[Pythia8-announce] Pythia 8.223 now available

[Torbjorn Sjostrand]

Dear All,

PYTHIA 8.223 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 8.223.

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)

Note that Nadine Fischer and Leif Lönnblad now join a co-authors,
while Jesper Roy Christiansen has left.

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Physics changes

* - A significantly extended machinery for resolved gamma-gamma
   collisions, such that now soft processes and MPIs can be simulated,
   also when embedded in l^+l^- collisions. (But not yet diffraction.)
   - Also direct-resolved and direct-direct processes are included.
   This involves new photon-parton-initiated subprocesses.
   - A new sample main program (main69.cc) illustrates how the
   different gamma-gamma interaction classes are combined.
   - The gamma-gamma kinematics have been revised to include all
   mass corrections and to handle also non-equal leptons.
   - New cuts for the kinematics of gamma-gamma interactions in l^+l^-
   collisions, with matching new kinematics output methods.
   -  A PDF for point-like photon is included.
   - In case of lepton PDFs, the photon contribution has now restricted
   virtuality and also more accurate lower limit for the virtuality.
   - A new option to use separate PDFs for hard processes, with photon
   PDFs obtained from LHAPDF5.
   - CJKL PDFs are modified so that, instead of freezing the scale below
   its minimum, the scale evolution is approximated with log(Q^2).
   - Also some further improvements have been introduced, mainly
   related to beam remnants and beam particles.

* - A new alternative "thermal hadronization" option is introduced,
   wherein an exponential exp(-pT/T) hadronic transverse momentum
   spectrum replaces the default Gaussian one, with a "temperature" T
   as free parameter. Given this pT, the next hadron (consistent with
   local flavour conservation) is picked among the possibilities with
   an exp(-mT/T) weight.
   - A new close-packing option allows to enhance the pT width in
   regions with a high density of partly overlapping strings. This works
   both for the default Gaussian and the new exponential pT description;
   in the latter case it will also enhance the rate of heavier particles.
   - A new simple model for hadronic rescattering is introduced,
   with two variants.
   - For further details, see N. Fischer and T. Sjöstrand,
   "Thermodynamic String Fragmentation", arXiv:1610.09818 [hep-ph]

* A first process for Dark Matter production, for the pair production
   of new Dirac DM particles through an s-channel vector-like mediator.

* New option with running coupling in  Hidden Valley scenarios,
   plus some other small fixes.

* Included rudimentary facilities to guess the process for merging.

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Administative changes

* New mode can be used to let original hadron remain as beam remnant,
   e.g. for coherent emission of photons off a proton.

* Added functionality to write Pythia events to an LHEF3-style string,
   e.g. for use in an external Pythia caller.

* Improved safety checks for the presence of LHE files.

* New status codes 49 and 59 introduced for ISR and FSR partons
   where E^2 - p^2 = m^2 is not fulfilled.

* New behaviour of Event::remove, where mother and daughter indices
   now are updated by default.

* New particle data method nQuarksInCode(int idQ) counts how many
   copies of the requested quark code idQ that a quark, diquark, meson
   or baryon code contains.

* New methods string Hist::getTitle() and void Hist::title(string) to
   get or set the title  of a histogram. Thanks to Roberto Franceschini.

* Two new Hist::rivetTable methods allow histograms to be written
   on file in a format that Rivet understands.

* Bring the FJcore package inside the Pythia8 namespace to avoid
   potential name clashes with user code. Thanks to Andy Buckley.

* Minor updates so that the main91 and main92 examples work also with
   ROOT 6, in addition to ROOT 5. Thanks to Li Huang.

* New #define PYTHIA_VERSION_INTEGER 82xx in Pythia.h matches already
   existing #define PYTHIA_VERSION 8.2xx, for more convenient matching
   using integers. Thanks to Andrii Verbytskyi.

* Some small clarifications here and there in the documentation.

* Year updated to 2017.

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

Bug fixes

* Fix in the setup of tunes with SpaceShower:rapidityOrder = off.
   The new (in 8.219) SpaceShower:rapidityOrderMPI then also ought to
   have been set off, but this was missed, giving small inconsistencies
   (around 2% reduction of the multiplicity). Thanks to James Monk.

* The handling of the particle decay meMode ranges 52-60 and 62-70
   were incorrect, insofar as checks or not against duplication of
   existing channels go. Thanks to Christopher West.

* Corrected behaviour for R-hadrons produced in sequential resonance
   decays. Thanks to Jinmian Li.

* Include correct mass suppression for excited fermion three-body
   decays. Thanks to Olya Igonkina and Oleg Zenin.

* Fixed a check in the construction of shower histories for  merging,
   which meant that not all histories were produced for squarks+jets.

* Minor bug fix in the TimeShower machinery to optionally enhance
   the rate of some shower branchings.

* A minor fix for BeamParticle::popBack() method.

* Minor particle data fixes and some other xml tag fixes.

* Small fix in the parsing code of LHEF3.h.

* Fixed flat_namespace issue for macOS. Thanks to Juergen Reuter.

* Ensure that bash shell is used in Makefiles. Thanks to Inga Strumke.

* Cleanup of unmatched xml tags and other xml inconsistencies.

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