DEPARTMENT OF PHYSICS / HIP JOINT COLLOQUIA / SEMINARS 2022

  • Tuesday 8 March 2022 at 10.15 in A315: Paul Hoyer (Helsinki)
    Bound States in Perturbative Quantum Field Theory
    Abstract:
    Precision calculations of atoms are based on a perturbative expansion of QED. The more familiar expansion for scattering amplitudes assumes free asymptotic (in-, out-) states, whereas bound state constituents always interact. Thus no Feynman diagram for e^+e^- scattering has a Positronium pole. Various perturbative methods for atoms have been developed, but are rarely discussed in QFT textbooks. The similarities of hadrons and atoms motivate a closer look. I discuss the bound state issues and present a new approach based on canonical quantization in temporal (A^0=0) gauge. This method is valid for any bound state momentum and is applicable also in QCD, where a possibility appears for including confinement.
  • Tuesday 29 March 2022 at 10.15 with Zoom: Lorenzo Bartolini (Henan)
    Neutron stars and phase diagram in hard-wall holography
    Abstract:
    Description of nuclear matter in the core of neutron stars eludes the main tools of investigation of QCD, such as perturbation theory and the lattice formulation of the theory. Recently, the application of the holographic paradigm (both via top-down and bottom-up models) to this task has led to many encouraging results, both qualitatively and quantitatively. In this talk, I will present our approach to the description of neutron star cores, relying on a simple model of the (double) hard-wall type: I will discuss results concerning the nature of homogeneous nuclear matter at high density emerging from the model including a quarkyonic phase, the mass-radius relation for neutron stars, as well as the rather stiff equation of state we have found. I will show how, despite the very simple model employed, for an appropriate calibration we were able to obtain neutron stars that only slightly fall short of the observational bounds on radius and tidal deformability. Finally, I will show results from simulations of mergers of neutron star pairs, including the characteristic gravitational waves signal and its frequency spectrum.
  • Thursday 31 March 2022 at 10.15 with Zoom: Ville Vaskonen (IFAE Barcelona)
    Bubble dynamics in (non-)interacting plasma
    Abstract:
    Cosmological first order phase transitions can have played an important role in formation of baryon asymmetry and dark matter, leaving a potentially observable gravitational wave signature. Such phase transitions proceed by expanding true vacuum bubbles that interact with the surrounding plasma. Typically, it is assumed that the plasma remains locally in equilibrium throughout the transition and the plasma is described as a perfect fluid. However, in some cases, in particular for very strong transitions, this description may not be accurate. In this talk I will present an approach based on particle dynamics, that can be used to describe the bubble evolution and its effects on the surrounding plasma in the case that the plasma does not remain locally in equilibrium. I will show results on the bubble wall terminal velocity, the distribution of energy between the bubble wall and the plasma, and the plasma profiles around the bubble wall, and a comparison of the results to the case where the perfect fluid description can be used. Finally, I will also discuss the possibility for primordial black hole formation in cosmological phase transitions.
  • Tuesday 5 April 2022 at 10:15 A315: Jeff Kost (Sussex)
    Massless Preheating and Electroweak Vacuum Metastability
    Abstract:
    Current measurements of Standard Model parameters suggest that the electroweak vacuum is metastable. This metastability has important cosmological implications because large fluctuations in the Higgs field could trigger vacuum decay in the early universe. For the false vacuum to survive, interactions which stabilize the Higgs during inflation—e.g., inflaton-Higgs interactions or non-minimal couplings to gravity—are typically necessary. However, the post-inflationary preheating dynamics of these same interactions could also trigger vacuum decay, thereby recreating the problem we sought to avoid. This dynamics is often assumed catastrophic for models exhibiting scale invariance since these generically allow for unimpeded growth of fluctuations. In this talk, we examine the dynamics of such “massless preheating” scenarios and show that the competing threats to metastability can nonetheless be balanced to ensure viability. We find that fully accounting for both the backreaction from particle production and the effects of perturbative decays reveals a large number of disjoint “islands of (meta)stability” over the parameter space of couplings. Ultimately, the interplay among Higgs-stabilizing interactions plays a significant role, leading to a sequence of dynamical phases that effectively extend the metastable regions to large Higgs-curvature couplings.
