• Tuesday 21 January 2020 at 10.15 in A315: Paul Saffin (Nottingham)
    Integrating the Path integral.
    Abstract: The path integral gives a very useful framework for understanding quantum mechanics, but actually evaluating it can be tricky. We shall review some recent progress on a method to perform the integration of the real-time path integral using Picard-Lefschetz techniques.
  • Tuesday 28 January 2020 at 10.15 in A315: Sampsa Vihonen (Guangzhou)
    China and the quest for the lepton flavour
    In recent years, China has invested heavily in its particle physics programs. Whereas the last decade belonged to the reactor neutrino experiments and the precision measurement of the mixing angle theta_{13}, the near future will entail efforts to search for CP violation in the leptonic sector and look for signatures of new physics in form of lepton flavour violation. In the future, we will hopefully see the last pieces fall in place in the lepton flavour puzzle. In this talk, I will briefly review the Chinese efforts in these questions and discuss their position in the global picture.
  • Tuesday 11 February 2020 at 10.15 in A315: Chris Rosen (Imperial College, London)
    Supersymmetric deformations and spatially modulated masses
    Deforming fixed point theories by couplings to operators which explicitly break Poincare invariance can provide an interesting perspective on the scale dependent properties of interacting gauge theories. These properties are encoded in the action of the Renormalisation Group, and many of them can be realised geometrically via applications of gauge/gravity duality. We appeal to this holographic perspective to construct infinite classes of spatially modulated deformations of the ABJM theory which break Poincare invariance but nonetheless preserve some supersymmetry. The resulting flows offer explicit field theory examples of novel RG trajectories, such as RG “boomerang flows”, whose phenomenology we also discuss.
  • Thursday 20 February 2020 at 10.15 in A315: Shinsuke Kawai (Sungkyunkwan University, Suwon, Korea)
    eV-scale sterile neutrinos from an extra dimension
    arXiv:1910.02936 [hep-ph]
  • Tuesday 25 February 2020 at 10.15 in A315: Alexander Krikun (Nordita)
    Non-metallic ground states from holography: features and observables.
    I will discuss the features of holographic models in case when the breaking of translations is relevant in the IR and the horizon geometry is not metallic. This can be achieved either via the strong explicit source or via the spontaneous symmetry breaking mechanism.  While in both cases the resulting model is power-law insulating, the features of AC transport are quite different. In case of strong explicit breaking I will also discuss the electric charge density response, which demonstrates “fake plasmon” feature, while the sound mode is absent in the neutral spectrum. Based on 1909.1224, 1812.08132.
  • Tuesday 25 February 2020 at 14.15 in D114: Arnaud Ferrari (Uppsala) Note time and place!
    Higgs boson pair production at the LHC: experimental overview
    Abstract: Searches for the simultaneous production of two Higgs bosons (HH) in proton-proton collisions are an important part of the LHC experimental program, as they allow to probe the shape of the Higgs potential in (and beyond) the Standard Model. HH pairs can also be produced in the decay of new resonances. This seminar will give an overview of the searches for HH pairs in ATLAS and CMS with 13 TeV, as well as their statistical combinations, with a focus on the non-resonant production mode. Prospects at the future LHC runs will also be discussed.
  • Thursday 27 February 2020 at 10.15 in A315: LISA day
    9:30-10:00: Coffee
    10:00-10:40: Martin Hewitson (AEI, Hannover) – The Status and Design of the LISA Mission
    10:40-11:20: Mark Hindmarsh (Sussex and Helsinki) – LISA Science
    11:20-12:00: Antoine Petiteau (APC, Paris) – Challenges of LISA Data Analysis
    12:00-13:30: Lunch
    13:30-13:50: Eija Tuominen (HIP, Helsinki) – Helsinki Detector Laboratory in the instrumentation of international physics experiments
    13:50-14:10: Elina Keihänen (Helsinki) – Planck and Euclid
