12 C++ matrix element interface

Chapter 12
C++ matrix element interface

Since version 10.1, MCFM offers a dedicated C++ interface to access its analytic one-loop amplitudes. Please cite ref. [69] in addition to the main MCFM references when using the C++ interface.

12.1 Processes with C++ interface

The following Standard-Model processes are available:


Process	Order EW	Order QCD


$pp \to ℓ^{+} ℓ^{−}$	2	1
$pp \to ℓ^{+} ℓ^{−} j$	2	2
$pp \to ℓ^{+} ℓ^{−} jj$	2	3

$pp \to ℓ^{±} ν_{ℓ}$	2	1
$pp \to ℓ^{±} ν_{ℓ} j$	2	2
$pp \to ℓ^{±} ν_{ℓ} jj$	2	3

$pp \to h$	1	2
$pp \to hj$	1	3
$pp \to hjj$	1	4

$pp \to hh$	2	2

$pp \to ℓ^{+} ℓ^{−} h$	3	1
$pp \to ℓ^{+} ℓ^{−} hj$	3	2

$pp \to ℓ^{±} ν_{ℓ} h$	3	1
$pp \to ℓ^{±} ν_{ℓ} hj$	3	2

$pp \to γj$	1	2
$pp \to γjj$	1	3

$pp \to γγ$	2	1
$gg \to γγ$	2	1
$pp \to γγj$	2	2

$pp \to γγγ$	3	1

$pp \to γγγγ$	4	1

$pp \to ℓ^{+} ℓ^{−} γ$	3	1
$pp \to ℓ^{±} ν_{ℓ} γ$	3	1
$pp \to ν_{ℓ} {\bar{ν}}_{ℓ} γ$	3	1

$pp \to ℓ^{+} ℓ^{' -} ν_{ℓ} {\bar{ν}}_{ℓ^{'}}$	4	1

$pp \to ℓ^{+} ℓ^{−} ν_{ℓ^{'}} {\bar{ν}}_{ℓ^{'}}$	4	1
$pp \to ℓ^{+} ℓ^{−} ν_{ℓ} {\bar{ν}}_{ℓ}$	4	1
$pp \to ℓ^{+} ℓ^{−} ℓ^{' +} ℓ^{' -}$	4	1
$pp \to ℓ^{+} ℓ^{−} ℓ^{+} ℓ^{−}$	4	1

$pp \to ℓ^{+} ℓ^{−} ℓ^{' \pm} ν_{ℓ^{'}}$	4	1
$pp \to ℓ^{±} ν_{ℓ} ν_{ℓ^{'}} {\bar{ν}}_{ℓ^{'}}$	4	1
$pp \to ℓ^{+} ℓ^{−} ℓ^{±} ν_{ℓ}$	4	1

$pp \to ℓ^{+} ℓ^{−} γj$	3	2
$pp \to ℓ^{±} ν_{ℓ} γj$	3	2

$pp \to ℓ^{+} ℓ^{' -} ν_{ℓ} {\bar{ν}}_{ℓ^{'}} j$	4	2

$pp \to ℓ^{+} ℓ^{−} ν_{ℓ^{'}} {\bar{ν}}_{ℓ^{'}} j$	4	2
$pp \to ℓ^{+} ℓ^{−} ν_{ℓ} {\bar{ν}}_{ℓ} j$	4	2
$pp \to ℓ^{+} ℓ^{−} ℓ^{' +} ℓ^{' -} j$	4	2
$pp \to ℓ^{+} ℓ^{−} ℓ^{+} ℓ^{−} j$	4	2

$pp \to ℓ^{+} ℓ^{−} ℓ^{' \pm} ν_{ℓ^{'}} j$	4	2

$pp \to t \bar{t}$	0	3

$pp \to jj$	0	3

In addition, the following HEFT processes are available (requires model=heft):


Process	Order EW	Order QCD


$pp \to h$	1	2
$pp \to hj$	1	3
$pp \to hjj$	1	4

All processes are crossing invariant.

Further processes may be implemented in the future. Please contact the authors if interested in a specific process.

12.2 Installation

To use the C++ interface, please enable compiling MCFM as a library by adding the following flag

cmake .. -DWITH_LIBRARY

This will create a shared library libMCFM.so in the lib/ directory.

12.3 Usage

Examples showing the basic usage of the interface and how to fill the complete list of parameters with default values are given in src/BLHA/text.cxx and src/BLHA/params.cxx, respectively.

The MCFM C++ interface is constructed as a C++ class

CXX_Interface mcfm;

included in the header

#include "MCFM/CXX_Interface.h"

It must be initialized on a std::map of std::string, containing all (standard-model) parameters

bool CXX_Interface::Initialize(std::map<std::string,std::string>& parameters);

Prior to use, each process has to be initialized in the interface

int CXX_Interface::InitializeProcess(const Process_Info &pi);

which takes a Process_Info object as input, which in turn contains the defining parameters of a given process, i.e. the PDG IDs, number of incoming particles, and QCD and EW coupling orders

Process_Info(const std::vector<int> &ids, const int nin,const int oqcd, const int oew);

Phase space points are defined using the FourVec struct, which represents four-vectors in the ordering $(E, p_{x}, p_{y}, p_{z})$

FourVec(double e, double px, double py, double pz);

Given a list of four-vectors in this format, one-loop matrix elements can be calculated either using the process ID returned by the InitializeProcess method

void CXX_Interface::Calc(int procID,const std::vector<FourVec> &p, int oqcd);

or using a Process_Info struct:

void CXX_Interface::Calc(const Process_Info &pi,const std::vector<FourVec> &p,int oqcd);

In the same way, the result of this calculation can be accessed either via the process ID

const std::vector<double>& CXX_Interface::GetResult(int procID);

or using the Process_Info struct:

const std::vector<double> &CXX_Interface::GetResult(const Process_Info &pi)

The result is returned as a list of Laurent series coefficients in the format

(𝒪 (𝜀^{0}), 𝒪 (𝜀^{- 1}), 𝒪 (𝜀^{- 2}), Born) .

However, by default only the $𝒪 (𝜀^{0})$ coefficient, i.e. the finite part, is returned.

The calculation of the pole terms and the Born can be enabled by setting the following switch to 1

void CXX_Interface::SetPoleCheck(int check);

12.4 Tests

A set of programs to test MCFM’s amplitudes against OpenLoops, Recola, and MadLoop can be compiled. For example to compile the OpenLoops test program an additional OpenLoops directory -DOLDIR=$HOME/OpenLoops must be specified that contains the header files in the include subdirectory. For Recola and MadLoops the variables RCLDIR and MLDIR must be specified, respectively.

Chapter 12C++ matrix element interface

12.1 Processes with C++ interface

12.2 Installation

12.3 Usage

12.4 Tests

Chapter 12
C++ matrix element interface