60
2 Phenomenology of Jet Substructure
actions and hadronisation. A full tune can only be successful in describing all aspects
of collider data if all free parameters are constrained simultaneously. A parametrisation tool called Professor [399] has been developed for this purpose, facilitating
the fast calculation of the event generator response to parameter variations. Various tunes [291–295] are available for the three common event generators Pythia,
Herwig and Sherpa, and are updated regularly when new measurements become
available or major modifications are made to the programs. Only once all relevant
measurements are described satisfactorily, an event generator can be used to make
reliable predictions for unknown regions of phase space. However, even then caution
is advised and some understanding is needed of the approximations made in event
generators and their limitations for successful use. This is especially true when event
generators are used to predict backgrounds in analyses searching for new physics,
where the interesting regions have usually not been explored so far. One recent example comes from a study using jet substructure to separate quark- from gluon-initiated
jets [400]. This study observes large differences in the quark/gluon discrimination
power under variations of generator settings, with some trends in opposite directions
when comparing different generators. A possible conclusion is that, contrary to common belief, e
+ e
− data does not constrain all aspects of final state showers sufficiently.
Gluon radiation patterns are largely unconstrained and new global tuning strategies
including more LHC measurements may be needed in the future.
2 Phenomenology of Jet Substructure
actions and hadronisation. A full tune can only be successful in describing all aspects
of collider data if all free parameters are constrained simultaneously. A parametrisation tool called Professor [399] has been developed for this purpose, facilitating
the fast calculation of the event generator response to parameter variations. Various tunes [291–295] are available for the three common event generators Pythia,
Herwig and Sherpa, and are updated regularly when new measurements become
available or major modifications are made to the programs. Only once all relevant
measurements are described satisfactorily, an event generator can be used to make
reliable predictions for unknown regions of phase space. However, even then caution
is advised and some understanding is needed of the approximations made in event
generators and their limitations for successful use. This is especially true when event
generators are used to predict backgrounds in analyses searching for new physics,
where the interesting regions have usually not been explored so far. One recent example comes from a study using jet substructure to separate quark- from gluon-initiated
jets [400]. This study observes large differences in the quark/gluon discrimination
power under variations of generator settings, with some trends in opposite directions
when comparing different generators. A possible conclusion is that, contrary to common belief, e
+ e
− data does not constrain all aspects of final state showers sufficiently.
Gluon radiation patterns are largely unconstrained and new global tuning strategies
including more LHC measurements may be needed in the future.