    14:10-14:30: Juhani Huovelin (Helsinki) – Activities for ongoing and planned space science missions at the University of Helsinki
    14:30-14:50: Tuomas Savolainen (Aalto) – Horizon-scale imaging of supermassive black holes
    14:50-15:10: Peter Johansson (Helsinki) – Simulating black hole dynamics and gravitational wave emission in galactic-scale simulations
    15:10-15:30: David Weir (Helsinki) – Detecting first-order phase transitions at the electroweak scale with LISA
  • Tuesday 3 March 2020 at 10.15 in A315: Carlos Hoyos (Oviedo)
    Scattering length from holographic duality
    Interesting theories with short range interactions include QCD in the hadronic phase and cold atom systems. The scattering length in two-to-two elastic scattering process captures the most elementary features of the interactions, such as whether they are attractive or repulsive. However, even this basic quantity is notoriously difficult to compute from first principles in strongly coupled theories. We present a method to compute the two-to-two amplitudes and the scattering length using the holographic duality. Our method is based on the identification of the residues of Green’s functions in the gravity dual with the amplitudes in the field theory. To illustrate the method we compute a contribution to the scattering length in a hard wall model with a quartic potential and find a constraint on the scaling dimension of a scalar operator Δ>d/4. For d<4 this is more stringent than the unitarity constraint and may be applicable to an extended family of large-N theories with a discrete spectrum of massive states. We also argue that for scalar potentials with polynomial terms of order K, a constraint more restrictive than the unitarity bound will appear for d<2K/(K−2).
  • Thursday 19 March 2020 at 10.15 in A315: Valery Khoze (Durham & St. Petersburg) Cancelled!
    Photon-induced processes at high energy hadron colliders
    I discuss the possibilities for using the high-energy hadron colliders as photon colliding machines. The photon parton density is quite precisely known and I consider the implications, first of all,  for the LHC phenomenology. In addition, the colour singlet nature of the photon means that it can readily lead to exclusive or semi-exclusive events, with limited or no extra particle production in the final state. I will show how such exclusive processes, in particular in combination with proton tagging measurements, provide a well understood environment in which to test the Standard Model and search for the BSM physics. I shall focus on a few topical examples, includingsearches for Dark Matter at the LHC in forward proton mode.
  • Thursday 26 March 2020 at 10.15 in A315: Tyler Gorda (Virginia) Cancelled!   TBA                                                                                       Abstract: TBA
  • Tuesday 7 April 2020 at 10.15 in A315: Toru Koju (Wuhan) Cancelled!
    Abstract: TBA
  • Tuesday 14 April 2020 at 10.15 in A315: Daiki Suenaga (Wuhan) Cancelled!
    Abstract: TBA
  • Tuesday 28 April 2020 at 10.15 using Zoom: Eemeli Annala (Helsinki)
    Neutron stars and quark matter
    Neutron stars are some of the densest known objects in the Universe. Therefore, they are ideal targets to study the properties of cold and dense matter. In this talk, latest observational and theoretical results will be used to constrain the neutron-star-matter equation of state, and especially, to examine the possibility that quark matter exists inside neutron stars.
  • Tuesday 5 May 2020 at 10.15 using Zoom: Rebeca Gonzalez Suarez (Uppsala)
    Long Live the Large Hadron Collider
    The LHC is the largest and most energetic particle physics experiment ever made. By colliding protons accelerated close to the speed of light to very high energies, the LHC experiments probe the smallest components of matter and study how they interact with each-other in controlled conditions that emulate the Universe a fraction of a second after the Big-Bang. The LHC is has been in operation for a decade. The two largest experiments, ATLAS and CMS, have plowed the data producing more than 900 publications each; achieving a milestone discovery, the one of the Higgs boson, in 2012.

