Tuple Variables: Jet, Clusters, and Missing E_t

1.Introduction and Disclaimers

This page contains some documentation for the root-tuple variables available for the SLAC Jet/MissingEt retreat March 2007. The tuples have been produced from csc11 production, which in large parts used Athena release 11.0.42 and some 11.0.5^†. The content of the tuples is selective, with a focus on the the calorimeter signal and jet algorithms. Generally, there are variables for vertices, tracks (TrackParticle objects), egamma object variables (not Electron!), sliding window and topological cluster variables for electron reconstruction, uncalibrated and (first order) calibrated topological clusters for jets, missing E_t blocks from various sources, and muons from the MuID package.
The data structure is flat, i.e. all variables independent of their type are leaves of only one tree in root. The the Athena Aware Ntuple (CBNTAA) technology has been used to book and fill the variables. The production code is based on release 12.0.3, with several new tags for jet reconstruction and CBNTAA jet code now available in the main development branch (toward 13.0.0). The main difference to 12.0.6 and the main branch may be a slightly different local hadronic calibration package lacking the recent improvements concerning dead material corrections, cluster tagging, and out-of-cluster corrections.

^†11.0.5 includes trigger information which is not included in the tuples available for this retreat.

2.Cluster Variables

The basis of the hadronic final state signal (jets, missing E_t) in the calorimeter are topological cell clusters, based on nearest neighbour clustering. Alternatively, calorimeter cell towers on a regular grid of Δη × Δφ = 0.1 × 0.1 are still considered for jet finding, mainly as a benchmark.
Topological clustering exploits the local cell geometry best by forming three-dimensional, topologically connected cell clusters (nearest neighbour strategy). Each cluster starts with a seed cell with |E| > 4σ, to which neighbouring cells with signals |E| > 2σ are added. A final "ring" of cells with |E| > 0 is added to collect even the smallest signals. If a cluster has more than one signal maxium, it is split. This leads to clusters sharing cell signals, as cells are typically not completely assigned to one or the other cluster. Rather, their signal is split between clusters i and j proportional to the cluster energy, with w_i = E_i / ( E_i + E_j ) and w_j = E_j / ( E_i + E_j ) being the corresponding cell signal weights.
Two index-parallel cluster collections are stored in the tuple. Cluster variables with a _rawtopo belong to uncalibrated (electromagnetic) energy scale clusters, while variables tagged _caltopo belong to calibrated (hadronic) energy scale clusters. Note that a cluster at index i in any of the uncalibrated cluster variable arrays has the same index in the calibrated cluster variable array - the cell composition of the cluster is independent of the calibration. A full list of cluster variables, together with some brief description, is available for both collections in this file.
Topological clusters actually have shape parameters (cluster moments), which are also available in the tuple. These are useful for cluster classification, (hadronic) calibration, and local dead material corrections. This brief note describes how some of these moments are actually calculated (starts at the bottom of page 54). Also, this Wiki page, maintained by Sven Menke (MPI, Munich), the principal author of the topological clustering, contains an up-to-date description of the algorithms.

3.Global Jet Variables

⟨Algorithm⟩	⟨Parameter⟩	Algorithm
C	4	Seeded Cone Algorithm with cone size R = 0.4
	7	Seeded Cone Algorithm with cone size R = 0.7
Kt	4	Kt Algorithm with distance parameter D = 0.4
	6	Kt Algorithm with distance parameter D = 0.6

The jet data available in the tuple reflects physics requirements and allows quite detailed performance studies for different jet algorithm and signal sources. To separate the various sources and algorithms, the following convention has been used:

The algorithm indicator ⟨Algorithm⟩ and the ⟨Parameter`⟩ are summarized in the table to the right. The algorithms and their parameter space considered for the tuple corresponds to the standard jet collections available in default reconstruction.

The available signal sources (⟨SignalSource⟩ parameter) are

Truth	jets from (stable) particles in the truth event in Monte Carlo;
Tower	jets from projective calorimeter cell towers (size Δη×Δφ = 0.1 × 0.1);
CalClus	jets from calibrated topological clusters (hadronic energy scale, corrected for e/h > 1 and some dead material), reconstructed using 4/2/0 cell signal significance cuts (in σnoise, as seed/neighbour/perimeter cell cut);
Clus	jets from uncalibrated topological clusters (electromagnetic energy scale, no further corrections), reconstructed using 4/2/0 cell signal significance cuts (in σnoise, as seed/neighbour/perimeter cell cut);

The global jet variables available for all jets, independent of the signal source, are (also see this file for a full listing):

VarName	Type	Index Range	Range	Comment
Num	UInt_t		0..500	number of jets in event
Eta	vector⟨double⟩	0..Num-1	[-5,5]	pseudo-rapidity η of jets
Phi	vector⟨double⟩	0..Num-1	[-π,+π]	azimuth φ of jets
E	vector⟨double⟩	0..Num-1		jet energy in MeV
Et	vector⟨double⟩	0..Num-1		jet transverse energy in MeV
M	vector⟨double⟩	0..Num-1		jet mass in MeV
Px	vector⟨double⟩	0..Num-1		jet momentum component px in MeV
Py	vector⟨double⟩	0..Num-1		jet momentum component py in MeV
Pz	vector⟨double⟩	0..Num-1		jet momentum component pz in MeV
Emf	vector⟨double⟩	0..Num-1	0..1	electromagnetic energy fraction in jet (see below)
Size	vector⟨long⟩	0..Num-1		number of constituents in jet

The electromagnetic energy fraction in a jet is calculated in different ways, depending on the signal source. For a truth particle jet, the energy fraction carried by photons, electrons, and positrons is used. For a calorimeter cluster jet or a calorimeter tower jet, the energy fraction deposited in the electromagnetic calorimeter samplings is calculated. Note that for jets from topological clusters the energy in tagged electromagnetic clusters can also be used as an indicator of electromagnetic energy fraction in jets. It is available through the jet constituent links described below.

4.Jet Structure Variables

The constituents for each jet are also available in the root tuple. These are basically the kinematic four-vector for all jets, indpendent of signal source or calibration. Note that for jets from uncalibrated towers and uncalibrated clusters these four-vectors are also not fully calibrated. Each constituent also has a kinematic weight with which it contributes to the jet (always 1 for the moment). Also, a reference to other constituent stores is provided by the index pointing into the corresponding tuple array, which for the moment only works for jets from topological clusters. This index gives access to more cluster variables for a given jet.
A detailed list of all jet variables is available. Jets from clusters have already more information for each constituent attached. This allows a quick evaluation of the effect of local hadronic calibration, for example. Also, the absolute cluster barycenter (x,y,z) is given for studies of the longitudinal and radial energy distribution in jets. In general it is quite possible to study the calorimeter signals for jets in quite some detail. The variable name pattern for constituent variables is^∗:

Note that VarNames ending in f denote variables calculated using the final calibration of the constituents, which for uncalibrated cluster and tower jet input means electromagnetic energy scale, while for locally calibrated clusters the variables are calculated on hadronic energy scale. In any case the names of variables calculated using the electromagnetic scale end with i.

^∗due to a bug in some cluster related jet variables, not all start with jetC - to be fixed for 13.0.0. The documentation here shows the "wrong" names for consistency with the data on disk, which follow a ⟨Algorithm⟩⟨Parameter⟩⟨SignalSource⟩jetC⟨VarName⟩ pattern.