# Faggruppemøte for subatomær fysikk og astrofysikk

Europe/Oslo
University of Stavanger

#### University of Stavanger

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Description

The main purpose of this topical section meeting is to bring together young researchers within the fields of subatomic-, medical- and astrophysics. Our goal is to provide a welcoming environment, where each participant can present their current work and get an update on what others within the community in Norway are working on.

All participants (master-students, PhD-students and Postdocs) are highly encouraged to give a presentation of 20-25 minutes about their work.

During the conference there will be a common lunch on Friday and Saturday, in addition we will have dinner together on the evenings of Thursday and Friday. This provides an excellent opportunity for networking with other students, researchers and scientific staff within the field. There is no conference fee.

We encourage you to register for the conference and submit a short abstract/summary of your presentation before the 30th of April 2021. Please remember to inform us about any food allergies or other special requirements at the time of registration.

The local organizers are looking forward to welcoming you all to Stavanger!

Alexander Rothkopf,
Anders Tranberg

Participants
• Aleksi Kurkela
• Alex Nielsen
• Alexander Rothkopf
• Amanda Lavinia Kjærnsmo
• Ann-Cecilie Larsen
• Are Raklev
• David McNutt
• Dieter Roehrich
• DIVYARANI C G
• Eirik Eik Svanes
• Erlend Aakvaag
• Francesco Pogliano
• Gaurang Parkar
• Georgios Filios
• Germano Nardini
• Hans Winther
• Helena Kolesova
• Håvard Helstrup
• Ida Storehaug
• Ingrid McKibben Lofnes
• Ismael Ferrero
• Jahed Abedi
• Johannes Hamre Isaksen
• Jonas El Gammal
• Jonas Tjemsland
• Karanvir Singh
• Lasse Lorentz Braseth
• Magdalena Eriksson
• Paolo Marcoccia
• Per Osland
• Per-Dimitri Sønderland
• Renate Mauland-Hus
• Simon Huiberts
• Stian Hartman
• sunniva siem
• Tomas Brauner
• Torsten Bringmann
• Vegard Undheim
• YILUN DU
• Thursday, June 24
• 1:45 PM 2:00 PM
Welcome 15m
Speakers: Are Raklev (University of Oslo), Håvard Helstrup (Høgskulen på Vestlandet)
• 2:00 PM 3:20 PM
Faggruppemøte Plenary: Session A
• 2:00 PM
Quark matter in neutron stars 20m

Neutron stars are the densest astrophysical objects in the universe. Cores
of neutron stars reach densities as high as those realized in ultrarelativistic
heavy-ion collisions studied in particle accelerators. In these collisions ordinary nuclear matter melts into a new phase of
elementary particle matter, quark matter. This naturally raises the question: does quark matter also exist
inside neutron stars? In my talk, I describe how recent advancements in
theory of superdense matter and in observations of neutron stars—such as the LIGO/Virgo
detection of gravitational waves arising from a merger of two neutron stars—can inform
us about what lies in the centers of neutron stars. I discuss how the different constraints
point to the existence of quark matter cores in large neutron stars.

Speaker: Aleksi Kurkela (UiS)
• 2:20 PM
Finite temperature effects on dense QCD matter in strong magnetic fields 20m

The ground state of QCD in sufficiently strong magnetic fields and at moderate baryon densities was shown to carry a crystalline condensate of neutral pions: the chiral soliton lattice. As proved previously, this phase of matter is stable under thermal fluctuations. Recently, the effects of quantum fluctuations and finite temperature were calculated in detail within chiral perturbation theory. The obtained results on the QCD phase diagram for varying temperature, baryon chemical potential and magnetic field will be presented.

Speaker: Helena Kolesova (University of Stavanger)
• 2:40 PM
Quenching effects in the cumulative jet spectrum 20m

The steeply falling jet spectrum induces bias on the medium modifications of jet observables in heavy-ion collisions. To explore this bias, we develop a novel analytic framework to study the quenched jet spectrum, and its cumulative. We include many energy-loss-related effects, such as soft and hard medium induced emissions, broadening, elastic scattering, jet fragmentation, cone size, coherence effects, etc. We show that, different jet spectrum-based observables are connected, e.g., the nuclear modification, spectrum shift, and the quantile procedure. We present the first predictions for the nuclear modification factor and the quantile procedure with cone size dependence. As an example, we compare dijet and boson+jet events to unfold the spectrum bias effects. We improve quark-, and gluon-jet classification using arguments based on the cumulative. Besides pointing out its flexibility, we apply our framework with other energy loss models such as the hybrid weak-, strong-coupling.

