Looking for Unusual Signatures of New Physics at Current and Future Colliders

Xu, Dong

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https://hdl.handle.net/2142/113965 Copy

Description

Title

Looking for Unusual Signatures of New Physics at Current and Future Colliders

Author(s)

Xu, Dong

Issue Date

2021-11-19

Director of Research (if dissertation) or Advisor (if thesis)

Shelton, Julia

Doctoral Committee Chair(s)

Yang, Liang

Committee Member(s)

• El-Khadra, Aida
• Hooberman, Benjamin

Department of Study

Physics

Discipline

Physics

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

Displaced vertex, LHC, dark shower, Hidden Valleys, exotic Higgs decay, future lepton colliders

Abstract

In this thesis, we explore new searches for beyond the Standard Model physics in three different scenarios: long-lived Higgsinos at the LHC, hidden valleys at the LHC, and a Higgs exotic decay channel at future electron-positron colliders. In Chapter 1, we briefly introduce the background knowledge of the three different scenarios. In Chapter 2, we introduce the Higgsino multiplets and their R-parity violating decay and discuss the parameter space (nearly pure-Higgsino scenario) that we will investigate. We recast ATLAS searches for one or two displaced vertices in the muon spectrometer to obtain results for long-lived Higgsinos. The resulting sensitivity limits are very good at long proper lifetime ranges compared with that of the other currently existing searches. In Chapter 3, we introduce the “dark shower” topology which is the signature of a high multiplicity of non-isolated dark particles. Then we discuss each of the three components (production, showering and hadronization, and decay) of a typical dark shower topology taking some theory priors into consideration. With these theory priors, we present a set of five hidden valley benchmark models that span a variety of experimental signatures. We believe these five benchmark models constitute a good basis for systematic experimental searches for the hidden valley model space. In Chapter 4, with the benchmark models in Chapter 3 as a guide, we identify and discuss important phenomenological features of hidden valley events at the LHC. Then we discuss a set of various triggers at length. The performances of these triggers are examined on 16 test models. Because the selection criteria of these triggers well represent the typical features of the dark shower events, these triggers should be effective and inclusive strategies for recording dark shower events at the LHC. In Chapter 5, we study the prospects of discovering the Higgs exotic decay channel h → ss → 4τ at future electron-positron colliders in the case where the particle s is a beyond-the-Standard-Model scalar. We show that the 95% C.L. upper limits on the Higgs exotic decay branching fractions of the channel h → ss → 4τ with various s masses between 5 GeV and 10 GeV are achieved at O(10^−4).

Graduation Semester

2021-12

Type of Resource

Thesis

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http://hdl.handle.net/2142/113965

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Copyright 2021 Dong Xu

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