DEPARTMENT OF PHYSICS / HIP JOINT COLLOQUIA / SEMINARS 2025
Cancelled/Postponed: Tuesday 14 January 2025 at 10:15 in A315 and using Zoom: Michael Campbell (CERN) Hybrid pixel detectors Abstract: Hybrid pixel detectors were first developed for the LHC as they provide noise hit free imaging of particle collisions even at extremely high event rates. The same feature can be used to provide detection and imaging solutions in numerous other fields. The Medipix Collaborations, which are coordinated at CERN, have pioneered the spread of hybrid pixel detector technology to fields as far apart as space-based dosimetry, industrial X-ray analysis and inspection, homeland security, molecular biology, medical imaging and classroom experiments. This presentation will explain how hybrid pixel detectors work and cover how the Medipix and Timepix readout chips can provide unique imaging solutions to many different fields. The recent introduction of sub-ns timing at the pixel level combined with novel interconnect technologies permitting almost seamless coverage of large detection areas should further extend the reach of the technology.
Cancelled/Postponed: Thursday 16 January 2025 at 10:15 in A315 and using Zoom: Michal Heller (Gent) New looks at nonequilibrium phenomena at high and low energies Abstract: I will discuss recent insights into nonequilibrium dynamics at high and low energies originating from the interplay of slow and fast processes. These theoretical developments are motivated, on one hand, by ultrarelativistic nuclear collisions at RHIC and LHC and neutron star mergers, and on the other, by cold atom experiments.
Tuesday 21 January 2025 at 10:15 using Zoom: Emidio Gabrielli (Trieste) Quantum Entanglement and Bell Inequality Violation at High Energies Abstract: I will present pioneering results demonstrating, for the first time, quantum entanglement and non-locality at high energies and in the presence of strong and weak interactions. Central to this discussion is Bell’s inequality relating the probabilities of different measurements on a given system. It is satisfied by local, deterministic theories and violated by quantum mechanics in the presence of entangled states. I will review the potential for detecting entanglement and testing Bell inequalities in collider experiments, focusing on diverse final states such as top-quark and tau-lepton pairs, Lambda baryons, massive gauge bosons, and vector mesons. Additionally, I will explore how entanglement offers a novel approach for constraining physics beyond the Standard Model, highlighting its most promising implications for future researches.
Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=363a6d95-fdf1-4b51-9ba4-452e7675bb5d
Link to pdf slides: Talk_Entanglement-Bell .
Tuesday 4 February 2025 at 10:15 in A315 and using Zoom: Laurids Jeppe (DESY) Searching for new scalars, pseudoscalars and top quark pair bound states at CMS Abstract: I will present a search for heavy pseudoscalar or scalar bosons decaying to a top quark pair ($t\bar{t}$) in final states with one or two charged leptons, using 138 fb$^{-1}$ of proton-proton collisions at $\sqrt{s} = 13$ TeV recorded by the CMS experiment. The invariant mass of the reconstructed $t\bar{t}$ system and variables sensitive to its spin state are used to discriminate against the standard model $t\bar{t}$ background and to infer spin quantum numbers. An excess of the data above the background prediction, as modeled using perturbative quantum chromodynamics (QCD) only, is observed with a significance of above five standard deviations. I will discuss three possible hypotheses to interpret the excess, which is located close to the $t\bar{t}$ production threshold: by production of an additional scalar or pseudoscalar boson, or by the existence of a color singlet pseudoscalar $t\bar{t}$ bound state, as predicted in a simplified model of nonrelativistic QCD. For the first two hypotheses, I will outline upper limits on the coupling of pseudoscalar or scalar bosons to top quarks in a mass range of 365-1000 GeV and relative widths of 0.5-25%. For the third scenario, I will present and discuss an extracted cross section of 7.1 pb with an uncertainty of 11%.
Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=10f833b4-012e-489c-8d92-0f6013c485fd
Thursday 20 February 2025 at 10:15 in A315 and using Zoom: Oleg Komoltsev (Stavanger) QCD in the cores of neutron stars Abstract:Rapid advancements in neutron-star (NS) observations allow unprecedented empirical access to cold, ultra-dense Quantum Chromodynamics (QCD) matter. The combination of these observations with theoretical calculations reveals previously inaccessible features of the equation of state and the phase diagram of QCD. In this talk, I demonstrate how perturbative-QCD calculations at asymptotically high baryon density provide robust constraints on the equation of state at neutron-star densities, based solely on causality and stability. By comparing the calculations to multimessenger neutron-star observations using a Bayesian framework, I show that QCD input softens the equation of state at neutron-star densities, supporting the hypothesis of a first-order phase transition or a crossover to quark matter cores in the most massive neutron stars.
Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=45c5502a-fa88-40df-9a52-45a1321ddb78
Tuesday 25 February 2025 at 10:15 in A315 and using Zoom: Samuli Autti (Lancaster) From a universe in a helium droplet to that droplet observing the Universe Abstract: Superfluid 3He is perhaps the most versatile macroscopic quantum system in the laboratory. It has touching points with seemingly distant fields such as particle physics and cosmology and supports practical realisations of many exotic theoretical concepts. In this presentation, I discuss some striking phenomena recently discovered in this system, for example time crystals and their interactions and a superfluid that behaves as if it is two dimensional. Superfluid 3He is also increasingly being used to observe and model the behaviour of our Universe. I will show the first results from a dark matter detector using 3He as the collision target and explain how this sheds light on the yet-unresolved homogeneous nucleation problem with possible implications for a first-order phase transition in the early Universe.
Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=f60519ab-e971-4833-b4d4-0e7b813175e5
Thursday 27 February 2025 at 10:15 in A315 and using Zoom: Adrita Chakraborty (Krakow) Holographic study of confining QCD-like theory on non-SUSY D2 brane and partial deconfinement Abstract: We elucidate the nonperturbative aspects of non-conformal 2+1D Yang-Mills-like gauge theories on the worldvolume of isotropic and anisotropic
non-supersymmetric D2 brane solutions of type IIA supergravity. Because of the lack of conformal invariance, these theories possess energy scale-dependent running coupling similar to the pure QCDs. We implement the holographic approach to study some salient QCD-like confining properties, e.g., flux-tube tension and glueball masses in the low-energy nonperturbative sector of these non-SUSY theories. For anisotropic D2 brane, the variation of the above QCD features with the anisotropy parameter in low-energy limit manifests a duality between Hawking-Page transition and QCD confinement-deconfinement phase transition. We also discuss the same from an empirical finite temperature scenario. The smooth crossover found at this transition point has been countered as the appearance of “partially deconfined” mixed QCD phases where glueballs and quark-gluon plasma coexist. As the transition from a black brane to large black hole occurs at the Gregory-Laflamme transition point, one can comprehend it as the dual of complete confinement/deconfinement phase transition in a confining gauge theory. In this context, we make some remark upon a possible holographic duality between the mixed phases and the small evaporating black holes.
Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=d451c33a-5d60-4d43-bb6d-0a9a219c00b9
Tuesday 4 March 2025 at 10:15 in A315 and using Zoom: Michael Campbell (CERN) Hybrid pixel detectors Abstract: Hybrid pixel detectors were first developed for the LHC as they provide noise hit free imaging of particle collisions even at extremely high event rates. The same feature can be used to provide detection and imaging solutions in numerous other fields. The Medipix Collaborations, which are coordinated at CERN, have pioneered the spread of hybrid pixel detector technology to fields as far apart as space-based dosimetry, industrial X-ray analysis and inspection, homeland security, molecular biology, medical imaging and classroom experiments. This presentation will explain how hybrid pixel detectors work and cover how the Medipix and Timepix readout chips can provide unique imaging solutions to many different fields. The recent introduction of sub-ns timing at the pixel level combined with novel interconnect technologies permitting almost seamless coverage of large detection areas should further extend the reach of the technology.