Speaker: Adam Takacs (University of Bergen)
• 3:00 PM
Finite-$N_c$ corrections to Wilson line correlators 20m

In high-energy heavy-ion collisions, a quark-gluon plasma is produced. One way to study this exotic type of matter is to analyze the effect it has on high-energy partons traversing it. This leads to a rich phenomenology that commonly is referred to as jet quenching. High-energy partons propagating in the hot and dense medium mainly experience transverse momentum broadening which leads to additional radiative energy loss. The theoretical description of the main mechanisms behind these phenomena has been well known for some time.
One effect the medium has on partons propagating through it, is that their color continuously rotates, an effect that is encapsulated in a Wilson line along their trajectory. When calculating observables, one typically has to deal with traces of two or more medium-averaged Wilson lines. These are usually dealt with in the literature by invoking the large-$N_c$ limit, but exact calculations have been lacking in many cases. In our work, we show how correlators of multiple Wilson lines appear, and develop a method to calculate them to all orders in $N_c$. Specifically, we will focus on the trace of four Wilson lines, which we develop a differential equation for. We will then generalize this calculation to a product of an arbitrary number of Wilson lines, and show how to do the exact calculation numerically, and even analytically in the large-$N_c$ limit.
These results are relevant for high-$p_T$ jet processes and initial stage physics at the LHC.

Speaker: Johannes Hamre Isaksen (University of Bergen)
• 3:20 PM 4:00 PM
Coffe Break 40m
• 4:00 PM 5:20 PM
Faggruppemøte Plenary: Session B
• 4:00 PM
Relativistic corrections to the binary inspiral using Feynman diagrams 20m

All the ~50 confirmed gravitational wave events observed by the LIGO and Virgo collaboration thus far have been of compact binary mergers. In this talk I will present a fairly new method for computing the wave form produced by such inspirals using field theoretical methods, including Feynman diagrams, to calculate these effects of classical gravity.

Speaker: Vegard Undheim (NTNU)
• 4:20 PM
Radiation hard pixel ‘3D’ sensors for use in ATLAS-ITk at LHC 20m

The High Luminosity LHC (HL-LHC) upgrade requires that the planned Inner Tracker (ITk) of the ATLAS detector must be tolerant to extremely high radiation doses. Specifically, the innermost parts of the pixel system will have to withstand radiation fluences above $1\times10^{16}n_{eq}cm^{-2}$. Novel 3D silicon pixel sensors offer a superior radiation tolerance compared to conventional planar pixel sensors, and are thus excellent candidates for the innermost parts of the ITk. This talk presents studies of 3D pixel sensors with pixel size $50 \times 50 \mu m^2$ mounted on the RD53A prototype readout chip. Test beam results are presented for unirradiated as well as heavily irradiated sensors. For particles passing at perpendicular incidence, it is shown that efficiencies above 96% are reached for sensors exposed to fluences of $1\times10^{16}n_{eq}cm^{-2}$ when biased to 80V.

Speaker: Simon Huiberts
• 4:40 PM
Scalar-field dark matter from CDM , and constraints from cosmological data 20m

Despite the success of LCDM in explaining cosmological observables on scales spanning many orders of magnitude in both time and space, there are ingredients central to the model that are completely unknown, such as dark matter (DM), the dominant matter component of our universe. A wide range of DM candidates have therefore been proposed over the years, motivated by fundamental theories beyond the Standard Model, and discrepancies between predictions of LCDM and observations. In this talk I will focus on scalar field candidates of DM, in particular the phenomenology of self-interacting scalar field DM that have been produced from an initial standard cold DM (CDM)-like phase. By solving the linear equations for such a DM model using a Boltzmann code, large-scale observables, such as the cosmic microwave background and the matter power spectrum, can be computed and used to place constraints on the phenomenology of the transition between the CDM and scalar field DM phases.

