Truth-matching ============== .. _mcmatching: ----------- MC matching ----------- .. important:: If you use MC matching for your analysis, please cite the following proceeding `Monte Carlo matching in the Belle II software `_ which describes the Belle II MC matching algorithm. ~~~~~~~~~~~~~~~~~~~~~~ First, you must run it ~~~~~~~~~~~~~~~~~~~~~~ MCMatching relates ``Particle`` and ``MCParticle`` objects. .. important:: Most MC matching variables will have non-trivial values only if the :b2:mod:`MCMatcherParticles` module is actually executed. It can be executed by adding the module to your path, there is a `modularAnalysis.matchMCTruth` convenience function to do this. ~~~~ Core ~~~~ MC matching at Belle II returns two important pieces of information: the true PDG id of the particle :b2:var:`mcPDG`, and an error flag :b2:var:`mcErrors`. Both variables will have non-trivial values only if the MCMatching module, which relates composite ``Particle`` (s) and ``MCParticle`` (s), is executed. :b2:var:`mcPDG` is set to the PDG code of the first common mother ``MCParticle`` of the daughters of this ``Particle``. .. b2-variables:: :variables: mcPDG,mcErrors :noindex: ~~~~~~~~~~~~~~~ Extra variables ~~~~~~~~~~~~~~~ There are several extra variables relating to MCMatching. Many are defined for convenience and can be recreated logically from :b2:var:`mcPDG` and :b2:var:`mcErrors`. Some extra variables are provided externally, for example :b2:var:`isCloneTrack` from the tracking-level MC matching. .. b2-variables:: :variables: isSignal,isSignalAcceptWrongFSPs,isSignalAcceptMissingNeutrino,isSignalAcceptMissingMassive,isSignalAcceptMissingGamma,isSignalAcceptMissing,isWrongCharge,isMisidentified,isCloneTrack,isOrHasCloneTrack,genNStepsToDaughter(i),genNMissingDaughter(PDG) :noindex: .. _Particle_status: ~~~~~~~~~~~~~~~~~~~~ Particle Status Flag ~~~~~~~~~~~~~~~~~~~~ The status flag :b2:var:`mcParticleStatus` is a bit set where each bit flag describes a specific property or simulation state of the corresponding ``MCParticle``. The individual flags are defined by the ``MCParticle::StatusBit`` enum. A value of ``mcParticleStatus`` equal to 0 means that none of the predefined status conditions are set for the particle. These flags can be used to identify special categories of particles, such as primary particles generated by the event generator, particles that left or stopped inside the detector volume, virtual particles that are not propagated to Geant4, or photons originating from ISR, FSR, or PHOTOS radiation processes. Since the status is encoded as a bit mask, multiple flags can be set simultaneously for a single particle. =============================== ============================================================================================ Flag Explanation =============================== ============================================================================================ c_PrimaryParticle = 1 Particle is a primary particle produced by the event generator. c_StableInGenerator = 2 Particle is stable in the generator, i.e. it does not decay during event generation. c_LeftDetector = 4 Particle left the detector simulation volume. c_StoppedInDetector = 8 Particle was stopped inside the detector simulation volume. c_IsVirtual = 16 Particle is virtual and is not propagated to Geant4 (e.g. exchange bosons or off-shell states). c_Initial = 32 Particle is an initial-state particle such as the incoming :math:`e^+` or :math:`e^-`. c_IsISRPhoton = 64 Particle is a photon produced by initial state radiation (ISR). c_IsFSRPhoton = 128 Particle is a photon produced by final state radiation (FSR). c_IsPHOTOSPhoton = 256 Particle is a radiative photon produced by PHOTOS. c_IsRadiativePhoton = 448 Combined flag identifying ISR, FSR, or PHOTOS photons. =============================== ============================================================================================ .. _Error_flags: ~~~~~~~~~~~ Error flags ~~~~~~~~~~~ The error flag :b2:var:`mcErrors` is a bit set where each bit flag describes a different kind of discrepancy between reconstruction and ``MCParticle``. The individual flags are described by the ``MCMatching::MCErrorFlags`` enum. A value of mcErrors equal to 0 indicates perfect reconstruction (signal). Usually candidates with only FSR photons missing are also considered as signal, so you might want to ignore the corresponding ``c_MissFSR`` flag. The same is true for ``c_MissingResonance``, which is set for any missing composite particle (e.g. :math:`K_1`, but also :math:`D^{*0}`). The behavior of :b2:var:`mcErrors` can be configured by decay string grammar with :func:`modularAnalysis.reconstructDecay`. For more information and examples how to use the decay strings correctly, please see :ref:`DecayString` and :ref:`Grammar_for_custom_MCMatching`. Full documentation of the MCMatching algorithm is described in `BELLE2-CONF-PROC-2022-004 `_. ============================== ================================================================================================ Flag Explanation ============================== ================================================================================================ c_Correct = 0 This Particle and all its daughters are perfectly reconstructed. c_MissFSR = 1 A Final State Radiation (FSR) photon is not reconstructed (based on MCParticle: :c_IsFSRPhoton). c_MissingResonance = 2 The associated MCParticle decay contained additional non-final-state particles (e.g. a rho) that weren't reconstructed. This is probably O.K. in most cases. c_DecayInFlight = 4 A Particle was reconstructed from the secondary decay product of the actual particle. This means that a wrong hypothesis was used to reconstruct it, which e.g. for tracks might mean a pion hypothesis was used for a secondary electron. c_MissNeutrino = 8 A neutrino is missing (not reconstructed). c_MissGamma = 16 A photon (not FSR) is missing (not reconstructed). c_MissMassiveParticle = 32 A generated massive FSP is missing (not reconstructed). c_MissKlong = 64 A Klong is missing (not reconstructed). c_MisID = 128 One of the charged final state particles is mis-identified (wrong signed PDG code). c_AddedWrongParticle = 256 A non-FSP Particle has wrong PDG code, meaning one of the daughters (or their daughters) belongs to another Particle. c_InternalError = 512 No valid match was found. For tracks, it indicates that there is not a true track related to the reconstructed one. Might indicate fake or background track or cluster. c_MissPHOTOS = 1024 A photon created by PHOTOS was not reconstructed (based on MCParticle: :c_IsPHOTOSPhoton). c_AddedRecoBremsPhoton = 2048 A photon added with the bremsstrahlung recovery tools (correctBrems or correctBremsBelle) has no MC particle assigned, or it doesn't belong to the decay chain of the corrected lepton mother ============================== ================================================================================================ ~~~~~~~~~~~~~~ Example of use ~~~~~~~~~~~~~~ The two variables together allow the user not only to distinguish signal (correctly reconstructed) and background (incorrectly reconstructed) candidates, but also to study and identify various types of physics background (e.g. mis-ID, partly reconstructed decays, ...). To select candidates that have a certain flag set, you can use bitwise and to select only this flag from :b2:var:`mcErrors` and check if this value is non-zero: ``(mcErrors & MCMatching::c_MisID) != 0`` . For use in a ``TTree`` selector, you'll need to use the integer value of the flag instead: .. code-block:: cpp ntuple->Draw("M", "(mcErrors & 128) != 0") You can also make use of ``MCMatching::explainFlags()`` which prints a human-readable list of flags present in a given bitset. Can also be used in both