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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 = {
  ps_point = add_dimension(),
  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);
ConfigurationReader configuration("example.lua", lua_parameters);

Makes them available in Lua, i.e.:

if question then
  print(test1)
else
  print(test2)
end

Would print "hello, world!".

---

1. T. Hahn, "Cuba - a library for multidimensional numerical integration" ↩