[Pythia8-announce] Pythia 8.219 now available

[Torbjorn Sjostrand]

Dear All,

PYTHIA 8.219 has been released today, and can be downloaded from
       http://home.thep.lu.se/Pythia

There are a few major physics news, several small improvements
and also several 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 8.219.

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

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Physics changes

* Included a new framework for automated parton-shower uncertainty bands.
   Variations of the QCD renormalisation scale for both initial- and
   final-state showers can now be computed on the fly, and are provided
   as a list of alternative weights for each event, representing the
   probability that the given event would have occurred under different
   shower assumptions. So-called "nonsingular terms" can also be added to
   the splitting kernels to estimate the possible effect of missing
   matrix-element corrections. A paper is due to appear shortly.

* Changes to the cross section handling in the presence of user
   vetoes/weights, containing three changes:
   1. The counter of selected event is now updated  immediately after
      the hard process generation.
   2. More fine-grained input settings to enforce that Pythia generates
      (or reads) exactly a fixed number of hard process events.
   3. The Pythia "internal cross section" and the event weight now
      directly include the effect of event vetoes and event reweighting
      that are applied by the leading-order ME+PS merging prescriptions.

* The gamma-gamma hard-process machinery has been extended to convolute
   the partonic PDFs in a photon with the flux of photons inside a lepton.
   The description is intended for the region of quasireal photons, but
   full kinematics is implemented.

* Changes to the merging classes to allow for a postponed CKKW-L event
   veto.

* The jet matching algorithm has been extended to better handle heavy
   quarks, heavy colored particles (such as squarks) and "other" partons
   (coloured but produced from an Electroweak vertex).

* New flag allows charged resonances, like the W^+-, to radiate photons.

* Four Pomeron PDF sets from the ACTW study now implemented.

* Increased possibilities to set the epsilon and alpha' parameters of
   the Pomeron trajectory for hard-diffraction Pomeron fluxes.

* Extrapolation of PDFs to small x values now extended to more cases.

* Added a new option which will enforce an ordering in rapidity for
   emissions off secondary scattering systems. The old switch now only
   refers to the hard(est) subprocess.

* Allow for lightest neutralino not to decay as a resonance.

* Do not switch off the Breit-Wigner width treatment of a resonance
   as easily as previously, but only if the width is below 1e-6 GeV.

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

Administative changes

* An interface to the Python programming language has been introduced,
   making all PYTHIA classes and methods available. Various minor changes
   in the C++ code have been done in order to permit the automatic
   generation of the interface.

* New constructors that take streams rather than files as input.
   Thus the contents of a file can be read and then broadcast to multiple
   instances of Pythia, eliminating the inefficiency of multiple jobs
   reading from the same initialization files. This facilitates running
   with MPIs (Message Process Interfaces).

* A new class allows PDF data files in the lhagrid1 format, with some
   restrictions, to be read and used. This does not replace all that you
   can do with a complete LHAPDF6 installation, but at least permits some
   simple studies without LHAPDF6 + Boost. New example main55.cc
   illustrates this, and event properties for an intermediate spinless
   resonance in gamma + gamma -> gamma + gamma at 750 GeV.

* The three Pomeron H1 Jets PDF data files have been joined to one.

* The external decays interface has been extended by a new method that
   allows sequential decays to be done in one call.

* Small changes to virtual parton shower functions, and to the merging
   classes, to prepare ME+PS merging with the Vincia parton shower.

* The HepMC interface has been modified such that the detection of
   unhadronized quarks or gluons leads to an exception being thrown,
   so that the user can decide what action to take. Thanks to James Monk.

* A class WVec has been introduced to store vectors of strings as part of
   the settings machinery. The delimiters { } are introduced to provide
   input with  embedded blanks, and broken up across several lines.

* The settings method used to convert a string to lowercase, and also
   trim it from initial or trailing blanks and special characters, is now
   moved to PythiaStdlib.h so it can be used more generally. Other code
   changes accordingly.

* Remove many rarely (if ever) used ostream& os = cout optional arguments
   in favour of hardcoded cout. Eliminates some redundancy of methods.

* Rename ...print(... methods to ...list(... to favour a more regular
   naming pattern.

* New --config option of pythia8-config echoes the arguments passed to
   configure.

* Make some SlowJet methods virtual to allow derived classes with
   modified properties.

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

Bug fixes

* When a final-state g -> g g branching happens with a massive recoiler,
   radiation in the recoiler direction is now by default further suppressed
   to respect the "dead cone" effect. Furthermore, the program more often
   attempts to guess the most relevant ME correction when the correct
   choice is not known or implemented. Thanks to Jesse Thaler, Michele
   Selvaggi and Fabio Maltoni.

* Small additions and fixes to the LHEF3 framework, to keep track of
   weight keys, and improve parsing.

* Minor configure and Makefile updates, to address potential linking
   problems on some platforms for boost, gzip and promc. Thanks to Dmitri
   Konstantinov.

* The description of POWHEG:vetoCount = 0 has been corrected. Thanks to
   Florian Koenig.

* Moved some misplaced info on parton-level choice of MPIs.

* Corrected typos where some bottomonium long-distance matrix element
   had been set larger than normally assumed.

* Fixed typo potentially giving incorrect colour flow in resonance decays.

* Fixed an out-of-bounds array access in HelicityMatrixElements.
   Thanks to Vittorio Zecca.

* Fixes in main80.cc and main89.cc.

* Fixed problem with the SLHAinterface not being zeroed-out when
   using repeated subruns.

* Minor fix for beam particles, that no default was set as to whether
   they are gammas or not.

* Cleaned up error printout for the PDF classes.

* Change a few true/false to on/off in the documentation to make the
   php version of the manual recognize them.

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