GridMedianBackgroundEstimator.hh

#ifndef __GRID_MEDIAN_BACKGROUND_ESTIMATOR_HH__
#define __GRID_MEDIAN_BACKGROUND_ESTIMATOR_HH__

//STARTHEADER
// $Id: GridMedianBackgroundEstimator.hh 2580 2011-09-13 17:25:43Z salam $
//
// Copyright (c) 2005-2011, Matteo Cacciari, Gavin P. Salam and Gregory Soyez
//
//----------------------------------------------------------------------
// This file is part of FastJet.
//
//  FastJet is free software; you can redistribute it and/or modify
//  it under the terms of the GNU General Public License as published by
//  the Free Software Foundation; either version 2 of the License, or
//  (at your option) any later version.
//
//  The algorithms that underlie FastJet have required considerable
//  development and are described in hep-ph/0512210. If you use
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//  FastJet is distributed in the hope that it will be useful,
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//  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
//  GNU General Public License for more details.
//
//  You should have received a copy of the GNU General Public License
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//----------------------------------------------------------------------
//ENDHEADER


#include "fastjet/tools/BackgroundEstimatorBase.hh"

FASTJET_BEGIN_NAMESPACE      // defined in fastjet/internal/base.hh

/// @ingroup tools_background
/// \class GridMedianBackgroundEstimator
/// 
/// Background Estimator based on the median pt/area of a set of grid
/// cells. 
///
/// Description of the method:
///   This background estimator works by projecting the event onto a
///   grid in rapidity and azimuth. In each grid cell, the scalar pt
///   sum of the particles in the cell is computed. The background
///   density is then estimated by the median of (scalar pt sum/cell
///   area) for all cells.
///
/// Parameters:
///   The class takes 2 arguments: the absolute rapidity extent of the 
///   cells and the size of the grid cells. Note that the size of the cell
///   will be adjusted in azimuth to satisfy the 2pi periodicity and
///   in rapidity to match the requested rapidity extent.
///
/// Rescaling:
///   It is possible to use a rescaling profile. In this case, the
///   profile needs to be set before setting the particles and it will
///   be applied to each particle (i.e. not to each cell). 
///   Note also that in this case one needs to call rho(jet) instead of
///   rho() [Without rescaling, they are identical]
///
class GridMedianBackgroundEstimator : public BackgroundEstimatorBase {
public:
  /// @name  constructors and destructors
  //\{
  //----------------------------------------------------------------
  ///   \param ymax   maximal absolute rapidity extent of the grid
  ///   \param requested_grid_spacing   size of the grid cell. The
  ///            "real" cell size could differ due e.g. to the 2pi
  ///             periodicity in azimuthal angle (size, not area)
  GridMedianBackgroundEstimator(double ymax, double requested_grid_spacing) :
    _ymin(-ymax), _ymax(ymax), 
    _requested_grid_spacing(requested_grid_spacing),
    _has_particles(false){setup_grid();}
  //\}


  /// @name setting a new event
  //\{
  //----------------------------------------------------------------

  /// tell the background estimator that it has a new event, composed
  /// of the specified particles.
  void set_particles(const std::vector<PseudoJet> & particles);

  //\}

  /// @name  retrieving fundamental information
  //\{
  //----------------------------------------------------------------

  /// returns rho, the median background density per unit area
  double rho() const;

  /// returns sigma, the background fluctuations per unit area; must be
  /// multipled by sqrt(area) to get fluctuations for a region of a
  /// given area.
  double sigma() const;

  /// returns rho, the background density per unit area, locally at the
  /// position of a given jet. Note that this is not const, because a
  /// user may then wish to query other aspects of the background that
  /// could depend on the position of the jet last used for a rho(jet)
  /// determination.
  double rho(const PseudoJet & jet);

  /// returns sigma, the background fluctuations per unit area, locally at
  /// the position of a given jet. As for rho(jet), it is non-const.
  double sigma(const PseudoJet & jet);

  /// returns true if this background estimator has support for
  /// determination of sigma
  bool has_sigma() {return true;}

  /// returns the area of the grid cells (all identical, but
  /// referred to as "mean" area for uniformity with JetMedianBGE).
  double mean_area() const {return _cell_area;}
  //\}

  /// @name configuring the behaviour
  //\{
  //----------------------------------------------------------------

  /// Set a pointer to a class that calculates the rescaling factor as
  /// a function of the jet (position). Note that the rescaling factor
  /// is used both in the determination of the "global" rho (the pt/A
  /// of each jet is divided by this factor) and when asking for a
  /// local rho (the result is multiplied by this factor).
  ///
  /// The BackgroundRescalingYPolynomial class can be used to get a
  /// rescaling that depends just on rapidity.
  ///
  /// Note that this has to be called BEFORE any attempt to do an
  /// actual computation
  virtual void set_rescaling_class(const FunctionOfPseudoJet<double> * rescaling_class);

  //\}

  /// @name description
  //\{
  //----------------------------------------------------------------

  /// returns a textual description of the background estimator
  std::string description() const;

  //\}


private:
  /// configure the grid
  void setup_grid();

  /// retrieve the grid cell index for a given PseudoJet
  int igrid(const PseudoJet & p) const;

  /// verify that particles have been set and throw an error if not
  void verify_particles_set() const;

  // information about the grid
  double _ymin, _ymax, _dy, _dphi, _requested_grid_spacing, _cell_area;
  int _ny, _nphi, _ntotal;

  // information abotu the event
  std::vector<double> _scalar_pt;
  bool _has_particles;

  // various warnings to let people aware of potential dangers
  LimitedWarning _warning_rho_of_jet;
  LimitedWarning _warning_rescaling;
};

FASTJET_END_NAMESPACE        // defined in fastjet/internal/base.hh

#endif // __GRID_MEDIAN_BACKGROUND_ESTIMATOR_HH__