[Pythia8-announce] Pythia 8.209 now available

[Torbjorn Sjostrand]

Dear All,

PYTHIA 8.209 has been released today, and can be downloaded from
       http://home.thep.lu.se/Pythia

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 8.205.

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

Wishing you an enjoyable experience with PYTHIA 8.209.

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

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

Publication change

The "An Introduction to PYTHIA 8.2", arXiv:1410.3012 [hep-ph],
has now been published in Comput. Phys.Commun. 191 (2015) 159.

The published CPC article is open for free download from
    http://authors.elsevier.com/a/1QvSs2OInJT9w
up until 11 June, whether your library has a CPC site licence
or not, so take this opportunity to download your copy.

 From now on, kindly refer to this article whenever you use
PYTHIA 8.2. We also remind of the MCnet guidelines, like:
"In addition, the original literature on which the program is
based should be cited to the extent that it is of relevance
for a study, applying the same threshold criteria as for other
literature."

Also note that the Doxygen code documentation, available from
the PYTHIA homepage, was significantly expanded some time ago.
It is updated for the current release.

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

Physics changes

* The new QCD-based colour reconnection machinery has been expanded
   with checks whether reconnecting dipoles are causally connected.
   This cannot be defined exactly, and therefore several different
   options are available. Also some other minor changes in this part
   of the code, including updates of parameter default values.

* New options for ColourReconnection:flipMode allows to sidestep
   the gluon-move handling but still retain the final flip step.

* The old SK I and SK II colour reconnection models are now
   available. These are specially aimed at the processes
   e^+ e^- -> W^+ W^- / Z^0 Z^0 -> q_1 qbar_2 q_3 qbar_4,
   but are also relevant for H^0 -> W^+ W^- / Z^0 Z^0.

* New ColourReconnection:forceResonance switch allows an additional
   colour reconnection step after late resonance decays. This is
   especially relevant for
   H^0 -> W^+ W^- / Z^0 Z^0  -> q_1 qbar_2 q_3 qbar_4,
   since the Higgs is so long-lived that its decay is well separated
   from the rest of the event.

* Colour reconnection also made possible, optionally, for the
   forceHadronLevel method.

* The LHAPDF6 PDF values are frozen at the respective x and Q^2
   boundary if the (x, Q^2) pair falls outside the fit region.

* New flag LesHouches:matchInOut, by default on, to recalculate
   the energies and longitudinal momenta of the incoming particles
   from the outgoing ones for Les Houches input. Reduces effect
   of numerical inconsistencies in input.

* Spin information for tau leptons now also set up for
   Z' and W' decays.

* New method AlphaStrong::setThresholds(...) allows to set the
   charm, bottom and top flavour-threshold masses used for the
   running of alpha_strong.

* Minor fix for polarization sign when the tau polarization
   is forced.

* Minor correction in the treatment of the highest multiplicity in
   FxFx jet matching.

* Minor correction in the interface to aMC at NLO for dijet and
   photon+jet processes.

* The meaning of the HiggsXX:parity options for CP mixing
   has been slightly modified and is better described.

* Fixed a factor sqrt(2) error in couplings for chargino + squark
   pair production. Thanks to Michihisa Takeuchi.

* Some minor bug fixes in the SUSY code, and some speed optimization
   suggested by Martin White.

* Fixed incorrect indexing in the SLHA interface.

* New machinery for hard diffraction now in place. Still being
   debugged and tested, so not yet ready for public usage.

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

Administrative changes

* New #define PYTHIA_VERSION 8.2xx in Pythia.h allows user-code
   preprecessors to make version-specific choices, and allows the
   Pythia class constructor to check that the header-file version
   number matches those of the source code and the XML files.
   Thanks to Pere Mato.

* The [] operator is implemented for the Vec4 class to return
   its components by index.

* New method string Settings::output(string key) returns
   the value of a variable as a string. In Pythia::readString()
   or readFile() calls this can now be used to print a
   current setting value by the command key = ?.

* New option for the Event::list() methods allows to show
   momenta with more decimal digits.

