# Events at Physics

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### Monday, July 11th, 2022

No events scheduled

### Tuesday, July 12th, 2022

Network in Neutrinos, Nuclear Astrophysics, and Symmetries (N3AS) Seminar
New signals and backgrounds in direct detection experiments
Time: 2:00 pm
Place: Meeting ID: 912 3071 4547
Speaker: Mukul Sholapurkar , UCSD
Abstract: Direct detection experiments have traditionally looked for nuclear recoils. In the last few years, experiments have also started looking for electron recoils, reaching thresholds of ~ eV. As experiments achieve these low thresholds, they also encounter new and previously unexplored backgrounds. In this seminar, I will discuss about new low energy backgrounds correlated with previously considered high energy backgrounds like cosmic rays and radiogenic photons in current as well as future low threshold detectors. In the end, I will also briefly comment about using direct detection experiments as neutrino detectors, and the possibilities of probing new neutrino physics with them. Meeting ID: 912 3071 4547
Host: Baha Balantekin
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### Wednesday, July 13th, 2022

Physics ∩ ML Seminar
Explainable deep learning models for cosmological structure formation
Time: 11:00 am
Place: Online Seminar: Please sign up for our mailing list at www.physicsmeetsml.org for zoom link
Speaker: Luisa Lucie-Smith, Max Planck Institute for Astrophysics
Abstract: According to our standard cosmological model, the formation of cosmic structures in the Universe is driven by the gravitational collapse of small matter density fluctuations present in the early Universe. The non-linear nature of gravitational collapse makes it difficult to develop a physical understanding of how complex late-time cosmic structures emerge from these linear initial conditions. In this talk, I will present an explainable deep learning framework for extracting new knowledge about the underlying physics of cosmological structure formation. I will focus on an application to dark matter halos, which form the building blocks of cosmic large-scale structure and wherein galaxy formation takes place. The goal is to use interpretable neural networks to discover the independent degrees of freedom in the density profiles of dark matter halos. I will show that the model is able to reproduce the known variations encapsulated by previous empirical approaches. The network then goes further and discovers an additional factor of variation in the outer profile, which we identify as related to infalling dark matter onto the halo (also known as the ‘splashback’ effect).
Host: Gary Shiu
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Physics Summer Fun
Weekly Recess
Time: 12:30 pm
Place: Bascom Hill or 5310 Chamberlin Hall
Speaker: Sharon Kahn
Abstract: We hope you’ll take a 20-30 minute break on (some/all) Wednesdays this summer (12:30-1pm) to come play together! For nice days, we’ve arranged to borrow some lawn games from the L&S dean’s office and will likely bring along a frisbee and/or a hackeysack, too. Meet us on Bascom Hill (between Birge/South Hall).
In case of rain, we’ll meet indoors (5310 CH) for board games. Feel free to come play or just hang out!
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### Thursday, July 14th, 2022

NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Recent results from the ANTARES and KM3NeT neutrino telescopes at the Mediterranean Sea
Time: 2:30 pm
Place:
Speaker: Rodri Gracia, ECAP/ University Erlangen-Nuremberg, Germany
Abstract: ANTARES was the first undersea neutrino telescope and has been taking data steadily since its completion in 2008, until early 2022 when it was decommissioned. Currently, the KM3NeT collaboration is building two additional detectors which will target two different energy ranges. The ORCA detector geometry is optimised of the detection of neutrinos in the GeV range while the ARCA detector geometry has been optimised for the detection of neutrinos in the PeV range and above. In this talk I will review the scientific output of the ANTARES telescope, and I will report on the status of the KM3NeT detectors as well as on the latest results.
Host: Lu Lu
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### Friday, July 15th, 2022

Thesis Defense
Exploring kinetic physics in space plasmas using PIC simulations
Time: 2:00 pm
Place: B343 Sterling or
Speaker: Harsha Gurram, Physics PhD Graduate Student
Abstract: This dissertation investigates two major long-standing problems in space physics with the aid of kinetic simulations. Our first study delves into magnetic reconnection, a fundamental process in magnetized plasmas wherein global magnetic topology is modified and the built-up magnetic stress is transformed into plasma flows and heating. This energy conversion process is thought to be an important mechanism for particle energization in space. Here, we analyzed fully kinetic simulation results of magnetic reconnection to study how the released energy and associated signatures propagate away from the reconnection site. In contrast to previous studies, where, the Hall magnetic field structures were carried away by kinetic Alfv\'{e}n waves (KAWs). Our kinetic simulation implemented at a large numerical domain and with open boundary conditions permits large-amplitude SAWs to be excited by the reconnection dynamics. Due to the dispersive nature of the KAWs, they eventually get damped, and SAWs become the main carrier of the energy away from the reconnection site. These reconnection-driven SAWs are observed to propagate distances $\gg 9R_e$ unattenuated carrying sufficient energy and may act as a primary energy source to drive the white aurora.

Our second study examines the effects of Coulomb collisions in solar-wind heating. The temperature of the solar wind plasma expanding from the hot solar corona does not decrease with the distance as fast as predicted by the adiabatic expansion law. The heating of the solar-wind electrons results from the energy exchange of the fast electrons propagating from the corona along the background magnetic field (the beam or strahl) and the electrons trapped between the electric potential and magnetic mirror walls (the core). The level of the trapped population is a result of two competing processes—particle influx from the streaming population due to pitch-angle scattering and particle losses through the boundary due to energy diffusion. As scattering rates are a free parameter in our study, we can determine how the scattering rates affect the electron distributions, and in turn temperature of the solar wind electrons. Using 1D cylindrical simulations we found the electron temperature profiles scaled with the ratio $\nu{ee}/\nu{ei}$, higher the $\nu{ee}/\nu{ei}$ higher the electron temperatures. The dependency of the electron temperatures and trapped electron population on $\nu{ei}/\nu{ee}$ in the kinetic simulations implies that the Coulomb collisions have a significant effect on the electron temperature profiles as suggested by the collisional model.
Host: Jan Egedal
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