2.1 Introduction to Jet Substructure
9
exclusive jet clustering using the particles inside a jet as input, or the maximisation
of the projection of the jet constituents’ momenta onto the desired number of axes,
as illustrated in Fig. 2.2.
Since the opening angle between the quarks depends on the momentum of the
parent particle and its mass, larger jets (R ∼ 1) than normally employed in LHC
analyses (R ∼ 0.4) are used to reconstruct boosted heavy particle decays. A larger
distance parameter is chosen to capture the full kinematics of the decay already
at moderate momenta of 200–400 GeV. The drawback of jets with large areas is
unwanted contributions from the underlying event and from multiple proton-proton
collisions in the same or adjacent bunch crossings (pileup). These lead to a worsening
of the resolution in quantities used to identify the substructure of jets, like the jet
mass. Jet grooming and pileup removal algorithms have been developed to mitigate
these effects. Grooming algorithms aim at removing soft and wide-angle radiation,
therefore not only reducing the effects from the underlying event but also reducing the
sensitivity to the details of fragmentation. Pileup removal algorithms are designed to
identify and subtract contributions from a different interaction vertex, by eliminating
uncorrelated radiation from jets. A combination of these techniques often leads to
the best overall performance and it is an ongoing effort to understand the interplay
of pileup removal, grooming and tagging algorithms.
The theoretical and algorithmic developments have been made possible thanks to
advances in experimental methods. New technologies, like silicon pixel detectors,
high-resolution tracking detectors in conjunction with strong magnetic fields, highly
granular calorimeters with low electronic noise and lightweight materials for detector structures with little dead material inside the active detector volume have enabled
increasingly precise jet measurements and studies of internal jet structure. Modern
particle detectors at the LHC are equipped with many layers of high-resolution tracking detectors, strong and very homogeneous magnetic fields and finely segmented
calorimeters with an excellent energy resolution. With these technologies, the ATLAS
and CMS detectors
2 are equipped to track and reconstruct individual particles produced in high energy collisions. On average about 60% of a jet’s momentum is
carried by charged hadrons, photons account for about 25% of the total jet momentum and the remaining 15% can be attributed to long-lived neutral hadrons [70].
With increasing jet energy, the particle multiplicity increases, and also the fraction
of the jet’s momentum carried by soft particles. For example, on average 50% of the
momentum of a 50 GeV jet is carried by particles with a momentum less than 5%
of the jet’s momentum. It is therefore crucial to ensure that particles with energies
down to O(100 MeV) can be reconstructed in order to retain the full information on
a jet’s kinematics and internal structure.
As important as the reconstruction of the total jet energy is the measurement of the
jet constituent multiplicity and their angular distributions. While charged particles
2 The ALICE and LHCb detectors are also well-equipped to perform jet substructure studies. While
these experiments do not have access to boosted massive particles due to their data rate (ALICE)
or acceptance (LHCb), they are performing many interesting QCD studies with jet substructure.
This review will be focused on ATLAS and CMS, but the future of jet substructure will involve key
contributions from all four LHC experiments.
9
exclusive jet clustering using the particles inside a jet as input, or the maximisation
of the projection of the jet constituents’ momenta onto the desired number of axes,
as illustrated in Fig. 2.2.
Since the opening angle between the quarks depends on the momentum of the
parent particle and its mass, larger jets (R ∼ 1) than normally employed in LHC
analyses (R ∼ 0.4) are used to reconstruct boosted heavy particle decays. A larger
distance parameter is chosen to capture the full kinematics of the decay already
at moderate momenta of 200–400 GeV. The drawback of jets with large areas is
unwanted contributions from the underlying event and from multiple proton-proton
collisions in the same or adjacent bunch crossings (pileup). These lead to a worsening
of the resolution in quantities used to identify the substructure of jets, like the jet
mass. Jet grooming and pileup removal algorithms have been developed to mitigate
these effects. Grooming algorithms aim at removing soft and wide-angle radiation,
therefore not only reducing the effects from the underlying event but also reducing the
sensitivity to the details of fragmentation. Pileup removal algorithms are designed to
identify and subtract contributions from a different interaction vertex, by eliminating
uncorrelated radiation from jets. A combination of these techniques often leads to
the best overall performance and it is an ongoing effort to understand the interplay
of pileup removal, grooming and tagging algorithms.
The theoretical and algorithmic developments have been made possible thanks to
advances in experimental methods. New technologies, like silicon pixel detectors,
high-resolution tracking detectors in conjunction with strong magnetic fields, highly
granular calorimeters with low electronic noise and lightweight materials for detector structures with little dead material inside the active detector volume have enabled
increasingly precise jet measurements and studies of internal jet structure. Modern
particle detectors at the LHC are equipped with many layers of high-resolution tracking detectors, strong and very homogeneous magnetic fields and finely segmented
calorimeters with an excellent energy resolution. With these technologies, the ATLAS
and CMS detectors
2 are equipped to track and reconstruct individual particles produced in high energy collisions. On average about 60% of a jet’s momentum is
carried by charged hadrons, photons account for about 25% of the total jet momentum and the remaining 15% can be attributed to long-lived neutral hadrons [70].
With increasing jet energy, the particle multiplicity increases, and also the fraction
of the jet’s momentum carried by soft particles. For example, on average 50% of the
momentum of a 50 GeV jet is carried by particles with a momentum less than 5%
of the jet’s momentum. It is therefore crucial to ensure that particles with energies
down to O(100 MeV) can be reconstructed in order to retain the full information on
a jet’s kinematics and internal structure.
As important as the reconstruction of the total jet energy is the measurement of the
jet constituent multiplicity and their angular distributions. While charged particles
2 The ALICE and LHCb detectors are also well-equipped to perform jet substructure studies. While
these experiments do not have access to boosted massive particles due to their data rate (ALICE)
or acceptance (LHCb), they are performing many interesting QCD studies with jet substructure.
This review will be focused on ATLAS and CMS, but the future of jet substructure will involve key
contributions from all four LHC experiments.