Topological cell clustering in the ATLAS calorimeters and its performance in LHC Run 1

The reconstruction of the signal from hadrons and jets emerging from the proton-proton collisions at the Large Hadron Collider (LHC) and entering the ATLAS calorimeters is based on a three-dimensional topological clustering of individual calorimeter cell signals. The cluster formation follows cell signal-significance patterns generated by electromagnetic and hadronic showers. In this, the clustering algorithm implicitly performs a topological noise suppression by removing cells with insignificant signals which are not in close proximity to cells with significant signals. The resulting topological cell clusters have shape and location information, which is exploited to apply a local energy calibration and corrections depending on the nature of the cluster. Topological cell clustering is established as a well-performing calorimeter signal definition for jet and missing transverse momentum reconstruction in ATLAS.

9 March 2016

Contact: PERF conveners internal

Eur. Phys. J. C 77 (2017) 490

e-print arXiv:1603.02934 - pdf from arXiv

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Figures | Tables

Figures

Figure 01

Cutaway view on the ATLAS calorimeter system.

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Figure 02a

The peak luminosities measured by the ATLAS online luminosity monitor system throughout the run years are shown in (a). The mean number of additional proton–proton interactions at the beginning of each LHC fill is shown in (b) for the same period in time.

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Figure 02b

Figure 03

The pulse shape in the ATLAS LAr calorimeters. The unipolar triangular pulse is the current pulse in the liquid argon generated by fast ionising particles. Its characteristic time is the drift time (charge collection time) t_d ≈ 450 ns in the example for the central EMB calorimeter shown here. The shaped pulse is superimposed, with a characteristic duration of t_signal ≈ 600 ns. The full circles on the shaped pulse indicate the nominal bunch crossings at 25 ns intervals. The figure has been adapted from Ref. [14].

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Figure 04a

The energy-equivalent cell noise in the ATLAS calorimeters on the electromagnetic (EM) scale as a function of the direction |η| in the detector, for (a) the 2010 configuration with μ = 0, (b) the 2011 configuration with μ = 8 (both plots from Ref.[16]), and (c) the 2012 configuration with μ = 30. The various colours indicate the noise in the pre-sampler (PS) and the three layers of the LAr EM calorimeter, the three layers of the Tile calorimeter, the four layers of the hadronic end-cap (HEC) calorimeter, and the three modules of the forward (FCAL) calorimeter.

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Figure 04b

Figure 04c

The energy-equivalent cell noise in the ATLAS calorimeters on the electromagnetic (EM) scale as a function of the direction |η| in the detector, for (a) the 2010 configuration with μ = 0, (b) the 2011 configuration with μ = 8 (both plots from Ref.[16]), and (c) the 2012 configuration with &mu = 30. The various colours indicate the noise in the pre-sampler (PS) and the three layers of the LAr EM calorimeter, the three layers of the Tile calorimeter, the four layers of the hadronic end-cap (HEC) calorimeter, and the three modules of the forward (FCAL) calorimeter.

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Figure 05a

Signal significance (ς_cell^EM) distributions for all cells (blue/cyan) and for cells after the noise suppression in the topological cell clustering is applied (red/yellow), in selected sampling layers of the LAr calorimeters: (a) the first sampling of the central electromagnetic LAr calorimeter (EMB), (b) the first sampling of the electromagnetic LAr end-cap calorimeter (EME), (c) the first sampling of the hadronic LAr end-cap calorimeter (HEC), and (d) the first module of the LAr forward calorimeter (FCAL). The spectra are extracted from 2012 zero-bias data at √8 TeV with an average number of pile-up interactions <μ> = 28. The dashed lines indicate S = ± 4, N = ± 2, and P = 0.

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Figure 05b

Figure 05c

Figure 05d

Figure 06a

Stages of topo-cluster formation in the first module (FCAL0) of the FCAL calorimeter for a simulated dijet event with at least one jet entering this calorimeter. Shown in (a) are cells with signal significance |ς_cell^EM| > 4 that can seed topo-clusters, in (b) cells with |ς_cell^EM| > 2 controlling the topo-cluster growth, and in (c) all clustered cells and the outline of topo-clusters and topo-cluster fragments in this module. All clusters shown in (c) which do not contain a seed cell from this module are seeded in other modules of the FCAL, or in other calorimeters surrounding it. Pile-up is not included in this simulation, but electronic noise is modelled. Cells not colour coded but inside a topo-cluster have a negative signal, while cells shaded grey are completely surrounded by clustered cells but not part of a topo-cluster themselves. The cell and cluster boundaries are displayed on a dimensionless grid using the polar angle θ and the azimuthal angle Φ. This view maintains the cell shapes and proportions. For the definition of the cell signal significance ς_cell^EM see Eq.(2).