    The Universe however, remains puzzling at its most fundamental level. The Standard Model of particle physics is clearly incomplete but nature does not seem to like to strand away much from it at the energies we are able to reach at the LHC so far. Although many theoretical models are being tested, most of the questions present before the discovery of the Higgs boson still remain, so we must keep searching. At the LHC and its upgrade, the HL-LHC, two lines of research stand out from the rest: Searches for Dark Matter, where the LHC provides a complementary sensitivity that cannot be tested with non-collider experiments; and the exploration of the Higgs sector: where the HL/LHC have exclusive options.

    This talk will highlight specific experimental signatures that can appear in both dark sectors and Higgs physics: new long-lived particles (LLPs) with discernible decay lengths giving rise to decays significantly displaced from the primary interaction point. LLPs have characteristic experimental signatures that, while making them distinct from other processes, also can make them invisible to current data-acquisition methods. The potential of LLP is therefore still untapped.

    Link to the slides:

  • Tuesday 12 May 2020 at 14.15 using Zoom (Note time): Chris Herzog (King’s college, London)
    Graphene and Boundary Conformal Field Theory
    The infrared fixed point of graphene under the renormalization group flow is a relatively under studied yet important example of a boundary conformal field theory with a number of remarkable properties.  It has a close relationship with three dimensional QED.  It maps to itself under electric-magnetic duality.   Moreover, it along with its supersymmetric cousins all possess an exactly marginal coupling — the charge of the electron.  I will review past work on this model and also discuss my own contributions which have focused on understanding the boundary contributions to the anomalous trace of the stress tensor and their role in helping to understand the structure of boundary conformal field theory.  I will also present some new results on the hemisphere partition function for supergraphene, its duality properties, and an exact computation of the conductivity at arbitrary values of the interaction strength.

