2 Method
The purpose of this section is to chalk out a strategy that can identify whether a jet is
consistent with a boosted top quark where the top quark decays to an electron. Broadly
speaking, the central part of this identification is broken down into following steps.
1. The first stage of this procedure is to groom the jet using “soft drop” (SD)
method [49], which allows us to identify the last stage of clustering with hard split-
ting, and to remove soft radiation making it somewhat robust under underlying event
and pileup [50].
2. The distribution of energy of the groomed jet, as well as of its subjets, over different
parts of the calorimeters (namely, the electromagnetic portion or the ECal and the
hadronic portion or the HCal) and the signature left at the tracker allow us to deter-
mine whether the jet contains an energetic electron within itself or not. We use the
notation V
e
to denote the set of all variables, which we employ to determine whether
the jet under consideration is consistent with a jet containing a hard electron.
3. We devise a set of steps that starts with the constituents of the groomed jet and
determines the momentum four-vectors, which may correspond to the momenta of
the b quark and the electron from top quark decay.
4. The ansatz that the jet contains the remnants of a boosted top implies that if a
momentum four-vector (representing the invisible neutrino), massless and roughly
collimated to the electron, is added to the electron or the full jet, the resultant
should reconstruct the momentum of a W or a top particle respectively. As we show
later in this section, this ansatz allows us to determine the energy carried away by the
neutrino and some aspects of its direction inside the jet; we develop few measurable
variables using this ansatz, and refer this set of variables based on neutrino properties
by V
ν
.
5. We construct two boosted decision trees (BDT) [51], denoted by B
t/b
e
and B
t/b
ν
which
use the variables V
e
and V
ν
respectively as input. Both these BDTs are optimized to
separate a sample of jets containing the decay products of top quarks from a sample
of jets initiated by b quarks. After the training, any jet gets characterized by two
BDT responses. In other words, the jet gets mapped to a point in a plane of BDT
responses. We identify and thereby veto regions in this plane which are dense in b
quark initiated jets. As we show later, this procedure yields an efficient tagger for
boosted tops decaying to electrons.
In the rest of this section we give brief account of all the five steps mentioned before.
2.1 Grooming
In the first stage, we groom the jet under consideration. Typically, input jets result
from infrared- and collinear-safe clustering algorithms, such as k
T
[52], Cambridge-Aachen
– 4 –