## DEPARTMENT OF PHYSICS / HIP JOINT COLLOQUIA / SEMINARS 2024

**Thursday 25 January 2024 at 10:15 in A315 and using Zoom: Saga Säppi****(Munich)**Mass effects and bulk viscosity in high-density QCD

*Abstract:*With gravitational wave observations probing neutron stars through their mergers in new ways, there is an increasingly pressing need for improved theoretical predictions. In this context, there are properties of extremely dense matter to which traditional measurements of quiescent neutron stars are insensitive to. One example of this is the effect of quark masses: In the study of the equation of state of neutron stars, masses can often be neglected. However, these mass effects can become unexpectedly important in the context of mergers. Looking at the problem through the point of view of perturbative high-density QCD, existing methods of taking masses into account often involve convoluted formulas and relatively delicate numerical integrals—this is particularly true at finite temperatures, also a necessary ingredient for realistic mergers simulations. I will explain a simpler approximate method of dealing with these finite-mass loop integrals in perturbative thermal field theory, vastly simplifying existing calculations while maintaining good accuracy. This method can be applied to the computation of bulk viscosity, a transport quantity which is particularly important for merger scenarios, and something which*vanishes*when mass effects are neglected. By doing so, we have obtained a state-of-the-art evaluation of the bulk viscosity of unpaired quark matter at extremely high densities, with the QCD bulk quantities contributing to it now well under control. The pQCD results can be compared with holography, applicable down to much lower densities, resulting in a beautifully compact approximate pocket formula usable in neutron star simulations.

Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=184a03a8-7fbb-4202-8afc-807323164f32**Tuesday 30 January 2024 at 10:15 in A315 and using Zoom: Elina Palmgren****(Helsinki)**

*Teaching (theoretical) physics for student deep learning: perspectives from physics education research*In studies of higher education, it has been shown that approaches to teaching are affected by discipline: instructors of natural sciences tend to have content-based approaches to teaching, while instructors of humanistic sciences are on average more inclined towards learning-focused approaches (cf. Lindblom‐Ylänne et al., 2006). The content-focused approaches emphasize conveying the taught content to students and are linked to surface learning, whereas the learning-focused approaches aim at fostering students’ active knowledge building and are linked to deep learning. It has been speculated that the variations in the teaching approaches depend on the academic traditions and historical teaching methods of disciplines. Moreover, the characteristics of the discipline affect the teaching approach, as the knowledge structure of the discipline guides the choice of the approach. More specifically in the case of physics, the knowledge structure is cumulative, hierarchical, and mathematically structured, which may lead to sequential and factual content-based teaching that often forgets to emphasize the holistic structures and interconnections between topics.In this talk, some ideas for more learning-focused approach to teaching (theoretical) physics are presented. Especially the role of mathematics as a holistic structure of physical theories, that could be utilized in teaching, is considered. The topic is discussed from the physics education perspective, reflecting on how to turn the mathematical nature of physics into an asset for deep learning.Abstract:

Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=a8137130-8967-4510-8138-6239eb1c4b81**Tuesday 20 February 2024 at 10:15 in A315 and using Zoom: Benoit Laurent****(McGill)**

First-order phase transitions in the early Universe are well-motivated events predicted by several BSM models. In the first part of this talk, I will derive the fluid equations needed to compute the bubble wall velocity from first principles. By treating the background and out-of-equilibrium perturbations in a consistent way, the resulting equations are free of the discontinuity at v_w=c_s that was observed in previous studies. I will show that the solutions can naturally be classified as deflagration/hybrid walls (v_w ~ c_s) or ultrarelativistic detonations. In the second part, I will explain how this calculation can be significantly simplified when local thermal equilibrium (LTE) is maintained in the plasma. Using this LTE assumption, the fluid equations can be reexpressed in terms of only four parameters that completely characterize a particle physics model. I will present an efficient algorithm to solve these equations and discuss the properties of their solutions. Finally, I will compute the kinetic energy fraction which is essential for predicting the gravitational wave spectrum produced during the phase transition*First principles determination of bubble wall velocity and local thermal equilibrium approximation.*

Abstract:**Thursday 22 February 2024 at 10:15 in A315 and using Zoom: Helena Kolesova****(Stavanger)**

Taking axion inflation as an example, we study the evolution of a non-Abelian dark sector coupled to the inflaton for different choices of the confinement scale. For confinement scales just a few orders of magnitude below the Planck scale, gravitational wave signal could be generated due to fluctuations in the thermal plasma. Another possible source of gravitational waves is the confinement phase transition; however, this signal might be strongly suppressed due to the presence of an early matter-dominated era. We also study the reheating of Standard Model fields and glueball dark matter within this scenario, which, on the other hand, seems to be possible at lower confinement scales.*Non-Abelian dark sectors: gravitational waves vs dark matter*

Abstract:Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=4b6fbdf1-1a09-4b2b-b17b-77fe6cbbd9b9

**Thursday 29 February 2024 at 15:15 in D106 (note location and time!) and using Zoom: Sarah Williams****(Cambridge)**The Future Circular Collider as a Higgs/top/EW Factory: status and plans for FCC-ee

*Abstract:*The 2020 update to the European Strategy for Particle Physics laid out several key priorities for the future of collider physics beyond the Large Hadron Collider (LHC). It identified an electron-positron Higgs factory as the next high-priority experiment and encouraged CERN and its international partners to investigate the technical and financial feasibility of a 100 TeV hadron collider. The integrated Future Circular Collider (FCC) project aims to combine these goals, and would involve a ~90km tunnel being used first for e+e- collisions (FCC-ee) and then for 100 TeV proton-proton collisions (FCC-hh) with additional opportunities for electron proton (FCC-eh) and heavy ion collisions. This seminar will discuss the physics opportunities of FCC-ee, and the status of the FCC feasibility study which is ongoing and has just undergone its mid-term review ahead of completion in 2025. I will also discuss the roadmap ahead which could enable FCC-ee to begin operation in the late 2040s.