Speaker: Stian Hartman (University of Oslo)
• Friday, June 25
• 10:00 AM 11:00 AM
Faggruppemøte Plenary: Session C
• 10:00 AM
Quantum corrections to slow-roll inflation 20m

Cosmic inflation is often described through the dynamics of a scalar field (called the inflaton), slow-rolling in a suitable potential. Quantum fluctuations during inflation can be shown to seed the temperature fluctuations of the CMB and following formation of structure in the Universe. The inflaton field must ultimately be identified with the expectation value of a quantum field, evolving in a quantum effective potential. The shape of this potential is determined by the underlying tree-level potential, dressed by quantum corrections from fluctuations both in the scalar field itself and the background metric. These corrections influence the relation between the slow-roll parameters, which quantify the conditions for slow-roll, and which also enter in observables in the sky. The magnitude of the corrections can be estimated for specific tree-level potentials, of which I discuss two examples.

Speaker: Magdalena Eriksson (UiS/NTNU)
• 10:20 AM
Pion Decay Constant in a magnetic field for QCD - Like theories with real quarks 20m

In this presentation, I will demonstrate how the dependence of the pion decay constant on the magnetic field can be calculated for QCD-Like theories with real quarks. This study is important, due to the fact that the dependence of the pion decay constant on the magnetic field plays an important role when investigating the properties of the phase diagram of QCD-Like theories, under the presence of an external magnetic field.

Speaker: Georgios Filios (University of Stavanger)
• 10:40 AM
Rindler horizons in Schwarzschild and Vaidya spacetimes 20m

We will discuss linear uniformly accelerating (LUA) trajectories in a background Schwarzschild spacetime thus modelling the trajectories in the vicinity of a static black hole. Also, an analogous model is proposed in the simplest dynamic spacetime that permits black holes, the Vaidya spacetime. These LUA trajectories give rise to Rindler horizons which have been studied extensively due to their association with the Unruh effect. We will also discuss numerical solutions to these LUA trajectories in Schwarzschild and Vaidya spacetimes.

Speaker: Karanvir Singh (Universitet i Stavanger)
• 11:00 AM 11:20 AM
Coffee break 20m
• 11:20 AM 12:20 PM
Faggruppemøte Plenary: Session D
• 11:20 AM
The physics of symmetries in the Higgs sector 20m

An extended Higgs sector allows for reparametrizations, or different bases. These do not affect the physics. In addition, it can accommodate several genuine symmetries. These were identified some ten years ago in terms of reparametrization-invariant quantities. We have expressed these criteria in terms of physical quantities, masses and couplings.

Speaker: Per Osland (University of Bergen)
• 11:40 AM
Accelerating Bayesian Inference of expensive Likelihoods with Gaussian Processes 20m

Numerically approximating multidimensional posterior distributions can be very expensive
when evaluating the likelihood function involves expensive numerical computation.
At the same time many likelihoods in physics show a "speed hierarchy"
between the different dimensions of the parameter space which means that recomputing
the likelihood function is much more expensive when changing some parameters
than others. This naturally arises when some of these parameters come from theoretical
models while others are associated to the data. Recently some attempts have
been made at fast Bayesian inference using Bayesian quadrature to reduce
the number of samples required for mapping the posterior distributions drastically.
While this approach works well in low dimensions it becomes prohibitively expensive
if the number of dimensions exceeds d~10. Additionally these approaches
cannot take advantage of the aforementioned speed hierarchy in the likelihood. In
this thesis we develop an algorithm which mitigates these problems and improves
on the current state of the art by (i) introducing a novel acquisition function which
is well suited to performing Bayesian quadrature of log-probability distributions (ii)
accelerating the Kriging believer batch acquisition algorithm with blockwise matrix
inversion and (iii) Proposing an algorithm which can take advantages of
speed hierarchies by marginalizing nuisance parameters with the PolyChord nested
sampling algorithm. We test these algorithms on gaussian toy likelihoods and
real cosmological likelihoods and report a decrease in wall clock time of up to several
orders of magnitude for mapping the posterior space.

Speaker: Jonas El Gammal (University of Stavanger)
• 12:00 PM
Gravitational waves from the first order phase transitions in the early universe 20m

Laser Interferometer Space Antenna (LISA), will be the first space based gravitational wave observatory, which is mainly funded by the European Space Agency (ESA). Planned for launch in the early 2030s, LISA is designed to detect GW sources spanning the whole history of the Universe. While the loudest sources will be resolved individually, the rest will combine into a stochastic GW background (SGWB).

