Welcome to NJet project wiki

These instructions are relevant for NJet version 2.

Fixed order NLO event generation with NJet+Sherpa

The following method has been used with Sherpa version 2.0.beta and version 2.0.beta2.

NOTE in Sherpa 2.0.0 the parameter INTEGRATOR 6 inside the (integration) section of the Sherpa run cards listed below MUST be removed.

Basic strategy

Use the basic template below for parts RS (real+subtraction), I (integrated subtraction) and B (born). For V (virtual) we will use the USE_DUMMY_VIRTUALS option for the grid.

Sherpa should be compiled with --enable-root, --enable-mpi, --enable-fastjet and --enable-lhole. For example:

../configure --enable-gzip --enable-lhapdf=<path-to-lhapdf> --enable-lhole 
--enable-mpi --enable-fastjet=<path-to-fastjet> --enable-rivet=<path-to-rivet> 
--enable-root=<path-to-root>

The procedure goes in 3 stages

1. Generate Comix process maps. Do this on a single core per NLO part. In (integration) section set the option Finish_Optmization=Off and set ERROR=5.0. This step can take a while, especially for RS.

Sherpa -f Run<NLOpart>.dat

2. Delete the result directory created in step 1. Use mpi to generate integration grid. Turn Finish_Optmization=On, keep the ERROR large around 0.3 .

mpiexec -np <cores> Sherpa -f Run<NLOpart>.dat

This step can also be run on a single core if mpi is not available.

3. Event generation specify output file using EVENT_OUTPUT=Root[<file>] which will store events in EVT_FILE_PATH/<file>. e.g.

Sherpa -f Run<NLOpart>.dat EVENT_OUTPUT=Root[event1] -R 1

Event sets maybe generated in parallel using different random seeds.

This can be set either by -R <seed> or -R "<seed1> <seed2>" from the command line.

NB: If the directory EVT_FILE_PATH is set in the Run card then ensure the directory exists.

Extra parameters for Virtuals

To link NJet to Sherpa add:

SHERPA_LDADD SherpaLHOLE njet

to the (run) parameters.

For steps 1 and 2 add USE_DUMMY_VIRTUALS=1 to the (run) parameters and Loop_Generator Internal to (process). For event generation switch to Loop_Generator LHOLE and make sure OLE_contract.lh exists. This means running Sherpa -f RunV.dat once after adding the LHOLE generator, this will issue some errors in the cross checks and create the OLE_order.lh file.

IR regularization scheme in OLE_order.lh MUST be changed to CDR when running V separately from B, I and RS.

For > 2j we recommend adding the NJetType option to the order files to make use of the faster de-symmetrized colour sums.

IRregularisation CDR
NJetType loopds

To create the OLE_contract.lh file run

njet.py -o OLE_contract.lh OLE_order.lh

Run Cards for Multi-jet Production

RunB.dat
RunV.dat
RunI.dat
RunRS.dat

Benchmark results for di-jets

To help validate the set-up we provide the following breakdown of NLO components for di-jet production using:

p_T^{1st jet} > 80 GeV, p_T^{all jets} > 60 GeV
|eta| < 2.8, anti-kt with R=0.4, MSTW2008 PDFs,
Mu_R = Mu_F = H_T/2,
IR scheme for V,I : CDR
alpha_dipole 0.1

MuR           H_T/4                      H_T/2                        H_T

LO      1271.1482 ( 1.518 ) nb      956.29871 ( 1.133 ) nb      735.58093 ( 0.868 ) nb
B       1030.0985 ( 1.218 ) nb      800.64112 ( 0.938 ) nb      633.34814 ( 0.738 ) nb
V      -102.76486 ( 0.883 ) nb      216.89281 ( 1.076 ) nb      153.42467 ( 1.186 ) nb
I      -251.45876 ( 0.361 ) nb     -275.40691 ( 0.212 ) nb     -52.369789 ( 0.131 ) nb
RS      639.37296 ( 4.288 ) nb      448.29426 ( 2.868 ) nb      322.79007 ( 1.994 ) nb

where LO uses MSTW2008lo68cl PDFs rather than MSTW2008nlo68cl for B,V,I and RS.

Total XS (LO)  =   956.29871 (1.132) ^(314.84949) _(220.71778) nb
Total XS (NLO) =  1190.42128 (3.210) ^(124.82656) _(133.22819) nb

NB: The scale choice of H_T/2 is not ideal for the di-jet cross section compared to data.