6. Tips and tricks

6.1. Shell completion

Sherpa will install a file named $prefix/share/SHERPA-MC/sherpa-completion which contains tab completion functionality for the bash shell. You simply have to source it in your active shell session by running

$ .  $prefix/share/SHERPA-MC/sherpa-completion

and you will be able to tab-complete any parameters on a Sherpa command line.

To permanently enable this feature in your bash shell, you’ll have to add the source command above to your ~/.bashrc.

6.2. Rivet analyses

Sherpa is equipped with an interface to the analysis tool Rivet. To enable it, Rivet and HepMC have to be installed (e.g. using the Rivet bootstrap script) and your Sherpa compilation has to be configured with the following options:

$ ./configure --enable-hepmc2=/path/to/hepmc2 --enable-rivet=/path/to/rivet

(Note: Both paths are equal if you used the Rivet bootstrap script.)

To use the interface, you need to enable it using the ANALYSIS option and to configure it it using the RIVET settings group as follows:

ANALYSIS: Rivet
RIVET:
  ANALYSES:
    - D0_2008_S7662670
    - CDF_2007_S7057202
    - D0_2004_S5992206
    - CDF_2008_S7828950

The ANALYSES list specifies which Rivet analyses to run and the histogram output file can be changed with the normal ANALYSIS_OUTPUT switch.

You can also use rivet-mkhtml (distributed with Rivet) to create plot webpages from Rivet’s output files:

$ source /path/to/rivetenv.sh   # see below
$ rivet-mkhtml -o output/ file1.yoda [file2.yoda, ...]
$ firefox output/index.html &

If your Rivet installation is not in a standard location, the bootstrap script should have created a rivetenv.sh which you have to source before running the rivet-mkhtml script.

6.3. HZTool analyses

Sherpa is equipped with an interface to the analysis tool HZTool. To enable it, HZTool and CERNLIB have to be installed and your Sherpa compilation has to be configured with the following options:

$ ./configure --enable-hztool=/path/to/hztool --enable-cernlib=/path/to/cernlib --enable-hepevtsize=4000

Note that an example CERNLIB installation bootstrap script is provided in AddOns/HZTool/start_cern_64bit. Note that this script is only provided for convenience, we will not provide support if it is not working as expected.

To use the interface, enable it using the ANALYSIS and configure it using the HZTool settings group:

ANALYSIS: HZTool
HZTOOL:
  HISTO_NAME: output.hbook
  HZ_ENABLE:
  - hz00145
  - hz01073
  - hz02079
  - hz03160

The HZ_ENABLE list specifies which HZTool analyses to run. The histogram output directory can be changed using the ANALYSIS_OUTPUT switch, while HZTOOL:HISTO_NAME specifies the hbook output file.

6.4. MCFM interface

Sherpa is equipped with an interface to the NLO library of MCFM for decdicated processes. To enable it, MCFM has to be installed and compiled into a single library, libMCFM.a. To this end, an installation script is provided in AddOns/MCFM/install_mcfm.sh. Please note, due to some process specific changes that are made by the installation script to the MCFM code, only few selected processes of MCFM-6.3 are available through the interface.

Finally, your Sherpa compilation has to be configured with the following options:

$ ./configure --enable-mcfm=/path/to/mcfm

To use the interface, specify

Loop_Generator: MCFM

in the process section of the run card and add it to the list of generators in ME_GENERATORS. Of course, MCFM’s process.DAT file has to be copied to the current run directory.

6.5. Debugging a crashing/stalled event

6.5.1. Crashing events

If an event crashes, Sherpa tries to obtain all the information needed to reproduce that event and writes it out into a directory named

Status__<date>_<time>

If you are a Sherpa user and want to report this crash to the Sherpa team, please attach a tarball of this directory to your email. This allows us to reproduce your crashed event and debug it.

To debug it yourself, you can follow these steps (Only do this if you are a Sherpa developer, or want to debug a problem in an addon library created by yourself):

  • Copy the random seed out of the status directory into your run path:

    $ cp  Status__<date>_<time>/random.dat  ./
    
  • Run your normal Sherpa commandline with an additional parameter:

    $ Sherpa [...] 'STATUS_PATH: ./'
    

    Sherpa will then read in your random seed from “./random.dat” and generate events from it.

  • Ideally, the first event will lead to the crash you saw earlier, and you can now turn on debugging output to find out more about the details of that event and test code changes to fix it:

    $ Sherpa [...] --output 15 'STATUS_PATH: ./'
    

6.5.2. Stalled events

If event generation seems to stall, you first have to find out the number of the current event. For that you would terminate the stalled Sherpa process (using Ctrl-c) and check in its final output for the number of generated events. Now you can request Sherpa to write out the random seed for the event before the stalled one:

$ Sherpa [...] --events <#events - 1> 'SAVE_STATUS: Status/'

(Replace <#events - 1> using the number you figured out earlier.)

The created status directory can either be sent to the Sherpa developers, or be used in the same steps as above to reproduce that event and debug it.

6.6. Versioned installation

If you want to install different Sherpa versions into the same prefix (e.g. /usr/local), you have to enable versioning of the installed directories by using the configure option --enable-versioning. Optionally you can even pass an argument to this parameter of what you want the version tag to look like.

6.7. NLO calculations

6.7.1. Choosing DIPOLES ALPHA

A variation of the parameter DIPOLES:ALPHA (see Dipole subtraction) changes the contribution from the real (subtracted) piece (RS) and the integrated subtraction terms (I), keeping their sum constant. Varying this parameter provides a nice check of the consistency of the subtraction procedure and it allows to optimize the integration performance of the real correction. This piece has the most complicated momentum phase space and is often the most time consuming part of the NLO calculation. The optimal choice depends on the specific setup and can be determined best by trial.

Hints to find a good value:

  • The smaller DIPOLES:ALPHA is the less dipole term have to be calculated, thus the less time the evaluation/phase space point takes.

  • Too small choices lead to large cancelations between the RS and the I parts and thus to large statisical errors.

  • For very simple processes (with only a total of two partons in the iniatial and the final state of the born process) the best choice is typically DIPOLES: {ALPHA: 1}. The more complicated a process is the smaller DIPOLES:ALPHA should be (e.g. with 5 partons the best choice is typically around 0.01).

  • A good choice is typically such that the cross section from the RS piece is significantly positive but not much larger than the born cross section.

6.7.2. Integrating complicated Loop-ME

For complicated processes the evaluation of one-loop matrix elements can be very time consuming. The generation time of a fully optimized integration grid can become prohibitively long. Rather than using a poorly optimized grid in this case it is more advisable to use a grid optimized with either the born matrix elements or the born matrix elements and the finite part of the integrated subtraction terms only, working under the assumption that the distibutions in phase space are rather similar.

This can be done by one of the following methods:

  1. Employ a dummy virtual (requires no computing time, returns a finite value as its result) to optimise the grid. This only works if V is not the only NLO_QCD_Part specified.

    1. During integration set the Loop_Generator to Dummy. The grid will then be optimised to the phase space distribution of the sum of the Born matrix element and the finite part of the integrated subtraction term, plus a finite value from Dummy.

      Note

      The cross section displayed during integration will also correspond to these contributions.

    2. During event generation reset Loop_Generator to your generator supplying the virtual correction. The events generated then carry the correct event weight.

  2. Suppress the evaluation of the virtual and/or the integrated subtraction terms. This only works if Amegic is used as the matrix element generator for the BVI pieces and V is not the only NLO_QCD_Part specified.

    1. During integration add AMEGIC: { NLO_BVI_MODE: <num> } to

      your configuration. <num> takes the following values: 1-B, 2-I, and 4-V. The values are additive, i.e. 3-BI. @b{Note:} The cross section displayed during integration will match the parts selected by NLO_BVI_MODE.

    2. During event generation remove the switch again and the events will carry the correct weight.

Note

this will not work for the RS piece!

6.7.3. Avoiding misbinning effects

Close to the infrared limit, the real emission matrix element and corresponding subtraction events exhibit large cancellations. If the (minor) kinematics difference of the events happens to cross a parton-level cut or analysis histogram bin boundary, then large spurious spikes can appear.

These can be smoothed to some extend by shifting the weight from the subtraction kinematic to the real-emission kinematic if the dipole measure alpha is below a given threshold. The fraction of the shifted weight is inversely proportional to the dipole measure, such that the final real-emission and subtraction weights are calculated as:

w_r -> w_r + sum_i [1-x(alpha_i)] w_{s,i}
foreach i: w_{s,i} -> x(alpha_i) w_{s,i}

with the function \(x(\alpha)=(\frac{\alpha}{|\alpha_0|})^n\) for \(\alpha<\alpha_0\) and \(1\) otherwise.

The threshold can be set by the parameter NLO_SMEAR_THRESHOLD: <alpha_0> and the functional form of alpha and thus interpretation of the threshold can be chosen by its sign (positive: relative dipole kT in GeV, negative: dipole alpha). In addition, the exponent n can be set by NLO_SMEAR_POWER: <n>.

6.7.4. Enforcing the renormalization scheme

Sherpa takes information about the renormalization scheme from the loop ME generator. The default scheme is MSbar, and this is assumed if no loop ME is provided, for example when integrated subtraction terms are computed by themselves. This can lead to inconsistencies when combining event samples, which may be avoided by setting AMEGIC: { LOOP_ME_INIT: 1 }.

6.7.5. Checking the pole cancellation

The following options are all sub-settings for AMEGIC and can be specified as follows:

AMEGIC:
  <option>: <value>
  ...

To check whether the poles of the dipole subtraction and the interfaced one-loop matrix element cancel phase space point by phase space point CHECK_POLES: 1 can be specified. In the same way, the finite contributions of the infrared subtraction and the one-loop matrix element can be checked by setting CHECK_FINITE: 1, and the Born matrix element via CHECK_BORN: 1. The accuracy to which the poles, finite parts and Born matrix elements are checked is set via CHECK_THRESHOLD: <accu>.