    Link to seminar    
  • Tuesday 19 May 2020 at 10.15 using Zoom: Ruth Pöttgen (Lund)
    Light in the Dark — Opening a new window to the Dark Sector
    The origin and observed abundance of Dark Matter in the Universe can be explained elegantly by the thermal freeze-out mechanism, leading to a preferred mass range of the Dark Matter particles in the MeV-TeV region. The GeV-TeV mass range is being explored intensely by the variety of experiments searching for Weakly Interacting Massive Particles. The sub-GeV region, however, in which the masses of most of the building blocks of stable matter lie, is hardly being tested experimentally to date. This mass range occurs naturally in Hidden Sector Dark Matter models. The Light Dark Matter eXperiment (LDMX) is a planned electron-beam fixed-target experiment, that has unique potential to conclusively test models for such light Dark Matter in the MeV to GeV range. This presentation will give an overview of the theoretical motivation, the main experimental challenges and how they are addressed as well as projected sensitivities.
  • Tuesday 26 May 2020 at 10.15 using Zoom:  Niels Schlusser (Darmstadt)
    Jet momentum broadening from the lattice
    The transverse momentum of a high-energy particle broadens in crossing a  strongly interacting medium. The transverse collision kernel, C(b),  quantifies that phenomenon and receives dominant infrared contributions which cannot be tackled perturbatively, although the coupling seems small. We calculated this contribution on the lattice in Electrostatic QCD as proposed by Caron-Huot and pioneered by Panero, Rummukainen, and Schäfer. Our continuum-extrapolated data, provided at four different temperatures, renders the common quadratic approximation of C(b) redundant.
    Link to seminar
  • Tuesday 2 June 2020 at 10.15 using Zoom:  Jere Remes (Helsinki)
    V-QCD and neutron stars
    In this talk, we consider a family of state-of-the-art holographic models called Veneziano-QCD, phenomenologically tuned to mimic QCD.These models are combined “continuously” with various ab-initio nuclear physics models used to describe low density nuclear matter, with V-QCD being used for high density nuclear matter as well as for quark matter. We examine the constraints from astrophysical observations on these hybrid equations of state and the implications on the structure of compact stars.
    Link to seminar: 
  • Tuesday 9 June 2020 at 10.15 using Zoom:  Veronica Sanz (Sussex and Valencia/CSIC)
    Theoretical interpretation of non resonant searches
    With null results in resonance searches at the LHC, the physics potential focus is now shifting towards the interpretation of non-resonant phenomena. An example of such shift is the increased popularity of the EFT programme. We can embark on such programme owing to good integrated luminosity and an excellent understanding of the detectors, which will allow these searches to become more intense as the LHC continues. In this talk I will explain a framework to perform this interpretation in terms of a diverse set of scenarios, including 1.) generic heavy new physics described at low energies in terms of a derivative expansion, such as in the EFT approach, 2.) very light particles with derivative couplings, such as axions or other light pseudo-Goldstone bosons, and 3.) the effect of a quasi-continuum of resonances, which can come from a number of strongly-coupled theories, extra-dimensional models, clockwork set-ups and their deconstructed cousins.  Slides are here:  SanzHelsinki-nonres
  • Tuesday 26 May 2020 at 10.15 in A315: Murray Moinester (Tel Aviv) Cancelled!
    Pion Polarizabilities
    Abstract: The electric απ and magnetic βπ charged pion polarizabilities characterize the induced dipole moments of the pion during γπ Compton scattering. Pion polarizabilities affect the shape of the γπ Compton scattering angular distribution. By crossing symmetry, the γπ→γπ amplitudes are related to the γγ→ππ amplitudes. Dispersion relations (DR) describe how charged pion polarizabilities contribute to both γγ → π+π- and γγ → π0π0 reactions. A stringent test of chiral perturbation theory (ChPT) is possible by comparing the experimental polarizabilities with ChPT predictions. The combination (απ-βπ) has been measured by: (1) radiative pion Primakoff scattering πZ → πZγ at CERN COMPASS, (2) two-photon pion pair production γγ→ππ at SLAC PEP Mark-II, (3) radiative pion photoproduction γp→ γπn at Mainz MAMI. COMPASS and Mark-II (but not Mainz) polarizabilities are in good agreement with ChPT predictions; and by DRs, with DESY Crystal Ball γγ → π0π0 data. A pion polarizability status report is presented, following the review by S. Scherer and M. Moinester for IJMPA.
  • Tuesday 24 March 2020 at 10.15 in A315: Harri Waltari (Southampton)Cancelled!
    Exploring neutrino physics through sneutrinos
    Supersymmetry relates neutrinos to their superpartners, sneutrinos. Unlike neutrinos, sneutrinos may decay visibly at collider experiments and offer a window to neutrino dynamics that neutrinos cannot. I shall discuss an explicit example of a case, where sneutrinos could be detected through a lepton number violating signature and how such a signal could be used to constrain neutrino Yukawa couplings.

  • Tuesday 1 September 2020 at 15.15 using Zoom: Aleksey Cherman (Minneapolis)
    Higgs-confinement phase transitions with fundamental matter
    I will discuss the conditions under which Higgs and confining regimes in gauge theories with fundamental representation matter fields can be sharply distinguished. It is widely believed that these regimes are smoothly connected unless they are distinguished by the realization of global symmetries. However, I will show that when a U(1) global symmetry is spontaneously broken in both the confining and Higgs regimes, the two phases can be separated by a phase boundary. The phase transition between the two regimes may be detected by a novel topological vortex order parameter. I’ll illustrate these ideas by explicit calculations in gauge theories in three spacetime dimensions, and then explain the generalization to four dimensions. One important implication of our results is that nuclear matter and quark matter are sharply distinct phases of QCD with an approximate SU(3) flavor symmetry. 
    Link to the seminar:

  • Tuesday 8 September 2020 at 10.15 using Zoom: Joona Havukainen (Helsinki)
    Machine learning at the LHC
    Modern machine learning methods can be used to process large amounts of high dimensional data, taking advantage of correlations among the input variables to accomplish their goal. While machine learning has been used in experimental particle physics for decades, the surge in popularity of deep neural networks during the last five or so years has led to a host of new applications at the LHC experiments. These include topics such as hardware level trigger decisions in data taking, correcting for detector effects in data and classifying measured events into categories for the final analysis. The detailed simulations in experimental particle physics combined with the enormous amount data collected by the detectors offers a unique environment for developing new and interesting machine learning solutions.
    An overview of various machine learning applications developed for the LHC and its experiments at CERN will be presented in this talk, with the emphasis on novel deep learning use cases of the recent years.
    Link to slides:

  • Tuesday 22 September 2020 at 10.15 using Zoom: Maurizio Piai (Coleg y Gwyddoniaeth, Abertawe, Cymru)
    Higgs Compositeness and Beyond the Standard Model Physics
    Appealing solutions to the hierarchy problem of the Standard Model involve assuming that the Higgs particle might be composite, and originates from new strong dynamics. This general scenario has a number of concrete possible realisations, and their study requires non-perturbative theoretical methods.  I illustrate a selection of three such possibilities, via examples of calculations  that use gauge-gravity dualities, lattice gauge theories and effective field theories, respectively.  
    Link to seminar:

  • Tuesday 29 September 2020 at 10.15 using Zoom: Harri Waltari (Southampton)
    Exploring neutrino physics through sneutrinos
    Supersymmetry relates neutrinos to their superpartners, sneutrinos. Unlike neutrinos, sneutrinos may decay visibly at collider experiments and offer a window to neutrino dynamics that neutrinos cannot. I shall discuss an explicit example of a case, where sneutrinos could be detected through a lepton number violating signature and how such a signal could be used to constrain neutrino Yukawa couplings.
  • Tuesday 6 October 2020 at 10.15 in D101: Saga Säppi (Helsinki).
    Thesis defence, opponent Guy Moore behind a Zoom connection.
    Maximum D101 attendance 50.
    Link to thesis:

  • Tuesday 20 October 2020 at 10.15 in Zoom: Niko Jokela (Helsinki)
    Holographic confinement confronts lattice Yang-Mills theory
    Confinement is one of the most intriguing phenomena that our fundamental quantum field theories exhibit. Due to its non-perturbative nature, it is defying all analytic approaches. At infinite-N, holography offers geometric explanations to features that echo confinement. In this talk I discuss salient details of holographic confinement and aim at making contact with ongoing work on lattice simulations. The key player throughout the talk will be entanglement entropy, connecting the two seemingly different topics: high energy physics and quantum information.
    Link to video:

  • Tuesday 3 November 2020 at 10.15 with Zoom: Jani Kastikainen (Helsinki, Paris)
    Holographic F-functions in ABJM theory with flavor on the 3-sphere
    Classifying possible RG flows between quantum field theories is an important open problem. Progress has been made by constructing c-functions (functions of coupling constants) that decrease monotonically along RG flows proving for example their irreversibility. In three-dimensional QFTs, the free-energy of the theory on a 3-sphere is proposed to be such a function and is hence called an F-function. In this talk, which is based on ongoing work, I use a recently proposed holographic method to construct F-functions from free-energy in ABJM theory coupled to massive flavor on a 3-sphere. The theory is holographically dual to Type IIA supergravity with dynamical D6-branes in ten dimensions. I describe the dynamics of D6-branes in the probe limit, compute their free-energy and argue that the corresponding F-functions are monotonic in this top-down setup.
    Link to video:

