[Pythia8-announce] Pythia 8.226 now available

[Torbjorn Sjostrand]

Dear All,

PYTHIA 8.226 has been released today, and can be downloaded from
       http://home.thep.lu.se/Pythia

There are a some 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 8.226.

Sincerely yours,
The Pythia 8 Collaboration
(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

* - Implementation of gamma-hadron collisions and photoproduction in
     lepton-hadron ones. The "Photon-photon Interactions" manual page
     has been expanded accordingly and renamed "Photoproduction".
   - Automatic mixing of resolved and unresolved photon-photon
     and photon-hadron interactions. A new method Info::photonMode()
     can output the type of the process.
   - A new partonic subprocess q gamma -> q gamma, mainly to
     study photon production in photoproduction.
   - Updated sample program main69.cc.

* - The merging machinery is redesigned to allow users to define their
     own ME+PS merging plugin, which can then be used by Pythia. This
     change does not affect the physics of Pythia's internal merging
     schemes. For further details see the new section on "Implementing
     an external ME+PS combination scheme and interfacing this plugin
     with Pythia" on the Matching and Merging manual page.
   - Added some new (optional) virtual functions to the timelike and
     spacelike showers, to ease ME+PS merging with shower plugin codes.
     The change does not impair the compatibility of existing shower
     plugin codes. See the description on how to Implement New Showers.

* Four central members from the NNPDF 3.1 sets are made available,
   as PDF:pSet (and equivalent) codes 17 - 20: the LO ones with
   alpha_s = 0.130 and 0.118, and the NLO and NNLO ones with the
   latter alpha_s. Note that these are rather different from the
   default NNPDF 2.3 ones; in particular the small-x behaviour is
   completely changed. Therefore retunes will be necessary before
   using NNPDF 3.1 in production. Thanks to Juan Rojo.

* Add options to use the DIS Q^2 variable as factorization and/or
   renormalization scale.

* Introduce possibility to make phase space cuts on the DIS Q^2
   variable.

* Modified kinematics methods for DIS and photoproduction physics,
   to take beam particle masses into account where important.

* Introduce some freedom to modify the default shape of the
   low-mass part of diffractive cross sections, and thereby also
   the integrated value.

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Administrative changes

* Modified configure and Makefiles automatizes the selection of whether
   to link static or shared libraries to the example main programs, and
   fixes an issue with linking shared libraries on a Mac.

* The new LHAupHelaconia class in include/Pythia8Plugins/LHAHelaconia.h
   provides an interface to the HelacOnia package for onium production,
   see further the new HelacOnia Processes page. The new main35.cc example
   shows how to use the interface, and how to compare with corresponding
   internal Pythia results.

* A new approach has been introduced to force settings values outside
   their allowed range, either by using the keyword FORCE after the key
   (= parameter name) in an readString() or readFile() command line,
   or by using a new optional third argument force = true (which is
   false by default) in the Settings methods used to change values.
   The latter methods also will add a new key to the database, if not
   already there, when force = true is set. The old special force
   methods are now redundant and will be removed in the next major
   release.

* New methods Settings::getReadHistory and ParticleData::getReadHistory
   return a vector with all strings that have been read in by the
   Settings::readString() and ParticleData::readString() methods,
   respectively, and thereby also the information set by the
   Pythia::readString() and Pythia::readFile() commands.

* New method Pythia::addUserHooksPtr(...) allows the simultaneous use
   of several User Hooks. When several hooks are applicable for a
   given task the net effect is multiplicative, in weights or in veto
   survival.

* New User Hooks added to set the space-time vertices for the ISR,
   FSR and MPI evolution process. New main65.cc example illustrates
   how to set it up. Thanks to Christian Bierlich.

* Upgrade from fjcore version 3.0.5 to 3.2.1. This removes the usage
   of the deprecated std::auto_ptr C++ feature. Thanks to Ivan Razumov.

* Two new Vec4 methods introduced: cross4 for cross product of three
   four-vectors, and getTwoPerpendicular to create to four-vectors
   perpendicular to two given ones.

* New main74.cc illustrates how the modified Mass Drop Tagger code in
   FastJet can be used to improve mass reconstruction of a resonance.

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

Bug fixes

* Bug fix the calculation of enhancement factor for the machinery
   with two hard processes. By mistake statistics was added once with
   the correct value for each event (accessible with Info::enhanceMPI()),
   and once with unity, leading to a dilution of the effect. The average
   enhancement factor is now also calculated at initialization, see
   Info::enhanceMPIavg() and Info::enhanceMPIoldavg().

* Bug fix in SusyLesHouches.cc, in which the unitary checks of SLHA
   mixing matrices previously ignored imaginary components, leading to
   failures when reading in spectra with explicit CP violation. Thanks
   to M. Noormandipur for pointing to this bug. Mixing-matrix output
   simultaneously updated so that the magnitudes, rather than the real
   parts, of mixing-matrix elements are printed.

* Bug fixes for the squark-gluino and gluino-chargino processes.
   The charge-conjugate processes were not handled correctly, since
   the PDF factors are different, and have now been separated.

* Minor division-by-zero bug fix in statistics calculation and
   harmless uninitialized bools in CoupSUSY. Thanks to Vittorio Zecca.

* Bug fix in HVStringFlav, which otherwise left some Hidden Valley
   particles massless. Thanks to Colleen Treado.

* A few minor fixes when Dark Matter is used as incoming beams in
   Les Houches input. Thanks to Roberto Ruiz.

* Bug fix in LED/Unparticle + Z^0 production: correct cross section
   for allowed Z^0 decay channels. Thanks to Andreas Albert.

* Minor bug fix where the process container for resonance decays
   (only) from LHE input is not initialized. This can lead to the
   problem where particle input without lifetimes are not corrected,
   even when the LesHouches:setLifetime mode is non-zero.

* Construct pi^+- PDFs so that dbar = u and d = ubar, shortcutting
   the returned d, dbar values since these are not always constructed
   correctly in LHAPDF. Thanks to Vincent Andrieux.

* When using the PhaseSpace:bias2Selection to reweight high-pT events
   the PhaseSpace:pTHatMinDiverge value is now used as ultimate
   fail-safe to avoid pT = 0.

* Fix that the MPI machinery did not work for the (infrequently used)
   MultipartonInteractions:bProfile = 0 option.

* Bug fix in the user hooks machinery for setting fragmentation
   parameters, and an extension of this framework also to junction
   topologies.

* Fixed memory leak in TimeShower, for the case when the ProcessLevel
   is off. Thanks to Ryosuke Sato.

* Bug fix in the TimeShower::findMEtype(...) for a few rare cases.

* Fix a number of harmless but annoying warnings issued in some
   versions of GCC 6.x, where the -Wmisleading-indentation
   flag appears to be on by default. Thanks to Ivan Razumov.

* Minor addition to Streams.cc includes to avoid problems on one
   platform. Thanks to Joshua Ellis.

* Minor numerical precision improvements in gamma-gamma kinematics
   and in tHat and uHat construction.

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