Physics

Title of thesis: Validating the dark matter origin of a LDMX signal with direct detection experiments
Abstract: The search for a particle candidate explaining the origin of dark matter is one of the central goals in modern astro-particle physics. Many experiments based on different measurement strategies are being built in an effort to detect such a candidate. In the event that a dark matter signal is seen, it is crucial to verify the signal with other experiments. In this thesis we explore the possibility of using a direct detection experiment to validate a hypothetical dark matter signal seen at the Light Dark Matter eXperiment (LDMX). We consider a representative light dark matter model, with masses ranging between ∼ 1 − 1000 MeV, that are assumed to be mediated by a massive vector dark photon. Hypothetical dark matter signals are simulated, both for a LDMX-type experiment and for a semiconductor based direct detection experiment. These signals are generated for varying values of the "true" dark matter mass mχ and dark photon kinetic mixing ε parameters, which are chosen under thermal relic abundance constraints. Validation is achieved by using the MultiNest Monte Carlo algorithm to estimate a marginal posterior of the mχ and ε parameters given the hypothetical direct detection signal. The parameter estimate is input into the LDMX-type simulation software and compared with that of the hypothetical LDMX result. The chi-square hypothesis test is used to conclude whether the two distributions are consistent with a single distribution function. A conclusion is drawn regarding the amount of exposure required for a direct detection experiment to validate a hypothetical LDMX signal for some choices of mχ. For instance, when mχ = 10 MeV the threshold exposure for validation was determined to be 0.05 kg-year.
Supervisor: Riccardo Catena Examiner: Riccardo Catena Opponent: Elsa Danielsson
Location of presentation: <a rel="noopener" href="https://maps.chalmers.se/#24d8b7be-1f59-4388-be28-a07861b429b2" target="_blank" data-anchor="#24d8b7be-1f59-4388-be28-a07861b429b2">PJ-salen</a>

Title of thesis: Dipole Dark Photon Emission at LDMX: Theory and Simulation
Abstract: The dark photon (DP) is a hypothetical massive mediator particle introduced to explain the present cosmological abundance of dark matter (DM) candidates that are lighter than nucleons, and that can thus be effectively searched for in fixed target experiments such as the Light Dark Matter eXperiment (LDMX). This thesis studies and compares higher order electromagnetic couplings between a DP, A', and the Standard Model (SM) electron. These couplings are expected to have interesting experimental signatures in missing energy and momentum experiments, but have so far only been explored within semi-analytical approaches that are not able to capture the full complexity of DP production at fixed target experiments. Motivated by this, simulations of the energy and transverse momentum distributions of a scattered electron in the production of a DP through a bremsstrahlung process have been performed by using the software MadGraph5 (MG5). These simulations have been made for the different electromagnetic interactions and a customary kinetic mixing (KM) coupling. The DP is added through a new gauge group, U'(1), with mass in the range 0.1-1 GeV. The results of the different interactions and masses is then compared, and the LDMX potential for differentiating them is discussed. Further, the numerical simulations have been compared with results obtained through the Weizsäcker-Williams (WW) approximation, where agreement was found for the energy but not for the transverse momentum. The reason for this is believed to be that the nucleus transverse momentum, which is neglected in this application of the WW approximation, is non-negligible. Finally, the parameter space of the different DP interactions is examined by calculating the relic abundance that can be produced for an example where the DP couples to complex scalar DM and the SM electron. This calculation has been performed using the program micrOMEGAs and validated with semi-analytical calculations.
Examiner: Riccardo Catena Supervisor: Riccardo Catena Opponent: Sixten Furenäs
Location of presentation: <a rel="noopener" href="https://maps.chalmers.se/#24d8b7be-1f59-4388-be28-a07861b429b2" target="_blank" data-anchor="#24d8b7be-1f59-4388-be28-a07861b429b2">PJ-salen</a>

Title of thesis: Dark Matter Interactions with Sun-like Celestial Bodies
Abstract: One interesting avenue in the search for dark matter is the possibility of dark matter interacting with celestial bodies, like our Sun, in such a way that the effects can be detected here on Earth. The purpose of this thesis is to investigate how sub-GeV, Dirac-type dark matter particles could interact with the electrons in the Sun via a heavy dark photon. This study also aims to account for the surrounding medium and its effects on these scattering rates through in-medium effects using the framework of linear response theory, specifically the density-density response function. The impact of the medium will be demonstrated by comparing the scattering rates calculated through linear response theory to those calculated without considering the medium. Using these calculated scattering rates, in both the heavy and light mediator limits, the resulting dark matter capture rates inside the Sun, for various dark matter masses and dark matter-electron reference cross sections are calculated. Furthermore, the calculated scattering rates are applied to the phenomenon of solar dark matter reflections, where in-medium effects are also demonstrated to be of some importance.
Supervisor: Riccardo Catena&nbsp; Examiner: Riccardo Catena Opponent: Andreas Lund
Location of presentation: <a rel="noopener" href="https://maps.chalmers.se/#24d8b7be-1f59-4388-be28-a07861b429b2" target="_blank" data-anchor="#24d8b7be-1f59-4388-be28-a07861b429b2">PJ-salen</a>

Title of thesis: Exploring flavor physics in extensions of the Standard Model
Abstract:The flavor puzzle refers to the unsolved mysteries surrounding the families of the Standard Model (SM), in particular the question why there is a mass hierarchy despite each flavor coupling int he same way to SM gauge bosons. Solving this requires a new theory without flavor universality.
In this thesis, the possibility of flavor deconstruction is explored, which is the idea of splitting the SM group into factors under which only certain families are charged. One such group is investigated in numerical detail, where the aim is to evaluate the suppression of unwanted flavor-changing neutral currents (FCNC) for a range of masses of new fermions and bosons.&nbsp;
The results show that tuning the parameters of the theory to fulfill FCNC constraints for all currents simultaneously is very challenging. The method serves as a proof-of-concept for numerical studies of novel theories within flavor deconstruction.&nbsp;
The concept of composite Higgs is briefly discussed, that is theories where the Higgs is a bound state of more fundamental particles, as a possible alternative solution.
Supervisor: Gabriele Ferretti&nbsp; Examiner: Gabriele Ferretti Opponent: Thomas Jerkvall
Location of presentation: <a rel="noopener" href="https://maps.chalmers.se/#24d8b7be-1f59-4388-be28-a07861b429b2" target="_blank" data-anchor="#24d8b7be-1f59-4388-be28-a07861b429b2">PJ-salen</a>

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Presentation of master thesis

Master thesis presentations at Physics

June 17 2024

9:00-10:00 Andreas Lund, MPPHS

11:00-12:00 Thomas Jerkvall, MPPHS

13:00-14:00 Gustav Strandlycke, MPPHS

14:00- 15:00 Elsa Danielsson, MPPHS