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Figure 06b

Figure 06c

Figure 07a

The number of reconstructed clusters for simulated charged and neutral single pions without actual pile-up added but with nominal pile-up noise used in the reconstruction. In (a) the distribution of the number of clusters N_clus is shown for neutral and charged pions injected into the ATLAS calorimeters at |η| = 0.3 with an energy of E = 100 GeV, together with the N_clus distribution for empty events (topo-clusters generated by electronic noise only). The distributions are individually normalised to unity. The dependence of the average ⟨N_clus⟩ on the generated η_gen is shown in (b), again for π⁰, π⁻ and empty events. The shaded area and the dashed lines indicate the spread (in terms of RMS) around the central value.

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Figure 07b

Figure 08

Schematic view of geometrical moments for topo-clusters.

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Figure 09

Overview of the local hadronic cell-weighting (LCW) calibration scheme for topo-clusters. Following the topo-cluster formation, the likelihood for a cluster to be generated by electromagnetic energy deposit (℘_clus^EM) is calculated. After this, the sequence of calibration and corrections indicated in the schematics is executed, each yielding cell signal weights for the two possible interpretations of the cluster signals. These weights are indicated in the figure. They are then used together with ℘_clus^EM to calculate the topo-cluster energy and barycentre from the contributing calorimeter cells, as described in the text.

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Figure 10

Distribution of the likelihood ℘_clus^EM(ρ_clus/E_clus^EM,λ_clus) for reconstructed topo-clusters to originate from an electromagnetic shower as a function of the shower depth λ_clus and the normalised cluster signal density ρ_clus/E_clus^EM, with ρ_clus = ⟨ρ_cell⟩ being the energy-weighted average of ρ_cell. The shown distribution is determined as described in the text, in a selected bin of the cluster energy E_clus^EM and the cluster direction η_clus. The red line indicates the boundary of the ℘_clus^EM > 50% selection, below which the topo-cluster is classified as mostly electromagnetic ("EM-like") and above which it is classified as mostly hadronic ("HAD-like"). The small EM-like area at the edge of the HAD-like region stems from neutral pions showering late. These areas are typical in regions of the detector where the second layer of the EM calorimeter is thinner and substantial parts of the shower are deposited in its last layer. The larger volume of the cells in this last layer leads to the reduced energy density while the position at the back of the EM calorimeter means a larger λ_clus.

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Figure 11a

The distribution of the longitudinal depth λ_clus of topo-clusters inside anti-k_t jets with R = 0.6, |y| < 2.8, and p_T > 20 GeV, for clusters classified as (a) electromagnetic (EM) and (b) hadronic (HAD), in 2010 data and MC simulations (no pile-up). Also shown is the average topo-cluster depth ⟨λ_clus⟩ as function of the cluster energy E_clus^EM for the same topo-clusters classified as (c) EM and (d) HAD, respectively. The figures are adapted from Ref.[38].

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Figure 11b

Figure 11c

Figure 11d

Figure 12a

Distributions of the cell energy E_cell^EM in the (a) central pre-sampler (PreSamplerB) and the cell energy density ρ_cell in the second sampling of (b) the central (EMB2) and (c) the end-cap (EME2) electromagnetic calorimeters in ATLAS, as observed inside anti-k_t jets with R = 0.6 in 2010 data and MC simulations (no pile-up) [38]. The data/MC ratio of the spectra is shown below the corresponding distributions.

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Figure 12b

Figure 12c

Figure 13a

Distributions of the cell energy density ρ_cell in various regions of the central (a), end-cap (b), and forward (d) hadronic calorimeters in ATLAS, as observed inside anti-k_t jets with R = 0.6 in 2010 data and MC simulations (no pile-up) [38]. The data/MC ratio of the spectra is shown below the corresponding distributions.

