Since the discovery of the Higgs boson in 2012, the search for new physics beyond
the Standard Model has been greatly intensified. At the CERN Large Hadron Collider
(LHC), ATLAS searches for new physics entail looking for new particles by colliding
protons together. Presented here is a search for a new form of quark matter called
Vector-like Quarks (VLQ), which are hypothetical particles that are expected to have
mass around a few TeV. VLQ can come in a variety of forms and can couple to
their Standard Model (SM) quark counterparts, particularly to the third generation.
They are necessary in several beyond the SM theories in order to solve the hierarchy
problem. This search uses 36.1 fb−1of proton-proton collision data collected with the
ATLAS detector at the LHC from August 2015 to October 2016. Only events with two
leptons of the same charge, or three leptons, plus b-jets and high missing transverse
energy are considered in the main analysis. This signature is rarely produced in the
SM, which means the backgrounds in this analysis are relatively low. This analysis
is sensitive to specific predicted decay modes from pair production of an up-type
VLQ with a charge of +2/3, T, an up-type VLQ with a charge of +5/3, T5/3, and
a down-type quark with a charge of −1/3, B, as well as single production of T5/3.
There is another theorized VLQ that this analysis is not sensitive to: B−4/3, due
to its primary decay mode, which is unable to produce the final-state signature of
interest. The results from this analysis suggest only a slight deviation of data from
SM backgrounds reaching as high as 1.89σ, which does not indicate evidence for
VLQ. A mostly frequentist statistical technique, called the CLS Method, is used to
interpret the data and set limits on the T, B, and T5/3 signal models. Using this
method, exclusion limits are set at the 95% confidence level, effectively excluding T
mass below 0.98 TeV, T5/3 mass below 1.2 TeV, and B mass below 1.0 TeV, assuming
singlet branching ratios. Also, branching ratio independent limits are set on the T
and B VLQ.
| Identifer | oai:union.ndltd.org:arizona.edu/oai:arizona.openrepository.com:10150/626353 |
| Date | January 2017 |
| Creators | Jones, Sarah, Jones, Sarah |
| Contributors | Johns, Kenneth, Johns, Kenneth, Varnes, Erich, Cheu, Elliott, Sarcevic, Ina, Toussaint, Doug |
| Publisher | The University of Arizona. |
| Source Sets | University of Arizona |
| Language | en_US |
| Detected Language | English |
| Type | text, Electronic Dissertation |
| Rights | Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. |
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