  • Tuesday 12 April 2022 at 10:15 A315: Pablo Navarrete Noriega (Bio Bio U, Chillan, Chile)
    Tackling the infamous g^6 term of the hot QCD pressure
    Abstract:
    The determination of the hot QCD pressure has a long history, and has — due to its phenomenological relevance in cosmology, astrophysics and heavy-ion collisions — spawned a number of important theoretical advances in perturbative thermal field theory applicable to equilibrium  thermodynamics. In particular, the long-standing infrared problem that obstructs the perturbative series has been overcome by a systematic use of dimensionally reduced effective theories, essentially mapping the problem of determining a full physical leading-order determination of the pressure to an extremely tough, but in principle doable, four-loop perturbative calculation in finite-temperature Yang-Mills theory. We present advances in organizing this challenging calculation, by classifying the distinct contributions, filtering out a large fraction of sub-diagrams that exhibit a factorized structure, and push ahead systematic simplifications of the remaining core sum-integral structures taking into account systems of linear relations that originate from symmetry- as well as integration-by-parts-relations. This will eventually allow us to gauge the grade of difficulty of a full determination of the physical leading-order QCD pressure, by analytic means.
  • Tuesday 26 April 2022 at 10:15 using Zoom: Wladyslaw Trzaska (HIP and Jyväskylä)
    Dark Matter searches in Finland: A glimmer of light at the end of the tunnel
    Abstract: First postulated a century ago, Dark Matter still defies our attempts to confirm its existence and determine its nature. Most terrestrial DM searches focus on direct detection via elastic scattering on cryogenic targets. These evermore elaborate experiments are done at an immense expense in the world’s best-equipped underground laboratories. However, each time the results are published, they are disappointing. Clearly, a new approach is needed. In my talk, I’ll show the puzzling outcome of the latest analysis of indirect DM searches from four underground experiments in Finland, the UK and Canada. These measurements, performed at various depths by different groups using diverse methods, yield persistent anomalous structures with comparable cross-sections, rates, and multiplicities. The nature of the anomalies remains unclear, but, in principle, they may be a signature of self-annihilation of a Weakly Interacting Massive Particle (WIMP) with a mass ~10 GeV/c2. With that assumption, the expected cross-section would be of the order of 10-42 cm2 for Spin-Dependent or 10-46 cm2 for Spin Independent interactions. We propose verifying this hypothesis by collecting an order of magnitude more data with an upgraded NEMESIS setup located in the Pyhäsalmi mine in Finland. Based on the statistical uncertainty, analysis of the event rate indicates that such a setup can probe cross-section limits for DM masses in the vicinity of 3-40 GeV/c2.
  • Tuesday 10 May 2022 at 10:15 A315 and using Zoom: Carlos Hoyos (Oviedo)
    Effective theories and scattering in holographic models
    Abstract:
    Effective theories can be used to capture the low energy behavior of strongly coupled systems such as QCD. A paradigmatic example is chiral effective theory, where the low energy degrees of freedom are pions, and whose Lagrangian is ultimately determined by demanding that it reproduces low energy scattering amplitudes. Holographic models of QCD should be able to reproduce the same physics, so it is interesting to determine low energy scattering amplitudes from the holographic dual. We develop a procedure to compute them in holographic duals of confining theories and apply it to pion scattering in the Sakai-Sugimoto model, where we determine for the first time coefficients of four derivative terms in the chiral Lagrangian, correcting previously reported values.
    Link to video: https://unitube.it.helsinki.fi/unitube/6ec71033-1b7d-4818-9486-e44dda128a6f
  • Tuesday 24 May 2022 at 10:15 in A315 and using Zoom: Miika Sarkkinen (Helsinki)
    Gravitational wave memory and its tail in cosmology
    Abstract:
    Gravitational wave memory effect is a displacement that persists in a gravitational wave detector even after the wave has passed. In this talk, I will examine the memory effect in the LambdaCDM model, a good approximation to reality. Since the background is curved, gravitational radiation develops a tail part arriving after the main signal that travels along the past light cone of the observer. First I discuss first order gravitational wave sourced by a binary system, and find that the tail only gives a negligible memory. Then I study the nonlinear memory effect coming from induced gravitational radiation sourced by first order gravitational radiation propagating over cosmological distances. In the light cone part of the induced gravitational wave one finds a novel term missed in previous studies of the cosmological memory effect. As the main result, I show that the induced gravitational wave has a tail part that slowly accumulates after the light cone part has passed and grows to a sizeable magnitude over a cosmological timescale.