C++ and python: .. code-block:: python import basf2 from ROOT import Belle2, gInterpreter gInterpreter.ProcessLine('#include "analysis/utility/MCMatching.h"') print(Belle2.MCMatching.explainFlags(a_weird_mcError_number)) If instead only binary decision (1 = signal, 0 = background) is needed, then for convenience one can use :b2:var:`isSignal` (or :b2:var:`isSignalAcceptMissingNeutrino` for semileptonic decays). .. code-block:: python from modularAnalysis import variablesToNtuple variablesToNtuple("X:mycandidates -> Y Z", variables = ["isSignal"] + other_interesting_variables) assuming you have reconstructed :code:`X -> Y Z` : .. code-block:: python from modularAnalysis import applyCuts applyCuts('X:myCandidates', 'isSignal==1') ------------------------------------------------------- MC decay finder module :b2:mod:`ParticleCombinerFromMC` ------------------------------------------------------- Analysis module to reconstruct a given decay using the list of generated particles ``MCParticle``. Only signal particles with `isSignal` equal to 1 are stored. The module can be used for: * Determination of the number of generated decays for efficiency studies, especially in the case of inclusive decays (e.g.: What's the generated number of :math:`B \to D^0 X` decays?). * Matched MC decays as input for a truth matching module. .. code-block:: python import basf2 # Create main path main = basf2.create_path() # Modules to generate events, etc. ... import modularAnalysis as ma # Load particles from MCParticle at first ma.fillParticleListFromMC('K+:MC', '', path=main) ma.fillParticleListFromMC('pi+:MC', '', path=main) ma.fillParticleListFromMC('e+:MC', '', path=main) ma.fillParticleListFromMC('nu_e:MC', '', path=main) ma.fillParticleListFromMC('gamma:MC', '', path=main) """ Example 1 Search for B+ decaying to anti-D0* e+ nu_e, where anti-D0* decays to [anti-D0 -> K+ pi- pi0] and pi0. Additional photons emitted are ignored. Charge conjugated decays are matched, too. """ # Reconstruct pi0 from gamma gamma at fist for convenience. Then reconstruct B+ with pi0:gg. ma.reconstructMCDecay('pi0:gg =direct=> gamma:MC gamma:MC', '', path=main) ma.reconstructMCDecay( 'B+:DstENu =direct=> [anti-D*0:D0pi0 =direct=> [anti-D0:Kpipi0 =direct=> K+:MC pi-:MC pi0:gg] pi0:gg ] e+:MC nu_e:MC ', '', path=main) # One can directly reconstruct pi0:gg in same decay string. # But in this case, one have to write sub-decay of pi0:gg only once. Otherwise same particles are registered twice. # ma.reconstructMCDecay( # 'B+:DstENu =direct=>\ # [anti-D*0:D0pi0 =direct=> [anti-D0:Kpipi0 =direct=> K+:MC pi-:MC [pi0:gg =direct=> gamma:MC gamma:MC]] pi0:gg ]\ # e+:MC nu_e:MC ', # '', # path=main) """ Example 2 Search for B+ decaying to anti-D0 + anything, where the anti-D0 decays to K+ pi-. Ignore additional photons emitted in the anti-D0 decay. Charge conjugated decays are matched, too. If there is a match found, save to ParticleList 'B+:testB' """ # Reconstruct B+ from [anti-D0 =direct=> K+ pi-] accepting missing daughters ma.reconstructMCDecay('B+:D0Kpi =direct=> [anti-D0:Kpi =direct=> K+:MC pi-:MC] ... ?gamma ?nu', '', path=main) ... For more information and examples how to use the decay strings correctly, please see :ref:`DecayString` and :ref:`Grammar_for_custom_MCMatching`. ---------------------------------------------- MC decay finder module :b2:mod:`MCDecayFinder` ---------------------------------------------- Analysis module to search for a given decay in the list of generated particles ``MCParticle``. The module can be used for: * Determination of the number of generated decays for efficiency studies, especially in the case of inclusive decays (e.g.: What's the generated number of :math:`B \to D^0 X` decays?). * Matched MC decays as input for a truth matching module. .. code-block:: python import basf2 # Create main path main = basf2.create_path() # Modules to generate