a matrix model. In this talk, I will provide a very simple way of encoding the geometry to matrices, along the line of Witten’s work on the effective action of D-branes and strings, and the Matrix Theory conjecture by Banks, Fischler, Shenker and Susskind. Roughly speaking, eigenvalues of matrices can be identified with the location of the D-brane probe or extended objects such as black hole.

    Actually there is a famous argument against such simple interpretation advocated by Polchinski in 1998. His argument used generic properties of large-N gauge theory to show that the ground-state wave function delocalizes at large N, leading to a conflict with the locality in the bulk geometry. We show that this argument is not correct: the ground-state wave function does not delocalize, and there is no conflict with the locality of the bulk geometry. In order to understand how Polchinski’s argument fails, recently-discovered connection between color confinement at large N and Bose-Einstein condensation is useful. This confinement-BEC connection has a striking consequence: in the SU(N) gauge theory, there is a partially-deconfined phase in which an SU(M)-subgroup is deconfined. Partial deconfinement gives a natural realization of the idea in BFSS Matrix Theory conjecture — extended objects, such as black hole, are realized as bound states of D-branes and strings, that look like non-commutative blocks in big matrices — in the Maldacena-type gauge/gravity duality.
    Link to video:
  • Tuesday 30 March 2021 at 10:15 with Zoom: Yago Bea (Queen Mary)
    New Insights from Real-time Dynamics
    Abstract: By using holography and numerical relativity I study the real-time dynamics of strongly coupled field theories and obtain insights that might be relevant for physical systems like the quark-gluon plasma formed in heavy-ion collisions or cosmological scenarios exhibiting phase transitions and bubble nucleation. I study the applicability of hydrodynamics in these systems.
    Links to videos: (part 1/2) and (part 2/2)
  • Tuesday 13 April 2021 at 10:15 with Zoom: Manuel Reichert (Sussex)
    Gravitational waves from first-order phase transitions with non-perturbative effective potentials
    Abstract: In this talk, I will present the gravitational-wave signals from the electroweak phase transition as well as from a dark SU(N) confinement-deconfinement phase transition. The common denominator is that the effective finite-temperature potential stems from a non-perturbative computation, in one case from the functional renormalisation group and the other case from the lattice.
    In the electroweak sector, I will detail the impact of non-perturbative higher-dimensional operators in the Higgs potential. Interestingly, the type of higher-dimensional operator seems to have no impact on the gravitational-wave signal, which leads to a universal relation between the mean-bubble separation and the strength parameter of the electroweak phase transition.
    In the dark sector, I will show how the confinement phase transition from a strongly coupled dark SU(N) sector can be computed via effective Polyakov loop models adapted to lattice data. This allows for an eagle view of the dark landscape which is spanned by the phase-transition temperature and the number of dark colours.
    The talk is based on
    – Universal gravitational-wave signatures from heavy new physics in the electroweak sector (
    – Testing the Dark Confined Landscape: From Lattice to Gravitational Waves (
  • Tuesday 20 April 2021 at 10:15 with Zoom: Mikko Laine (Bern)
    How well do we know the thermal photon rate?
    Abstract: The ALICE experiment at LHC has proposed to replace itself during Long  Shutdown 4 through a new experiment, aimed e.g. at studying the production  of very soft electromagnetic radiation (pT < 100 MeV). I review what 40+  years of theoretical research have taught us about the production of photons from a thermal QCD plasma. In particular, the prospects for constraining the  thermal photon rate through lattice QCD are discussed, pointing out that  there may still be room for new theoretical ideas in this field.
    Link to seminar:
  • Tuesday 18 May 2021 at 10:15 with Zoom: Hans-Werner Hammer (Darmstadt)
    Un-nuclear Physics
    Abstract: I will discuss a nonrelativistic version of Georgi’s “unparticle physics”. An “un-nucleus” is a field in a nonrelativistic conformal field theory characterized by a mass and a scaling dimension. Unlike the relativistic unparticle, which remains a hypothetical object, the un-nucleus is realized approximately in high-energy nuclear reactions involving emission of a few neutrons with relative energies between about 0.1 MeV and 5 MeV. Combining this observation with the known universal properties of fermions at unitarity in a harmonic trap, we predict a power-law behavior of the inclusive cross section in this kinematic regime. We compare our predictions with previous theoretical calculations of nuclear reactions and point out opportunities to measure un-nuclei at radioactive beam facilities.
    Link to seminar:
  • Tuesday 25 May 2021 at 11:15 with Zoom: Mitesh Patel (Imperial College) Note time!
    B-anomalies at LHCb
    Abstract: An update on the so-called B-anomalies will be presented, putting the recent evidence for lepton flavour universality breaking into the wider context of such measurements. The future of this field will also be discussed.