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Figure 13b

Figure 13c

Figure 14

Illustration of the assignment scheme for cells inside the calorimeter with true signal not captured in a topo-cluster in the context of the out-of-cluster correction (see Sect. 5.4) and for dead material cells outside the calorimeter for the dead material correction discussed in Sect. 5.5. The deposited energy in cells inside the topo-cluster is used to determine the hadronic calibration described in Sect. 5.3. A schematic depiction of a typical section of the ATLAS end-cap calorimeter with four highly granular electromagnetic samplings and four coarser hadronic samplings is shown in a view with η as the horizontal and the depth z as the vertical coordinate. The boxes at small z in front of the EM calorimeter symbolise upstream energy losses collected into dead material cells.

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Figure 15a

The distribution of the isolation moment f_iso in (a) clusters classified as electromagnetic, and (b) clusters classified as hadronic. The average isolation ⟨f_iso⟩ as a function of the cluster signal E_clus^EM is shown in (c) for electromagnetic and in (d) for hadronic topo-clusters. The figures are taken from Ref.[38].

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Figure 15b

Figure 15c

Figure 15d

Figure 16

The average energy loss in the virtual dead material cells for charged 100 GeV pions. The numbers 1 to 7 indicate the different regions, with region 8 (not displayed) being everywhere outside regions 1−7. The dead material cells are superimposed on a schematic (r,z) view showing a quarter of the ATLAS calorimeter system with its read-out segmentation.

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Figure 17a

The average ratio of reconstructed to EM-scale energy after each calibration step, as a function of the cluster energy E_clus^EM ((a), (b), and (c)) for topo-cluster in anti-k_t jets with R =0.6 and p_T > 20 GeV and with rapidities |y_jet| < 0.3. The corresponding average ratios as a function of η_clus are shown in (d), (e), and (f). Data recorded in 2010 is compared to the corresponding MC simulations. The figures are adapted from Ref.[38].

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Figure 17b

Figure 17c

Figure 17d

Figure 17e

Figure 17f

Figure 18a

The distribution of E/p, the ratio of the calorimeter energy E and the track momentum p, for (a) central tracks with 1.2 GeV < p < 1.8 GeV and (b) forward-going tracks with 2.8 GeV < p < 3.6 GeV, for data and MC simulations of proton−proton collisions at √s = 900 GeV and no pile-up (from Ref.[38]).

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Figure 18b

Figure 19a

In (a), the likelihood ℘_{E = 0}(d_dm) to find no matching energy in the calorimeter (E = 0) for reconstructed isolated charged-particle tracks is shown as a function of the thickness d_dm of the inactive material in front of the calorimeter, for data and MC simulations in proton−proton collisions at √s = 900 GeV. The thickness of the inactive material is measured in terms of the nuclear interaction length λ_nucl. The tracks are reconstructed within |η| < 1. The likelihood to reconstruct E = 0 as a function of the incoming track momentum is shown for the same data and MC simulations in (b), for reconstructed tracks within |η| < 0.6. Both figures are taken from Ref.[38].

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Figure 19b

Figure 20a

The average ⟨E/p⟩ ratio as a function of the track momentum p, for (a) tracks within |η| < 0.6 and (b) tracks within 1.9 < |η| < 2.3. Data from isolated tracks recorded in 2010 and 2012 with insignificant pile-up are shown together with MC simulations employing two different hadronic shower models.

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Figure 20b

Figure 21a

The distribution of the transverse momentum of topo-clusters reconstructed on the EM scale (p_T,clus^EM) in various bins of η_clus for an inclusive Z→μμ event sample recorded in 2012. Data are compared to distributions from MC simulations including fully simulated pile-up (upper row) and with pile-up overlaid from data (lower row) for all topo-cluster within ((a), (d)) |η_clus| < 0.2, ((b), (e)) 2.0 < |η_clus| < 2.2, and ((c), (f)) 3.8 < |η_clus| < 4.0. The ratio of the distribution from data to the one from MC simulation is evaluated bin-by-bin and shown below the respective set of distributions. The shaded bands indicate the statistical uncertainties from MC simulations for both the spectra and the ratios.

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Figure 21b

Figure 21c

Figure 21d

Figure 21e

Figure 21f

Figure 22a

The average ⟨Σp_T,clus^EM⟩ of positive-energy clusters at the EM scale as a function of η, for (a) all clusters, and for clusters with (b) p_T,clus^EM > 100 MeV, (c) p_T,clus^EM > 250 MeV, (d) p_T,clus^EM > 500 MeV, (e) p_T,clus^EM > 1 GeV, and (f) p_T,clus^EM > 2 GeV. Results are obtained from a 2012 Z→μμ sample without jets with p_T > 20 GeV in data and MC simulation. The ratios of ⟨Σp_T,clus^EM⟩(η) from data and MC simulations are shown below each plot.

