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BUGS
Known bugs
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1. For N log(N) strategies requiring CGAL, in CGAL versions 3.7, 3.8,
and 3.9-beta1, in rare case of near degeneracies, the program may
abort with the following error message:
terminate called after throwing an instance of
'CGAL::Assertion_exception'
what(): CGAL ERROR: assertion violation!
Expr: false
File: .../include/CGAL/Triangulation_2.h
This is due to an excessively conservative check in versions 3.7,
3.8, and 3.9-beta1 of CGAL that has been fixed in CGAL-3.9. Note
also that versions<=3.6.1 do not show that behaviour. If you
encounter this problem, please switch to an appropriate version of
CGAL. [Thanks to the CGAL development team and to Olivier Devillers
in particular for the quick fix.]
2. With JetMedianBackgroundEstimator, the computation of sigma()
implicitly assumes that all jets have the same area. The fact that
they don't introduces a bias at the level of a couple of tens of
percent. GridMedianBackgroundEstimator should not suffer from this
problem. Thanks to Christopher Young for bringing this to our
attention.
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Other issues to be aware of
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1. Some algorithms can have ambiguities in the clustering steps and we
do not guarantee that different internal clustering strategies (or
different fastjet versions) will always resolve the ambiguities
identically. This issue doesn't arise with normal particle inputs.
However:
- Some (older) versions of Pythia 6 sometimes produce massive
particles with pt=0; phi is then ill-defined, while the rapidity
is finite and we make no guarantees about how we treat the
particle.
- inputs that lie on a perfect grid in rapidity-phi (e.g. one
interpretation of a calorimeter) cause many interparticle
distances to be identical. In the kt, Cambridge/Aachen and anti-kt
algorithms, many dij can therefore also be identical. The choice
of which dij to take is then ambiguous and what fastjet actually
does may depend on: the compiler, the machine architecture
(because of rounding errors), the fastjet version and the
clustering strategy.
Physically this issue should not change the jets
much. Nevertheless, one might choose to break any degeneracy
explicity, e.g. using information from the location of energy
deposits in each calorimeter tower, so that the inputs are not on
a perfect grid.
2. For some of the plugins (listed below), the result of the clustering
depends on how "sort" treats equal entities. Since this is
supposedly undefined, FastJet has no control over the different
results that one may obtain using those plugins.
- The ATLAS-Cone plugin: At the beginning of the stable-cone search,
the input particles are sorted in Et. In that sort, particles with
an Et difference below 0.001 are considered as having the same Et
(see line 80 of Jet.hh), resulting in the undefined behaviour
mentioned above.
A consequence of this is that, if 2 input particles have too
similar an Et, the stable cone search will consider them in an
undefined order. If the 2 resulting stable cones are too close
(deta<0.05, dphi<0.05) one will be accepted and the other
rejected. Which one depends on the ordering and is thus
undefined. If the 2 stable cones do not have the same constituents
this could affect the result of the clustering.
- In the TrackJet plugin, input particles are sorted in Et. If two
particles have the same Et (within machine precision), the order
in which they will be considered is undefined. Note however that to
have exactly the same pt, the particles are very likely to be
back-to-back. In that case, only the order of the clustering will
be affected but not the final jets.
Relative to the original code, we have replaced the use of 'sort'
with 'stable_sort' so that in case of such a degeneracy the order
will be the same as that of the original particles and no
random behaviour is to be expected. Note however that the issue
could still arise if one changes the ordering of the input
particles.
3. Copy of ClusterSequenceArea (and the other CS area-related classes)
is not fully implemented.
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Probably solved
---------------
1. Compilation issues have been reported under windows with the VC7.1
compiler. We have incorporated fixes proposed by Ivan Belyaev for
this, but do not have access to an appropriate machine to test it
ourselves. (NB: it in any case works with cygwin).