    Link to video: https://unitube.it.helsinki.fi/unitube/c2c791fb-5d86-4fa3-8747-7f13707ec226
  • Thursday 2 June 2022 at 10:15 in A315 and using Zoom: Esko Keski-Vakkuri (Helsinki)
    Sequences of entanglement monotones and Landauer inequalities

    Abstract: I construct two infinite sequences of entanglement monotones, which can be computed from the R’enyi entropies, involving cumulants of the modular Hamiltonian K = -log \rho. The monotones yield infinite sequences of inequalities that must be satisfied in majorizing state transitions. We demonstrate this for information erasure, deriving an infinite sequence of “Landauer inequalities” for the work cost, bounded by arbitrarily high cumulants along with the variance. We also find constraints for some state conversions in periodic spin chains associated with CFTs, and identify “phases of entanglement” when the relative size of the subsystem rules out the conversion.
    Link to video: https://unitube.it.helsinki.fi/unitube/86cae6fb-1673-4a89-9f97-a718ed82410f
  • Tuesday 7 June 2022 at 10:15 in A315 and using Zoom: Oscar Henriksson (Helsinki)
    Bubbles at strong coupling from holographic effective actions

    Abstract: The quantum effective action of a field theory is a powerful object; among other things, it allows us to map out a QFT’s phases and phase transitions. I will discuss the computation of effective actions at strong coupling using holographic duality, focusing on a simple two-parameter gravity+scalar theory which admits first and second order phase transitions. With the effective action in hand, I apply it to study bubble nucleation with an eye towards gravitational wave production in the early universe.
    Link to video: https://www2.helsinki.fi/fi/unitube/video/23697b64-8d25-494c-903f-3a137f2f2c30
  • Tuesday 14 June 2022 at 10:15 in A315 and using Zoom: Jan Heisig (Université Catholique de Louvain)
    Bound state effects on dark matter coannihilation – pushing the boundaries beyond the WIMP paradigm

    Abstract: The nature of dark matter poses one of the most pressing questions in fundamental physics today. Thermal freeze-out of a weakly interacting massive particle (WIMP) has proved to be a successful framework for explaining the measured dark matter abundance in the Universe. However, the sizeable couplings of dark matter to the Standard Model particles required in its simplest realizations have been put under severe pressure by experimental null-results at colliders, direct and indirect detection experiments. Hence, fulfilling the relic density constraint often requires the exploration of ‘exceptional’ regions, e.g. the region where coannihilation effects increase the effective annihilation rate. In this talk, we revisit the coannihilation scenario and discuss bound-state formation effects of the coannihilating particle. They can have a large impact on the dynamics of dark matter freeze-out in the early Universe. Exploiting similarities to the physics of hydrogen recombination, we present a general formalism to include an arbitrary number of excitations of bound states in terms of an effective annihilation cross section. For a coannihilator in the fundamental representation of SU(3)c, we discuss radiative bound-state formation, decay, and electromagnetic transition rates among them. We then assess the impact of bound states within a minimal dark sectors considering the well-known coannihilation regime as well as the regime of conversion-driven freeze-out (or coscattering), where the relic abundance is set by the freeze-out of conversion processes. We find that the latter is considerably enhanced into the multi-TeV region due to bound-state effects. This has far-reaching implications for the search strategies at the upcoming LHC runs as conversion-driven freeze-out predicts a long-lived coannihilator.
