[Pythia8-announce] PYTHIA 8.204 now available

[Torbjorn Sjostrand]

Dear All,

PYTHIA 8.204 has been released today, and can be downloaded from
       http://home.thep.lu.se/Pythia
a new shorthand for the old
       http://home.thep.lu.se/~torbjorn/Pythia.html

There are no major discontinuities in this version, but hopefully
several new features and improvements that will make it worth the
small effort to upgrade from 8.201.

The list of changes below has been split in three sections:
physics, administrative, and other.

Wishing you an enjoyable experience with PYTHIA 8.204.

Sincerely yours,
The Pythia 8 Collaboration
(Torbjörn Sjöstrand, Jesper Roy Christiansen, Nishita Desai,
Philip Ilten, Stephen Mrenna, Stefan Prestel,
Christine Rasmussen, Peter Skands)

================================================================

Physics changes

* Fifteen new tunes have been added, the MonashStar tune from CMS
and fourteen A14 tunes from ATLAS. The latter correspond to central
tunes for four different PDF sets and ten variations in five
(approximate) eigenvector directions.

* The compositeness section has been expanded with six further
processes, describing the pair production of excited leptons or
neutrinos. Three-body contact-interaction decay modes of these
excited states have been introduced. A bug has been fixed that
gave the wrong helicity in decays for excited quarks, leptons and
neutrinos. Further, the gamma^*/Z^0/Z'^0 can decay to a pair
of excited fermions provided that the channels are added to the
list of allowed ones.

* New options 3 and 4 for TimeShower:pTdampMatch and
SpaceShower:pTdampMatch, with new default 3 for the latter.
The main effect is that, by default, t tbar production (as a
2 -> 2 process) obtains damped radiation above the process scale.

* Change in the setup of final-state-shower colour dipoles for
the non-default case of no interleaving, whereby it becomes less
likely to pick a colourless final-state particle as recoiler.
New option TimeShower:allowMPIdipole gives more flexibility.

* The chosen Tune:pp option now implies the Tune:ee value to which
it is related, and thus the latter need not be set separately.

* Default settings values have been updated to agree with the
(default) Monash 2013 tune. Thus typically the list of changed
settings is significantly reduced.

* A new interface to the EvtGen decay package, primarily intended
for bottom and charm decays, has been implemented. It is available
in include/Pythia8Plugins/EvtGen.h and an example how to set it up i
s found in main48.

* The ProMC input-output file format is now implemented among the
libraries that can be configured to run with PYTHIA. An examples
is provided in main46.

* Introduction of a new mode LesHouches:setLeptonMass, such that
by default final-state charged leptons acquire sensible masses,
even when the matrix-element calculations have been performed
with massless leptons.

* New parameter BeamRemnants:reducedKTatHighY introduced to reduce
technical problems with low-mass MPIs produced at high rapidities
when primordial kT is introduced.

* Small bug fixes for string and ministring fragmentation, for the
case when a low-mass (order 2 GeV) system contains at least three
partons, which fail to define a unique direction for the final
string region.

* Bug fix in check for colour sextets and transfer of such colour
information.

* Small bug fix in the global-recoil option for timelike showers.

* Two bug fixes in the new colour reconnection model, one for
diquarks at the ends of junction strings, and another to check that
coloured resonances are processes with early resonance decays option.

* Bug fix such that the valence content of a pi^0, K^0_S, K^0_L and
Pomeron is reselected for each new event.

* Bug fix in new beam remnant model, so that it basically
operates like the old one for e^+e^- annihilation.

* Bug fix for multiple Pythia::init() calls, where beam contents
were not properly reset.

* Fix typo in constants of the tau -> 3 pi current  for the
amplitudes of the rho, rho(1450), and f2.

================================================================

Administrative changes

* The examples directory has been moved back from the share/Pythia8/
subdirectory to the main directory, as was the case in PYTHIA 8.1,
to make it more visible to newcomers. The optional make install
step will create a copy of examples in share/Pythia8/. The rarely
used examples/outref subdirectory is moved to share/Pythia8/outref.

* Initialization will now abort if a mode, defined with a fixed
set of options, has been chosen with a non-allowed value.

* In LesHouches.cc the read-in of LHEF headers containing tags that
open and close on the same line has been enabled.

* Minor fixes in the LHEF version 3 reader. A new matching writer
of LHEF version 1 or 3 files.

* New file include/Pythia8Plugins/execinfo.h contains trivial copies
of three backtrace methods needed to be able to compile PYTHIA under
Cygwin. The README file and the worksheet updated with brief
information on three (non-supported) ways of working with PYTHIA
under Windows.

* In main24 the example spectrum file format has been updated
from SLHA1 to SLHA2, obtained from SoftSusy 3.5.1. The two new
slha1-example.spc and slha2-example.spc files replace the older
cmssm.spc, snowmass2.spc and softsusy.spc ones.

* A new example main30 shows how to create a tailormade copy of
the ordinary event record, here with a history tracing of the
hard process closer to the PYTHIA 6 conventions.

* main61 has been removed, since now LHAPDF can be loaded
dynamically for main42, so that the two become equivalent.

* main62 has been renamed main43 to gather HepMC-related examples.

* Added getChannels() in SusyResonanceWidths to dynamically create
the decay table for SUSY particles and thereby to remove duplication
in the XML file.

* New file SusyWidthfunctions. Added new class WidthFunction
to handle calculation of three- and four-body decay widths.

* Improved handling of stray characters in the SUSY Les Houches
code. The check on the consistency of decay tables has been removed.
Improved warning/error printing in the SLHA interface.

* testlhef3.lhe has been renamed wbj_lhef3.lhe.

* Bug fix in the conversion from the xml settings files to their
php radio-button equivalents. Previously the text describing some
options was not set properly.

================================================================

Other changes

* Update year to 2015, remove tabs and superfluous blanks, break
long lines where meaningful, and some further minor changes.

* http://home.thep.lu.se/Pythia has been introduced as a simpler but
equivalent address to http://home.thep.lu.se/~torbjorn/Pythia.html,
and various documentation has been updated accordingly.

* Also the webpages themselves have been updated, with a typography
similar to that of the PYTHIA html manual. Adjusted split of material
between Past - Recent - Present - Future, and a new Contact.
A Doxygen representation of the code has been made available.

================================================================