Vertexing Algorithms with the ATLAS Experiment (SULI) Strong Cosmic Censorship (Part III Essay) Invisible Higgs decays (BLUR) Massive electrodynamics (honors thesis)

VBF Higgs→invisible with the ATLAS Experiment

Berkeley Lab Undergraduate Research (BLUR) program, Lawrence Berkeley National Laboratory (January 2018–April 2018).

Mentor: Maurice Garcia-Sciveres, LBNL Physics Division and ATLAS Collaboration.
Associate mentor: Ben Nachman, ATLAS Collaboration.

A possible decay of the Higgs boson (\(H\), dashed line) into dark matter particles (\(\chi^0\), dotted lines). In this diagram, the Higgs is produced by the "vector boson fusion" (VBF) process, where two vector bosons (\(W^\pm/Z\), wavy lines) combine to form a Higgs.

Introduction

The Higgs boson is the keystone of the Standard Model of particle physics. Its long-awaited discovery in 2012 experimentally confirmed the mechanism by which a broad class of elementary particles gain their masses. However, the Higgs also remains the only new fundamental particle discovered at the Large Hadron Collider (LHC) to date. In light of this, physicists are examining the properties of the Higgs boson more closely to look for new physics. One way to do this is by using accelerators like the LHC to search for exotic decays of the Higgs into unknown particles.

Motivation

When a Higgs boson is produced at the LHC, sometimes it may decay into particles which pass through but do not actually deposit their energy in the detector. Because they do not leave tracks in the detector, these particles are practically invisible to us. However, we can still indirectly study these invisible decays by looking at the other products of the proton collisions, which provide clues about the missing particles.

Invisible decays are very rare in the Standard Model, so observing such decays would be a clear indication of new physics beyond the Standard Model. In principle, the Higgs boson can decay into dark matter particles, such as "weakly interacting massive particles," or WIMPs. Studying invisible Higgs decays may therefore give us new insights into the fundamental nature of dark matter.

Tasks and Skills

During this internship, I used the data analysis framework ROOT to analyze the vector boson fusion (VBF) Higgs production mechanism for ATLAS in the context of invisible Higgs decays. I determined the signal-to-noise ratio between these invisible decays and other background events. Using this analysis, I made a projection for the expected sensitivity to these invisible decays once data collection from the High-Luminosity LHC (HL-LHC) upgrade is completed.

I presented progress updates on my research at weekly group meetings at Berkeley Lab, as well as giving short (15–20 minute) presentations to various upgrade physics groups via teleconference. I also prepared a research poster for presentation at the spring intern poster session run by Workforce Development & Education at Berkeley Lab.

Results and Broader Impacts

The preliminary work from this project suggests that the final dataset may be sensitive to invisible decays with branching ratio (relative frequency) as small as 1%, a great improvement over the previous limit of ~28% reported in the completed ATLAS Run 1 and Run 2 searches. Projections for the HL-LHC are particularly interesting because the full dataset will be thirty to forty times larger than the current dataset, and the extended detectors planned in the upgrade may benefi t our analysis by allowing us to collect more forward tracks from close to the beamline. Because of the high signal-to-noise ratio of this process, we expect VBF Higgs→invisible to be a powerful tool for probing the dark sector of physics beyond the Standard Model.

Documentation