    Link to video: https://www2.helsinki.fi/fi/unitube/video/a0b1f38b-cf95-4104-938f-3e179f212fcb
  • Tuesday 21 June 2022 at 10:15 in A315 and using Zoom: Javier G. Subils (Nordita)
    Boost-invariant superfluid flows

    Abstract: Hydrodynamics has proven to be a powerful tool to investigate heavy ion collisions. One could wish to incorporate into the hydrodynamic equations the Goldstone modes associated to the spontaneously broken chiral symmetry (the pions), even though this symmetry is only approximate. Motivated by this, I will present new analytical solutions to relativistic superfluid hydrodynamics and explain the symmetry assumptions that allowed us to find them.
    Link to video: https://www2.helsinki.fi/fi/unitube/video/6ed6453e-be7d-40e9-9849-a6b8557c2dc9
  • Tuesday 23 August 2022 at 11:00 in A315 and using Zoom: Jani Penttala (Jyväskylä)  Note time!
    Exclusive vector meson production at next-to-leading order in the Color Glass Condensate framework

    Abstract: Exclusive vector meson production is a powerful process to probe the small Bjorken-$x$ structure of protons and nuclei, as such processes are especially sensitive to gluonic structure and also provide access to the spatial distribution of small-$x$ gluons in nuclei. A powerful theoretical framework to study such high-energy processes is the Color Glass Condensate (CGC) effective field theory. So far, most calculations in the CGC framework have been done at the leading order. Recent theoretical developments on the NLO heavy vector meson wave function [1] and the NLO virtual photon light-front wave function [2,3] have made it possible to go beyond the leading order in exclusive vector meson production, allowing us to calculate this process at NLO in the dipole picture for the first time. In this talk, I will discuss the calculation of the NLO corrections to heavy vector meson production in the nonrelativistic limit [4,5], and to light vector meson production in the limit of large photon virtuality [6].

    [1] M. Escobedo and T. Lappi, Phys.Rev.D 101 (2020) 3, 034030, arXiv:1911.01136 [hep-ph][2] G. Beuf, T. Lappi and R. Paatelainen, Phys. Rev.D 104 (2021) 5, 056032, arXiv:2103.14549 [hep-ph][3] G. Beuf, T. Lappi and R. Paatelainen, arxiv:2204.02486 [hep-ph][4] H. Mäntysaari and J. Penttala, Phys. Lett.B 823 (2021), 136723, arXiv:2104.02349 [hep-ph][5] H. Mäntysaari and J. Penttala, arXiv:2204.14031[6] H. Mäntysaari and J. Penttala, Phys.Rev.D 105 (2022) 11, 114038, arXiv:2203.16911 [hep-ph]
  • Tuesday 6 September 2022 at 10:15 in A315 and using Zoom: Thomas Ward (Santa Fe)
    Dark Matter, Dark Energy in Radiation Gauge extended Standard Model

    Abstract: The electroweak symmetry breaking (EWSB) radiation gauge structure of the Standard Model (SM) is investigated using spin-dependent gauge boson mixing with complimentary SM Yang-Mills (Y-M) fields within a proposed Radiation Gauge Model (RGM) framework. A series of spin-dependent 2×2 mixing matrices featuring off‑diagonal mixing of scalar (Higgs), vector (EW) and tensor (graviton) radiation gauge fields with complimentary SM Y-M fields breaks isospin symmetry and removes degeneracy of the SU(2)_L x U(1)_Y neutral isotopic spin gauge boson states. The extracted spin-dependent off-diagonal isospin mixing matrix elements for scalar, vector and tensor matrices compare well with those derived from dynamical EW SM formulations. A massive neutral Y-M tensor gauge boson, m = (7.93+-0.12)GeV,  generated in tensor matrix analysis is a candidate for WIMP dark matter.  Dark energy, identified as the EW vector mixing interaction energy, is a SM self-interaction term of the Higgs vacuum.  Four Indirect neutron multiplicity DM searches are reviewed.  Four observed Indirect DM-like anomalous transitions are analyzed using RGM spin-dependent theory and found to be in good agreement with the theory.  The Direct XENONnT experiment will also be reviewed with respect to projections by RGM.
