Top Jets at the LHC Page: 3 of 48
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The Large Hadron Collider (LHC) is expected to uncover some of the most interesting
mysteries of nature. We expect to probe the underlying principles of electroweak symmetry
breaking (EWSB) and what stabilizes the weak scale against radiative corrections from
unknown microscopic dynamics. Due to its large mass, the top quark induces the most
severe contributions to the Higgs quadratic divergence. Furthermore, in almost every known
natural model of EWSB, the top sector plays a crucial role in breaking the EW symmetry.
Thus, the top sector might hold a key to a new physics (NP) discovery. Many interesting
models of EWSB predict new particles with mass - TeV scale. In several known examples,
the new particles decay into highly boosted top quark pairs (pp - X -- t), or other decay
chains containing a single top quark (pp - X -- t Y). In addition, the Standard Model
(SM) predicts that the LHC will produce more than 105 top quarks with PT > 1 TeV,
significantly enhancing our ability to study high PT tops and resolving beyond the SM
Top quarks decay dominantly into hadronic final states (t -- bW -- bqq) with a
branching ratio - 2/3, providing potentially enhanced statistics. In the present work,
we focus on highly boosted top quarks (decaying through the hadronic channel), and on
the dominant QCD jet background. We refer to a top quark that decays hadronically as
a hadronic top. For moderately boosted top quarks (PT - 500 GeV), conventional top
quark reconstruction methods, which exploit the decay chain topology, remain adequately
efficient (see e.g. ). As the top quark PT approaches 1 TeV, the situation significantly
changes [2, 3, 4, 5]. The average separation of the top quark decay products approaches
the limits of reliable jet reconstruction (cone size R - 0.4), and starts to encroach upon
the detector resolution (R - 0.1). As a result, the efficiency of conventional reconstruction
methods drops quickly. The performance of b-tagging and light jet rejection is expected
to drop substantially in this kinematic regime. At present, there is very little published
data on b-tagging at PT - 1 TeV . We perform our analysis without accounting for the
possible benefits of b-jet identification.
We turn our focus away from this family of "conventional" reconstruction methods. We
examine the situation where the decay products of at least one top quark are reconstructed
as a single jet, or top-jet. In semileptonic t events, for example, the leptonic top may still be
reconstructed via semi-conventional reconstruction methods, giving up on lepton isolation
cuts , see also [4, 8]. These methods call for further extensive study due to expected
reducible backgrounds and uncertainties related to the ability to measure the collimated
semi-leptonic top mass (dileptonic tt events are also analyzed in ). Hadronic top, on the
other hand, will give rise to a top-jet. There will still be some small, but non-negligible,
number of tt events where one of the top quarks reconstructs as a top-jet, but the other top
SFor earlier works in the case of boosted EW bosons see also .
1The possibility of b-tagging jets, when the top quark reconstructs to 2 (or more) jets, one of which has
a mass ~ Mw and the sum of the two jets has a mass ~ mt, is outside the main focus of this paper.
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Almeida, L.G.; Lee, S.J.; Perez, G.; Sung, I. & Virzi, J. Top Jets at the LHC, article, October 6, 2008; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc900892/m1/3/: accessed December 13, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.