8.3. External one-loop ME

Sherpa includes only a very limited selection of one-loop matrix elements. To make full use of the implemented automated dipole subtraction it is possible to link external one-loop codes to Sherpa in order to perform full calculations at QCD next-to-leading order.

In general Sherpa can take care of any piece of the calculation except one-loop matrix elements, i.e. the born ME, the real correction, the real and integrated subtraction terms as well as the phase space integration and PDF weights for hadron collisions. Sherpa will provide sets of four-momenta and request for a specific parton level process the helicity and colour summed one-loop matrix element (more specific: the coefficients of the Laurent series in the dimensional regularization parameter epsilon up to the order epsilon^0).

An example setup for interfacing such an external one-loop code, following the Binoth Les Houches interface proposal [B+10] of the 2009 Les Houches workshop, is provided in Zbb production. To use the LH-OLE interface, Sherpa has to be configured with -DSHERPA_ENABLE_LHOLE=ON.

The interface:

  • During an initialization run Sherpa stores setup information (schemes, model information etc.) and requests a list of parton-level one-loop processes that are needed for the NLO calculation. This information is stored in a file, by default called OLE_order.lh. The external one-loop code (OLE) should confirm these settings/requests and write out a file OLE_contract.lh. Both filenames can be customised using LHOLE_ORDERFILE: <order-file> and <LHOLE_CONTRACTFILE: <contract-file>. For the syntax of these files and more details see [B+10].

    For Sherpa the output/input of the order/contract file is handled in LH_OLE_Communicator.[CH]. The actual interface is contained in LH_OLE_Interface.C. The parameters to be exchanged with the OLE are defined in the latter file via

    lhfile.AddParameter(...);
    

    and might require an update for specific OLE or processes. Per default, in addition to the standard options MatrixElementSquareType, CorrectionType, IRregularisation, AlphasPower, AlphaPower and OperationMode the masses and width of the W, Z and Higgs bosons and the top and bottom quarks are written out in free format, such that the respective OLE parameters can be easily synchronised.

  • At runtime the communication is performed via function calls. To allow Sherpa to call the external code the functions

    void OLP_Start(const char * filename);
    void OLP_EvalSubProcess(int,double*,double,double,double*);
    

    which are defined and called in LH_OLE_Interface.C must be specified. For keywords and possible data fields passed with this functions see [B+10].

    The function OLP_Start(...) is called once when Sherpa is starting. The function OLP_EvalSubProcess(...) will be called many times for different subprocesses and momentum configurations.

The setup (cf. example Zbb production):

  • The line Loop_Generator: LHOLE tells the code to use the interface for computing one-loop matrix elements.

  • The switch SHERPA_LDADD has to be set to the appropriate library name (and path) of the one-loop generator.

  • The IR regularisation scheme can be set via LHOLE_IR_REGULARISATION. Possible values are DRED (default) and CDR.

  • Per default, Sherpa generates phase space points in the lab frame. If LHOLE_BOOST_TO_CMS: true is set, these phase space points are boosted to the centre of mass system before they are passed to the OLE.

  • The original BLHA interface does not allow for run-time parameter passing. While this is discussed for an updated of the accord a workable solution is implemented for the use of GoSam and enabled through LHOLE_OLP: GoSam. The LHOLE_BOOST_TO_CMS is also automatically active with this setup. This, of course, can be adapted for other one-loop programs if need be.

  • Sherpa’s internal analysis package can be used to generate a few histograms. Thus, then when installing Sherpa the option -DSHERPA_ENABLE_ANALYSIS=ON must be include on the command line when Sherpa is configured, see ANALYSIS.