Multiplicity and centrality selection in O2

Concept

The multiplicity and centrality selection in O2 is based on the concept of derived tables created in dedicated tasks from available AOD contents:

o2-analysis-multiplicity-table task Analysis/Tasks/multiplicityTable.cxx stores relevant multiplicity values (V0A, V0C, ZNA, ZNC) and their dynamic sums (V0M) in Mults table joinable wih Collisions table.
o2-analysis-multiplicity-qa task Analysis/Tasks/multiplicityQa.cxx creates multiplicity distributions in minimum bias triggers necessary for centrality calibration.
o2-analysis-centrality-table task Analysis/Tasks/centralityTable.cxx takes multiplicity values from the Mults table and stores centrality values in Cents table joinable with Collisions table. Relevant cumulative multiplicity distributions are stored in CCDB. At the moment, centrality calibration objects are available only for LHC15o. The centrality calibration relies on 90% anchor points but doesn't take into account vertex dependence yet. The difference with AliPhysics centrality calibration doesn't exceed 0.5%.
o2-analysis-centrality-qa task Analysis/Tasks/centralityQa.cxx creates centrality distributions for minimum bias triggers and can be used for control and QA purposes.

Note that o2-analysis-multiplicity-qa and o2-analysis-centrality-qa tasks rely on the minimum bias trigger selection therefore one has to run event selection in stack with these tasks, see here for more details.

Usage in user tasks

One can check o2-analysis-centrality-qa task for example usage: Analysis/Tasks/centralityQa.cxx. Usually, analysers perform event selection before the centrality selection therefore one has to consider the following steps:

add EventSelection.h and Centrality.h headers:

  #include "Analysis/EventSelection.h"
  #include "Analysis/Centrality.h"
 

join Collisions, EvSels and Cents tables and use corresponding iterator as an argument of the process function:
```
  void process(soa::Join<aod::Collisions, aod::EvSels, aod::Cents>::iterator const& col, ...)
```
check if your trigger alias is fired if you run over Run1 or Run2 data (or future triggered Run3 data):
```
  if (!col.alias()[kINT7])
    return;
```
Bypass this check if you analyse MC or future continuous Run3 data.
apply further offline selection criteria:
```
  if (!col.sel7())
    return;
```

apply centrality selection, for example:

  // analyse 0-20% central events
  if (col.centV0M()>20)
    return;
 

run your tasks in stack with timestamp, event-selection, multiplicity and centrality tasks:
```
  o2-analysis-timestamp --aod-file AO2D.root -b | o2-analysis-event-selection -b | o2-analysis-mulitplicity-table -b | o2-analysis-centrality-table -b | o2-analysis-user-task -b
```
o2-analysis-timestamp task Analysis/Tasks/timestamp.cxx is required to create per-event timestamps necessary to access relevant CCDB objects in the event selection and/or centrality tasks.