# Parameters

## Defining parameters¶

In MoMEMta, once the ConfigurationReader has been "frozen" to give a Configuration object, it is not possible anymore to modify anything in the integration. However, some parameters can be modified in the ConfigurationReader from the C++ code (allowing e.g. to carry out mass scans easily). In order to do that, the parameters have to be put in the special parameters table in Lua:

parameters = {
my_mass = 173.,
my_width = 2.5
}


These parameters can then be linked with the modules' parameters using the parameter() Lua function:

BreitWignerGenerator.m_prop = {
mass = parameter('my_mass'), -- Access the value through the "parameter" function, allowing it to be modified from the C++ code
width = parameters.my_width -- Access the value as a regular Lua table. It will NOT be possible to modify it later on
}


Danger

Accessing the parameter entries directly, as parameters.my_width or parameters['my_width'], makes it impossible to modify them from the C++ code. The latter is only possible when using the parameter() function.

### List of existing parameters¶

In addition to parameters accessible to modules, some parameters are read directly by MoMEMta and can influence its behavior. You can find the full list below:

Parameter Allowed values Default value Description
export_graph_as Any file name None Set this parameter to any value to export the computation graph built by MoMEMta. This is useful to visually see how your weight is computed.

## Configuring the integration algorithm¶

The integration algorithm can be configured from within Lua using the cuba table, for instance:

cuba = {
algorithm = "divonne", -- default is "vegas"
relative_accuracy = 0.01
}


Four different integration algorithms are available, and each has several parameters than can be tweaked to adjust their behaviour, tweak the precision of the calculations, ... The integration parameters can be changed from the C++ in a manner similar to the "global" parameters (see previous section), using the method ConfigurationReader::getCubaConfiguration(). The available algorithms and corresponding options are briefly listed here; for a full description, please consult the documentation of the numerical integration library used in MoMEMta: Cuba1.

Bear in mind the default values for these parameters are not guaranteed to be optimal for your particular case! It's your job to ensure the algorithm is tweaked to work well for the events and the functions you'll be integrating.

Integration algorithm:

Parameter Allowed values Default value
algorithm vegas, suave, divonne, cuhre vegas

Common arguments:

Parameter Allowed values Default value
relative_accuracy floats 0.005
absolute_accuracy floats 0.
seed integers 0
min_eval integers 0
max_eval integers 500000
grid_file paths
verbosity integers 0
subregion booleans false
retainStateFile booleans false
levels integers 0
ncores integers 0
pcores integers 1000000

Vegas-specific arguments:

Parameter Allowed values Default value
takeOnlyGridFromFile booleans true
n_start integers 25000
n_increase integers 0
batch_size integers min(n_start, 50000)
grid_number integers 0
smoothing booleans true

Suave-specific arguments:

Parameter Allowed values Default value
n_new integers 1000
n_min integers 2
flatness floats 0.25

Divonne-specific arguments:

Parameter Allowed values Default value
key1 integers 47
key2 integers 1
key3 integers 1
maxpass integers 5
border floats 0.
maxchisq floats 10.
mindeviation floats 0.25

Cuhre-specific arguments:

Parameter Allowed values Default value
key integers 0

## Passing arguments to the configuration¶

Once the ConfigurationReader has parsed the Lua configuration file, it is not possible anymore to change the resulting computation flow (the only changes possible are the above parameters). It is however possible to pass arguments from C++ to the Lua script when it is read by the ConfigurationReader, to influence its execution.

Defining the arguments in C++:

ParameterSet lua_parameters;
lua_parameters.set("text1", "hello, world!");
lua_parameters.set("text2", "always look on the bright side of life!");
lua_parameters.set("question", true);

if question then