84
3 Jet Substructure at the LHC
to make this channel complementary to H → bb. In addition to H analyses, tagging
W W → 4q jets may also become important in searches for new physics, for example
in boosted diboson resonances [519].
3.5.4 Top Tagging 9
The three-prong decays of highly boosted top quarks in the fully hadronic decay channel offer richer phenomenology for their identification than the two-prong decays
of W , Z and H bosons. This has been exploited in a number of algorithms, which
usually aim at an optimal performance
10 in a particular kinematic regime. Flavour
tagging also plays a key role for top tagging, which offers its own challenges because
the jet from the b quark may not be isolated from the radiation resulting from the
associated W boson decay. Due to the heavier mass of the top quark compared with
the electroweak bosons, top tagging must also operate in a moderate boost regime
where the decay products may not all be contained inside a single jet with R 1.0.
The techniques for tagging boosted top quarks have evolved as fairly complex
methods in comparison to the V and H taggers. Some commonly used techniques
include the CMSTT and HTT. While the CMSTT is only studied in CMS [240, 241,
243], the HTT is studied by ATLAS and CMS on 7 and 8 TeV data [243, 433, 520].
Efficiencies of the HTT of 10% with misidentification rates of 0.5% for jets with
200 < p T < 250 GeV are observed. The efficiency increases with increasing jet p T ,
where a plateau is reached for p T > 400 GeV, with efficiencies of approximately 40 at
3% misidentification rate, very similar to the performance achieved with the CMSTT.
One of the best performing taggers for top quark tagging is shower deconstruction,
with efficiencies from 30 to 45% at misidentification rates from 1 to 4% for jets
from p T = 350 to 700 GeV [520]. Recently, the shower deconstruction algorithm
was optimised for top quarks with p T > 800 GeV in context of the W
to tb hadronic
search [521] by using exclusive k T subjets.
In addition to the dedicated techniques described above, simpler algorithms using
grooming and substructure similar to V tagging methods have been investigated by
ATLAS. A performance study at 7 TeV [433] investigated a variety of performance
metrics relating to the usage of groomed jets. Different grooming algorithms were
investigated for their resilience to pileup and mass resolution. It was concluded
that trimmed anti-k T jets with R = 1.0 and trimming parameters of R sub = 0.3 and
f cut = 0.05 were the best option. This jet definition became standard in ATLAS
for W , Z , H and top quark tagging in 7 and 8 TeV analyses. The ATLAS choice
of R = 1.0 jets compared to CMS with R = 0.8 jets results in an earlier rise of
the tagging efficiency with increasing jet p T . A later ATLAS study [520] inves9 The text in this subsection has been taken from [26] and has been written by the author. It has been
adjusted to fit this book.
10 The text in this subsection has been taken from [26] and has been written by the author. It has
been adjusted to fit this book.
3 Jet Substructure at the LHC
to make this channel complementary to H → bb. In addition to H analyses, tagging
W W → 4q jets may also become important in searches for new physics, for example
in boosted diboson resonances [519].
3.5.4 Top Tagging 9
The three-prong decays of highly boosted top quarks in the fully hadronic decay channel offer richer phenomenology for their identification than the two-prong decays
of W , Z and H bosons. This has been exploited in a number of algorithms, which
usually aim at an optimal performance
10 in a particular kinematic regime. Flavour
tagging also plays a key role for top tagging, which offers its own challenges because
the jet from the b quark may not be isolated from the radiation resulting from the
associated W boson decay. Due to the heavier mass of the top quark compared with
the electroweak bosons, top tagging must also operate in a moderate boost regime
where the decay products may not all be contained inside a single jet with R 1.0.
The techniques for tagging boosted top quarks have evolved as fairly complex
methods in comparison to the V and H taggers. Some commonly used techniques
include the CMSTT and HTT. While the CMSTT is only studied in CMS [240, 241,
243], the HTT is studied by ATLAS and CMS on 7 and 8 TeV data [243, 433, 520].
Efficiencies of the HTT of 10% with misidentification rates of 0.5% for jets with
200 < p T < 250 GeV are observed. The efficiency increases with increasing jet p T ,
where a plateau is reached for p T > 400 GeV, with efficiencies of approximately 40 at
3% misidentification rate, very similar to the performance achieved with the CMSTT.
One of the best performing taggers for top quark tagging is shower deconstruction,
with efficiencies from 30 to 45% at misidentification rates from 1 to 4% for jets
from p T = 350 to 700 GeV [520]. Recently, the shower deconstruction algorithm
was optimised for top quarks with p T > 800 GeV in context of the W
to tb hadronic
search [521] by using exclusive k T subjets.
In addition to the dedicated techniques described above, simpler algorithms using
grooming and substructure similar to V tagging methods have been investigated by
ATLAS. A performance study at 7 TeV [433] investigated a variety of performance
metrics relating to the usage of groomed jets. Different grooming algorithms were
investigated for their resilience to pileup and mass resolution. It was concluded
that trimmed anti-k T jets with R = 1.0 and trimming parameters of R sub = 0.3 and
f cut = 0.05 were the best option. This jet definition became standard in ATLAS
for W , Z , H and top quark tagging in 7 and 8 TeV analyses. The ATLAS choice
of R = 1.0 jets compared to CMS with R = 0.8 jets results in an earlier rise of
the tagging efficiency with increasing jet p T . A later ATLAS study [520] inves9 The text in this subsection has been taken from [26] and has been written by the author. It has been
adjusted to fit this book.
10 The text in this subsection has been taken from [26] and has been written by the author. It has
been adjusted to fit this book.