[Pythia8-announce] PYTHIA 8.200 released

[Torbjorn Sjostrand]

Dear All,

PYTHIA 8.200 has been released today, and can be downloaded from
http://home.thep.lu.se/~torbjorn/Pythia.html
as usual. As the number indicates, this marks the start of a new
series. The release is coordinated with the publication of a new
article "An Introduction to PYTHIA 8.2", still without an arXiv
number, to be submitted to Computer Physics Communications, and
also an updated Worksheet for tutorials.

The list of changes is sufficiently long that it has been split
in three sections: administrative, physics and other. Still it
should be emphasized that, in most respects, there is direct
continuity. User-written programs that worked with 8.186 are also
likely to work with 8.2, or at most require modest adjustments.

Given the low threshold, we expect the advantages with the new
version to be sufficiently large that the transition should
happen rapidly. However, should the need arise, a bug update
could still be issued for the 8.1 series.

Wishing you an enjoyable experience with PYTHIA 8.2.

Sincerely yours,
The Pythia 8 Collaboration
(Torbjörn Sjöstrand, Jesper Roy Christiansen, Nishita Desai,
Philip Ilten, Stephen Mrenna, Stefan Prestel,
Christine Rasmussen, Peter Skands)

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

Administrative changes

* A new share/Pythia8 directory collects all documentation and
example code. The examples, htmldoc, phpdoc and xmldoc directories
have been moved here. The main-directory files AUTHORS, COPYING,
GUIDELINE and README are also copied here during installation.

* A new share/Pythia8/pdfdoc directory collects pdf documents that
are linked from the htmldoc and phpdoc directories. Over time it
will provide more in-depth descriptions of various physics aspects
than offered in the html/php-formatted documentation. In addition
to the official main publication and the worksheet, currently
notes on LO vs. NLO PDFs and on the g -> q qbar branching kernel
are included.

* A new include/Pythia8Plugins directory collects code that does
not form part of the core PYTHIA functionality but still has a
general usefulness. Code in this directory will not be compiled
as part of the Pythia library, but can be linked where needed.
This new directory contains
   - the jet matching classes in CombineMatchingInput.h,
     GeneratorInput.h and JetMatching.h, moved from the examples
     directory;
   - the PowhegHooks user hook, to veto shower emissions above the
     POWHEG scale, formerly found in examples/main31.cc;
   - the Pythia8ToHepMC interface for output of PYTHIA events
     into the HepMC format, combining the code previously in
     include/Pythia8ToHepMC.h and pythia8tohepmc/Pythia8ToHepMC.cc
     into a new HepMC2.h file;
   - the FastJet3.h interface of PYTHIA particles to the FastJet 3
     library of jet finders, formerly found in include/FastJet3.h;
     and
   - the LHAPDF5.h and LHAPDF6.h files for interfaces to the LHAPDF
     library (see further below).

* The pythia8-config.in script has been replaced by a new
bin/pythia8-config script. See the README file for details.

* The configure/make structure has been considerably rewritten.
Now all external libraries to be linked are specified in the
main-directory configure step, along with other options, so there
is no longer an examples/configure. The make step will, as before,
compile and install libraries inside the current directory, such
that the main programs in the examples directory can be run. One
small difference is that also the archive libraries are installed
in lib and not in lib/archiv. The Makefiles have been updated to
take into account the changed structure of the HepMC interface.

* A new optional make install step allows you to copy files to more
convenient locations. The default option, with no directories
specified in the configure step, requires you to have superuser
privileges. Then files will be copied to standard locations as
follows:
   lib/ -> /usr/lib/
   include -> /usr/include/
   share -> /usr/share/
   pythia-config -> /usr/bin/

* The interface to LHAPDF is now dynamically loaded when requested,
and can be either to version 5 or 6 of the library. The dummy code
previously in lhapdfdummy/LHAPDFDummy.cc, to be linked when LHAPDF
is not, is no longer required. The two new files LHAPDF5.h and
LHAPDF6.h in the include/Pythia8Plugins directory contain the
necessary interface code. The selection of PDF sets, notably for
the proton, has been extended to simplify mixing of internal and
external PDF sets, and it is now possible to specify different PDFs
for the two incoming protons at the LHC, see the PDF Selection
description.

* The new LHEF3.h file contains a generic interface for reading
Les Houches Event Files of versions 1.0, 2.0 and 3.0. This allows
more information to be read and studied by the author. Currently
PYTHIA itself makes little use of the information beyond the one
in 1.0, but it is available among the Event Information.
Examples are found in main37.cc and main38.cc.

* The executable built from examples/mainxx.cc is now named
examples/mainxx, while previously it was named examples/mainxx.exe.

* The rootexamples directory has been removed, and the two programs
examples/main91 and examples/main92 now illustrate how ROOT can be
used in conjunction with PYTHIA.

* The rudimentary support for compilation on Windows platforms,
present in PYTHIA 8.1, has not yet been updated for 8.2 and so
is omitted. Also the README.HepMC file is omitted for now.

* The ProMC interface is broken, and has been removed for now.