  • Tuesday 4 October 2022 at 10:15 in A315 and using Zoom: Alexandr Pimikov (JINR, Dubna)
    Nonlocal condensates in OPE of vacuum correlators

    Abstract: Nonlocal gluon condensates are vacuum expectations of the product of gluon field strength tensors. We present our recent results [PRD106(2022)5,056011] on short-distance expansions of two-, three-, and four-gluon nonlocal condensates up to dimension-8 local operators. We propose a method for calculating the Wilson coefficients based on the presented expansions and the Feynman diagram technique in the background field approach. The method is demonstrated using the glueball current correlators. We confirm the results for Operator Product Expansion (OPE) of the two-gluon  glueball current correlators and calculate additional contributions coming from dimension-six four-quark condensate and dimension-eight mixed quark-gluon condensates. The OPEs used in QCD sum rules for three-gluon  glueballs are revisited up to dimension-six order. QCD sum rules results on masses of C-odd scalar glueballs are briefly reviewed.
  • Tuesday 11 October 2022 at 10:15 in A315 and using Zoom: José Correia (Helsinki)
    On multi-tension string networks

    Abstract: Topological defects are possible by-products of phase transitions in the early Universe, predicted to have formed via the Kibble mechanism [Kibble, 1976]. An example of a defect are the line-like cosmic strings, which are an expected and generic prediction of several Beyond the Standard Model theories. As such, detection of cosmic string networks is a primary target for current and next-gen observational probes. However, the need for more accurate forecasts and the description of exotic types of strings often requires more compute power and the means to exploit it. This led us to the development of cosmic string simulation capable of leveraging accelerator hardware [Correia & Martins, 2021]. Superstrings [Tye & Sarangi, 2002] stretched to horizon-scales can be studied using the analytical and computational tools from the study of cosmic string networks. In this talk we will focus on using multi-GPU field theory simulations of interconnected strings to study the properties of cosmic superstring networks, as shown in [Correia & Martins, 2022].
  • Tuesday 18 October 2022 at 10:15 in A315 and using Zoom: Jack Holden (Southampton)
    Partial deconfinement
    Abstract:
    In large-$N$ gauge theories, evidence has emerged recently that between confined and deconfined phases a partially-deconfined phase can appear, in which only a subset of colours deconfine. The existence of such a phase has implications for the map between degrees of freedom under gauge/gravity duality and black hole phase diagrams, where a counterpart to the partially-deconfined saddle should be present. We investigate properties of partial deconfinement on the field theory side, first considering the partially-deconfined saddle of large-$N$ pure Yang-Mills theory. Here, the colour degrees of freedom split into confined and deconfined sectors. We argue with the use of numerical simulations that a linear confinement potential is generated in the confined sector, implying the formation of flux tubes, whereas the potential is screened in the deconfined sector and behaves instead according to the perimeter law. Furthermore, we find that the onset of partial deconfinement coincides with the breaking of chiral symmetry breaking, providing an order parameter for the partially-deconfined phase. We conjecture that partial deconfinement is accompanied by a unique signature of global symmetry breaking that can serve as an order parameter. As another, cleaner example of this, we show that CP symmetry breaking coincides precisely with the emergence of the partially-deconfined phase in supersymmetry-broken $\mathcal{N} = 1$ super-Yang-Mills with a theta-angle $\theta = \pi$, for both large finite $N$ and the formal large-$N$ limit. Finally, we discuss consequences of these findings for holography, the QCD crossover, and the internal phase structure of neutron stars.