It is generally believed that several phase transitions have taken place in the early universe. The effects of cosmological phase transitions may well have been crucial for the evolution of the universe.

First-order cosmological PTs are predicted in many scenarios beyond the Standard Model of particle physics. It is quite possible that these PTs may lead to the baryon asymmetry, one of the most important properties of our universe. They also provide a particularly compelling source of GWs in the early universe. At a first-order cosmological PT, bubbles of a new phase begin to nucleate and expand as the universe cools. The region inside the bubbles contains the new phase, typically characterized by a vacuum expectation value (VEV) of a scalar field that differs from its value outside the bubbles. The collision of the bubbles and the resulting motion of the ambient cosmic fluid sources a stochastic GW background that can be observable at GW interferometers like LISA.

Speaker: Ms Divyarani Chandrababu Geetha (Institute of Mathematics and Physics, University of Stavanger)
• 12:20 PM 1:30 PM
Lunch 1h 10m
• 1:30 PM 2:30 PM
Faggruppemøte Plenary: Session E
• 1:30 PM
Supersymmetry Cross Section Predictions with Bayesian Neural Networks 20m

Exploring the use of Bayesian neural networks for the purpose of evaluating supersymmetric cross sections at next-to-leading order with reliable uncertainty estimates. Calculating cross sections is a central part of the search for new physics beyond the Standard Model, however, it is a time consuming endeavour. Machine learning methods can speed up the production process by learning the relationships between physical parameters and cross sections, however, the uncertainty on their predictions is for most methods unavailable. A variational Bayesian neural network was trained on cross section data produced by Prospino 2.1 for electroweakino pair production in proton-proton collisions. To improve its performance deterministic pretraining was implemented and various annealing schemes were tested. We found that with the additional implementations the Bayesian neural network consistently produced reliable uncertainties and favourable predictions with relatively short run-times.

Speaker: Mr Per-Dimitri Per-Dimitri B. Sønderland
• 1:50 PM
Distinguishing wormholes from black holes with curvature invariants 20m

In general relativity, an apparent horizon is commonly used as a quasi-local boundary around a black hole region instead of the event horizon, which is a teleological surface. Apparent horizons have many useful applications in numerical relativity, however, they can be difficult to determine and are foliation dependent, i.e., observer dependent. Recently it has been proven that stationary black holes and dynamical black holes admitting isolated horizons can be determined using the zero-set of a collection of curvature invariants which defines the so-called geometric horizon of these black hole solutions. Motivated by this fact, a set of conjectures have been introduced which propose that a geometric horizon exist in every dynamical black hole spacetime and act as an invariant boundary around the black hole.

While the geometric horizon conjectures have now been verified for other dynamical black hole solutions, there are still ambiguities with the definition of a geometric horizon. To illustrate this problem, I will discuss a spherically symmetric line element which admits either a dynamical black hole geometry or a dynamical wormhole geometry and show that in both cases the apparent horizon and the wormhole throat is partially characterized by a single curvature invariant. In order to distinguish the wormholes from the black holes further conditions must be appended to the original definition. I will show what conditions are needed and discuss how these conditions might be extended beyond spherical symmetry.

Speaker: David McNutt (University of Stavanger)
• 2:10 PM
The charge sensitivity calibration of the upgraded ALICE Inner Tracking System 20m

The ALICE detector is undergoing an upgrade for Run 3 at the LHC. A new Inner Tracking System is part of this upgrade. The upgraded ALICE ITS features the ALPIDE, a Monolithic Active Pixel Sensor. Due to IC fabrication variations and radiation damages, the threshold values for the ALPIDE chips in ITS need to be measured and adjusted periodically to ensure the quality of data-taking. The calibration is implemented within the O$^2$ system, thus it runs in the same framework as the normal operation. This paper describes the concept and implementation of the calibration for the upgrade ALICE ITS.

Speaker: Shiming Yuan
• 2:30 PM 2:45 PM
Closing 15m
Speakers: Are Raklev (University of Oslo), Håvard Helstrup (Høgskulen på Vestlandet)