    Slides and the seminar can be found in
  • Tuesday 8 June 2021 at 10:15 with Zoom: Kenneth Österberg (Helsinki)
    Observation of odderon exchange from proton-proton and proton-antiproton elastic scattering at TeV scale
    Abstract: In addition to the dominant C-even gluonic exchanges, may C-odd gluonic exchanges (“odderon”) occur in high energy elastic hadron scattering.  Contrary to C-even exchanges, C-odd exchanges are not invariant for proton-proton (pp) and proton-antiproton (ppbar) scattering. Therefore,  any significant difference in the elastic pp and ppbar cross section at the same energy at TeV scale, where gluonic exchanges are dominant, would be evidence for C-odd exchanges. We make a model-independent extrapolation of the TOTEM 2.76, 7, 8, and 13 TeV pp elastic cross sections to 1.96 TeV. We observe a difference with a 3.4σ significance between the extrapolated pp elastic cross section and the D0 ppbar elastic cross section at 1.96 TeV in the region of the diffractive minimum and the second maximum of the pp cross section. We combine these results with a TOTEM analysis of the same C-odd exchange based on forward pp elastic scattering at TeV scale. The combined significance is larger than 5σ and is interpreted as the first observation of the exchange of an odderon.
    Link to seminar
  • Tuesday 28 September 2021 at 10:15 with Zoom: Timo Kärkkäinen (Budapest)
    Super-weakly coupled U(1)_z and GeV neutrinos
    Abstract: Super-weak force is a minimal extension of the standard model,  which adds three massive sterile Majorana neutrinos, a complex singlet scalar boson and a neutral massive vector boson to the particle spectrum. It aims to explain dark matter, accelerating expansion of the universe, neutrino mass generation, vacuum metastability, cosmic inflation and baryonic asymmetry of the universe. In the talk I will discuss the neutrino sector and phenomenology of this model. The model exhibits suppressed nonstandard neutrino interactions and potential to discover the disappearance of active neutrino flavours to sterile flavours via SHiP and MATHUSLA experiment. In addition, I will discuss the sub-leading corrections to neutrino masses arising from one-loop contribution to light neutrino self-energies.References: arXiv 2104.14571, 2105.13360.
  • Tuesday 2 November 2021 at 10:15 with Zoom: Mikel Sanchez Garitaonandia (Helsinki)
    Gravitational Waves from first order phase transitions with holography
    Abstract: First order phase transitions during the history of the universe are  known to lead to the nucleation, expansion and collision of bubbles that  source gravitational waves. This emission could be detected by next generation interferometers, opening an exciting window into physics  beyond the Standard Model. The GW production depends on non-equilibrium  properties, such as the wall speed, challenging to compute by conventional techniques. We use holography to perform fully non-linear,  time simulations of expanding and colliding bubbles in strongly coupled  theories with a thermal first order phase transition. We explore qualitative features of the wall and its velocity, the applicability of  hydrodynamics to the systems and some preliminary results on GW production.
    Link to seminar:
  • Tuesday 9 November 2021 at 10:15 with Zoom: Yago Bea (Helsinki)
    Evolutions in first-order viscous hydrodynamics.
    Abstract: We perform real-time evolutions using the first-order viscous relativistic hydrodynamic equations formulated by Bemfica, Disconzi, Noronha and Kovtun (BDNK) in three-dimensional conformal theories. For comparison, we also perform evolutions using the ideal and viscous BRSSS equations of hydrodynamics. Moreover, motivated by the physics of the quark-gluon plasma, we use holography to obtain the microscopic dynamical evolution of a system relaxing to equilibrium in a strongly coupled field theory that we use to study the applicability of hydrodynamics.
    Link to video:
  • Friday 12 November 2021 at 14:15 in A315 + Zoom: Harri Waltari (Uppsala)
    News from the ILCX workshop
    The European particle physics strategy update emphasizes the need of a future Higgs factory as the most important next collider facility. The ILC is one of the proposed future electron-positron colliders to be located in Japan and it would satisfy the need for the Higgs factory. I shall summarize contents of the ILCX workshop held virtually in Japan in October (, where the current status and neartime future plans of the project were discussed. I hope that my presentation will then lead to an interesting discussion. I especially want to encourage the early career researchers to participate as this collider could well be operational for a large fraction of your scientific career.
  • Tuesday 23 November 2021 at 10:15 in A315 and with Zoom: Matti Järvinen (Pohang & APCTP)
    Strong coupling models for Dense and Hot QCD
    Gauge/gravity duality (in combination with other approaches) can help to constrain the QCD equation of state at finite temperature and density, in particular in the region relevant for core-collapse supernovae and neutron star mergers. I present a new framework where I use the holographic V-QCD model to describe both nuclear and quark matter at high densities, with temperature dependence estimated by employing a simple van der Waals hadron gas model. At low densities, I use DD2 Hempel-Schaffner-Bielich model, i.e., a statistical model for nuclear matter with relativistic mean field interactions. I obtain predictions for the mixed quark-hadron phase and the location of the critical endpoint of the quark-hadron transition.
    Link to video:
  • Thursday 25 November 2021 at 10:15 with Zoom: Misha Shaposhnikov (Lausanne) Note day!
