[Pythia8-announce] Pythia 8.303 now available

[Torbjörn Sjöstrand]

Dear All,

PYTHIA 8.303 has been released today, and can be downloaded from
       http://home.thep.lu.se/Pythia
as usual.

This version includes several new physics modules, that extend the
usefulness of the code. There is also a sprinkling of other news,
and the traditional set of bug fixes.

While the transition from 8.2 to 8.3 was intended to be smooth
for the user, many technical changes were made under the hood.
Therefore the 8.301 and 8.302 versions were still deemed slightly
experimental, even if fully functional. With 8.303 we believe
that the 8.3 series now offers sufficient advantages to motivate
a transition from the 8.2 one. The one limitation is that some
recent matching-and-merging fixes still have to find their way
into 8.3. As a consequence, the Pythia webpage now lists 8.3
as the present main line, and 8.2 as the recent one, instead of
future and present ones, respectively.

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 8.303.

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

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

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

* Marius Utheim and Leif Gellersen join the author team.

* A new framework for Hadronic Rescattering has been developed.
   To enable rescattering, use HadronLevel:Rescatter = on, or see
   example programs main151.cc through main156.cc for use examples.
   Note that while the framework is ready to use, it is still being
   actively developed, and changes are likely in the near future.
   The older and much simpler rescattering models have been removed,
   including the related pp tunes 33 and 34, and there has been some
   file reorganization in code and documentation.

* A new class HadronWidths has been added to handle hadrons with
   mass-dependent widths. This has been done primarily because of its
   importance in resonance formation in rescattering, but the class
   is also used for decays of hadrons produced in other processes.

* A new class LowEnergySigma has been added to calculate cross sections
   of hadron-hadron interactions at low energies. These are used in
   rescattering and when performing primary collisions at low energies.
   Cross sections can be accessed using Pythia::getLowEnergySigma.

* Several changes in the LowEnergyProcess and MiniStringFragmentation
   frameworks, in particular with several new approaches to produce
   nondiffractive inelastic events as close as possible to the
   kinematical threshold. New method StringFlav::combineToLightest
   attempts to form the lightest hadron for a given flavour content.
   Also new possibility to disable popcorn baryon production, for use
   in small systems.

* The "decay"/mixing time of K^0/K^0bar -> K^0_S/K^0_L has been changed
   from 1e-15 to 1e-6 mm, such that it only happens after hadronic
   rescattering, but still well before K^0_S/K^0_L decays.

* The Parton Vertex impact-parameter-plane assignment machinery has
   been completely rewritten and expanded with new options and
   capabilities. The default proton radius has been change from 0.7 to
   0.85 fm.

* Limit the amount by which transverse smearing of the space-time
   quark-antiquark string breakup vertices can give large shifts, see
   further the new HadronVertex:maxSmear parameter. The new
   HadronVertex:maxTau gives the possibility to reject hadrons of
   suspected misreconstructed systems and try again.  The introduction
   of MPI impact parameters is compensated by a reduced transverse
   hadronization smearing. Also some other related changes.

* The internal generation of photon-induced processes in hadronic
   collisions has been extended. This can be enabled with the new
   options PDF:beamA2gamma and PDF:beamB2gamma, which supersede the
   previous PDF:leptonA2gamma option. Some minor updates also for other
   related options, see PDF Selection for details.

* New photon fluxes from hadrons implemented, see PDF Selection for
   details.

* New example main program main78.cc to generate elastic, single- and
   double-dissociative photon-induced processes in proton-proton
   collisions, using options PDF:beamA2gamma  and PDF:beamB2gamma.
   Also the sample programs main69.cc and main70.cc have been updated
   to use the new options.

* For asymmetric beams (e.g. electron-proton collisions) in Les
   Houches event files, a boost to the event CM frame is now defined and
   applied for outgoing particles, as required for successful event
   processing.

* The problem of "dangling" gluons causing changing color flows in
   SimpleTimeShower for top decays is now fixed. The fix is not fully
   validated for RPV.

* For LHE processes, the width of the primordial kT distribution is
   now set solely by the BeamRemnants:primordialKThard  parameter,
   ignorning BeamRemnants:primordialKTsoft.  This change is so that
   LHE events with low-pT extra jets (e.g., in the context of
   POWHEG-style merging) are given the same primordial kT as their
   Born-level counterparts.

* Improved HepMC3 multiweight output.

* Added automated renormalization scale variations in CKKW-L type
   merging schemes.