    Link to video: https://www2.helsinki.fi/fi/unitube/video/fd1d5add-21ae-4325-87c4-f8ffbaa9f649
  • Tuesday 25 October 2022 at 10:15 in A315 and using Zoom: Hannu Siikonen (Helsinki)
    Profile Likelihood Methods in Top Quark Mass Measurements at the CMS
    Abstract:
    Top quark mass measurements at the LHC yield exceptionally precise results. However, while more and more data is accumulated, statistical uncertainties have become less important than the systematic ones. Using traditional approaches the systematic uncertainties tend to be  irreducible, meaning that the measurement precision cannot be further improved by recording more data.This pessimistic view has been revolutionized by the increasing popularity of profile likelihood methods. Here, the systematic uncertainties are included into the full likelihood function describing the measurement. In consequence, some of the systematic uncertainties may be further constrained in-situ by the data of the measurement at hand. The likelihood approach also streamlines the combination of different data-taking periods, adding prospects for LHC Run2 + Run3 combinations.A profile likelihood approach in the measurement of the top quark mass was recently made public for the CMS 2016 data [1]. Similar techniques have also been applied for the CMS 2017-2018 data – a topic which is handled in more depth in the thesis defence of the speaker later this year.[1] https://cds.cern.ch/record/2806509
    Recording and slides in https://indico.cern.ch/event/1211706/
  • Tuesday 15 November 2022 at 10:15 in A315 and using Zoom: Shudhashil Bhartuar (Helsinki)
    Prototype evaluation and radiation hardness studies of position-sensitive and timing detectors for the Future CMS detector
    Abstract: 
    Silicon detectors have played a pivotal role in particle tracking and vertex reconstruction for high energy physics experiments, such as the Large Hadron Collider (LHC) experiment. Further developments on the detector prototypes based on Low-Gain Avalanche Detector (LGAD) technology with intrinsic charge multiplication, have shown a potential to improve the signal-to-noise ratio and time resolution of the detector. During the Phase-2 operation of the LHC, also known as High Luminosity-LHC (HL-LHC), the CMS Inner Tracker will be exposed to extreme radiation levels. In addition, to cope up with high pile-up rates during the HL-LHC run, the CMS detector will install a precision Minimum Ionising Particle Timing Detector (MTD). The endcap part of the MTD, called the Endcap Timing Layer (ETL), will consist of LGADs providing a timing resolution of∼30 ps.
    As a solution to the increment in radiation-induced leakage currents, AC-coupled design aids in enhancing the radiation hardness of pixel detectors for future collider experiments suitable for tracking purposes.Themotivation of the talk is characterisation of high resistivity p-type Magnetic Czochralski silicon (MCz-Si) based detectors that implement thin films of negative oxides with good dielectric constant grown by Atomic Layer Deposition (ALD) method, such as aluminum oxide and hafnium oxide, as field insulators. A comparison study was performed to analyse the impact of radiation on the effective negative oxide concentration of the dielectric layer in metal-oxide-semiconductor (MOS) devices. Further, electrical characterisation and optimisation of the gain termination region within LGADs were performed as well in the series of studies.
    Combination of various electrical characterisation studies demonstrate that an optimisation in the inter-pixel and edge termination design of AC-coupled pixel detectors in the Tracker and LGADs in the ETL of MTD is crucial for optimum operation of the future CMS detector. In addition, results based on the effect of radiation induced damage on position-sensitive and timing detectors will be presented in the talk.
  • Tuesday 22 November 2022 at 10:15 in A315 and using Zoom: Amelia Drew (Cambridge)
    Massless and Massive Radiation from Global Cosmic Strings with Adaptive Mesh Refinement
    Abstract:
    Global cosmic strings are topological defects that arise from field theories with a spontaneously broken global symmetry. They are a potential source of axion and gravitational radiation and have complex dynamics, making them challenging both to model analytically and to simulate numerically. This has knock-on effects for experimental predictions of the axion mass and gravitational wave spectra.  In this talk, we present results from adaptive mesh refinement (AMR) simulations of individual, sinusoidally-displaced global cosmic strings. We first present a quantitative investigation of the massless (axion) radiation from strings with a range of string energy densities $\mu \propto \ln{\lambda}$, where the string width parameter $\lambda$ spans over two orders of magnitude (https://journals.aps.org/prd/abstract/10.1103/PhysRevD.105.063517).  We determine the eigenmode decomposition for different $\lambda$ and string oscillation amplitudes by extracting the axion radiation on a diagnostic cylinder. We further characterise the effects of radiation backreaction, demonstrating that modelling in the thin-string (Nambu-Goto) limit provides an appropriate cosmological description.We also present a detailed investigation of the massive radiation from global strings. For the sinusoidal configurations studied, we establish that the massive radiation is exponentially suppressed with particle mass, with strings radiating in distinct harmonics of the fundamental frequency. We finally discuss the advantages and complexities of using AMR for string simulations.