Publication Date

2021

Document Type

Dissertation/Thesis

First Advisor

Martin, Stephen P.

Degree Name

Ph.D. (Doctor of Philosophy)

Legacy Department

Department of Physics

Abstract

Although the Standard Model (SM) is now complete with the spectacular discovery of the Higgs boson at the Large Hadron Collider (LHC), there are many unsolved puzzles such as the electroweak hierarchy problem, dark matter, neutrino masses, etc., that need to be addressed. Theories proposed to solve these issues often involve new and exotic particles at the TeV scale and beyond. It is therefore of great importance to devise novel strategies to extract new physics signals with small rates lurking in huge backgrounds. My research detailed here is broadly along those lines.

Vectorlike leptons (VLL) are an intriguing possibility for physics beyond the SM. In my first project, we study the reach for discovering (at 5$\sigma$ significance) or excluding (at 95\% confidence limit) models of charged VLL that mix predominantly with the tau, using multi-lepton signatures at various future proton-proton collider options: a high-luminosity LHC with $\sqrt{s} =$ 14 TeV, a high-energy LHC with $\sqrt{s} =$ 27 TeV, and possible new longer-tunnel colliders with $\sqrt{s} =$ 70 or 100 TeV. For weak isodoublet VLL, we estimate that a 27 TeV high-energy LHC with 15 ab$^{-1}$ could exclude masses up to about 2300 GeV, or discover them if the mass is less than about 1700 GeV, while a 100 TeV collider with 30 ab$^{-1}$ could exclude masses up to about 5750 GeV, or make a discovery if the mass is less than about 4000 GeV. However, we find that weak isosinglet VLL present a much more difficult challenge, with some reach for exclusion, but not for discovery at any of the collider options considered. The electronic input files relevant for VLL models have been used by both CMS and ATLAS collaborations in their LHC searches, and are also available for anyone upon request.

In my second project, we study the interference between the amplitudes for $gg \rightarrow X \rightarrow gg$, where $X$ is a new heavy digluon resonance, and the QCD background $gg \rightarrow gg$, at the LHC. The interference produces a large low-mass tail and a deficit of events above the resonance mass, compared to the naive pure resonance peak. For a variety of different resonance quantum numbers and masses, we evaluate the signal-background interference contribution at leading order, including showering, hadronization, and detector effects. The resulting new physics dijet mass distribution may have a shape that appears, after QCD background fitting and subtraction, to resemble an enhanced peak, a shelf, a peak/dip, or even a pure dip. We argue that the true limits on new digluon resonances are likely to differ significantly from the limits obtained when interference is neglected, especially if the branching ratio to $gg$ is less than 1.

The projected discovery and exclusion capabilities of particle physics and astrophysics/cosmology experiments are often quantified using the median expected $p$-value or its corresponding significance. Finally, in my third project, we argue that this criterion leads to flawed results, which for example can counterintuitively project lessened sensitivities if the experiment takes more data or reduces its background. We discuss the merits of several alternatives to the median expected significance, both when the background is known and when it is subject to some uncertainty. We advocate for standard use of the ``exact Asimov significance" $Z^{\rm A}$ detailed in this work.

Extent

174 pages

Language

eng

Publisher

Northern Illinois University

Rights Statement

In Copyright

Rights Statement 2

NIU theses are protected by copyright. They may be viewed from Huskie Commons for any purpose, but reproduction or distribution in any format is prohibited without the written permission of the authors.

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