* Several methods have been removed from the Event class since
the properties now instead can be accessed from the individual
Particle instance, if this particle belongs to an event. These
include iTopCopy, iBotCopy, iTopCopyId, iBotCopyId,motherList,
daughterList, sisterList, sisterListTopBot, isAncestor, statusHepMC
and undoDecay.

* A number of deprecated Pythia::init(...) methods with varying
arguments have been removed. Instead call init() without any
arguments and use Beam Parameters settings to specify beams and
energies in different ways.

*The deprecated Pythia::statistics(...) method has been removed;
instead use Pythia::stat(...).

*Several settings in the Main: series have been removed. Most of
these have already found replacements in the Init:, Next: and
Stat: ones, and have been marked as deprecated. Four further ones
were deemed so peripheral that they were removed altogether,
but of course the underlying functionality remains.

* A few aliases for (parts of) settings names have been removed.
Previously "Multiple" was mapped to "Multiparton", "MI" to "MPI"
and "minBias" to "nonDiffractive" if a settings name was not
found for the original input string.

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

Physics changes

* The default tune has been changed from 4C to Monash 2013, meaning
Tune:ee = 7 and Tune:pp = 14. The old 4C tune that was default in
8.1 can be recovered with Tune:ee = 3 and Tune:pp = 5. Also most
other older tunes are based on Tune:ee = 3.

* Two new CMS underlying-event tunes and the ATLAS AZ tune have
been added as options.

* The default handling of the g -> q qbar splitting kernel
has been changed, affecting in particular heavy-flavour production.
TimeShower:weightGluonToQuark has been changed from 1 to 4 to do
this. All old tunes are with the 1 value but, since the tunes are
not probing the detailed g -> q qbar behaviour, this is not set
as part of the tune options.

* A new model for the handling of beam remnants as an option to
the old one, which remains as default for now.

* Two new models for colour reconnection, one quite sophisticated
(J.R. Christiansen and P. Skands, in preparation) and one simpler
(S. Argyropoulos and T. Sjöstrand, arXiv:1407.6653 [hep-ph], to
apper in JHEP). This involves several new classes and files.
It also includes some changes in the hadronization framework,
notably for the handling of junctions. The old model remains
as default for now. The BeamRemnants:reconnectColours flag
to switch on/off reconnection has been renamed
ColourReconnection:reconnect, the main parameter
BeamRemnants:reconnectRange of the old model has been renamed
ColourReconnection:range, and several new settings have been
introduced, notably ColourReconnection:mode to switch among the
three models.

* A new include/Pythia8Plugins/ColourReconnectionHooks.h makes
available an even larger selection of toy colour reconnection
models, via user hooks. Some of them are only intended for top
decays, for top mass uncertainty studies, whereas others can be
used more generally. The examples/main29.cc program illustrates
how the different options should be set up.

* Several new features and improvements in the matching/merging
machinery. Notably the aMC at NLO matching scheme has been implemented,
see the aMC at NLO Matching description. To this end the global-recoil
option of timelike showers has been improved, and security checks
have been introduced for inaccurate LHEF input. A new main89.cc
example has been introduced, where different .cmnd files show how
to set up either CKKW-L, FxFx, MLM, UMEPS or UNLOPS merging.

* Improved capability for the LHAup Les Houches interface to read
SLHA information embedded in the input file or stream.

* The new Beams:strictLHEFscale switch can be used to restrict
parton showers in resonance decays to be below the input
Les Houches scale, not only the hard process itself. The new
Beams:setProductionScalesFromLHEF switch can be used to restrict
the emission off each separate parton to be below its specific
scale.

* The Z' production process has been updated to optionally allow
decay to a fourth generation of fermions, with universal or
non-universal couplings.

* Introduction of a new Higgs CP-mixing parametrization via a mixing
angle phi as described for Higgs Processes. The choice of the Higgs
CP-mixing parametrization now also affects the distributions of the
tau decay products from the processes H0 -> tau+ tau-.

* Bug fix in H^0 -> W^+ W^- -> 4 f matrix element for mixed CP-state
case.

* Various improvements and finer grain control for the determination
of tau decay correlations and <ei>tau</ei> polarizations. By default
the decays of tau pairs from known resonance decays in Les Houches
input are now correlated. The ParticleDecays:sophisticatedTau mode
has been renamed TauDecays:mode, as well as all tau-related
ParticleDecay options, with two new options of using only the
internal machinery to determine correlations and polarizations,
and only using the provided SPINUP digit from Les Houches input.
The option TauDecays:externalMode has been introduced to control
the interpretation of the SPINUP digit.

*For Les Houches Event input the energy of a particle is recalculated
from its three-momentum and mass, in order to limit mismatches from
limited numerical precision in the input values.

* Bug fix in the two-loop running alpha_s, for the matching to six
flavours at the top mass.

* Eliminate harmless compiler warnings for FJcore.

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

Other changes

* Christine Rasmussen joins as new PYTHIA collaboration member.

* Peter Skands and Stefan Prestel have updated affiliations and
e-mail addresses.

* Updated Introduction (= the official 8.2 article) and Worksheet.

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