  • Tuesday 24 November 2020 at 10.15 with Zoom: Saga Säppi (ECT*, Trento)
    Thermal pQCD and the progress towards the NNNLO pressure at zero temperature
    The behaviour of QCD at large baryochemical potentials has seen a surge of interest in recent years, partly thanks to the increased information from neutron star observations calling for a better theoretical understanding. In my talk, I will discuss the theoretical progress on the high-density side from the point of view of high-order loop calculations in perturbative QCD. I will first discuss the qualitative behaviour of the pQCD expansion of the pressure at finite density, contrasting it with the behaviour at finite temperature, and cover the historical progress in the calculation of the pQCD pressure in the past decades. My main focus will be in an ongoing program of computing the NNNLO pressure at zero temperature, and the developments that have enabled it. At high orders, resummations are needed to reduce the pressure IR-finite in the soft secor: I will explain how using the HTL effective theory makes this possible at high-orders and the challenges that nevertheless arise. I will also cover the roles the different scales of the theory play in various contributions to the pressure, focussing in particular on the contribution from soft scales.
    Link to video:
  • Tuesday 1 December 2020 at 10.15 with Zoom: Matti Järvinen (APCTP, Pohang, Korea)
    Holographic neutron stars and transport in dense QCD
    Gauge/gravity duality can be used to study QCD at high densities and low temperatures where many other theoretical tools do not work and uncertainties are therefore large. I will review recent progress on this topic, including modeling of both the nuclear and the quark matter phases, which may appear in neutron star cores and in neutron star mergers. Holographic results for viscosities and conductivities differ significantly from perturbative QCD predictions in the dense quark matter phase, highlighting the differing transport properties of weakly and strongly interacting systems. I also discuss applications of the holographic equation of state to isolated (static and rotating) neutron stars, as well as to neutron star mergers.
    Link to video:
  • Tuesday 8 December 2020 at 10.15 with Zoom: Jeff  Kost (Sussex)
    Deciphering the Archaeological Record: Cosmological Imprints of Non-Minimal Dark Sectors
    Many proposals for physics beyond the Standard Model give rise to a dark sector containing many degrees of freedom. In this talk, we discuss the cosmological implications of the non-trivial dynamics which may arise within such dark sectors, focusing on decay processes which take place entirely among the dark constituents. First, we demonstrate that such decays can leave dramatic imprints on the resulting dark-matter phase-space distribution. In particular, this distribution need not be thermal—it can even be multi-modal, exhibiting a non-trivial pattern of peaks and troughs as a function of momentum. We then proceed to show how these features can induce modifications to the matter power spectrum. Finally, we assess the extent to which one can approach the archaeological inverse problem of deciphering the properties of an underlying dark sector from the matter power spectrum. Indeed, one of our main results is a remarkably simple conjectured analytic expression which permits the reconstruction of many of the important features of the dark-matter phase-space distribution directly from the matter power spectrum. Our results therefore provide an interesting toolbox of methods for learning about, and potentially constraining, the features of non-minimal dark sectors and their dynamics in the early universe.
  • Monday 14 December 2020 at 14.15 in Zoom: Jere Remes (Helsinki).
    Thesis defence, opponent Umut Gürsoy behind a Zoom connection.
    Link to thesis:
  • Tuesday 15 December 2020 at 10.15 with Zoom: Francesco Bigazzi (INFN, Florence)
    Dark holograms and gravitational waves
    First order cosmological phase transitions in beyond the Standard Model theories may trigger the production of a stochastic background of gravitational waves (GW) which could be probed by near future experiments. Since many proposals for dark sectors involve QCD-like, strongly interacting theories, it is crucial to study this issue under the lens of a non-perturbative approach. We use the holographic correspondence to study finite temperature phase transitions and the related GW spectra within the top-down model which mostly resembles large N QCD at low energy. The model displays both a confinement/deconfinement and a chiral symmetry breaking/restoration first order transition. Depending on the model parameters, these can happen either at the same or at different critical temperatures. We study the nucleation of bubbles of true vacuum in the metastable phase in both cases and determine the relevant parameters entering the expressions of the GW spectra due to bubble collisions and sound waves. A large region in the parameter space of the model provides GW signals which may be accessible by near future experiments of different classes.
    Link to video: