High Energy Physics Seminars This Term

The nature and origin of dark matter are among the most compelling mysteries of contemporary science. LUX-ZEPLIN (LZ) is a dark matter direct detection experiment located at the Sanford Underground Research Facility in Lead, South Dakota. The experiment consists of a dual-phase xenon Time Projection Chamber with an active volume of 7 tonnes, surrounded by an active liquid xenon skin region and a gadolinium-loaded liquid scintillator neutron detector. With an exposure of 60 live days using a fiducial mass of 5.5 tonnes, LZ has achieved world-leading sensitivity to Weakly Interacting Massive Particles (WIMPs). This talk will give an overview of the LZ experiment and present results from LZ's first dark matter search.

The nature of dark matter poses one of the most pressing questions in fundamental physics today. Thermal freeze-out of a weakly interacting massive particle (WIMP) has proved to be a successful framework for explaining the measured dark matter abundance in the Universe. However, the sizeable couplings of dark matter to the Standard Model particles required in its simplest realizations have been put under severe pressure by experimental null-results at colliders, direct and indirect detection experiments. Hence, fulfilling the relic density constraint often requires the exploration of ‘exceptional’ regions, e.g. the region where coannihilation effects increase the effective annihilation rate. In this talk, we revisit the assumptions commonly made within the coannihilation scenario and discuss a new variant of dark matter freeze-out, dubbed conversion-driven freeze-out (or coscattering). In this scenario, the relic abundance is set by the freeze-out of conversion processes requiring significantly smaller couplings of dark matter to the standard model. While this parameter region is largely immune to direct detection constraints, it predicts an interesting signature of disappearing tracks or displaced vertices at the LHC. We will also discuss the effect of bound state formation of the coannihilating particle which considerably enhanced the valid parameter space into the multi-TeV region, making it a prime target for upcoming long-lived particle searches at the LHC.

The discovery of coherent elastic neutrino-nucleus scattering (CEvNS) was made by the COHERENT Collaboration in 2017 using a CsI[Na] scintillating crystal at Oak Ridge National Laboratory's Spallation Neutron Source (SNS). This observation was made over 40 years after the theoretical prediction by Freedman et al. and consists of a neutral-current neutrino-nucleus interaction on an entire nucleus as a whole. The CEvNS process is the dominant scattering mode below 100 MeV and serves as a fertile testbed for precisely examining the Standard Model and has applications in nuclear reactor monitoring and stellar astrophysics. COHERENT built out the CEvNS physics program to include detector systems using liquid argon (CENNS-10), germanium (GeMini) and NaI[Tl] crystals (NaIvEte)--of which liquid argon successfully yielded an additional CEvNS measurement. The variety of media studied within the COHERENT detector suite works towards establishing the predicted N^2 dependence of the CEvNS cross section. Additionally, the COHERENT Collaboration houses several detector systems aimed at elucidating inelastic neutrino-nucleus interactions in support of CEvNS searches and as standalone investigates of neutrino-induced nuclear reactions. This presentation will consist of both the published results and the ongoing work being done on new detectors to further advance the physics goals of the collaboration. 

The Standard Model of particle physics has been a crowning achievement of fundamental physics, culminating in the discovery of the Higgs boson in 2012. As a quantum theory of the building blocks of matter and forces, it has been one of the most successful theories in science. The recent measurement of the mass of the W boson disagrees with the theory prediction. This upset to the Standard Model may point towards exciting new discoveries in particle physics in the coming years. We will discuss the Standard Model, the crucial role of the W boson, and how it has become the harbinger of new laws of nature.

Despite the successes of the Standard Model of particle physics, it remains a challenge to understand the dynamics of the strong interaction among the building blocks of hadronic matter, namely quarks and gluons. At small distance scales or at high energies, the underlying theory, the Quantum Chromodynamics (QCD), is well tested and understood. Our understanding of the strong interaction deteriorates dramatically at larger distances scales such as the size of the nucleon. This so-called "strong QCD regime” exhibits spectacular effects such as the generation of hadron masses and color confinement. Moreover, the nature of QCD implies the existence of gluon-rich hadrons, such as glueballs and hybrids, multi-quark states, and molecules. This presentation will highlight a future research program that aims to provide precision data exploiting collisions of an intense beam of cooled anti-protons with protons or nuclei. This experiment, called PANDA at FAIR, has the ambition to carry out comprehensive spectroscopy studies of hadrons in the strange, charm, and gluon-rich regimes, covering topics in the field of nuclear, hadron, and particle physics.

The axion is a hypothetical particle that may solve two problems in particle physics & cosmology, the Strong-CP problem and the nature of dark matter. The Axion Dark Matter Experiment (ADMX) is the DOE Flagship search for these particles as part of the “Generation-2” direct dark matter search program in High Energy Physics. The experiment uses a tunable resonant cavity in a large static magnetic field to enhance the conversion of axions to detectable microwaves. Quantum-limited amplifiers based on superconducting Josephson Junction circuits are critical to allow the search to be sensitive enough to rapidly scan the frequencies where the axion may exist.  Here I will describe the status of the search along with an update on the planning and prototyping for the next phase of ADMX.”