    QFT without infinities and hierarchy problem
    The standard way to do computations in Quantum Field Theory (QFT) often  results in the requirement of dramatic cancellations between  contributions induced by a “heavy” sector into the physical observables  of the “light” (or low energy) sector – the phenomenon known as  “hierarchy problem”. This procedure uses divergent multi-loop Feynman  diagrams, their regularisation to handle the UV divergences, and then renormalisation to remove them. At the same time, the ultimate outcome  of the renormalisation is the mapping of several finite parameters  defining the renormalisable field theory into different observables  (e.g. all kinds of particle cross-sections). In this paper, we first  demonstrate how to relate the parameters of the theory to observables  without running into intermediate UV divergences. Then we go one step  further: we show how in theories with different mass scales, all physics  of the “light” sector can be computed in a way which does not require dramatic cancellations induced by physics of the “heavy” sector. The  existence of such a technique suggests that the “hierarchy problem” in renormalisable theories is not really physical, but rather an artefact  of the conventional procedure to compute correlation functions. If the QFT is defined by the “divergencies-free” method all fine-tunings in  theories with well separated energy scales may be avoided.
    Link to video:
  • Tuesday 30 November 2021 at 10:15 in A315: Andreas Ekstedt (Desy)
    First-order Phase Transitions: Jumping Over the Barrier
    In this talk I discuss how to consistently describe first-order phase transitions within perturbation theory. By working with a dimensionally reduced theory, I show how to push calculations to three loops and beyond. Doing so I compare observables, for example the surface tension and various condensates, with recent Lattice data. FurtherI discuss how the nucleation rate can be consistently calculated to higher orders using EFT methods.These methods are then used to calculate the rate for a radiatively generated barrier to NNLO.
    Link to seminar:
  • Tuesday 7 December 2021 at 10:15 in A315: Francesco Nitti (APC Paris)
    Curved holographic RG flows, quantum phase transitions and AdS vacuum decay
    I will discuss recent work on AdS-to-AdS vacuum decay mediated by maximall-symmetric Coleman-De Luccia (CDL) instantons, using the results from holographic Renormalization Group flows (RG-flows) of quantum field theories on curved space-times. On the field theory side, a CDL instanton is a state corresponding to a non-trivial RG flow on a sphere. On the gravity side, this is an asymptotically-AdS solution of Einstein-dilaton gravity, whose metric has spherical slicing transverse to the radial (holographic) direction. I will discuss under which conditions (which, as it turns out, are non-generic) these instantons exist, in which case they mediate the decay of the “false” AdS vacuum by bubble nucleation. I will discuss the subsequent evolution of the solution to real-time, and the interpretation in terms of the dual quantum field theory.
    Link to video:
  • Tuesday 14 December 2021 at 10:15 in A315: Jani Kastikainen (Helsinki & APC Paris)
    Quantum Information Geometry of Virasoro States
    The idea of quantum information geometry is to apply tools of Riemannian geometry to state spaces of quantum mechanical systems. In this talk, I explore information geometry of quantum states in two-dimensional conformal field theories. The space of density matrices I consider is an infinite-dimensional space of Virasoro states obtained by acting with unitary representations of the Virasoro group on a thermal state. I compute the relative entropy between Virasoro states and show how it induces an information metric, the Kubo-Mori metric, on these states. As an application, I derive the corresponding geodesic equation and explain its connection to a fluid equation describing liquid crystals.
    Link to video:
  • Thursday 16 December 2021 at 10:15 in A315: Oleg Komoltsev (Stavanger)
    How perturbative QCD constrains the Equation of State at Neutron-Star densities
    The rapid evolution of neutron-star astronomy in recent years is for the first time giving us empirical access to the physics of the cores of neutron-stars, the internal structure of which is determined by the equation of state (EoS) of strongly interacting matter. In this talk I demonstrate in a general and analytic way how high-density information about EoS of strongly interacting matter obtained using perturbative Quantum Chromodynamics constrains the same EoS at densities reachable in physical neutron stars. The results can be used to propagate the pQCD calculations reliable around 40ns to lower densities (starting from 2.2ns) in the most conservative way possible. These purely theoretical results are independent of astrophysical neutron-star input and hence they can also be used to test theories of modified gravity and BSM physics in neutron stars.
  • Tuesday 21 December 2021 at 10:15 using Zoom: Karapet Mkrtchyan (Imperial College)
    Democratic formulation for non-linear electrodynamics and manifest electric-magnetic duality
    In this talk, I will provide a manifestly Poincaré invariant Lagrangian formulation for arbitrary non-linear electrodynamics that features electric and magnetic degrees of freedom in equal footing. This formulation uses auxiliary fields, which are pure gauge on-shell. In this approach, discrete and continuous electric-magnetic duality symmetries can be imposed in a simple and straightforward manner (they are manifest!). Celebrated (generalized) Born-Infeld and recently discovered (conformal) ModMax theories are recast in a manifestly duality-symmetric form, demonstrating the power of this result. Time permitting, I will mention ongoing work on generalizations of the presented work.
    Link to video: