The Heavy Photon Search (HPS) is a new experiment at Jefferson Lab that will search for heavy U(1) vector bosons (heavy photons, dark photons or A') in the mass range of 10 MeV/c^2 to 1 GeV/c^2 that couple weakly to ordinary matter. Heavy photons in this mass range are theoretically favorable and may also mediate dark matter interactions. The heavy photon couples to electric charge through kinetic mixing with the photon, in turn, inducing an effective gauge coupling of the A' to electric charge, which is suppressed relative to the electron charge by a factor of ε ~ 10^-2 - 10^-12. Since heavy photons couple to electrons, they can be produced through a process analogous to bremsstrahlung radiation, subsequently decaying to narrow e+e- resonances which can be observed above the dominant QED trident background. For suitably small couplings, dark photons travel detectable distances before decaying, providing a second signature.

HPS will utilize this production mechanism to probe heavy photons with relative couplings of ε^2 ~ 10^-5 - 10^-10 and search for the e+e- decay of the heavy photon via two signatures: invariant mass and displaced vertex. Using Jefferson Lab’s high luminosity electron beam incident on a thin tungsten target along with a compact, large acceptance forward spectrometer consisting of a silicon vertex tracker and lead tungstate electromagnetic calorimeter, HPS will access unexplored regions in the mass-coupling phase space.

The HPS engineering run took place in spring of 2015 using a 1.056 GeV, 50 nA beam. This dissertation will present the results of a resonance search for a heavy photon in the mass range between 20 MeV/c^2 to 60 MeV/c^2 using a portion of the unblinded engineering run data which amounts to a luminosity of 74 nb^-1 (.4671 mC of charge).

The next generation of particle detector systems will require sensors that can simultaneously record position and time to great precision. The low gain avalanche detector (LGAD) is a thin n-on-p silicon sensor implementing internal gain which results in a temporal resolution on the order of 10s of picoseconds. However, due to gain layer segmentation, position resolution in LGADs is limited to the millimeter scale. AC-coupled LGADs (AC-LGAD) improve on the position resolution of traditional LGADs through the implementation of a thin dielectric AC-coupling layer between the ?$ implant and readout electrodes, allowing for the use of continuous, planar, ?$ and gain layers. This leads to intrinsic charge sharing between readout electrodes, resulting in a spatial resolution on the order of 10μm. In this thesis, the spatial and temporal resolutions of several AC-LGAD wafers, differing in either ?$ concentration or dielectric thickness, were characterized using the Transient Current Technique with an IR laser set to replicate a MIP. Position resolutions were found to range from 6.9-8.6μm depending on the wafer studied and the position between pads. A straightforward trend relating the wafer fabrication parameters to position resolution was not possible. Timing jitters were found to range from 8.5-13.9ps depending on wafer and position. Overall, it was shown that increasing ?$ concentration (decreasing sheet resistance) and increasing dielectric thickness (decreasing capacitance) both lead to lower timing jitter.

A method of generating energy loss distributions for particles of unit charge with a specific incident momentum through a thickness of silicon is developed. Monte Carlo methods are used to examine the significance of separation between kaon and pion energy loss distributions of the same incident momentum. Several detector configurations, including two homogeneous detectors each composed of 10 layers of 20 micron and 320 micron silicon, and a heterogeneous detector composed of 5 layers of 20 micron silicon followed by 5 layers of 320 micron silicon are simulated. The heterogeneous detector is inspired by the baseline concept of the innermost tracking system at the SiD. For the heterogeneous configuration, the maximum momentum range for a significance of separation of two (S=2) is found to be 700 MeV/c, while the maximum range for S=1 is found to be 940 MeV/c for the heterogeneous configuration. The effect of electronic noise on the kaon pion separation is briefly explored, and found to be small over most of the range of incident momentum for which there is a significant separation between kaons and pions.

Supersymmetry models in which the lightest particle is pure-wino or pure-higgsino have chargino next-to-lightest-particles (NLSP) nearly mass-degenerate with the lightest neutralino, resulting in a lifetime long enough to produce hits in the Pixel Detector but not long enough to produce a full track; this is called the ``disappearing track signature". Electrons and muons which traverse the Pixel Detector and Coulomb scatter at a steep angle and therefore fail to be reconstructed as electrons or muons are a significant background to this signature; this is the ``lepton scatter background". This lepton scatter background was characterized using $36 \, \mathrm{fb}^{-1}$ of integrated luminosity of proton-proton collisions at $\sqrt{s} = 13 \, \mathrm{TeV}$ center-of-mass energy in the ATLAS Detector. Electrons were found to dominate at lower transverse momentum whereas at above about $100 \, \mathrm{GeV}$ the contributions are similar. Comparison with a representative pure-wino Monte Carlo model suggests a minimum threshold of $p_{T} > 100 \, \mathrm{GeV}$ should be used in the full analysis.

This thesis presents a measurement of collinear ?-boson radiation from high-momentum jets using the full Run-2 dataset collected by the ATLAS collaboration at √? = 13 TeV, with an integrated luminosity of 140 fb−1 . The W+ jets signal, identified through leptonic decays in the electron or muon channels, focuses on events with small angular separation between the jet and lepton, often involving numerous QCD emissions. Accurate modeling of this phase-space requires sophisticated multi-jet merged setups. Over 86,000 ?-boson candidates were measured across various differential distributions with a precision of10-20%, and the data was compared against newly developed state-of-the-art generator predictions using Sherpa 2.2.11 and MadGraph5_aMC@NLO with FxFx multi-jet merged setups, which are accurate to next-to-leading order in the strong and weak coupling constants.

This thesis presents progress made on an experimental study of radiative B⁰

meson decays to the final state &rho⁰ γ using data collected at the &UPSILON(4S) resonance at

the BABAR detector at the PEP-II e⁺e^- collider at SLAC. Making use of the full

data set collected at the BABAR detector of 465 million BB pairs we have done a

preliminary measurement of the time-dependent CP -violation parameters S and

C in B⁰ &rarr &rho⁰ &gamma decays, finding S = 0.38 ± 0.54 ± 0.38 and C = 0.79 ± 0.51 ± 0.37, where the uncertainties are statistical and systematic, respectively.

As accelerator breakthroughs progress, Accelerator Physicists are greatly concerned with managing the so-called emittance of a particle beams. Emittance is defined as the momentum and position phase space area of a particle beam and is associated with the ability to focus and concentrate a beam. In general, lower emittance means higher beam quality. We care about emittance as it dictates many other key parameters that predict the success of an accelerator project. Collider experiments desire low emittance for higher luminosity, yielding more collisions to record. For free electron lasers (FEL), emittance directly determines how efficiently the electron beam microbunches, as microbunching is required to start the exponential growth of x-ray production in undulator magnets.

As FELs become more popular and scientifically in demand, the FEL user community is pushing for higher x-ray energies and higher repetition rates to enable new science. This presents a huge challenge for the accelerator community as the high repetition rate requirement coupled with the need for an ultra-low emittance beam is currently limiting the range of FELs. This dissertation explores this bottleneck for high repetition rate FEL light sources with the focus on a notable case study: SLAC's Linac Coherent Light Source II (LCLS-II). Simulations predict that decreasing the emittance at the undulators from the current estimate of 0.4 mm mrad to 0.1 mm mrad would expand SLACs LCLS-II x-ray energy upper bound from 15 keV to 22 keV. The benefit from lower emittance is even more pronounced for further linac energy upgrades.

With the LCLS-II emittance goals in mind, in this dissertation, I investigate emittance improvements for an electron injector system for FEL-like applications that would satisfy a high repetition rate requirement. I focused on two areas of potential improvement: using a Superconducting RF (SRF) gun cavity with a higher RF gradient on the cathode, or a higher quality cathode that produces a lower initial electron momentum distribution. I started with a detailed simulation study that decouples gun cavity improvements from cathode improvements. To do this, I utilized a genetic algorithm to optimize various injector lattices at different cathode qualities. I demonstrate that to realistically meet the demands of the FEL community, we will need both the higher gun gradients of SRF guns as well as improvements from the cathode.

I next present work on developing the solenoid component for a SRF gun developed at KEK in Japan to advance SRF gun cavity technology. I lay out the steps taken to design a superconducting solenoid that accommodates the unique requirement of a SRF gun cavity to have negligible magnetic field on the gun cavity wall material. The solenoid was tested and the operational results were compared to the expected performance of the solenoid design field with an acceptable agreement between the fields found. The solenoid will be used in a SRF gun test stand to add to experimental research for SRF gun cavities that can accommodate a higher RF gradient on the cathode.