How to run the code

In the following we provide detailed instructions on how to use the code in its different running modes and on how to analyse the results.

Runcard specifications

The basic object required to run the code is a runcard. In this section we document the different parameters which have to be specified here. As example we will refer to the runcard to reproduce smefit2.0, available from the repository smefit_database together with the files containing experimental data and the corresponding theory predictions. After cloning the repository, run

python update_runcards_path.py -d /path/to/runcard/destination/ runcards/NS_GLOBAL_NLO_NHO.yaml

This will create in /path/to/runcard/destination/ a smefit2.0 runcard ready to be used on the local machine of the user, pointing to the experimental data and an theory tables in the repository smefit_database. In the folder smefit_database/runcards the input runcards for MC and NS fits with both linear (NHO) and linear+quadratic corrections (HO) are available.

Input and output path

The path to where the experimental data and the corresponding theory tables are stored is automatically set to those contained in smefit_dayabase by the script update_runcards_path.py. The user can change them manually if other set of data are desired. The folder where the results will be saved can be set using result_path. The file containing the posterior of the fitted Wilson coefficient will be saved in resulth_path/result_ID. If result_ID is not provided, it will be automatically set to the name of the runcard (and any already existing result will be overwritten).

result_ID:
result_path:
data_path:
theory_path:

Theory specifications

The perturbative order of the QCD theory prediction (LO or NLO) should be specified using order. use_quad should be set to True for a fit with quadratic corrections, use_t0 controls the use of the t0 prescription and use_theory_covmat specifies whether or not to use the theory covariance matrix which can be specified in the theory files.

order: NLO
use_quad: False
use_t0: False
use_theory_covmat: True

Minimizer specifications

The different parameters controlling the minimizer used in the analysis are specified here. If single_parameter_fits is set to True, the Wilson coefficient specified in the run-card will be fit one at time, setting all the others to 0. See here for more details. If pairwise_fits is set to True, the minimizer carries out an automated series of pair-wise fits to all possible pairs of Wilson coefficients that are specified in the run-card. Pairwise fits are supported only with |NS|.

pairwise_fits: False
single_parameter_fits: False
bounds: Null

# NS settings
nlive: 400 # number of live points used during sampling
lepsilon: 0.05 #  Terminate when live point likelihoods are all the same, within Lepsilon tolerance.
target_evidence_unc: 0.5 # target evidence uncertanty
target_post_unc: 0.5 # target posterior uncertanty
frac_remain: 0.01 # Set to a higher number (0.5) if you know the posterior is simple.
store_raw: false # if true strare the raw result and enable resuming the job.


#MC settings
mc_minimiser: 'cma' # Allowed options are: 'cma', 'dual_annealing', 'trust-constr'
restarts: 7 # number of restarts (only for cma)
maxiter: 100000 # max number of iteration
chi2_threshold: 3.0 # post fit chi2 threshold

#A settings
n_samples: 1000 # number of the required samples of the posterior distribution

Datasets to consider and coefficients to fit

The datasets and Wilson coefficients to be included in the analysis must be listed under datasets and coefficients respectively.

datasets:

  - ATLAS_tt_8TeV_ljets_Mtt
  - ATLAS_tt_8TeV_dilep_Mtt
  - CMS_tt_8TeV_ljets_Ytt
  - CMS_tt2D_8TeV_dilep_MttYtt
  - CMS_tt_13TeV_ljets_2015_Mtt
  - CMS_tt_13TeV_dilep_2015_Mtt
  - CMS_tt_13TeV_ljets_2016_Mtt
  - CMS_tt_13TeV_dilep_2016_Mtt
  - ATLAS_tt_13TeV_ljets_2016_Mtt
  - ATLAS_CMS_tt_AC_8TeV
  - ATLAS_tt_AC_13TeV
  ...
  ...

# Coefficients to fit
coefficients:

  OpQM: {'min': -10, 'max': 10}
  O3pQ3: {'min': -2.0, 'max': 2.0}
  Opt: {'min': -25.0, 'max': 15.0}
  OtW: {'min': -1.0, 'max': 1.0}
  ...
  ...

As exemplified above, the syntax to specify the Wilson coefficient corresponding to the operator O1 is O1 : {'min': , 'max':} where min and max indicate the bounds within the sampling is performed.