* New Particle::isFinalPartonLevel() method to tell whether
   a particle belonged to the final state on the parton level of
   generation or not. New main73.cc example illustrates usefulness.

* The iTopCopyId() and iBotCopyId() methods now scan all
   mothers/daughters, rather than only the first and last,
   when searching for a unique flavour match.

* Updated initialization of LHE files in LesHouches.cc
   to ignore file contents enclosed in comment tags,
   <!-- .. --> and/or the special CDATA statement, <![CDATA[ .. ]]>.
   The latter occurs, e.g., in LHEF files produced by CalcHep.
   (It is generally used to store content that contains XML-illegal
   characters like "<" or "&", such as JavaScript source.) Thanks
   to A. Belyaev and A. Pukhov for help with this update.

* Updated EvtGen interface, to allow a pointer to an FSR engine
   to be passed. Thanks to Torben Ferber.

* The need to link to the Boost library to read gzipped files
   has been eliminated by including iostream classes wrapping the
   zlib compression library, see new files Streams.h and
   Streams.cc.

* Linkage to LHAPDF6 so far has been based on the LHAPDF5
   compatibility mode, which requires no Boost headers. The new
   configure option --with-lhapdf6-plugin=LHAPDF6.h uses native
   mode, and then requires Boost headers.

* New functions to extract the fit boundaries, the current
   alpha_s value, and the quark masses from LHADPF6.

* When reading in particle data from the SLHA interface, changes
   done by the user takes precedence over the SLHA input ones.
   To be more specific, particle data changes by the
   Pythia::readString and readFile methods are buffered and
   repeated after the SLHA initialization.

* New option in the HepMC interface, whereby PYTHIA particles can be
   appended to an existing HepMC event. Thanks to Mikhail Kirsanov.

* A new runtime interface to the POWHEGBOX matrix element programs,
   bypassing the need for intermediate LHE files. The new files
   include/Pythia8Plugins/LHAPowheg.h and
   include/Pythia8Plugins/PowhegProcs.h contain the LHAup class
   wrapper used to build the POWHEG plugin libraries and the simple
   class that facilitates loading the POWHEG plugins, respectively.
   The new examples/main33.cc demonstrates how to use these plugins,
   and examples/main33.cmnd contains the commands needed for
   POWHEGBOX to run the example.

* A new switch POWHEG:QEDveto has been introduced to steer the
   treatment of non-QCD radiation in the POWHEG implementation
   of include/Pythia8Plugins/PowhegHooks.h.

* User-defined semi-internal processes can now be accompanied by
   user-defined phase-space generators, via a second optional
   argument to Pythia::setSigmaPtr(SigmaProcess*, PhaseSpace* = 0).
   Default is the old behaviour, with PYTHIA selecting the phase-space
   generator itself, based on the process type. Alternatively, the user
   may provide a pointer to an instance of an object inheriting from
   the PhaseSpace class. Sufficiently well-tested and general
   such generators could be communicated to the PYTHIA authors
   for possible inclusion in a future release.

* The role of the pythia.forceTimeShower(...) method is better
   explained in the hadron-level standalone documentation, and
   main21.cc has been extended with an example.

* The TimeShower::enhancePTmax() method is made virtual.

* Fixed possible unwarranted destruction of pointer to external
   timelike shower by the Pythia destructor.

* Removed misleading flavour setup for unused Pomeron.

* New example main62.cc illustrates how a user hook can
   steer the selection of angles in a resonance decay.

* Clarification in the documentation that impact-parameter-enhancement
   factor calculation for two hard processes does not work for the
   x-dependent impact-parameter profile option.

* Inserted missing endtag that corrupted the Tunes.php page.
   Thanks to Tim Martin.

* Initialized pointers to NULL in the Info class, to avoid some
   problems. Thanks to Keno Fischer.

* Minor Makefile improvements for LHAPDF linking.

* Minor improvement to the main89.cc example program.

* Mildly modified warning/error messages when junction splitting
   fails.

* Minor fixes in LHAFortran.h, which is also moved to
   include/Pythia8Plugins to better reflect its peripheral role.

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