[Pythia8-announce] Pythia 8.230 now available

[Torbjorn Sjostrand]

Dear All,

PYTHIA 8.230 has been released today, and can be downloaded from
       http://home.thep.lu.se/Pythia

Some completely new physics capabilities are introduced. There is
also the usual mix of physics extensions, technical improvements
and bug fixes. Therefore we encourage users to upgrade.

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

Wishing you an enjoyable experience with PYTHIA 8.230.

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

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Physics changes

* A new Heavy Ions machinery has been added, that allows PYTHIA to
   generate pA and AA collisions within a simple model. This entails
   significant additions to the PYTHIA code, and some changes.
   Notably a new HeavyIons base class is introduced, which exists
   inside Pythia but itself can contain several Pythia instances to
   handle different hadronic subcollision types. A new derived class
   Angantyr provides the default heavy-ion description. It is inspired
   by the old Fritiof model, with recent improvements. A number of new
   utilities are used by Angantyr, but could also be used by an
   external Heavy ion generator, such as the nucleon distribution
   inside a nucleus. Particle data have been introduced for a few
   heavy ions, notably 208Pb, code 1000822080. The new capabilities
   are illustrated in main111.cc, main112.cc and main113.cc, for pp,
   pPb and PbPb collisions, respectively.

* A new Rope Hadronization framework is made available. This introduces
   the possibility to enable string shoving and flavour ropes. Both
   models attempt to model collective effects at a microscopic level,
   with inspiration from lattice QCD and the dual superconductor picture.

* A new dipole-shower option for initial-state radiation contributed
   by Baptiste Cabouat. More specifically, a unified description of
   initial-final and final-initial dipole ends is introduced. This
   allows a description of showers in Deeply Inelastic Scattering,
   illustrated by main36.cc.

* New processes for 3S1 charmonium or bottomonium production
   in association with a photon.

* Further processes have been added for Dark Matter production,
   either by a scalar or by a vector <ei>s</ei>-channel mediator.
   Also a new main75.cc example.

* The machinery for automated shower variations has been extended to
   also take into account the PDF variations inside a PDF family,
   using the LHAPDF6 machinery for this. It is now also possible to
   stop variation uncertainty evaluation below some scale, so as to
   better correlate the event weighting with the harder part of the
   event evolution. The new main121.cc example illustrates how to
   set up the variations.

* Implementation of nuclear PDFs for hard processes.

* A new proton PDF added, which sets out to combine a NNLO behaviour
   at high x values with a sensible LO low-x on. Thanks to Zahari
   Kassabov.

---------------------------------------------------------------------

Technical and administrative changes

* Christian Bierlich joins as co-author.
   Philip Ilten has new affiliation.
   PYTHIA author list rearranged alphabetically.

* A new search function introduced in the html documentation. You can
   type a word or phrase in the new "Search" box near the top of the
   left-hand index field, and immediately get up a list of links to
   places where it occurs.

* The bibliography now comes with anchor points for each article,
   and is directed to these from the text references.

* One more digit for PDG identity codes in event listings, at the
   expense of a shorter separation to the particle name.

* A new framework for setting partonic production vertices in MPI,
   FSR and ISR. Still at a primitive stage, and currenly only used for
   the rope hadronization framework. It replaces a previous setup with
   UserHooks. The main65.cc example has been removed.

* The MixMax random number generator is made available as a plugin
   distributed with PYTHIA, by agreement with the authors K. Savvidy
   and G. Savvidy. Some minor modifications are needed for linking,
   see examples/Makefile and the main23.cc example. The latter has been
   updated to include a comparison of execution time between the default
   and the MixMax generator.

* In configure the plugin option for LHAPDF6 is removed,  such that
   now LHAPDF5 uses LHAPDF5.h and LHAPDF6 always uses LHAPDF6.h.
   Previously LHAPDF6 used the LHAPDF5.h Fortran wrapper by default.
   Since LHAPDF 6.2 no longer requires BOOST this dependency has been
   removed. People using earlier LHAPDF versions must now explicitly
   enable BOOST. The "-std=c++98" flag has been removed to simplify
   compilation together with programs using later C++ standards.
   An "--enable-optdebug" alternative has been added for debug with
   optimization allowed.

* Makefile is updated to handle the removal of the LHAPDF plugin
   option, slightly improved for better compatibility across platforms,
   and an old shared library will be removed also when a compilation
   only generates a new static library.

* Fix up the extrapolation procedure of external PDFs, notably the
   ones accessed by the LHAPDF5 interface (previously also available
   for LHAPDF6). PDFs are now explicitly frozen at borders, except that
   PDF:extrapolate can be switched on to allow extrapolation to low x.
   Note that, as a consequence, results can change if you have used
   external PDFs for the MPI description. The native LHAPDF6 interface
   already froze at borders, but now optionally allows low-x
   extrapolation. Thanks to Radek Zlebcik.

* The main51.cc, main52.cc and main53.cc PDF examples have been
   modified to use LHAPDF6 by default rather than LHAPDF5.

* A new method to provide an external photon flux to study
   photoproduction with different fluxes. Still optimized for lepton
   beams, but also other fluxes can be studied. See main70.cc for an
   example.

* A new interface simplifying the usage of Rivet is included, and its
   usage illustrated in the main111.cc example.

* A new utility to keep track of progress and time remaining in a run,
   also illustrated in main111.cc.

* New particle methods y( double mCut) and
   y(double mCut, RotBstMatrix& M) to calculate rapidity assuming
   a minimum mass, and optionally after a rotation/boost operation.

* New option in the single-particle gun in main21.cc, to allow the
   input particle have a lifetime and thus decay some distance away
   from the origin. Thanks to Graham W. Wilson.

* Maximal number of histogram bins increased to 10000 and a warning is
   printed if this limit is exceeded. Thanks to Roberto Franceschini.

* Several minor updates and improvements of the documentation.

---------------------------------------------------------------------

Bug fixes

* Improved junction colour tracing to fix some problems e.g. in
   processes with Baryon Number Violation (BNV). Specifically, it is
   ensured that in- and out-colours are labelled differently for a
   particle with a BNV vertex both in production and at decay, to
   avoid colour lines being shortcut. Further examples have been added
   to main25.lhe. Thanks to A. Monteyx, M. Buckley and F. Jimenez.

* Fix minor (order 5%) normalization error of the impact-parameter
   enhancement factor for two preselected hard processes in the MPI
   framework. Thanks to Jonathan Gaunt.

* Correct so that the TimeShower:MEafterFirst option takes effect in
   g -> q qbar branchings. Does not change the default behaviour.
   Thanks to Keith Hamilton.

* Ensure that the "thermal string fragmentation" is not inadvertently
   used for Hidden Valley fragmentation.

* Minor fix in pythia8-config to solve some parsing issues.
   Thanks to Gavin Salam, Dmitry Konstantinov and Emanuel Hoogeveen.

* Fix typo in reweighting machinery in SpaceShower.cc.

* Several minor fixes to protect from rare occasions of division by
   zero. Thanks to Steffen Weber.

* A minor bug fix to set up correctly the internal indices for the
   initiators from emitted photons. Thanks to Aaron Angerami.

* Dummy class added in Streams.h and .cc to avoid harmless warning.

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