Constrains between coefficients

Some Wilson coefficients are not directly fit, but rather constrained to be linear combinations of the other ones. Taking as example some coefficient of the Higgs sector considered in smefit2.0, this can be specified in the runcard in the following way

  OpWB: {'min': -0.3, 'max': 0.5}
  OpD: {'min': -1.0, 'max': 1.0}
  OpqMi: {'constrain': [{'OpD': 0.9248},{'OpWB': 1.8347}], 'min': -30, 'max': 30}
  O3pq: {'constrain': [{'OpD': -0.8415},{'OpWB': -1.8347}], 'min': -0.5, 'max': 1.0}

In this example the coefficients CpqMi and C3pq correspondong to the operators OpqMi and O3pq will be set to

CpqMi = 0.9248*CpD + 1.8347*CpWB
C3pq = -0.8415*CpD -1.8347*CpWB

Fit in different basis

It is possible to run a fit using a different basis. In this case the coefficients specified in the runcard should be the ones to fit, and a rotation basis defining the new basis in terms of the Warsaw basis should be given as input. This can be done using the option below.

rot_to_fit_basis: /path/to/rotation/rotation.json

In addition, it is possible to perform a fit in a PCA rotated basis. This corresponds to the basis spanned by the eigenvectors of the Fisher information matrix at the linear level in the EFT expansion. To carry out such a fit, add the following flag

smefit NS --rotate_to_pca path/to/the/runcard/runcard.yaml

Adding custom likelihoods

SMEFiT supports the addition of customised likelihoods. This can be relevant when an external likelihood is already at hand and one would like to combine it with the one constructed internally in SMEFiT. To make use of this feature, one should add the following to the runcard:

external_chi2:
  'ExternalChi2': /path/to/external/chi2.py

Here, ExternalChi2 is the name of the class that must be defined in the referenced python file as follows:

import numpy as np


class ExternalChi2:
    def __init__(self, coefficients):
        """
        Constructor that allows one to set attributes that can be called in the compute_chi2 method
        Parameters
        ----------
        coefficients:  smefit.coefficients.CoefficientManager
            attributes: name, value
        """
        self.example_attribute = coefficients.name

    def compute_chi2(self, coefficient_values):
        """
        Parameters
        ----------
         coefficients_values : numpy.ndarray
            |EFT| coefficients values

        """

        # example
        chi2_value = np.sum(coefficient_values**2)
        return chi2_value

One is free to set custom attributes in the constructor. The coefficient values during optimisation are accesible via coefficient_values in the compute_chi2 method. In order for the external chi2 to work, it is important one does not change the name of the compute_chi2 method!

Running a fit with NS

To run a fiy using Nested Sampling use the command

smefit NS path/to/the/runcard/runcard.yaml

This will generate a file named posterior.json in the result folder, containing the posterior distribution of the coefficients specified in the runcard.

Running a fit with MC

Disclaimer: the MC mode is only supported for linear fits.

The basic command to run a fit using Monte Carlo is

    smefit MC path/to/the/runcard/runcard.yaml -n replica_number

This will produce a file called replica_<replica_number>/coefficients_rep_<replica_number>.json in the result folder, containing the values of the Wilson coefficients for the replica. Once an high enough number of replicas have been produced, the results can be merged into the final posterior running PostFit

    smefit POSTFIT path/to/the/result/ -n number_of_replicas

where <number_of_replicas> specifies the number of replicas to be used to build the posterior. Replicas not satisfying the PostFit criteria will be discarded. If the final number of good replicas is lower than <number_of_replicas> the code will output an error message asking to produce more replicas first. The final output is the file posterior.json containing the full posterior of the Wilson coefficients.

Solving the linear problem

In case only linear cocrrections are used, one can find the analytic solution to the linear problem by

    smefit A path/to/the/runcard/runcard.yaml

This will also sample the posterior distribution according to the runcard.

Single parameter fits

Given a runcard with a number of Wilson coefficients specified, it is possible to fit each of them in turn, keeping all the other ones fix to 0. To do this add to the runcard

single_parameter_fits: True

and proceed as documented above for a normal fit. For both NS, MC and A the final output will be the file posterior.json containing the independent posterior of the fitted Wilson coefficients, obtained by a series os independent single parameter fits.

Individual parameter scan

The code can also be used to produce 1-dimensional scans of the chi2 function. The command

    smefit SCAN /path/to/the/runcard/runcard.yaml

will produce in the results folder a series of pdf files containing plots for 1-dimensional scans of the chi2 with respect to each parameter in the runcard.