Sherpa 2.2.0 (Cho Oyu)

Version 2.2.0 is a feature-release based on the second NLO version of Sherpa. The program now supports the UFO model format, it allows to compute scale uncertainties for one and the same event sample on-the-fly, and it hosts a new parton shower. A more detailed changelog can be found below.

If you spot any problem, please send us an email. This will allow us to reach a mature version of the generator, suitable for all physics analyses. If you would like to be informed about patches and newer releases, please subscribe to our ​announcement mailing list.

Download

The Sherpa package can be downloaded here:

• ​SHERPA-MC-2.2.0.tar.gz

In can also be accessed via git using

• the web interface
• or git clone -b rel-2-2-0 https://gitlab.com/sherpa-team/sherpa.git for a local clone.

A Manual is included in the Manual/ directory of the tarball. It is available in the HTML and INFO format and provides extensive documentation of all parameters. It can also be found online.

References

The main reference describing the physics of Sherpa is ​JHEP02 (2009) 007. Please cite this publication if you have used Sherpa for your studies. Additionally, each run will produce a file called Sherpa_References.tex which contains information about the modules used in this run and the corresponding publications which should be referred. If in doubt, please seek the advice of the Sherpa authors.

Examples

You can find a large collection of example setups for using SHERPA in various different colliders/processes in the Examples/ directory of the tarball. They are also described in the manual.

Tests

A test suite is now available for Sherpa. It can be obtained from svn

• https://phab.hepforge.org/source/sherpasvn/browse/branches/tests-2-2-0

Tests are prepared using 'scons sherpa=/path/to/sherpa', and run using './run.py'.

Changelog

Interface changes

• New convention for Model names and coupling order specification. Most notably, the old Order_EW and Order_QCD switches have been replaced with a more generic Order switch, cf. our manual.
• New default PDF: NNPDF3.0(NNLO) with NNPDFDriver version 1.0.5
• New FSR/hadronisation tuning
• New MPI/ISR tuning for default PDF
• New and more convenient steering options for the enabling/disabling of hard decay channels, replacing the HDH_DECAY_ONLY and HDH_NO_DECAY options. Please refer to the manual for HDH_STATUS and HDH_WIDTH.

New features

• BSM models are supported via UFO, as described in ​EPJC75(2015)135.
• The specification of coupling orders has changed to accomodate arbitrary BSM models, cf. Manual. This also allows to calculate interference terms directly. Most notably, the old Order_EW and Order_QCD switches have been replaced with a more generic Order switch, cf. our manual.
• The full top-mass dependent gluon-Higgs interactions are implemented as described in ​JHEP02(2015)038.
• The OpenLoops interface for one-loop contributions has been enhanced to support QCD-loop calculations with differing masses and Yukawa couplings, as far as they are supported by OpenLoops.
• On the fly scale and PDF variations are supported for LO, LOPS, MEPS, NLO and NLOPS calculations, for details see Manual. Similarly, the weight components to perform arbitrary a posteriori scale variations can be written into HepMC's WeightContainer object, cf. Manual.
• New switch HEPMC_TREE_LIKE that triggers straightening of GenEvent output to a more tree like structure by disconnecting particles coming in/out of parton shower vertex (id=4) to production/end vertices of type 1,2 or 3.
• A new parton shower is implemented, as described in ​arXiv:1506.05057, but is not connected to the full matching/merging implementation and thus not used by default, yet.
• Update of OpenLoops interface to remain compatible with future releases of OpenLoops.
• Restore FixedDecays functionality.

Bugfixes

• A bug in the simulation of multiple parton scattering has been fixed. This will affect the rapidity spectra of low-pT jets.
• A bug in the simulation of QED bremsstrahlung off W->lnu decays has been fixed. This affected the cut-off dependence of the radiative energy loss of the lepton.
• A bug in the cluster hadronisation has been fixed. It lead to the very rare production of charge violating final states.
• A bug has been fixed which was leading to infinite loops (stalling event generation) when tau spin correlations have been enabled simultaneously with multiple parton interactions.
• Several bugs related to flavour remapping have been fixed in Amegic. These did previously lead to fatal error messages when using e.g. multi-vector-boson production processes in NLO+PS matching.
• A significant difference between the integration results obtained with the METS and STRICT_METS scale setter in V+jets production has been fixed.