Changes between Initial Version and Version 1 of Systematics

Timestamp:

Nov 4, 2016, 11:15:12 PM (8 years ago)

Author:

Olivier Mattelaer

Comment:

--

Systematics

@@ +1 @@
+[[PageOutline]]
+
+= Introduction
+
+In order to compute the systematics error, one need to recompute the cross-section/differential cross-section 
+for various PDF and scale choice. Running all those computation is parallel is a waste of time and a more appropriate 
+method is to use re-weighting approach to re-use the same sample generated to estimate those systematics.
+For full efficiency, it requires to store some additional information in the lhe file at the time of the generation such that 
+the computation of those systematics can take a very small amount of time.
+
+For more information concerning the implementation in our framework (such method exists in all MC code), please read:
+[[https://arxiv.org/pdf/1110.4738.pdf R. Frederix, S. Frixione, V. Hirschi, F. Maltoni, R. Pit-
+tau, P. Torrielli, JHEP 02, 099 (2012).]]
+
+= LO
+
+At LO, you have two code able to compute the systematics: SysCalc and the "systematics" module of MG5aMC.
+They have basically the same feature but use different interface approach to perform the computation
+ 1. SysCalc is a standalone C++ code.
+ 2. "systematics" is a python module which need to have lhapdf6 compiled for python support. Requiring the Python development headers to be installed (e.g. via the python-dev Ubuntu package).
+
+None of the those code is able to handle correctly cases with interference only generation.
+
+== How to run the code
+
+To use either of those code, you need first to set in the run_card the parameter ('''use_syst''' on true) which is going to ask MG5aMC to add the additional information
+in the lhef such that those module can read the information and add the weights in the event file.
+Note that use_syst=T is actually the default but for interference type of generation where it is by default set to False since those information are unlikely to be correct.
+
+If '''use_syst is True''' then by default we will automatically run SysCalc. If SysCalc is not installed on your machine then we will run "systematics" instead.
+You can change this behavior via the (hidden) parameter of the run_card
+'''systematics_program''' you give it one of the three following value: 
+   1. none: no program will be used (but the event will still have the additional information written inside the file)
+   2. syscalc: use the syscalc program
+   3. systematics: use the systematics program [even if SysCalc is installed]
+
+Note that both program can also be runned in standalone
+
+Various parameter of the run_card allow you to control which type of variation you want to perform:
+{{{
+0.5 1 2 = sys_scalefact  # factorization/renormalization scale factor
+None = sys_alpsfact  # \alpha_s emission scale factors
+auto = sys_matchscale # variation of merging scale
+# PDF sets and number of members (0 or none for all members).
+NNPDF23_lo_as_0130_qed = sys_pdf # separate by && if more than one set.
+# MSTW2008nlo68cl.LHgrid 1  = sys_pdf
+#
+}}}
+
+== SysCalc
+
+Description of SysCalc is available here: https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/SysCalc
+ 
+== Systematics (python) module
+
+The "systematics" module can be called externally.
+It is callable via the Madevent interface (either accessible via launch -i from the MG5aMC interface or via ./bin/madevent)
+Here is the details on the syntax (as given by the help command)
+
+{{{
+INFO: syntax: systematics RUN_NAME [OUTPUT] [options]
+INFO: -- Run the systematics run on the RUN_NAME run.
+INFO:    RUN_NAME can be a path to a lhef file.
+INFO:    OUTPUT can be the path to the output lhe file, otherwise the input file will be overwritten
+INFO:
+INFO: options: (values written are the default)
+INFO:
+INFO:    --mur=0.5,1,2    # specify the values for renormalisation scale variation
+INFO:    --muf=0.5,1,2    # specify the values for factorisation scale variation
+INFO:    --alps=1         # specify the values for MLM emission scale variation (LO only)
+INFO:    --dyn=-1,1,2,3,4 # specify the dynamical schemes to use.
+INFO:                     #   -1 is the one used by the sample.
+INFO:                     #   > 0 correspond to options of dynamical_scale_choice of the run_card.
+INFO:    --pdf=errorset   # specify the pdfs to use for pdf variation. (see below)
+INFO:    --together=mur,muf,dyn # lists the parameter that must be varied simultaneously so as to
+INFO:                           # compute the weights for all combinations of their variations.
+INFO:    --from_card      # use the information from the run_card (LO only).
+INFO:
+INFO:    Allowed value for the pdf options:
+INFO:        central  : Do not perform any pdf variation
+INFO:        errorset : runs over the all the members of the PDF set used to generate the events
+INFO:        244800   : runs over the associated set and all its members
+INFO:        244800@0 : runs over the central member of the associated set
+INFO:        CT10     : runs over the associated set and all its members
+INFO:        CT10@0   : runs over the central member of the associated set
+INFO:        CT10@X   : runs over the Xth member of the associated PDF set
+INFO:        XX,YY,ZZ : runs over the sets for XX,YY,ZZ (those three follows above syntax)
+}}}
+
+Note that the run_card do not have any entry for the type of dynamical scale (since SysCalc does not support it).
+So it is default all the possible value.
+One can also specify the argument of the "systematics" module by putting in the run_card an entry for '''systematics_arguments'''
+(as a list of string
+For example 
+{{{
+   ['--pdf=central', '--dyn=-1,3', '--together=mur,muf'] = systematics_arguments
+}}}
+
+
+
+= NLO
+
+At NLO, you have two options to compute the systematics error.
+'''on the flight''' this methods performs the re-weighting during the generation of the events. The output file contain the additional weight but '''NOT''' the information to perform an additional re-weighting  (by default) forbidding to perform a different re-weighting later (different pdf/scale choice/...)
+The second method is the "systematics" module of MG5aMC, where additional information will be written in the file in order to compute systematics weight independently of the event generation.
+
+== On the flight method
+
+In order to use this mode, you can set in the run_card those two parameters:
+{{{
+ True = reweight_scale ! Reweight to get scale variation using the
+            ! rw_rscale and rw_fscale factors. Should be a list of
+            ! booleans of equal length to dynamical_scale_choice to
+            ! specify for which choice to include scale dependence.
+ False = reweight_PDF  ! Reweight to get PDF uncertainty. Should be a
+            ! list booleans of equal length to lhaid to specify for
+            !  which PDF set to include the uncertainties.
+#**************************************************************
+}}}
+
+You can specify inside the run_card the list of pdf requested/ the list of dynamical scale /...
+
+For example in order to have multiple dynamical_scale you have to have the line in the run_card
+{{{
+ -1,2,3 = dynamical_scale_choice ! Choose one (or more) of the predefined
+           ! dynamical choices. Can be a list; scale choices beyond the
+           ! first are included via reweighting
+}}}
+The option 2 and 3 for the dynamical scale will then be include via the re-weighting approach
+
+
+== Systematics (python) module
+  
+In order to use this module you have to set the run_card parameter "store_reweight_info" to True (i.e. having the following line in the run_card) 
+{{{
+ True = store_reweight_info
+}}}
+This is not the default value of this parameter.
+If you want that this module is directly called after the event generation you have to add the following line in the run_card:
+{{{
+  systematics = systematics_program
+}}}
+parameter of the systematics module can be specified in '''systematics_argument''' for example with the following line in the run_card:
+{{{
+   ['--pdf=central', '--dyn=-1,3', '--together=mur,muf'] = systematics_arguments
+}}}
+The list of available options are the same as the one at LO (see above)
+
+As for the LO case, a dedicated function exists in the aMC@NLO interface (available via either launch -i or via the ./bin/aMC@NLO script)
+
+
+