[Pythia8-announce] Pythia 8.215 now available

[Torbjorn Sjostrand]

Dear All,

PYTHIA 8.215 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 8.215,
and a Happy New Year!

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

* A new machinery for gamma-gamma collisions is now available.
   So far only hard processes can be generated, along with parton
   showers and hadronization, but without multiparton interactions.
   The CJKL parton distributions of the photon have been implemented.

* Double production of charmonium and bottomonium 3S1 states is now
   available, but with only the colour-singlet processes included.

* Weak merging implemented, i.e. W gauge bosons can be produced
   either as part of the hard matrix element or in the parton shower,
   and a proper treatment merges these two possibilities consistently.

* Improved interface to external parton showers, such as VINCIA and
   DIRE, so that these now also can use the various matching and merging
   frameworks implemented in Pythia.

* New options in the jet matching framework, such that expert users
   can use their own veto code for Madgraph-style jet matching.

* New convenient possibility to run Madgraph5_aMC at NLO from within
   Pythia, by wrapping the Madgraph5_aMC at NLO executable inside a new
   LHAup-derived class, see examples/main34.cc. Still at an experimental
   stage.

* By default the program will now assign the PYTHIA mass for massless
   c and b quarks in Les Houches input. The same new machinery is also
   used to give mass to massless charged leptons.

* Some further new options and minor additions in the machinery for
   hard diffraction, and some default values changed.

* New switches regulate whether the first shower branching before/after
   the hard process can correlate with the hard-process event plane.

* Changed default setting in main89mlm.cmnd, to better agree with
   common practice.

* Minor improvements (and fixes) in the shower weighting facilities.

* Minor update in the beam-remnant handling for DIS.

* Minor improvements in the handling of resonance mass selection.

* Changed off-shell behaviour for squark pair production.

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Administative changes

* A new constructor for the Pythia class takes references to a Settings
   and a ParticleData object as inputs. In cases where multiple Pythia
   copies are created this allows the xmldoc files to be read only once.

* New method Particle::daughterListRecursive() that can trace
   consecutive generations of decay products.

* New method Particle::intPol() returns the polarization as an integer
   if the stored double is very close to 0, +-1, +-2 or 9, and else -9.

* New Vec4 friend method pShift(...) to transfer four-momentum between
   two four-vectors so as to bring them to have specified new masses.

* Initialize the random number generator earlier, so a non-default seed
   choice also could benefit early external initialization.

* The GetDJR function of the JetMatchingMadgraph class is renamed
   getDJR to adhere to standard naming conventions.

* Several minor configure and Makefile improvements.

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Bug fixes

* Significant bug found in timelike and spacelike showers, whereby
   the azimuthal anisotropy from gluon polarization in the past has been
   overestimated. This does not affect multijet rates, but can influence
   distributions sensitive to angular correlations, although checks have
   not revealed any appreciable effects. Thanks to Radek Zlebcik.

* Bug fix in the selection of masses in resonance decays. In rare
   situations this could give wrong masses for particles. Thanks to
   Are Raklev and Anders Kvellestad.

* Introduce protection against (close-to-)zero-energy partons in string
   length calculations, and against topologies with extremely small
   angles between two junction legs. Thanks to Jan Fiete
   Grosse-Oetringhaus.

* The StringFlav::combine( int, int bool) method is renamed combineId
   to avoid a potential incorrect overloading. Thanks to James Monk.

* Copy vertex information when a long-lived particle decays to three
   quarks, whereof two have such a small invariant mass that they
   collapse to a diquark. Thanks to Cristiano Alpigiani.

* Bug fix for excited quarks q^* and leptons l^*. If new decay channels
   were introduced they could incorrectly make use of the matrix element
   expressions for the existing decay modes. Thanks to Simone Amoroso.

* Bug fix in the kinematics of four or more resonance decay products
   when  kinematics is redone owing to matrix-element corrections.
   Thanks to Simone Amoroso.

* Bug fix for information on the pdf value chosen for the hardest MPI,
   which was reported a factor 9/4 too large for an incoming gluon.
   Does not affect the event generation itself.

* Correct BeamRemnants:primordialKThard from 2.0 to 1.71  for ATLAS
   tune AZ. Thanks to Christian Bauer.

* Minor fix for random number start-up in the PowhegBox interface.

* Minor fixes and technical changes in the LHEF3 machinery.

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Other

* Ilkka Helenius joins as new PYTHIA co-author.

* Updated address for Philip Ilten.

* Year updated to 2016.

* Several documentation updates, including a few in the bibliography.

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