events, etc. ... import modularAnalysis as ma # Search for B+ decaying to anti-D0 + anything, where the anti-D0 decays to K+ pi-. # Ignore additional photons emitted in the anti-D0 decay. Charge conjugated decays # are matched, too. If there is a match found, save to ParticleList 'B+:testB' ma.findMCDecay('B+:testB', 'B+ =direct=> [anti-D0 =direct=> K+ pi-] ... ?gamma ?nu', path=main) # Modules which can use the matched decays saved as Particle in the ParticleList 'B+:testB' ... For more information and examples how to use the decay strings correctly, please see :ref:`DecayString` and :ref:`Grammar_for_custom_MCMatching`. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Compact decay strings per event ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The `PrintMCParticles` module (used via `modularAnalysis.printMCParticles`) can store a compact, single-line representation of the generated decay tree of each event in the ``__MCDecayString__`` branch, by setting ``storeCompact=True`` instead of using the default indented multi-line tree:: import modularAnalysis as ma ma.printMCParticles(storeCompact=True, onlyPrimaries=True, suppressPrint=True, path=main) A typical value of the resulting string for a generic :math:`\Upsilon(4S) \to B\bar{B}` event looks like:: Upsilon(4S) -> [B+ -> mu+ nu_mu gamma] [B- -> pi- [D0 -> pi- pi+]] For non-:math:`\Upsilon(4S)` events, e.g. continuum :math:`e^+e^- \to c\bar{c}` events, the top-level particles are instead the initial-state beam particles and the intermediate boson they produce, since these also have no mother particle in the generated event, for example:: e- e+ Z0 -> [D*+ -> [D0 -> [K_S0 -> pi+ pi-] pi+ pi- pi+ pi-] pi+] [pi0 -> gamma gamma] [rho- -> pi- [pi0 -> gamma gamma]] pi+ [D- -> K_L0 pi-] Here ``->`` separates a particle from its daughters, ``[...]`` groups a composite daughter together with its own descendants, and ``~`` prefixes secondary particles (i.e. particles created during the detector simulation rather than by the generator). A particle whose PDG code is not known to ``TDatabasePDG`` is named ``UNKNOWN()``. Radiative photons are also given a distinct name so that they can be told apart from generator-level photons when parsing the string: * ``gammaI`` for initial state radiation (based on MCParticle: ``c_IsISRPhoton``), * ``gammaF`` for final state radiation (based on MCParticle: ``c_IsFSRPhoton``), * ``gammaP`` for photons added by PHOTOS (based on MCParticle: ``c_IsPHOTOSPhoton``), For example, if the muon in the decay above also radiated a PHOTOS photon:: Upsilon(4S) -> [B+ -> mu+ nu_mu gamma gammaP] [B- -> pi- [D0 -> pi- pi+]] By default ``onlyPrimaries`` is ``False``, so secondary particles are also included in the string, for example if the primary :math:`\pi^-` above instead interacted in the detector material:: Upsilon(4S) -> [B+ -> mu+ nu_mu gamma] [B- -> [pi- -> ~gamma ~gamma] [D0 -> pi- pi+]] Setting ``onlyPrimaries=True`` (as in the first example) omits such secondary particles and their descendants entirely, which is usually the more useful setting for studying the generated decay itself. If ``maxLevel`` limits the depth of the tree, a particle whose daughters are cut off by this limit has ``-> ...`` appended after it instead of showing its daughters, mirroring the ``→ …`` indicator used in the default indented tree. For example, applying ``maxLevel=1`` to the first example above would collapse it to:: Upsilon(4S) -> ... This truncation does not happen by default, since the default ``maxLevel=-1`` means the tree is never cut off. Because this string is written out per event, it can be stored alongside the other analysis variables (e.g. via `modularAnalysis.variablesToNtuple`) and then parsed offline, for example with a simple recursive-descent parser or even just string/regular-expression matching on the particle names, to study which decay chains actually occurred