@tableofcontents

The event data model provided in this package supports the reconstruction of missing transverse energy in ATLAS, and provides the
data containers to represent the results of this reconstruction.

@section met_intro Introduction

Missing transverse momentum (MET, @f$ E_{\rm T}^{\rm miss}) @f$ reconstruction in ATLAS is actually represented by the three 
kinematic components @f$ (p_{x}^{\rm miss},p_{y}^{\rm miss},\Sigma E_{\rm T}) @f$, which are the missing momentum components
in the transverse plane, and the scalar sum of transverse momenta contributing to MET. In addition to the full event MET, 
 MET @a terms represent contributions from 
certain physics and signal objects. In general, all reconstructed hard physics objects like individual particles and particle 
jets contribute their (fully calibrated) kinematics to their specific MET term (e.g., @f$ E_{\rm T}^{\rm miss,electron} @f$,
 @f$ E_{\rm T}^{\rm miss,jet} @f$, ...). In addition, signal objects (topological calorimeter cell clusters and reconstructed tracks) 
not used by hard objects are collected into a dedicated soft term MET contribution (SoftTerm). The sum of all terms reconstructed 
within the context of a given MET configuration is then the full event MET.

@subsection met_intro_scope MET reconstruction scope and basics

The scope of MET reconstruction is to use all hard objects passing dedicated kinematic and topological cuts, and having an acceptable
reconstruction quality indicator, for the individual MET terms. As there is likely signal overlap between these objects, an event signal
ambiguity resolution strategy is applied which is based on tagging common calorimeter signal use and rejecting object contributions if 
the signal they are based on is already used in a previously scheduled MET contribution. After all hard objects are considered this way, 
the residual unused signals are collected into the MET SoftTerm. 

This strategy makes the resulting total MET dependent on the order with which the hard objects are considered. This order is by default
defined by expectations for the reconstruction quality and precision, leading to the sequence (1) electrons, (2) photons, (3) taus,
(4) muons, (5) particle jets, and (6) SoftTerm.    

@subsection met_intro_edm Event data model basics

The MET event data model (EDM) reflects the needs introduced by the MET reconstruction discussed above. It consists of two basic data objects and
their respective containers, in addition to the corresponding auxiliary storage objects. The two basic data objects respectively provide storage of the 
@ref met_intro "kinematic variables" related to a MET term 
(@link xAOD::MissingET_v1 MissingET @endlink object), and storage of information related to the contributions 
to this MET term (@link xAOD::MissingETComponent_v1 MissingETComponent @endlink object). With this approach the kinematic data, characterized by 
basic internally used data types and structures, is clearly separated from the composition data providing detailed information, which typically
requires to provide more support for data management and search capabilities. 

The @link xAOD::MissingET_v1  MissingET @endlink is considered universal, as it is used to store individual MET terms as well as the final 
full event MET. The objects representing the terms and the full event MET are stored in a container object 
(@link xAOD::MissingETContainer_v1 MissingETContainer @endlink). Standard MET reconstruction tools also create a MET component object for
each MET term, but not for the full event MET, as this means duplication of information. This 
@link xAOD::MissingETComponent_v1 MissingETComponent @endlink object is stored in its own container 
(@link xAOD::MissingETComponentMap_v1 MissingETComponentMap @endlink, one per MET term).

@section met_intro Introduction

The MET EDM is tailored to support a dynamic (MET reconstruction in the precision reconstruction and analysis context) and static (mostly 
MET related data retrieval in an analysis context) phase. This means that the data objects in this EDM need to (among others) 
@li provide storage for the variables characterizing a MET term; 
@li support easy and efficient access to these variables for a given term;
@li support searching for a specific MET term and its components in their respective containers;  
@li implement the algebra correctly calculating the kinematic variables in the dynamic phase; 
@li support event signal ambiguity resolution;  
@li provide access to the individual contributions to a given MET term; 
@li support re-reconstruction of the full MET for a given configuration, or of selected MET terms only.

@note The MET EDM does not provide any implementation directly reconstructing a MET term. There is no functionality provided which fills e.g.
      the basic data object and the component object at the same time. There is also no implementation provided which e.g. extracts underlying
      basic signals like topological calorimeter cell clusters or reconstructed charged tracks from a given physics object. These are tasks which
      need to be implemented in the dedicated tools reconstructing a MET term from a list of given objects. On the other hand, the MET EDM provides 
      the book-keeping to keep track of use clusters and tracks, with the idea that the corresponding data has to be provided by the tools again 
      (used signal objects have to be explicitly set in the book-keeping caches).  

@subsection met_edm_data MissingET and MissingETContainer

The basic kinematic data object for an individual MET term, any combination of those, and the final (summed) MET is the 
@link xAOD::MissingET_v1 MissingET @endlink object. It provides all functionality and support for the correct algebra applicable for MET 
reconstruction. The relevant stored kinematic variables are @f$ (p_{x}^{\rm miss}, p_{y}^{\rm miss}, \Sigma E_{\rm T}) @f$.

