46
2 Phenomenology of Jet Substructure
jet mass, which is visually confirmed by the structure of jets best representing the
set of jets in a given bin.
2.4.5 Pileup Mitigation
While jet grooming methods target the rejection of soft and wide-angle radiation and
particles from the underlying event, pileup mitigation techniques are constructed to
remove particles produced in additional proton-proton collisions in the same bunch
crossing as the hard interaction. These also contribute to the hard jets in an event
and can in principle be distinguished from particles of the leading primary interaction due to their different production vertices. While jet grooming methods also
mitigate the effects of pileup, dedicated algorithms have been developed for this
purpose. These rely either on the diffuse energy distribution introduced by pileup or
on the identification of a pileup vertex as the source of additional particles. Applying
pileup mitigation techniques results in an improved resolution of jet substructure
observables and a stable performance of taggers even in very dense environments.
A more detailed description of some specific pileup mitigation techniques is given
in Sect. 3.3, with a focus is on experimental effects.
2.5 (Semi-)Analytical Calculations
Accurate quantitative predictions of quantities related to jets represent a serious
theoretical challenge due to the complex structure of QCD, the theory of strong
interactions. Besides the usual complexities of higher order calculations in perturbative QCD, additional complications arise for jet observables due to the appearance of
multiple, disparate scales, reaching from the high collision energy through the electroweak scale down to hadron masses. This hierarchy of scales renders perturbative
expansions unreliable at any fixed order. In order to obtain reliable predictions, allorder reorganisations of the perturbative expansion are necessary. Individual terms
in this expansion can often be expressed as closed analytical expressions, but once
an all-order resummation is performed, numerical methods need to be employed.
Also non-perturbative effects, such as hadronisation and the underlying event, can
be estimated using analytic QCD models. While these estimations will never achieve
the ultimate theoretical precision, their usefulness lies in attaining an understanding
of the behaviour of physical observables in presence of non-perturbative effects.
2.5.1 Perturbative Effects
Different ways of obtaining all-order results for parton level predictions exist in the
literature. These can be separated in two classes, one known as direct QCD in the soft
2 Phenomenology of Jet Substructure
jet mass, which is visually confirmed by the structure of jets best representing the
set of jets in a given bin.
2.4.5 Pileup Mitigation
While jet grooming methods target the rejection of soft and wide-angle radiation and
particles from the underlying event, pileup mitigation techniques are constructed to
remove particles produced in additional proton-proton collisions in the same bunch
crossing as the hard interaction. These also contribute to the hard jets in an event
and can in principle be distinguished from particles of the leading primary interaction due to their different production vertices. While jet grooming methods also
mitigate the effects of pileup, dedicated algorithms have been developed for this
purpose. These rely either on the diffuse energy distribution introduced by pileup or
on the identification of a pileup vertex as the source of additional particles. Applying
pileup mitigation techniques results in an improved resolution of jet substructure
observables and a stable performance of taggers even in very dense environments.
A more detailed description of some specific pileup mitigation techniques is given
in Sect. 3.3, with a focus is on experimental effects.
2.5 (Semi-)Analytical Calculations
Accurate quantitative predictions of quantities related to jets represent a serious
theoretical challenge due to the complex structure of QCD, the theory of strong
interactions. Besides the usual complexities of higher order calculations in perturbative QCD, additional complications arise for jet observables due to the appearance of
multiple, disparate scales, reaching from the high collision energy through the electroweak scale down to hadron masses. This hierarchy of scales renders perturbative
expansions unreliable at any fixed order. In order to obtain reliable predictions, allorder reorganisations of the perturbative expansion are necessary. Individual terms
in this expansion can often be expressed as closed analytical expressions, but once
an all-order resummation is performed, numerical methods need to be employed.
Also non-perturbative effects, such as hadronisation and the underlying event, can
be estimated using analytic QCD models. While these estimations will never achieve
the ultimate theoretical precision, their usefulness lies in attaining an understanding
of the behaviour of physical observables in presence of non-perturbative effects.
2.5.1 Perturbative Effects
Different ways of obtaining all-order results for parton level predictions exist in the
literature. These can be separated in two classes, one known as direct QCD in the soft