in each generated event. Unlike the numbered generated decay modes described below, this does not require every mode of interest to be enumerated in advance. .. important:: When running on all events, it is important to also set ``suppressPrint=True`` (as in the example above). Otherwise the default indented tree is still built and printed via ``B2INFO`` for every event, in addition to being stored in ``storeCompact`` format, which can cause the log files to explode in size. .. _GeneratedDecayModes: --------------------- Generated decay modes --------------------- A tool has been developed which analyzes the array of MC particles and determines the generated decay mode of the event. Compared to storing the full decay string (as described above), storing this decay mode number is a much more efficient way to record the generated decay of an event, since only a single integer needs to be saved instead of a string. However, this efficiency comes at the cost of flexibility: only the decay modes enumerated below are recognized, and this hardcoded mapping is limited to the decay modes explicitly generated by EvtGen for B decays. It does not cover decay modes generated by Pythia. A total of 825 :math:`B^+` decay modes have been defined: * The first 80 numbers are semi-leptonic decays (:math:`B^+ \to h l^+ \nu_l`), e.g. * 1001: :math:`B^+ \to \bar{D^{*0}} e^+ \nu_e` * 1002: :math:`B^+ \to \bar{D^0} e^+ \nu_e` * 1003: :math:`B^+ \to \bar{D_1^0} e^+ \nu_e` * The numbers 1085 to 1092 are radiative decays. * There are 17 :math:`B^+ \to X l^+ l^-` decay modes. * The numbers 1113 to 1608 are hadronic charmless decays. * Then there are 70 charmonium decays with :math:`J/\psi`, :math:`\Psi(2S)`, :math:`\eta_c`, etc. * This is followed by decays involving two or one charm meson. * The last 17 decay modes involve baryons. The same number of :math:`B^-` decay modes have been implemented. They have negative tags. .. b2-variables:: :variables: BplusMode,BminusMode The same general structure is used for the 1000 :math:`B^0` and :math:`\bar{B^0}` modes accessible via the variables .. b2-variables:: :variables: B0Mode,Bbar0Mode For :math:`B_s^0` and :math:`\bar{B_s^0}` a total of 264 decay have been defined. .. b2-variables:: :variables: Bs0Mode,Bsbar0Mode Besides the B meson decays three :math:`D^{*\pm}`, 84 :math:`D_s^\pm`, 84 :math:`D^\pm`, and 136 :math:`D^0` charm meson decays are implemented. For each category the tag number starts again at 1001. .. b2-variables:: :variables: DstplusMode,DstminusMode,DsplusMode,DsminusMode,DplusMode,DminusMode,D0Mode,Dbar0Mode The full list of all decay modes can be found in the technical Belle note `BELLE2-NOTE-TE-2021-002 `_. .. _TauDecayMCModes: ------------------ Tau decay MC modes ------------------ A special case is the decay of generated tau lepton pairs. For their study, it is useful to call the function ``labelTauPairMC`` in the steering file. .. code-block:: python from modularAnalysis import labelTauPairMC labelTauPairMC() .. b2-variables:: :variables: tauPlusMCMode,tauMinusMCMode,tauPlusMCProng,tauMinusMCProng Using MC information, `labelTauPairMC` identifies if the generated event is a tau pair decay. The variables `tauPlusMCProng` and `tauMinusMCProng` store the prong (number of final state charged particles) coming from each one of the generated tau leptons. If the event is not a tau pair decay, the value in each one of these variables will be 0. The channel number will be stored in the variables `tauPlusMCMode`, and `tauMinusMCMode` (one for the positive and the other for the negative) according to the following tables. ~~~~~~~~~~~~~~~~~~~~~~~~ TauolaBelle2 Decay Modes ~~~~~~~~~~~~~~~~~~~~~~~~ The table for TauolaBelle2 can be found in the note `BELLE2-NOTE-PH-2020-055 `_. ~~~~~~~~~~~~~~~~~~~~~~~ TauolaBelle Decay Modes ~~~~~~~~~~~~~~~~~~~~~~~ ============ =============================================== ============ ================================================== MC mode Decay channel MC mode Decay channel ============ =============================================== ============ ================================================== -1 Not a tau pair event 24 :math:`\tau^- \to \pi^- \omega \pi^0 \nu` 1 :math:`\tau^- \to e^- \nu \bar{\nu}` 25 :math:`\tau^- \to \pi^- \pi^+ \pi^- \eta \nu` 2 :math:`\tau^- \to \mu^- \nu \bar{\nu}` 26 :math:`\tau^- \to \pi^- \pi^0 \pi^0 \eta \nu` 3 :math:`\tau^- \to \pi^- \nu` 27 :math:`\tau^- \to K^- \eta \nu` 4 :math:`\tau^- \to \rho^- \nu` 28 :math:`\tau^- \to K^{*-} \eta \nu` 5 :math:`\tau^- \to a_1^- \nu` 29 :math:`\tau^- \to K^- \pi^+ \pi^- \pi^0 \nu` 6 :math:`\tau^- \to K^- \nu` 30 :math:`\tau^- \to K^- \pi^0 \pi^0 \pi^0 \nu` 7 :math:`\tau^- \to K^{*-} \nu` 31 :math:`\tau^- \to K^0 \pi^- \pi^+ \pi^- \nu` 8 :math:`\tau^- \to \pi^- \pi^+ \pi^- \pi^0 \nu` 32 :math:`\tau^- \to \pi^- \bar{K}^0 \pi^0 \pi^0 \nu` 9 :math:`\tau^- \to \pi^- \pi^0 \pi^0 \pi^0 \nu` 33 :math:`\tau^- \to \pi^- K^+ K^- \pi^0 \nu` 10 :math:`\tau^- \to 2\pi^- \pi^+ 2\pi^0 \nu` 34 :math:`\tau^- \to \pi^- K^0 \bar{K}^0 \pi^0 \nu` 11 :math:`\tau^- \to 3\pi^- 2\pi^+ \nu` 35 :math:`\tau^- \to \pi^- \omega \pi^+ \pi^- \nu` 12 :math:`\tau^- \to 3\pi^- 2\pi^+ \pi^0 \nu` 36 :math:`\tau^- \to \pi^- \omega \pi^0 \pi^0 \nu` 13 :math:`\tau^- \to 2\pi^- \pi^+ 3\pi^0 \nu` 37 :math:`\tau^- \to e^- e^- e^+ \nu \bar{\nu}` 14 :math:`\tau^- \to K^- \pi^- K^+ \nu` 38 :math:`\tau^- \to f_1 \pi^- \nu` 15 :math:`\tau^- \to K^0 \pi^- K^0bar \nu` 39 :math:`\tau^- \to K^- \omega \nu` 16 :math:`\tau^- \to K^- K^0 \pi^0 \nu` 40 :math:`\tau^- \to K^- K^0 \pi^+ \pi^- \nu` 17 :math:`\tau^- \to K^- \pi^0 \pi^0 \nu` 41 :math:`\tau^- \to K^- K^0 \pi^0 \pi^0 \nu` 18 :math:`\tau^- \to K^- \pi^- \pi^+ \nu` 42 :math:`\tau^- \to \pi^- K^+ \bar{K}^0 \pi^- \nu` 19 :math:`\tau^- \to \pi^- \bar{K}^0 \pi^0 \nu` 20 :math:`\tau^- \to \eta \pi^- \pi^0 \nu` 21 :math:`\tau^- \to \pi^- \pi^0 \gamma \nu` 22 :math:`\tau^- \to K^- K^0 \nu` 23 :math:`\tau^- \to \pi^- 4\pi^0 \nu` ============ =============================================== ============ ================================================== -------------- Track matching -------------- A reconstructed track can be: 1) **matched**, the reconstructed track is matched to a true track, and it is its best description. 2) **clone**, the reconstructed track is matched to a true track, but there is another reconstructed track that better describes the true track (this second reconstructed track will therefore be matched) 3) **fake**, the reconstructed track is not matched to any true track. It can be a beam-background track or a track built out of noise hits in the detector, or a mixture of these two. .. note:: In case of matched or clone tracks, the charge of the reconstructed track is **not checked** against the charge of the true track. The charge check is anyway included in the MCMatching that sets the :ref:`Error_flags`. More details on the track matching can be found in the :ref:`trk_matching` section of the Tracking chapter, in particular :ref:`trk_matching_analysis`. Here is a table to translate the matching status at tracking level with the one at analysis level: ================= =============== tracking-level analysis-level ================= =============== matched matched wrongCharge matched clone clone cloneWrongCharge clone background fake ghost fake ================= =============== .. _ecl-mcmatching: -------------------------- MCMatching of ECL clusters -------------------------- To understand the method of ECL cluster matching, a basic introduction to the ECL objects used during the reconstruction of simulated data is required. From the ``ECLSimHits`` taken from the GEANT4 simulation, ``ECLDigits`` are created and then calibrated to make ``ECLCalDigit`` objects which store the energy and time of a single ECL crystal. The ``ECLCalDigits`` are then grouped to make ``ECLShower`` objects. This grouping is represented with weighted relations between an ``ECLShower`` and a subset of ``ECLCalDigits``. The energies of the shower objects are corrected such that, for simulated photons, the peak of the reconstructed energy distribution coincides with the generated photon energy. The shower objects are also used to calculate shower-shape quantities and certain particle likelihoods (these calculations are derived using information stored in subsets of the ``ECLCalDigits`` that form the shower). Following this, track matching is performed between reconstructed tracks and shower objects. The last step is the conversion of the ``ECLShower`` object into the mdst ``ECLCluster`` object which is the highest level ECL reconstruction object. Each ``ECLShower`` object (and by extension each ``ECLCluster`` object) with a photon hypothesis holds weighted **relations** to a maximum of twenty-one ``ECLCalDigits``, with the weights calculated using the fraction of energy each ``ECLCalDigit`` contributes to each shower. In addition to this, the ``ECLCalDigit`` can itself have a weighted relation to none, one or many ``MCParticles``. This is calculated using the total energy deposited by the ``MCParticle`` in each ``ECLCalDigit``. A diagram that visualises these relations is given in :numref:`photon_matching`. Please note, that ``ECLShower`` with a neutral hadron hypothesis can have more than twenty-one ``ECLCalDigits`` related to them and are neither corrected nor calibrated. Besides that, the MCMatching works in the same way. .. _photon_matching: .. figure:: figs/photon_matching.png :width: 45em :align: center Schematic diagram showing the weighted relations between ECL reconstruction objects and simulated particles. The overall weight for the relation between an ``ECLCluster`` object and a ``MCParticle`` is then given by the product of the weight between the corresponding ``ECLShower`` and ``ECLCalDigit`` and the weight between the ``ECLCalDigit`` and ``MCParticle``. For example, the weight of the relation between the first ``ECLCluster`` in :numref:`photon_matching` and MCParticle :math:`\gamma_2` is given by :math:`1.0\times 0.8=0.8` GeV. When then a neutral ``Particle`` is reconstructed from a ``ECLCluster`` (so there is no reconstructed track matched to the cluster), it will be MCMatched in the modular analysis if the following conditions are true: 1) :math:`\mathrm{weight}/{E_\mathrm{rec}} > 0.2` 2) :math:`\mathrm{weight}/{E_\mathrm{true}} > 0.3` where :math:`{E_\mathrm{rec}}` is the reconstructed Energy of the ``ECLCluster``, and :math:`{E_\mathrm{true}}` is the generated ``MCParticle`` energy. The *weight* here refers to the relation with the largest weight. This means that if multiple relations between a given ``ECLCluster`` and ``MCParticles`` exist, only the relation with the largest weight will be used, and the corresponding ``MCParticle`` with this relation will be used to decide the MCMatching. A photon match is made if `mcErrors` == 0 and the ``MCParticle`` has a `mcPDG` == 22. If the chosen ``MCParticle`` does not correspond to a true photon, then the `mcErrors` :math:`\neq` 0 and no correct match will be made (even if another one of the smaller-weighted relations for the particle is correct). Information regarding these weights can be accessed on a user-analysis level using the following variables: * `mcMatchWeight` * `clusterTotalMCMatchWeight` * `clusterBestMCMatchWeight` * `clusterMCMatchWeight` .. note:: These weight variables can be used to help isolate photons that originate from **beam background processes**. Such photons typically have a very low total weight. This information has been extracted from `this talk `_. More details about MC matching for photons can be found there and from the references therein. .. _TopologyAnalysis: .. include:: TopoAna.rst