**Friday 8 March 2024 at 10:15 in A315 and using Zoom: Michael Doser****(CERN)**

The seminar will provide a glimpse of some elements of the rapidly evolving field of quantum sensing, specifically focusing on particle physics. Specific approaches involving quantum systems, such as low-dimensional systems or manipulations of ensembles of quantum systems, hold great promise for improving high-energy particle physics detectors, particularly in areas like calorimetry, tracking, and timing. The use of quantum sensors for high-precision measurements, as well as the development of new quantum sensors based on superconducting circuits, ion and particle traps, crystals, and nanomaterials, are equally relevant for low energy particle physics and for fundamental physics.However, significant advances and improvements in existing or future quantum technologies will be necessary to address such topics related to the dark universe, the detection of relic neutrinos, precision tests of symmetries and of the standard model and probing general foundational issues in physics. The seminar will thus also feature discussions of the Quantum Sensing Initiatives at CERN and the ECFA R&D Roadmap on Quantum Sensing and Advanced Technologies and will discuss options for future collaborations in the context of the imminent implementation of this roadmap.*Quantum Sensing for (low and high energy) particle physics*

Abstract:**Tuesday 12 March 2024 at 10:15 in A315 and using Zoom: Juska Pekkanen**The Future Circular Collider (FCC) is CERN’s next flagship project which fulfills the physics goals of the 2020 update of the European strategy for particle physics set by the European Committee for Future Accelerators. The proposed accelerator would be placed in a 91 km long tunnel to be bored down to 560 m deep under the Geneva region, with the first phase being an electron-positron collider (FCC-ee), followed by an update to a hadron machine (FCC-hh) with proton-proton collision energies of up to 100 TeV. ALLEGRO is a proposed general-purpose experiment for FCC-ee, with a high-granularity noble-liquid electromagnetic calorimeter as a central feature. In this seminar I will give an overview to the status of the FCC project and introduce the ALLEGRO detector concept, with focus on the progress in the development of the next-generation noble-liquid calorimetry technology.**(CERN)**FCC project and ALLEGRO detector concept

Abstract:

Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=d4617bca-0dc6-41fe-8ce4-41873076acec*Tuesday 19 March 2024 at 10:15 in A315 and using Zoom: Michal Heller (Ghent)*Geodesics and extremal hypersurfaces play a critical role in the contemporary understanding of gravity. They compute certain correlation functions, various notions of entropy and, at least sometimes, complexity. I will discuss a new look at this venerable idea motivated by a lack of real extremal surfaces due to de Sitter expansion in cosmologies or timelike separation of boundary points (subregions) in anti-de Sitter holography. Based on 2305.11280 and work in progress with Fabio Ori and Alex Serantes.

Beyond (real) geodesics and extremal hypersurfaces in gravity

Abstract:

Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=92ceb299-98b5-449c-8da5-fb0cf8792a63**Thursday 4 April 2024 at 12:15 in A315 and using Zoom: Karapet Mkrtchyan**I will summarize recent progress on the democratic covariant Lagrangian formulation of (chiral) p-form dynamics, including arbitrary abelian interactions. Applications include democratic formulations for the gauge sector of maximal supergravities in 10d/11d, with on-shell values of their actions consistent with AdS/CFT predictions.**(London)**Democratic approach to p-forms and electric-magnetic duality

Abstract:

Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=17ffea06-cea6-414d-b5a7-21f5c84f2ca2**Tuesday 9 April 2024 at 10:15 in A315 and using Zoom: David Benisty****(FIAS, Frankfurt and KICC, Cambridge)**

In this talk I’ll show a method to constrain the Cosmological Constant \Lambda from binary galaxies, focusing on the Milky Way and Andromeda galaxies. We provide an analytical solution to the two-body problem with \Lambda and show that the ratio between the Keplerian period and T_\Lambda = 2\pi/(c \sqrt{\Lambda}) \approx 63.2 Gyr controls the importance of effects from the Cosmological Constant. The Andromeda-Milky Way orbit has a period of \sim 20 Gyrs and so Dark Energy has to be taken into account. Using the current best mass estimates of the Milky Way and Andromeda galaxies, we find the Cosmological Constant value based only on the Local Group dynamics to be lower than 5.44 times the value obtained by Planck. With future astrometric measurements, the bound on the Cosmological Constant can be reduced to \left(1.67 \pm 0.79\right) \Lambda_{\rm PL}. Our results offer the prospects of constraints on \Lambda over very different scales than previously. The Local Group provides also a completely novel platform to test alternative theories of gravity. We illustrate this by deriving bounds on scalar-tensor theories of gravity over Megaparsec scales.*Constraining Dark Energy from the Local Group dynamics*

Abstract:

Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=0f27d7f5-3cd2-4a45-bb32-8b2b0db9f9a2*Thursday 11 April 2024 at 10:15 in A315 and using Zoom: Christian Ecker (Frankfurt)*

Determining the phase structure of Quantum Chromodynamics (QCD) and its Equation of State (EoS) at densities and temperatures realised inside neutron stars and their mergers is a long-standing open problem. I will present a framework for the EoS of dense and hot QCD that describes the deconfinement phase transition between a dense baryonic and quark matter phase via the holographic V-QCD model. This model is then used to study the consequences on the formation of quark matter in binary neutron star mergers in the prompt and non-prompt collapse regime.*Exploring the Phase Diagram of QCD with Neutron Star Mergers in the Prompt and Non-Prompt Collapse Regime*

Abstract:

Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=621819fb-c411-49d5-ab9f-6c1eb7bdc932