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GenericSubtractor is part of the fastjet-contrib project.
The GenericSubtractor class allows one to subtract pile-up
contamination from jet shapes as described in
Pileup subtraction for jet shapes
Gregory Soyez, Gavin P. Salam, Jihun Kim, Souvik Dutta, Matteo Cacciari.
arXiv:1211.2811
Basic Usage
-----------
This code introduces a class named GenericSubtractor. The basic usage
of this class is as follows:
GenericSubtractor gensub(& _some_background_estimator);
FunctionOfPseudoJet<double> shape;
double subtracted_shape_value = gensub(shape, jet);
By default, this only subtracts the transverse momentum density rho
(obtained from the background estimator provided to the class). Two
options allow also for the inclusion of the "particle mass"
contribution to the background density (the "rho_m" term): one can
either instruct the GenericSubtractor class to compute rho_m from the
same background estimator using
gensub.set_common_bge_for_rho_and_rhom(); (1)
or explicitly construct gensub with two backgroundestimators
GenericSubtractor gensub(& _background_estimator_for_rho,
& _background_estimator_for_rhom); (2)
Note that since FastJet 3.1, the option (1) will work with any
background estimator that internally computes rho_m (and has that
computation enabled). For FastJet 3.0, the first option is only
available for JetMedianBackgroundEstimator.
For the option (2), 'gensub' will obtain rho_m using
background_estimator_for_rhom->rho() [NOT rho_m()!]
unless one calls gensub.set_use_bge_rhom_rhom() (it requires FJ>=3.1)
in which case it will be obtained using
background_estimator_for_rhom->rhom()
The right choice between these two procedures for option (2) depends
on the details of 'background_estimator_for_rhom'.
If the background transverse momentum density rho (and optionally
rhom) are known, the subtractor can be constructed directly as
GenericSubtractor gensub(rho,rhom);
Extra information can be retrieved using
GenericSubtractorInfo info;
double subtracted_shape_value = gensub(shape, jet, info);
[see the class definition in GenericSubtractor.hh for details]
See example.cc for an illustration of the usage of GenericSubtractor.
Advanced Usage
--------------
In some cases, it is possible to use clever tricks to improve the
subtraction or its efficiency.
- The first case is the one where the calculation of a shape first
partitions the jet into subjets before proceeding to an actual
calculation of the shape. To avoid recomputing that partition
everytimes GenericSubtraction varies the ghost scale, one can derive
such a shape from the ShapeWithPartition class [see
ShapeWithPArtition.hh]. To do that, one implements separately the
partitioning and the calculation fron the partition by overwriting
// compute the partition, returning a conposite jet
PseudoJet ShapeWithPartition::partition(const PseudoJet &jet)
// compute the shape from the partition
double ShapeWithPartition::result_from_partition(const PseudoJet &partit)
For such shapes, GenericSubtraction will only compute the partition once.
- The second case is the one when a shape is made of several
ingredients and one wishes to subtract each of these ingredients
independently. A tyical case would be N-subjettiness ratio, say tau21,
where one wishes to subtract tau2 and tau1 independently and then take
their ratio. To do that, one would derive the shape from
ShapeWithComponenents [see ShapeWithComponents.hh] and overwrite the
following methods:
// returns the number of components
unsigned int n_components() const;
// returns a vector (of size n_components()) containing the
// different components which go into the calculation of the shape.
virtual std::vector<double> components(const PseudoJet &jet) const;
// given a vector of components, determine the result of the shape
virtual double result_from_components(const std::vector <double> &) const = 0;
An example using a ShapeWithComponent is provided in example_with_components.cc.
Examples
--------
The code example.cc illustrates how to use the class in a simple case.
It can be built using
make example
example.cc makes use of an input datafile which is provided in the
/data directory, and it should be run with
./example < ../data/Pythia-Zp2jets-lhc-pileup-1ev.dat
The expected output can be found in example.ref
A more advanced example (shapes made up of several components) is
given in example_with_components.cc, which can be built and run with
make example_with_components
./example_with_components < ../data/Pythia-Zp2jets-lhc-pileup-1ev.dat
and compared to the expected output example_with_components.ref

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