Version 1.2.0 is a major update of Sherpa which contains many physics improvements and also some minor user interface changes. Correspondingly, the input structure has been improved, cf. the online manual and changelog below for a summary of updates.

The Sherpa package can be downloaded here:

• SHERPA-MC-1.2.0.tar.gz

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

• SHERPA-MC-1.2.0 (split pages)
• SHERPA-MC-1.2.0.html (all in one page)

A publication describing the physics of the SHERPA Monte-Carlo can be found in JHEP02 (2009) 007. This is meant to be the standard reference, please cite this publication if you have used SHERPA for your studies.

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 briefly described in the manual. The release has been tested for various processes at hadron colliders, e.g. production of Z+jets, W+jets, pure QCD, di-boson + jets, Higgs through effective couplings, top-pairs, Higgs associated with a top-pair, and also various BSM processes. You can also find examples for non-hadron collider setups.

A Changelog for important differences between the previous version and this version is provided below.

If you would like to be kept up-to-date about new versions or important updates of Sherpa, please subscribe to our announcement mailing list.

Please note that you still have to apply any patches provided below after unpacking the tarball.

• Sherpa was not compiling with GNU Compiler Collection Version 4.4.0 and higher. Fix: Apply patch file via patch -p0 < gcc-4.4.x.patch in the top level directory of your Sherpa installation, and recompile with make install.
• The NLO example referenced in the Manual was missing in the tarball. Fix: Apply patch file via patch -p0 < nlo-example.patch in the top level directory of your Sherpa installation, and recompile with make install.
• Multi-parton interactions were not working for most BSM models. Fix: Apply patch file via patch -p0 < MPIforBSM.patch in the top level directory of your Sherpa installation, and recompile with make install.
• The status code of some incoming partons was set incorrectly. Fix: Apply patch file via patch -p0 < ISPartonStatusCode.patch in the top level directory of your Sherpa installation, and recompile with make install.
• Newer autotools do not allow for duplicate header entries leading to the abort of the installation procedure. Fix: Apply patch file via patch -p0 < dupl_header_remove.patch in the top level directory of your Sherpa installation and recompile with autoreconf -i; ./configure [options...]; make install.
• Vertices for the AGC model are switched off by mistake if certain anomalous couplings are set to zero. Fix: Apply patch file via patch -p0 < agc_fix.patch in the top level directory of your Sherpa installation, and recompile with make install.
• For event generation with disabled METS merging the cross section error estimate might return nan‘s. Fix: Apply patch file via patch -p0 < xs-error-nan.patch in the top level directory of your Sherpa installation, and recompile with make install.
• In event generation runs with METS merging using AMEGIC, the parton shower might abort with the error message “Missing colour partner”. Fix: Apply patch file via patch -p0 < amegic_cluster_1.patch in the top level directory of your Sherpa installation, and recompile with make install.

• Online Manual
  The online manual described above is also contained in the Manual/ subdirectory of the Sherpa package.
• Matrix-Element generators and processes
  With multiple automated Matrix-Element generators available, SHERPA can use any combination of them in the same run, see the description of ME_SIGNAL_GENERATOR in the Manual. To alleviate read-in and to generalise the handling of the various ME generators many of the input parameters in the (processes) section of the input card were renamed. Please refer to the examples and to the Processes section of the online manual for possible changes to your run card.
• Matrix-Element Parton-Shower merging
  The merging algorithm is now initialised by the keyword CKKW directly in the merged process specification, followed by the value of the merging cut to be used. The separate specification of a JetFinder selector is not appropriate anymore and will eventually lead to wrong results. Cf. the description of the CKKW keyword in the Manual.
• Scales and couplings
  The setting of factorisation and renormalisation scales has been made clearer and more flexible. Cf. the description of SCALES and COUPLINGS in the Manual.
• Citation recommendations
  At the end of each Sherpa run, a file called Sherpa_References.tex is written to disk, containing links to recommended references for the user. Please follow the on-screen instructions to generate a bibtex list of references from this file.
• Internal event record
  In line with switching to the new dipole-like Parton-Shower module CSS (see below), the structure of the internal event record has changed such that now only one Blob accomodates the complete Parton-Shower evolution. Initial and final state emissions cannot be disentangled.
• New include structure
  If you are maintaining an interface between SHERPA and another software, you will have to make small adjustments to your #include directives. SHERPA sorts its header files into separate directories now, so e.g. #include “Sherpa.H” becomes #include “SHERPA/Main/Sherpa.H”. Please contact us, if you need any help with this.
• Example setups
  A wide range of example setups is available in the Examples/ directory of the tarball. See also Examples in the Manual for a short description of each of them.

• New Matrix-Element generator
  The automated tree-level Matrix-Element generator Comix is included. It is described in detail in JHEP12 (2008) 039 with a brief summary contained in the online Manual.
• New Parton-Shower module
  A dipole-like Parton Shower is made the new default shower module of Sherpa, superseeding the old virtuality ordered shower module APACIC++. It is described in detail in JHEP03 (2008) 038 with a short summary given in the online Manual.
• Improved Matrix-Element + Parton-Shower Merging
  A new version of the merging algorithm is implemented. It is described in JHEP05 (2009) 053 and also summarised in the Manual together with usage instructions.
• Dipole subtraction for NLO ME’s
• FeynRules interface
  An interface to new physics model implementations in the FeynRules package is provided. For details on FeynRules, the Sherpa interface and some reference results cf. arXiv:0906.2474 [hep-ph]. Further details can be found in the Model section of the Manual.
• Hidden Valley parton shower
• YFS for hard leptons
  QED corrections to leptons in the hard process are now effected using a YFS type resummation supplemented with a first order correction for hard emissions, as described in JHEP12 (2008) 018. See also Parameters → QED Corrections in the Manual.
• Rivet interface
  It is now possible to run Rivet analyses on top of Sherpa events directly from the Sherpa run card. See Tips and Tricks in the Manual.
• LesHouches interface for one-loop-amplitudes
• Hadronisation improvements
• Output of weighted events
  It is now possible to output weighted events such that they can be used for an external analysis. This also allows for the consistent inclusion of enhancement factors/functions, e.g. to increase statistics for higher jet multiplicities or in high pT tails.
• Total cross section programmatically accessible
  The total cross section for a given simulation is printed at the end of each Sherpa run and can be retrieved online by the user, either through HepMC or directly from the Sherpa instance. Details are given in the Manual.