The result of MET reconstruction is represented by three kinematic variables @f$ (p_{x}^{\rm miss},p_{y}^{\rm miss},\Sigma E_{\rm T}) @f$, which are 
constructed as
@anchor met_sum_rule
@f{eqnarray*}{
   p_{x}^{\rm miss} & = & - \sum_{i = 1}^{N_{\rm contrib}} p_{x,i} \\ 
   p_{y}^{\rm miss} & = & - \sum_{i = 1}^{N_{\rm contrib}} p_{y,i} \\
   \Sigma E_{\rm T} & = &   \sum_{i = 1}^{N_{\rm contrib}} p_{{\rm T},i} .
@f}
Here @f$ N_{\rm contrib} @f$ is the number of contributions to the MET term, and 
@f$ p_{x,i} @f$ and @f$ p_{y,i} @f$ are the transverse momentum components of contribution @f$ i @f$, and @f$ p_{{\rm T},i} @f$ is the transverse
momentum of this component. These three MET variables can be accessed, set, and manipulated using methods provided by the 
@link xAOD::MissingET_v1 MissingET @endlink object.  

In addition, each @link xAOD::MissingET_v1 MissingET @endlink object provides accessors and setters for the name of the MET term it represents, 
and a source indicator (bitmask).

   @subsubsection met_edm_basics_construct Construction

   The @link xAOD::MissingET_v1 MissingET @endlink can be instantiated in three principal states. The <i>default state</i>
   is a data object without even a data store, which generally is unusable for reconstruction or data presentation, but is needed for some internal I/O
   and data container instantiation support. The <i>empty state</i> is represented by a MET object with a name and (default or client provided) source
   indicator. The kinematic variables are set to @f$( p_{x}^{\rm miss} = 0, p_{y}^{\rm miss} = 0, \Sigma E_{\rm T} = 0) @f$. The last option is the 
   <i>loaded state</i>, which has the same data content as the empty date, except that the kinematic variables are loaded from a given contributing
   object, or a corresponding set of numbers.     

   The dynamic phase is supported by @link xAOD::MissingET_v1::add methods @endlink implementing the summing convention given in this 
   @ref met_sum_rule "set of equations". Other operations like re-scaling to the kinematic variables and removing a kinematic contribution 
   from a given physics or signal object are implemented as well.

   @note It is suggested that clients interact with the @link xAOD::MissingET_v1 MissingET @endlink object using the available 
         methods and operators. This allows maintaining  the consistency of the kinematic data. In addition, the correct sum rules
         are applied.   

   @subsubsection met_edm_basics_storage Storage 

   @link xAOD::MissingET_v1 MissingET @endlink objects are initially constructed as individual, non-storable data objects - either in the default or 
   one of  the plain-old-data (POD) states (empty or loaded). Independent of the initial state, if the pointer to a 
   @link xAOD::MissingET_v1 MissingET @endlink is stored in a @link xAOD::MissingETContainer_v1 MissingETContainer @endlink, and this container 
   has a valid reference to an auxiliary data store provided  by the @link xAOD::MissingETAuxContainer_v1 MissingETAuxContainer @endlink, the 
   @link xAOD::MissingET_v1 MissingET @endlink object becomes fully useable and storable. If its initial object state is default, the newly attached 
   data store has default content, while for the other two states the (private) data store content is transfered to the auxiliary store. 

   While there are no implementations enforcing any rules on the sequence of the @link xAOD::MissingET_v1 MissingET @endlink pointers

As mentioned above, each of these data objects typically contains the contribution from a set of same type final state objects like particles and jets, and is identified as a specific contribution by a name and a source indicator (bit pattern). In general all MET objects from the various sources contribute to a given MET reconstruction configuration and their sum is the overall MET for the event. For storage, and to indicated that a given xAOD::MissingET_v1 object represents a certain MET term in a given MET reconstruction configuration, the individual MET objects are collected into a xAOD::MissingETContainer_v1 object. Typically, the insertion sequence (e.g. the index of an individual MET object) in this container is determined by the tool sequence with which the MET terms are reconstructed, there is no particular enforcement of this rule in the EDM. Safe (random) accessors using the name or the source indicator of the requested MET object are provided by the MET object.

   The @link xAOD::MissingET_v1 MissingET @endlink object can be instantiated as a plain-old-data (POD) object. To store it, it needs to be
   collected into a @link xAOD::MissingETContainer_v1 MissingETContainer @endlink with an attached auxiliary
   @link xAOD::MissingETAuxContainer_v1 MissingETAuxContainer @endlink store. This auxiliary store provides the underlying storable (and optimized) 
   data structures.

@subsection met_edm_composition Contributions and composition    

The contributions to a given MET term are characterized by (1) a list of contributing objects, (2) a list of the kinematic weights associated with
each contributing object,  and (3) a list of status-words characterizing each of the contributions. The data object storing this information is
@link xAOD::MissingETContribution_v1 MissingETContribution @endlink. This object can be instantiated as POD, and it provides all functionality 
to manipulate the mentioned stores.

For storage, and to describe a fully reconstructed MET, the @link xAOD::MissingETContribution_v1 MissingETContribution @endlink objects are collected
into a @link xAOD::MissingETCompositionMap_v1 MissingETCompositionMap @endlink object, which together with its auxiliary 
@link xAOD::MissingETAuxCompositionMap_v1 MissingETAuxCompositionMap @endlink store providing the underlying storage structures, can be persistified. 

Most of the functionality connected with the access and management of the @link xAOD::MissingETCompositionMap_v1 MissingETCompositionMap @endlink
is provided by static functions in the @link xAOD::MissingETComposition MissingETComposition @endlink structure.