[Pythia8-announce] Pythia 8.302 now available

[Torbjörn Sjöstrand]

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Dear All,

PYTHIA 8.302 has been released today, and can be downloaded from
       http://home.thep.lu.se/Pythia
specifically from the "Future (v 8.3)" frame of the left-hand index.

The new version is mainly aimed at technical improvements of the
PYTHIA 8.301 release. With this step the PYTHIA 8.3 series should
offer a stable replacement of the PYTHIA 8.2 series.

You find a summary of changes below, which is a slightly abbreviated
list of the one in the Update History, found e.g. in the html manual.

Wishing you an enjoyable experience with PYTHIA 8.302.

Sincerely yours,
The Pythia 8 Collaboration
(Christian Bierlich, Nishita Desai, Ilkka Helenius, Philip Ilten,
Leif Lönnblad, Stephen Mrenna, Stefan Prestel, Torbjörn Sjöstrand,
Peter Skands, Marius Utheim)

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Physics changes

* None.

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Technical and administrative changes

* Bug fix to correct units for automated shower uncertainty weights
   for lhaStrategy = +-4 (similarly to 8.244 fix). In addition, the
   combination of weight variations, within the context of uncertainty
   bands, was not previously well documented and was therefore prone to
   misunderstandings. Now, all methods related to cross sections and
   weights are documented together on a single "Cross Sections and
   Weights" page. Further, the documentation on shower "Variations" has
   been improved, and the behaviour of Info::getGroupWeight() has been
   made more transparent. The main121.cc example has been modified
   accordingly.

* Significant speedup of parton distribution handling, and thereby of
   the whole PYTHIA execution, since PDF is such a major part of the
   total execution time. This has been achieved by careful work by
   Dmitri Konstantinov and Grigory Latyshev. In detail, the main
   changes are:
   - In the LHAPDF6 interface the evaluation of the x and Q2 limits is
     extremely slow, such that a caching of these numbers increases
     execution speed by about a factor of 2 when LHAPDF  is used.
   - A caching of info on the amount of x already used up helps speed
     up MPI, ISR and FSR execution times, giving gains of order 20%.
   - Optimization of array handling in LHAGrid1 give a slight speedup.
     This internal implementation of interpolation in LHAPDF6 grids
     runs more than 50% faster than the same PDF evaluated by LHAPDF6,
     at the cost of not offering facilities like PDF error bands.
   - The special internal routines for the NNPDF 2.3 series have been
     replaced by an LHAGrid1 implementation, which gives some speedup.

* New base class ShowerModel introduced, as a top-level handler of
   parton-shower models. The default, Vincia and Dire showers inherit
   from it. New public method Pythia::getShowerModelPtr() is intended to
   provide access to public ShowerModel methods for diagnostics purposes.

* A HepMC 3 interface is now included in Pythia8Plugins/HepMC3.h.
   All HepMC examples have been migrated from HepMC 2 accordingly,
   except for main44.cc and main93.cc, which are kept for legacy
   purposes.

* New particle property directive tauCalc allows for a particle's
   proper lifetime to be set independently of its width.  This is useful
   for studying the effect of lifetime for a particle when the particle
   width is otherwise negligible.

* Fix for subscripting issue for photon beams, courtesy of Vittorio
   Zecca.

* Extra protection against unexpected input in e+e- Les Houches events,
   specifically where x >= 1 by roundoff errors. Thanks to Alexander
   Belyaev, Alexander Pukhov and Marco Zaro.

* Bug fix in SusyLesHouches::checkSpectrum(), to ensure uniform error
   handling when one or more spectrum self-consistency checks fails.

* Replaced use of fabs() by abs() in several places (PythiaStdLib.h
   ensures appropriate overload).

* Improvements to Vincia's handling of error messages; now issued via
   Info::errorMsg().

* Removed share/Pythia8/tunes directory, with Vincia reverting to the
   in-code way of setting tunes used elsewhere.

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