AliPhysics  d0c3a41 (d0c3a41)
The basic multiplicity code

Module: AOD

# Introduction

The code in this section defines methods to generate a fully corrected 2-dimensional histogram of the charged particle multiplicity in the forward regions per event. This histogram is then stored in an object of class AliAODForwardMult on a seperate branch on the AOD tree. This object can then be used for sub-sequent detail analysis, like $$dN_{ch}/d\eta$$, $$v_n{m}$$, $$P(N_{ch})$$, and $$Corr(n_f,n_b)$$.

The input to this analysis is the ESD information from the FMD, plus some corrections stored in AliROOT (should be migrated to OADB).

Also defined in this module, is code to produce a similar object AliAODCentralMult, but this time the input information comes from the SPD clusters on the first layer. The two analysis are similar in methodology.

# Structure

There are classes for containing data, classes that represent analysis tasks, and classes that perform calculations, as well as specialized classes for analysis of simulation (MC) output.

## Data structures

The classes AliAODForwardMult and AliAODCentralMult each contain a 2-dimensional $$(\eta,\phi)$$ map of the charged particle multiplicity per event. The data is fully corrected for acceptance and secondary particle production, though this can be turned off if needed.

For the forward analysis, there are two tasks: AliForwardMultiplicityTask and AliForwardMCMultiplicityTask - both of which derive from the base class AliForwardMultiplicityBase. AliForwardMultiplicityTask is intented for analysis of collision data, while AliForwardMCMultiplicityTask is intented simulation data, and the AddTaskForwardMult script function automatically selects the appropriate one. Both of these tasks uses worker classes to do the actual computations.

For the analysis of the SPD cluster, there are also two tasks: AliCentralMultiplicityTask and AliCentralMCMultiplicity task, where the latter derives from the first. Again, the AddTaskCentralMult script function automatically choses the appropriate implementation based on the set-up of the train.

The special task AliForwardMCCorrectionsTask is used to generate the various simulation based corrections needed by AliForwardMultiplicityBase and it's derivatives, and the task AliFMDEnergyFitterTask generates corrections for both real and simulated input used by AliForwardMultiplicityTask and AliForwardMCMultiplicityTask respectively.

## Workers

AliFMDEventInspector
A worker that inspects the global event properties, such as location of the interaction point, the centrality (if applicable), and the fired triggers (both on-line and off-line). The result of the inspection is returned to the caller task and diagnostics histograms are filled.
AliFMDSharingFilter
A worker that merges signals in the FMD silicon strips to correct for the situation where a charged particles energy loss is shared over 2 or 3 strips as a consquence of it's incident angle. The worker returns a new AliESDFMD object with the corrected signals and fills diagnostic histograms.
AliFMDDensityCalculator
This worker calculates the number of charged particles per $$(\eta,\varphi)$$ bin using one of two methods: Energy loss deconvolution or Poissonian statistics. The user selects which method to use at start-up but both methods are fired and compared in the diagnostics histograms. The output is 5 histograms - one for each sub-ring - of the event charged particle multiplicity in $$(\eta,\varphi)$$ bins.
AliFMDCorrector
Applies various corrections, such as $$\phi$$ acceptance, correction for secondary particle production, merging efficiency, and so on. The exact corrections applied is selected by the user. The output is 5 histograms of the event charged particle multiplicity in $$(\eta,\varphi)$$ bins corrected for the selected effects.
AliFMDHistCollector
Collects the 5 histograms of the event charged particle multiplicity in $$(\eta,\varphi)$$ bins into one `super' histogram of the event charged particle multiplicity, taking care of overlaps and differences in $$\varphi$$ acceptance of the 5 sub-rings.
AliFMDEventPlaneFinder
Calculates the reaction plane of the event from the FMD data