MultiVariateSelections

The purpose of these selections in LHCb is to improve the data selection performance compared to those obtained from applying a set of cuts one after the other.

The current (first) implementation is for selecting events in the Bs2DsK channel. It is expected that, this implementation will be expanded to include other channels.

Software to use for the selections

The selection is implemented in a DaVinci based package named MvaSelections. To access it and try out a test run, one may do:
  getpack Phys/MvaSelections
  cd ../Phys/MvaSelections
  make
  gaudirun.py ../Phys/MvaSelections/Bs2DsKSelection.py
 

Input and Output of the selections

The input can be a DST or a microDST. In the example .py file mentioned above, a microDST is used as input. This was created by applying standard preselection cuts to a signal DST.

The output is a flag setFilterPassed=true for selected events and false for others.

Adapting the usage of this package

This package uses a Davinci Tool named Bs2DsKFisherDSelectionTool to test if a given Bs candidate is to be selected or not. This Tool is used by a DaVinci Algorithm named Bs2DsKMvaSelectionAlg. In principle this Tool can be directly accessed any DaVinci algorithm in a similar way.

The Bs2DsKMvaSelectionAlg searches through all Bs candidates in the event and if there is at least one candidate selected, the event is set to be selected. It also selects the candidate with largest Fisher discriminant as the best candidate and sends that candidate to histogram monitoring within the package. Any of these manipulations with Bs candidates may be modified for different purposes.

All the options for running this can be modified by editing the Bs2Ds2Selection.py file or by copying the relevent part of this file into your options file area. This file contains an option for selecting events with/without L0 Trigger selection. It also has an option to activate histogram monitoring to plot quantities like the Bs mass before/after selection. By default these are commented out, which means by default , it activates the usage of neither L0 nor histograms.

Current Selection Strategy

A sample of DC06 simulated data are used to create the TMVA-Fisher discriminants which are stored in a 'weights' file. This 'weights' file is used by the MvaSelections package to perform the selections on preselected data. For now, the fisher cut is applied only to obtain about 92% background rejection and the rest of the background rejection is achieved by applying a small set of cuts. For details of these, one may refer to the talk S.Easo gave on 02-July-2009 at the CP-WG meeting.

-- SajanEaso - 06 Aug 2009

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Topic revision: r1 - 2009-08-06 - SajanEaso
 
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