## 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-6c1eb7bdc932The cores of the most massive neutron stars contain the densest matter in the universe. What is possible to learn about the behavior of matter near these densities? In this talk, I will discuss two recent papers that study this question. In the first half, I focus on the interplay between information from perturbative QCD calculations and astrophysical data and how these jointly constrain the behavior of matter in the vicinity of the maximum neutron-star density. In the second half, I turn to the post-merger phase of a binary neutron-star merger and highlight a new way to pinpoint the maximum pressure and densities probed in nature.**Tuesday 14 May 2024 at 10:15 in A315 and using Zoom: Tyler Gorda (Frankfurt)**Probing the behavior of strongly interacting matter at the highest densities

Abstract:

Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=c7b058cf-b5a4-4480-9c7e-9602ab48371e*Tuesday 21 May 2024 at 10:15 in A315 and using Zoom: David Milstead (Stockholm)*

*Search for baryon number violation and other phenomena with the HIBEAM experiment at the European Spallation Source*

The violation of baryon number is an essential ingredient for the preferential creation of matter over antimatter needed to account for the observed baryon asymmetry in the Universe. However, such a process has yet to be experimentally observed. The HIBEAM/NNBAR program is a proposed two-stage experiment at the European Spallation Source to search for baryon number violation. The program will include high-sensitivity searches for processes that violate baryon number by one or two units: free neutron–antineutron oscillation via mixing, neutron–antineutron oscillation via regeneration from a sterile neutron state and neutron disappearance; the effective process of neutron regeneration is also possible. The program can be used to discover and characterize mixing in the neutron, antineutron and sterile neutron sectors. Furthermore, the intense neutron beam can be used to search for phenomena beyond neutron mixing, such as axions. The experiment addresses topical open questions such as the origin of baryogenesis and the nature of dark matter, and is sensitive to scales of new physics substantially in excess of those available at colliders. A key goal of the program is to open a discovery window to neutron conversion probabilities (sensitivities) by up to three orders of magnitude compared with previous searches. The opportunity to make such a leap in sensitivity tests should not be squandered. The experiment pulls together a diverse international team of physicists from the particle (collider and low energy) and nuclear physics communities, while also including specialists in neutronics and magnetics. A conceptual design report, funded by European Union, for the NNBAR stage has been published. A similar paper for the HIBEAM stage will be complete in 2024.*Abstract:**Thursday 23 May 2024 at 14:15 in E204: Hannu Kurki-Suonio (Helsinki)*

*Farewell lecture; Hannu retires on 1 July 2024*

*Tuesday 28 May 2024 at 10:15 in A315 and using Zoom: Priyotosh Bandyopadhya (IIT Hyderabad)*

The most exciting new physics that we are hoping to see are heavy neutrinos and dark matter which can address a few unsolved issues in the Standard Model. However, the regular searches for these new physics scenarios did not give us any hint of the observation. Certainly, we now look for more exootic new physics scenarios, one of which is the displaced vertex scenarios. In this talk we address Type-I, Type-III seesaw and Vector like leptons which can be probed via the displaced heavy neutrinos, displaced Higgs production, displaced double recoil, and displaced deccays doubly charged leptons. These prospects are discussed for the CMS, ATLAS and the proposed MATHUSLA detectors at the LHC, and for the proposed multi-TeV muon collider.*Displaced heavy leptons at colliders*

Abstract:

Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=98f8d2e1-766a-4131-b371-6e77a285f68a*Thursday 30 May 2024 at 10:15 in A315 and using Zoom: Carlos Hoyos (Oviedo)*

*Spinodal slowing down and scaling in a holographic model*

We construct a holographic model of a strongly coupled system with two equilibrium homogeneous phases separated by a first order phase transition at nonzero temperature. We consider a temperature evolving in time and driving the system through the transition point. In the holographic model bubble nucleation is suppressed, so that the system can in principle remain close to a equilibrium phase up to the spinodal temperature where the phase disappears. We study the behaviour close to the spinodal point and show that it shares qualitative features with critical points of second order phase transitions, namely critical slowing down of perturbations. Similarly to critical points, this leads to the breakdown of quasistatic evolution close to the spinodal point with a characteristic scaling depending on the critical exponents.*Abstract:*

Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=88ea6572-34c9-47ac-af26-526cd6975f1e*Tuesday 4 June 2024 at 10:15 in A315 and using Zoom: Kirtiman Ghosh (IOP Bhubaneswar)*

*A minimal model to explain non-zero neutrino masses/mixings, muon anomalous magnetic moment, and dark matter relic density*

The Standard Model (SM) of particle physics is a highly successful framework, as all its predictions have been experimentally verified. However, there are several experimental results, such as neutrino oscillation data, anomalous magnetic moments of charged leptons, and the dark matter relic density, etc., that cannot be explained within the SM framework. These discrepancies motivate the search for new physics models at the TeV scale. In this talk, I will discuss a few simple extensions of the SM that can address some of its inadequacies. I will mainly focus on our recently proposed minimal framework for a radiative seesaw model, which can simultaneously explain neutrino oscillation data, and anomalies in muon g-2, and provide a cosmologically viable candidate for dark matter.*Abstract:*

Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=fa7299a5-d1a2-4ea9-9930-6de402ad9102*Thursday 6 June 2024 at 10:15 in A315 and using Zoom: Lusaka Bhattacharya (Bhubaneswar)*

*Probing QGP Properties at LHC-HIC Using Photons and Heavy SM Particles*The quark-gluon plasma (QGP), a state of deconfined quarks and gluons, emerges as a many-body aspect of quantum chromodynamics (QCD). Heavy-ion collision (HIC) experiments at the LHC and RHIC explore the QCD phase diagram at ultra-high temperatures where QGP is expected. While indirect QGP signatures like jet quenching have been observed, many of its properties remain unclear. This talk discusses using photons to probe QGP characteristics such as isotropization and thermalization. With increased center-of-mass energy in Pb-Pb collisions at the LHC, a new regime of heavy-ion physics allows the study of massive Standard Model (SM) particles like W/Z bosons, the Higgs boson, top quarks, and hard jets. The presentation covers ongoing work on searching for these SM resonances at the LHC-HIC.

Abstract:

Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=70716f50-a80a-4eba-b2d7-dc4936ccfcb9I will discuss the QCD equation of state (EoS) at high baryon density. The talk is mainly divided into two parts.In the first part, I explain two consequences from the recent lattice QCD calculations at finite isospin chemical potential, known to be free of the sign problem. I first explain the uses of rigorous QCD inequalities and input from the lattice QCD that put robust bounds on the symmetric nuclear matter EoS at finite baryon density. Then, I compare the weak-coupling results with the lattice data. In particular, I address the following issues: applicability of the weak-coupling results and color superconductivity.In the second part, I explain a duality between dense baryon and quark matter implied from the large-Nc limit of QCD; it is known as Quarkyonic duality. We construct an explicit dual description of Quarkyonic matter using an exactly solvable ideal gas model and demonstrate that the duality can be responsible for the rapid stiffening in the EoS.**Thursday 13 June 2024 at 10:15 in A315 and using Zoom: Yuki Fujimoto (Seattle)**Dense QCD equation of state: Lessons from neutron stars, lattice, and large-Nc QCD

Abstract: