[Pythia8-announce] Pythia 8.304 now available

[Torbjörn Sjöstrand]

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Dear All,

PYTHIA 8.304 has been released today, and can be downloaded from
       http://home.thep.lu.se/Pythia
as usual.

The main news is a significantly updated version of the Vincia shower
framework. There is also a sprinkling of other news, and the traditional
set of bug fixes.

We remind that Pythia 8.3 is to be considered the main branch.
As for 8.303, however, some recent matching-and-merging fixes
in 8.245 still have to find their way into 8.3.

You find a summary of changes below, based on the list in the
Update History, found e.g. in the html manual. It starts with the
major new features and then proceeds towards smaller news and
bug fixes.

Wishing you an enjoyable experience with PYTHIA 8.304.

Sincerely yours,
The Pythia 8 Collaboration
(Christian Bierlich, Nishita Desai, Leif Gellersen, Ilkka Helenius,
Philip Ilten, Leif Lönnblad, Stephen Mrenna, Stefan Prestel,
Christian Preuss, Torbjörn Sjöstrand, Peter Skands, Marius Utheim,
Rob Verheyen)

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Summary list of changes, starting with the major physics news and
then proceeding towards bug fixes:

* Christian Preuss and Rob Verheyen join the author team,
   and Christine Rasmussen leaves it.

* Significant updates to the Vincia shower implementation.
   - New "sector" antenna showers implemented and made default.
   - Minor changes to a few of the default Vincia hadronisation
     parameters, reflecting the new sector shower default.
   - New dedicated LO merging scheme for the sector antenna showers.
     The scheme is similar to CKKW-L in spirit, but exploits the
     bijective nature of the sector shower to vastly reduce the
     number of histories that need to be taken into account.
   - New full-fledged electroweak shower option implemented. The
     splitting kernels are based on collinear limits of corresponding
     helicity matrix elements, so this EW shower model requires Born
     partons with assigned helicities. Currently these must either be
     input via LHEF or (for experts) runtime via Vincia's interface
     to MG5 matrix elements.
   - Introduced tools to automatically generate MadGraph plugin
     libraries.
   - New options for interleaved resonance decays implemented and
     made default.

* Interleaved resonance decays are also introduced for the standard
   simple showers, but is not made default there.

* Revised internal handling of shower variations and splitting
   enhancements, both for regular and trial emissions. Enhancements
   of rare shower splittings are now steered by settings instead of
   UserHooks, and the necessary weights are automatically included
   in the shower and merging weights.

* A number of updates have been made to sophisticated tau decays.
   - External SPINUP is now always interpreted as a helicity state in
     the lab frame to ensure consistent treatment between tau
     decays and helicity showers.
   - The default external mode for tau decays has been changed to
     handle helicity showers.
   - gamma gamma -> tau^+ tau^- processes are now handled.
   - Helicity states for initial beams can be specified and used in
     tau decays.

* Added HDF5 support.

* Added support for lepton-photon initiated hard processes generated
   internally or externally. Thanks to Liu Yao-Bei.

* Minor code changes to allow hadronic rescattering in nuclear
   collisions (remove limit on number of rescatterings, increase maximum
   transverse vertex location).

* Double quarkonia production, if enabled, is now also included in
   both MPI and second hard processes.

* Added some new functionality to the Hist class.

* Moved the event information in the Info class to an appropriate
   diffractive system for hard diffraction. This way also the
   hard-process scale is properly propagated for primordial kT sampling.

* The stopMassNow in StringFragmentation.cc is made to increase faster
   for charm and bottom end quarks, to allow an increased few-body
   fraction further above threshold. Thanks to Andy Buckley.

* Added ResonanceDecayFilterHook and example main103.cc to demonstrate
   how to select specific final states from resonance decays.

* Fixed vertex smearing in heavy ion events.

* Fixed remnant handling for e+e- collisions for photon-initiated
   processes, thanks to Murat Köksal.

* Minor fixes in xml/html QCD process links and text.

* Several small bug fixes based on error reports by Vittorio Zecca.
   - Fixed a bug where some elastic cross sections for pi K at low
     energies were set equal to the total ones.
   - Protect against too high constituent masses inside a hadron in
     LowEnergyProcess.cc, already at an earlier stage of the process.
   - Protect against exp(-infinity) in HadronLevel.cc.
   - Two fixes for first step in the trial emissions in
     StringFragmentation.cc, used primarily in the thermal model option.
   - Properly set the photon-in-lepton PDF to vanish at very large x,
     rather than give unphysical values there.
   - Better protection against weird results for the rap and eta methods
     in Basics.h, and y in Event.cc.
   - Fixed pointer logic in Dire getXPDF.
   - Migrated from log(1 + x) to log1p(x) for numerical stability.

* Several small fixes in response to a static code analysis,
   provided by Dmitri Konstantinov.
   - Fix inconsistency in the usage of the third argument of the dsigmaEl
     method in the classes derived from SigmaTotAux.
   - Flawed and unused LowEnergySigma::sigmaPartial method removed.
   - Fix inconsistent type for varWidthIn in ParticleDataEntry class.
   - Introduce minor changes, e.g. to protect against out-of-bounds
     addressing or insert missing initializer, even though there is no
     evidence that it is needed.
   - Small speedup of the Vec4 [] operator using switch() instead of if
     to find element to be returned.
   - Several (mostly minor) optimisations in Vincia.

* Bug fixed in the new possibility to let particles with narrow widths
   (such as onia states) obtain a simple Breit-Wigner distribution,
   specifically for MPIs, introduced in 8.240. This bug led to excessive
   rejection of low-invariant-mass interactions. Thanks to Albert De Roeck.

* Default event record checks switched off for main21.cc, to avoid issues
   with junction mother-daughter relations not being fully set. Also change
   so that showers are not called twice. Now all available cases are looped
   over, but easy to change back to study only one.

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