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3 Jet Substructure at the LHC
the particle origin of jets are a much more recent development, though. While the
detectors have not been specifically designed for measurements of jet substructure,
ATLAS and CMS have the specifications necessary to efficiently identify the origin
of jets using substructure observables. In what follows, the term ‘tagger’ indicates
the use of one or more of these observables, sometimes together with a grooming
algorithm, for jet identification. The performance of taggers is classified in terms
of their efficiencies and misidentification rates. Experimental use of jet substructure
taggers hinges upon detailed measurements of these numbers and the verification of
a similar performance in data as in simulation. In addition, care has to be taken to
exactly define how these numbers are evaluated, as differences in the definition of
efficiencies and misidentification rates can lead to very different conclusions. Systematic uncertainties play an important role in these studies, as performance gains
obtained by the use of more advanced taggers can be negated by larger uncertainties
relative to a less complex approach.
3.5.1 Quark/Gluon Discrimination
Experiments at the LEP, Tevatron and HERA colliders have already used jet substructure to distinguish quark-initiated (quark) versus gluon-initiated (gluon) jets.
In these early studies, the dynamics of quark and gluon scattering were probed. At
the LHC, quark versus gluon jet (q/g) tagging also serves the purpose of separating
signal from background, where often signal jets are quark jets (for example in production processes through vector-boson scattering) and gluon jets are background.
The probability for a g → gg splitting is enhanced by a factor of C A /C F = 9/4 over
the probability q → qg at the same opening angle and energy fraction [460]. As a
result, gluon jets tend to have more constituents and a broader radiation pattern than
quark jets, which can be used to discriminate between them. There are also more
subtle differences due to quark and gluon electric charges and spins, which can be
exploited when constructing substructure variables for q/g tagging.
A complication arising when studying q/g tagging is the ambiguous definition of
a quark and gluon jet. Quarks and gluons carry colour charges, but jets reconstructed
in the detector result from sprays of colour-singlet particles such that these can
not be connected unambiguously. A number of definitions have been suggested,
ranging from matching jets to outgoing partons at the level of the hard scattering,
to parsing the entire parton shower and jet clustering history [461, 462], to using
operational definitions at the level of observable distributions [463, 464]. In practice,
differences in these definitions have a small impact on the experimental performance
of q/g taggers as long as used consistently throughout an analysis. However, different
choices may render results from experimental studies incommensurable. Note also
that quark and gluon jet radiation depends on the production mechanism [465], such
that the calibration and application of q/g taggers must be treated with additional
care compared to other taggers.
3 Jet Substructure at the LHC
the particle origin of jets are a much more recent development, though. While the
detectors have not been specifically designed for measurements of jet substructure,
ATLAS and CMS have the specifications necessary to efficiently identify the origin
of jets using substructure observables. In what follows, the term ‘tagger’ indicates
the use of one or more of these observables, sometimes together with a grooming
algorithm, for jet identification. The performance of taggers is classified in terms
of their efficiencies and misidentification rates. Experimental use of jet substructure
taggers hinges upon detailed measurements of these numbers and the verification of
a similar performance in data as in simulation. In addition, care has to be taken to
exactly define how these numbers are evaluated, as differences in the definition of
efficiencies and misidentification rates can lead to very different conclusions. Systematic uncertainties play an important role in these studies, as performance gains
obtained by the use of more advanced taggers can be negated by larger uncertainties
relative to a less complex approach.
3.5.1 Quark/Gluon Discrimination
Experiments at the LEP, Tevatron and HERA colliders have already used jet substructure to distinguish quark-initiated (quark) versus gluon-initiated (gluon) jets.
In these early studies, the dynamics of quark and gluon scattering were probed. At
the LHC, quark versus gluon jet (q/g) tagging also serves the purpose of separating
signal from background, where often signal jets are quark jets (for example in production processes through vector-boson scattering) and gluon jets are background.
The probability for a g → gg splitting is enhanced by a factor of C A /C F = 9/4 over
the probability q → qg at the same opening angle and energy fraction [460]. As a
result, gluon jets tend to have more constituents and a broader radiation pattern than
quark jets, which can be used to discriminate between them. There are also more
subtle differences due to quark and gluon electric charges and spins, which can be
exploited when constructing substructure variables for q/g tagging.
A complication arising when studying q/g tagging is the ambiguous definition of
a quark and gluon jet. Quarks and gluons carry colour charges, but jets reconstructed
in the detector result from sprays of colour-singlet particles such that these can
not be connected unambiguously. A number of definitions have been suggested,
ranging from matching jets to outgoing partons at the level of the hard scattering,
to parsing the entire parton shower and jet clustering history [461, 462], to using
operational definitions at the level of observable distributions [463, 464]. In practice,
differences in these definitions have a small impact on the experimental performance
of q/g taggers as long as used consistently throughout an analysis. However, different
choices may render results from experimental studies incommensurable. Note also
that quark and gluon jet radiation depends on the production mechanism [465], such
that the calibration and application of q/g taggers must be treated with additional
care compared to other taggers.