Thursday 17 April 2025 at 10:15 in A315 and using Zoom: Jacopo Ghiglieri (Nantes) Thermal axion production in the early universe
Abstract: I will introduce the axion as a solution to the strong CP problem and show how axion models give rise to two separate cosmological populations of axions: a condensate of zero-momentum modes acting as dark matter and a “hot” ensemble of thermally-produced axions acting as dark radiation and thus subject to present and future constraints from the effective number of neutrinos Neff. I will concentrate on this latter population and show how the contribution to Neff and its uncertainty can be quantified from thermal QCD dynamics above its crossover transition. In more detail, I will show how the leading-order thermal axion production rate has been computed in the past for axion momenta of the order of the temperature and I will show how different schemes for the incorporation of the collective dynamics of soft gluons extrapolate differently into the regime of softer axion production, thus giving us a first quantitative handle on the theory uncertainty of the rate. I will also show how popular gauge-dependent resummations in the thermal gravitino and axion literature are actually plagued by IR divergences stemming from an improper handling of the chromomagnetic sector. Finally, I will show how, upon solving the Boltzmann equation, the theory uncertainty on the axion rate translates to an uncertainty on Neff.
Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=33c49a66-162b-4a60-988c-5d0c92ed17d6
Tuesday 6 May 2025 at 10:15 in A315 and using Zoom: Andreas Schmitt (Southampton) Dense QCD matter and neutron stars from holography
Abstract: Phases of matter under extreme conditions – high temperatures and baryon densities – are difficult to understand on theoretical grounds, although the underlying theory, QCD, is known. This is due to the strongly coupled nature of the problem and the so-called sign problem of lattice gauge theory at nonzero baryon densities. I will introduce the main open questions and how the gauge-gravity duality (“holography”) can help to solve them. I will focus on recent work within the Witten-Sakai-Sugimoto model and will review the latest progress on the holographic descriptions of pion condensation, isospin-asymmetric baryonic matter, and neutron stars.
Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=2f641a6a-94b0-430f-8ace-7024f0b77d5a
Thursday 8 May 2025 at 10:15 in A315 and using Zoom: Björn Garbrecht (Munich) CP conservation in the strong interactions
Abstract: We examine possible effects of the topological term in the QCD-Lagrangian. For the path integral in zero-temperature Euclidean space, topological quantization only emerges in the infinite-volume limit, so that it must be taken prior to summing over winding numbers. This procedure is consistent with the construction of the path-integral contour from steepest-descent flows. As a result, CP violation is absent from observables, in contrast to calculations that rely on singular deformations leading to an inequivalent integration contour with the opposite order of limits. We also address theta-vacua in temporal gauge, which in the conventional picture are not properly normalizable, in contradiction with the axioms of quantum mechanics. However, we show that the possible wave-functionals, given the canonical commutation relations and without introducing extra gauge constraints, are properly normalizable states that do not exhibit CP violation.
Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=a4e11ce6-59c1-404d-8250-5d42dd3f74df
Tuesday 13 May 2025 at 10:15 in A315 and using Zoom: Viatcheslav Grishin (European Spallation Source, Lund) European Spallation Source: the beam diagnostics and current beam commissioning results
Abstract: The European Spallation Source (ESS) in Lund, Sweden is a pulsed neutron source based on a linac. The ESS linac has been designed to accelerate protons to 2 GeV with a peak current of 62.5 mA and deliver a 5 MW beam at 14 Hz to a rotating tungsten target for neutron production. The ESS beam instrumentation includes many the different types of beam diagnostics devices . April 9th 2025 was a historical day at ESS, as the first proton beam was transported from the Ion Source to the Tuning Beam Dump, some 542.5 meters down the accelerator tunnel.
Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=12ff990f-2e09-4ec9-89d1-1ebf199166b5
Tuesday 20 May 2025 at 10:15 in A315 and using Zoom: Helime Ruotsalainen (Helsinki) Entanglement-based holographic measures
Abstract: Holographic entanglement entropy is an elegant and alternative way of studying the confinement in QCD-like gauge theories. The gauge/gravity duality provides an efficient method for computing entanglement entropy geometrically. I will describe how to use the entanglement entropy as a tool to construct measures along renormalization group flow to find the effective number of degrees of freedom. With confining and gapped field theory examples, I will demonstrate both the power and the limitations of entanglement-based measures.
Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=12eee11c-a695-48b0-9433-dacc02dfb510
Friday 31 May 2025 a full day seminar in connection with the meeting of European Committee for Future Accelerators, Helsinki University main building, https://indico.cern.ch/event/1492840/timetable
Tuesday 3 June 2025 at 10:15 in A315 and using Zoom: Zoltan Fodor (Wuppertal) The magnetic moment of the muon: a status report
Abstract: Recently there have been stronger and stonger disagreements between direct measurements of the anomalous magnetic moment of the muon and the standard-model prediction. Such a large discrepancy should signal the discovery of interactions or particles not present in the standard model. However, two independent determinations of the most uncertain contribution to the standard-model prediction display significantly less tension with the measurement. Here we present a new first-principle calculation of this contribution. We reduce uncertainties compared to our earlier (2020) BMW computation by 40%. To reach this unprecedented level of precision we improve on many aspects of the calculation. Namely, we perform large-scale lattice QCD simulations on finer lattices than in 2020, allowing for an even more accurate continuum extrapolation. We also include a small, long-distance contribution obtained using input from experiments in a low-energy regime where they all agree. Combined with the extensive calculations of other standard model contributions, our result leads to a prediction that differs from the direct measurement by only 0.9 standard deviations. This provides a remarkable validation of the standard model to 0.37ppm.
Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=bf7d6199-11b7-498b-9977-f56592019ffd
Tuesday 10 June 2025 at 10:15 in A315 and using Zoom: Reijo Keskitalo (Berkeley) Next ten years of CMB observations from the Chilean high desert
Abstract: I will discuss aspects of observational cosmology in the context of ground-based CMB experiments in the Chilean Atacama desert. The talk will include overviews of the Simons Observatory and CMB Stage 4 (CMB-S4) with an eye on survey design, simulations and data reduction.
Canceled/postponed: Thursday 12 June 2025 at 10:15 in A315 and using Zoom: Marcel Niedermeier (Aalto) Re-interpreting tensor networks: quantum-inspired algorithms for hard problems in quantum physics and beyond Abstract: Solving problems in many-body quantum physics is hard – as soon as we consider quantum systems made up of multiple constituents, the complexity of the problem grows exponentially with the size of the system. A well-established tool for tackling these systems are different tensor network algorithms, which may reduce the parameter count of the problem to scaling only linearly with the system size, if the entanglement in the system is low. Recently, it has been recognised that the same tensor network algorithms have yet a much wider range of applications to offer. While they are rooted in quantum many-body physics, ultimately, expressing a given quantity as a tensor network can be understood as analogous to the compression of information contained in an image. In this talk, I will discuss the quantics prescription, which allows us to represent arbitrary numerical functions as tensors – and thus make them amenable to a tensor network representation, using the tensor cross interpolation algorithm. This idea is ultimately due to the fact that functions in physics typically exhibit a lot of structure – smoothness, organisation in length scales, etc. – and structure implies compressibility. After a thorough introduction, I will illustrate this idea with two examples. First, I will discuss the solution of non-linear partial differential equations with quantics tensor cross interpolation methods, using the Gross-Pitaesvkii equation as an example. By transforming the initial state and the time-evolution operator into quantics tensor trains, we are able to resolve physical phenomena across length scales separated by seven orders of magnitude within one hour on a single-core PC, thus greatly surpassing the capabilities of conventional numerical methods. Second, I will present an improved way of performing self-consistent mean-field calculations in the context of super-moiré tight-binding models. Here, the attainable system size is in principle severely restricted by the number of mean-field parameters to be calculated in each iteration for every system site. Using a quantics tensor train representation of the mean-field Hamiltonian, we could however show that a high-fidelity approximation of the Hamiltonian can be obtained while reducing the necessary number of parameters up to hundred-fold. Solving mean-field systems with tens of millions of sites is now feasible, thus surpassing the previous state of the art by orders of magnitude. I will conclude by giving an outlook on some ongoing research.
A. Fumega, M.Niedermeier and J. Lado, 2025 2D Mater. 12 015018.
Y. Sun, M. Niedermeier, T. Antao, A. Fumega and J. Lado, arxiv:2503.04373 (2025).
M. Niedermeier, A. Moulinas, T. Louvet, J. Lado and X. Waintal, in preparation (2025).
