[Pythia8-announce] Pythia 8.235 now available

[Torbjörn Sjöstrand]

Dear All,

PYTHIA 8.235 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 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 8.235.

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)

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

Physics changes

* The machinery for total, elastic and diffractive cross sections
   has been rewritten and expanded. Article is in preparation.
   - The ABMST model has been introduced, and extended from total,
     elastic and single diffraction to also include double and central
     diffraction.
   - The RPP 2016 parametrization for total and elastic cross sections
     has been introduced.
   - The set-it-yourself machinery has been extended and systematized.
   - The descriptions on the TotalCrossSections and the Diffraction
     webpages have been regrouped. The former now describes both
     integrated and differential cross sections of all scenarios,
     whereas the latter describes parameters related to the hadronization
     of diffractive systems, and additionally the machinery for hard
     diffraction.
   - In the process of this regrouping, several parameters have been
     renamed.
   - The phase space generation machinery has been rewritten to allow
     for the extended set of models, to be more general.
   - Also the cross section code itself has been restructured.

* A scenario for hard diffraction with gamma beams has been introduced,
   applicable for both gamma + gamm and gamma + p processes. Only a
   resolved photon, currently always associated with a virtual rho
   state, is assumed to contain a Pomeron flux. The framework can be
   applied also for photoproduction with lepton beams. Article is in
   preparation.

* A new machinery has been introduced to handle soft diffraction in
   gamma-p and gamma-gamma collisions, based on VMD (Vector Meson
   Dominance) states assumed present in a photon beams. VMD states
   have been introduced as possible beam particles inside a photon beam.
   The standard beam machinery has been extended to pick VMD states when
   required. Possibility to rescale the pion PDF has been implemented,
   e.g. when used to represent a VMD state.

* It is now possible to obtain space-time production points for hadrons
   as predicted within the string fragmentation model, see further the
   Hadron Production Vertices section of the Hadron Scattering
   description. Physics and code developed by Silvia Ferreres-Sole;
   article is in preparation.

* The Angantyr model for Heavy Ion Collisions has been updated with
   a few extra options and new defaults. To recover old results one
   needs to set Angantyr:SASDmode = 1, Angantyr:impactMode = 0, and
   HIPDF:PomHixSupp = 4..

* The flavour part of the rope hadronization implementation has been
   updated. The option to always go to the highest possible SU(3)
   multiplet, relevant when junction CR is used in addition to ropes,
   has been added. The possibility of a simpler impact parameter model
   a la Buffons needle has also been added.

* An option is introduced to have a logarithmic parametrization for
   the energy dependence of pT0, used for the cross section
   regularization in the MPI framework. This is used as a default for
   gamma-gamma collisions, with new default parameters listed in
   the Photoproduction description.

* A new parameter ContactInteractions:etaRL is introduced into the
   differential cross section for q qbar -> l+ l- to distinguish a
   right-left contact interaction between quarks and leptons.
   Previously, the same parameter was used for both.

* A new Dark Matter production process f fbar -> Z' H has been added,
   with Z' decaying to a DM pair. Some other DM processes have been
   modified.

* The shower variations have been extended so that the calculation
   can be made for an individual member of a PDF family, instead of
   calculating the effect when averaged over all members.

* Possibility introduced to enhance shower rate of gluon branchings
   to charm and bottom independently from light flavours.

* The central NNPDF 3.1 QCD NLO and NNLO fits at alpha_s = 0.118,
   PDF:pSet = 19 and 20, have been replaced by the more recent
   corresponding NNPDF 3.1 QCD + QED (luxqed) fits.

* A new nuclear PDF set, EPPS16, is added.

* The GKG-DPDF Fit A and Fit B (central member) diffractive PDF sets
   have been added.

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Technical and administrative changes

* Up until now, PYTHIA has been licenced under GNU GPL v2 or later.
   This is extended so that alternatively any later version can be used,
   specifically version 3. The MCnet GUIDELINES are updated accordingly,
   as are many file headers. Thanks to Christophe Delaere.

* A new class HistPlot is introduced to generate PDF plots from the
   histograms of the Hist class in PYTHIA. More specifically, the
   histogram contents are written on files and Python code is generated
   that contains appropriate Matplotlib/Pyplot commands, that can then
   be run to produce the plots. The main03.cc, main07.cc and main51.cc
   programs have been modified to illustrate the new setup. Histograms
   can now also be booked with a logarithmic x scale. Thanks to Johannes
   Bellm.

* It is now possible to specify a given impact parameter when generating
   minimum bias events, using a new functions in the UserHooks.

* It is now possible to specify the a and <z> values of the Lund
   fragentation function, as an alternative to the conventional a and b
   selection.

* A Docker container for PYTHIA 8 and DIRE can be obtained from
   http://dire.gitlab.io/Docker/ . You will there also find some
   Jupyter notebooks and notes e.g. how to use Rivet analyses.

* Updated main89.cc to allow use also without employing matching or
   merging. This allows to use this main program as an option to read
   from LHE file input and write HepMC file output. Furthermore,
   main89.cc has been updated to also allow Pythia to guess the hard
   process used for merging through the Merging:process = guess setting.

* An example, main93, is added, which provides an updated ROOT interface,
   as well as options to run RIVET and output HepMC files, all steered
   from run cards and the command line.

* New simple facility introduced in PythiaStdlib for integrating a
   function by Gaussian quadrature or findig its roots. In part lifted
   out from SusyWidhtFunctions.

* When a particle is read in by the SLHA interface, the minimal and
   maximal allowed masses for the Breit-Wigner distribution can be set
   by the user but, if not, the maximal deviation from the nominal mass
   is now set to be the smaller of five times the width and half the mass.

* A modification in the strategy to get or set particle data saves
   having to search a map twice. New methods introduced to return
   an iterator of an entry to the map as part of this.

* New method PartonSystems::popBackOut(iSys) helps to remove the most
   recently added outgoing parton of a given parton system.

* Handling of multiple PDFs via the LHAPDF6 interface is rewritten
   for more flexibility. For instance, not all members of a set are
   loaded unless uncertainty variation studies need them.

* New xml tags introduced to group parameters visually in conversion
   to html, by omitting the paragraph break before a tag that contains
   "more", e.g. parmmore.

* Year has been updated to 2018.

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

Bug fixes

* Updates for non-diffractive photoproduction with externally provided
   photon flux, to fix the normalization and to solve issue with process
   initialization when using a heavy-ion specific flux.

* Updated remnant kinematics for 2 -> 1 processes with direct photons
   from leptons.

* A bug was fixed in the Hidden Valley model to allow copies of the
   hidden valley quark to properly shower and hadronize.

* A bug fix in the q g → Leptoquark l</ei> process, whereby
   the role of tHat</ei> and uHat</ei> was incorrectly swapped.
   Thanks to Yiming Zhong and Martin Schmaltz.

* An LHAPDF6 destructor problem is solved for the case that a
   Pythia object is made global. Thanks to Florian Herrmann.

* A bug fix in the q g -> onium q</ei> processes in the MPI
   machinery, whereby it was not correctly symmetrized. Thanks to
   Steffen Webber.

* Minor updates for the print format in MPI initialization.

* Corrected issue in timelike and spacelike showers that meant that
  the inclusive cross section was changed slightly when enabling
  shower variation.

* Ensure that all matrix-element corrections in timelike showers are
   switched off when that option is set.

* Fixed beam-remnant kinematics of scattered leptons when using
   PDF:lepton=on with lepton beams.

* Bug fix in the Alpgen interface for parton shower matching of
   non-QCD partons. Previously the matching would hang when
   encountering a resonance.

* Fix for PDF setup when second incoming hadron is a pion or other
   meson.

* Les Houches events with incoming top quarks are skipped, since
   the ISR and beam-remnant machineries are not set up to handle top.

* partonSystems vector cleared in beginning of Pythia::forceTimeShower.

* Modification in the parsing of a wvec, to avoid the removal of the
   first = sign in the first entry.

* Updated examples/Makefile to address recent problem that libz is
   not properly linked if the GZIP flags do not appear at the end of
   the compile command, experienced with recent gcc versions on Ubuntu.

* Fix typo in bin/pythia8-config.

* Remove obsolete --with-lhapdf6-plugin option in the configure
   script. Thanks to James Monk.

* Minor per-nucleon mass update in main70.cc.

* Minor fix to avoid division by zero in resonance mass selection.

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