# DEPARTMENT OF PHYSICS / HIP JOINT COLLOQUIA / SEMINARS

## HIP seminar room A315

**Tuesday 20 March 2018 at 10.15 in A315: Markus Seidel (CERN)**

*Top mass and subtle QCD effects*

Abstract: I give an overview of the most precise top mass measurements at CMS and discuss their theoretical interpretation. Future improvements require measurements of radiation and non-perturbative effects for a better modeling of ttbar events. I will present a new measurement of jet substructure that can lead to a more precise determination of the effective alpha_s in final-state radiation for different jet flavors.**Thursday 22 March 2018 at 10.15 in A315: Bogdan Stefanski (CU London)**

*The protected closed string spectrum of AdS3/CFT2 from integrability*

Abstract: I will review the recent progress in computing the exact spectrum of closed strings in AdS3/CFT2 using integrability. The spectrum is determined through a set of all-loop Bethe Equations, from which one can find, for example, the protected closed string states and compare with supergravity results. Exact agreement is found in the case of AdS3xS3xT4. For AdS3xS3xS3xS1 past supergravity calculations predicted too many BPS states, many of which are not protected in string theory. I will discuss the current status of these states.**Tuesday 27 March 2018 at 11.00 in A315: K. Kajantie (Helsinki) Note start delayed by 45 min!**

*Memory effect, supertranslations and symmetries at null infinity*

Abstract: First I compute the memory effect, permanent displacement of relative positions of two detectors caused by a passage of a burst of gravitational radiation from a coalescing binary. Particular attention is focussed on null memory, that sourced by the gravitational radiation from the binary in the first place. The analogous physical effect can be studied in electrodynamics with its U(1) gauge invariance both in a gauge covariant way or fixing the gauge. In the context of gravity the relevant symmetry group of null infinity contains the Lorentz group but also direction dependent time translations, supertranslations. They correspond to U(1) gauge transformations. I discuss how the magnitude of these supertranslations can be related to the memory effect. Finally I summarize attempts to supertranslate a configuration with a black hole, dress the horizon with soft hair.**Thursday 12 April 2018 at 10.15 in A315: Alessandro Calandri (Marseille)**

*Search for the Standard Model Higgs boson produced in association with top quarks in the H->bb final state – b-jet idenfication and reconstruction in ATLAS*

Abstract: The measurement of the top-quark Yukawa coupling is possible using the Higgs production in association with a pair of top quarks (ttH). The bb final state features the largest branching ratio among the various ttH decay modes, however its understanding is very challenging in light of the overwhelming non-resonant ttbb background. ATLAS results in the H->bb final state [1] will be presented in this seminar with a particular focus on the flavour-tagging algorithm developments and optimisations as well as the typical b-jet reconstruction and identification performance achieved.**Tuesday 17 April 2018 at 11.00 in A315: Paul Hoyer (Helsinki)**

*Bound states and QCD*

Abstract: TBA**Tuesday 8 May 2018 at 10.15 in A315: Tommi Markkanen (Imperial College, London)**

*Evaporation of de Sitter space*

Abstract: The exponentially expanding de Sitter solution is frequently encountered in cosmology due to its significance for the late and early Universe. However, its stability in a quantised theory has for a long time been the source of debate. In this talk we discuss this issue in the framework of semi-classical gravity. Based on a first principle calculation, I argue that when one considers the experiences of a local observer, the quantum back reaction destabilizes de Sitter space leading to a gradual continuous increase of the dS horizon. This implies that the cosmological ‘constant’ decays over time, with flat space as the asymptotic late time solution.**Tuesday 7 August 2018 at 10.15 in A315: Mikko Laine (Bern)**

*TBA*

Abstract: TBA

Hopefully the up to **50 min + 10 min discussion long **seminar/colloquium will be understandable to a wide audience.

Contacts: Keijo Kajantie (keijo kajantie at helsinki fi) [department / HIP seminar],

Francesco Montanari (francesco montanari at helsinki fi) [cosmo seminar]

**Other related seminars**

Friday 10-12 seminar series in D117: Astrophysics seminar.

Mathematical Physics Seminar and Workshop series Wed 14-16 in Exactum C123.