* Added UNLOPS merging scheme variations.

* Added warning messages in UNLOPS merging.

* Added flexible runtime plugin loading for Dire and Vincia matrix
   element corrections from Madgraph. These corrections can be enabled
   with the --enable-mg5mes flag during configuration and with the
   Dire:MEplugin and Vincia:MEplugin settings.

* Introduced common runtime plugin loading and unloading structure.

* Removed PDF set tracking for LHAPDF6 which breaks thread safety.
   In some cases this might cause a small penalty in memory.

* Removed SLHA:keepSM as it duplicates the functionality provided by
   SLHA:minMassSM.

* Some new methods in the Hist and HistPlot classes. Hist::plotFunc
   creates a histogram from the function provided as first argument,
   by evaluating its value in the middle of each histogram bin.
   Hist::normalize can rescale a histogram to an intended sum of bin
   contents. Methods to get x limits and lowest and highest y value.
   Hist::smallestAbsValue renamed Hist::getYAbsMin for consistency.
   HistPlot::addFile can add a file of data points, with or without
   errors, on to an existing histogram, for comparison purposes.
   A new variant of HistPlot::plot can set the frame borders.

* New LinearInterpolator class in MathTools.

* New RotBstMatrix methods toSameVframe and fromSameVframe sets up
   transformation to/from a frame where two particles move towards
   each other with equal-size velocities along the +-z direction.

* Some new functionality accessible via PythiaStdlib.h.

* It is now possible to use range-based for loops to iterate over
   particles in an event, e.g. for (Particle& p : pythia.event).
   Similarly, the particle data table can be iterated over, with each
   entry being a pair< int, ParticleDataEntry&>.

* New possibility to redirect particle data listing output.

* A configure option --obj-common='FLAGS' to add additional compiler
   flags to object (.o) compilation (i.e. no effect on linking).

* Compilation flag -DGCCFPDEBUG enables floating point exceptions
   on a gcc compiler.

* New index() method in the Py8Particle class of the Fastjet3 interface
   ensures that the location of the particle in the PYTHIA event record
   is returned correctly. Thanks to Gavin Salam and Priyotosh
   Bandyopadhyay.

* A minor fix for kinematics of zero-virtuality photons to retain
   physical kinematics.

* Protect against diffractive masses exceeding phase space bounds for
   hard diffraction.

* A bug for propagation of space-like partons in string shoving.
   Thanks to Dong Jo Kim.

* Several indexing bugs for the "guess" option used in CKKW-L
   merging have been fixed. However, this does not yet mean that
   "guess" is guaranteed to be stable for any BSM process.

* Several fixes for problems discovered in a static code analysis,
   including one bug fix in LowEnergyProcess.cc. Thanks to Dmitri
   Konstantinov.

* Mixup of ASecondHardProcess and SecondHardProcess labels resolved.
   Thanks to Anton Karneyeu.

* Number of weights in info reset to 1 for each new init call.

* Fixed issue where the dark matter mass was reset even after
   explicitly being set by the user, e.g. 57:m0 = X. This affects
   particle IDs 57 (DM+), 58 (DM2), and 59 (DM++).

* An indexing bug for the scattering lepton in DIS events with
   photon radiation allowed has been fixed.

* The redefinition of dlsym for plugin loading has now been removed
   as this is no longer required by modern compilers. Thanks to
   Chris Burr.

* Several small bug fixes based on error reports by Vittorio Zecca.
   - In ProcessContainer.cc the error calculation for Les Houches
     strategy +-3 should not be renewed for each file when the
     event sample is split across several files.
   - In ProcessContainer::trialProcess() a call to
     phaseSpacePtr->rescaleMomenta( sHatNew); should not be
     done since the momenta have not been set yet.
   - In the CT14qed PDF grid (and others?) the first two x grid values
     coincide, so one must not divide by their difference.
   - Default initialization of hasOwnEventAttributes in Info.h is
     necessary.
   - Protected against explicit evaluation of alphaS(LambdaQCD) in
     VinciaISR. Also added explicit member-variable initialisations
     for the QEDemitElemental class in VinciaQED.
   - Protected against floating point exceptions caused by dividing
     by zero in PhaseSpace for zero-width resonances.
   - Corrected a floating point exception in Ropewalk.cc.
   - Protected against out of range renormalization scale access for
     diffractive systems.

* Some further replacements of fabs by abs.

* Fix missing-pointer bug when colour reconnection is switched off
   for diffraction. Thanks to Valentina Zaccolo.