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Figure 22b

Figure 22c

Figure 22d

Figure 22e

Figure 22f

Figure 23a

The average transverse momentum flow ⟨Σp_T,clus^EM⟩ is shown as function of the pile-up activity, measured by the number of proton−proton interactions per bunch crossing μ, in several detector regions of ATLAS. Two different selections are applied to topo-clusters contributing to ⟨Σp_T,clus^EM⟩. In (a)−(c) all clusters with p_T,clus^EM > 0 contribute, while in (d)−(f) only those with p_T,clus^EM > 1 GeV are included. Results are obtained from a 2012 Z→μμ sample without jets with p_T > 20 GeV in data and MC simulations. The narrow shaded bands around the results for MC simulations indicate statistical uncertainties, both for ⟨Σp_T,clus^EM⟩(μ) and the corresponding data-to-MC simulation ratios shown below each plot.

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Figure 23b

Figure 23c

Figure 23d

Figure 23e

Figure 23f

Figure 24a

Average topo-cluster number density ⟨∂N_clus/∂η⟩ as a function of η_clus, for clusters with p_T,clus^EM > p_T,min, for various p_T,min values. Results are obtained from a 2012 Z→μμ sample without jets with p_T > 20 GeV in data and MC simulations. The corresponding data-to-MC simulation ratios are shown below each figure

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Figure 24b

Figure 24c

Figure 24d

Figure 24e

Figure 24f

Figure 25

The reconstructed average transverse momentum flow on EM scale, measured with topo-clusters in bins of η using ⟨p_T,clus^EM⟩(η) in Eq.(27) and with all calorimeter cells in the same η-bins using ⟨p_T,cell^EM⟩(η) given in Eq.(28), in 2012 MB data.

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Figure 26a

The dependence of the average number of positive-energy topo-clusters on the pile-up activity measured in terms of the number of proton−proton collisions per bunch crossings (μ, see Sect. 2.3.2) in several regions of the detector is shown in ((a), (d)) for η_clus < 0.2, in ((b), (e)) for 2.0 < |η_clus| < 2.2, and in ((c), (f)) for 3.8 < |η_clus| < 4.0. The plots (a)−(c) in the upper row show the results for counting all clusters with p_T,clus^EM > 0, while (d)−(f) in the lower row show the results for only counting clusters with p_T,clus^EM > 1 GeV. The corresponding ratio of data to MC simulations is shown below each plot. All results are obtained from a 2012 Z→μμ sample without jets with p_T > 20 GeV in data and MC simulations. The narrow shaded bands indicate the statistical uncertainties associated with the results from MC simulations for the mean values and the ratios.

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Figure 26b

Figure 26c

Figure 26d

Figure 26e

Figure 26f

Figure 27a

The distribution of the topo-cluster depth location, measured in terms of log₁₀(λ_clus/λ₀), for clusters in various bins of η_clus for an inclusive Z→μμ event sample recorded in 2012. Data is compared to distributions from MC simulations including fully simulated pile-up for all topo-clusters within (a) |η_clus| < 0.2, (b) 2.0 < |η_clus| < 2.2, and (c) 3.8 < |η_clus| < 4.0. The corresponding distributions for MC simulations with pile-up overlaid are depicted in (d), (e), and (f). The ratios of the distributions for data and MC simulations are shown below the respective distributions. The shaded bands indicate the statistical uncertainties for MC simulations.

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Figure 27b

Figure 27c

Figure 27d

Figure 27e

Figure 27f

Figure 28a

The distribution of the topo-cluster depth location, measured in terms of log₁₀(λ_clus/λ₀), for selected topo-clusters within |η_clus| < 0.2 and with a transverse momentum p_T,clus^EM, evaluated on the EM scale, larger than various thresholds. Results are shown for an inclusive Z→μμ event sample recorded in 2012. Data are compared to distributions from MC simulations including fully simulated pile-up for all topo-clusters with (a) p_T,clus^EM > 1 GeV, (b) p_T,clus^EM > 2 GeV, and (c) p_T,clus^EM > 5 GeV. The corresponding distributions for MC simulations with pile-up overlaid are depicted in (d), (e), and (f). The shaded bands indicate the statistical uncertainties for the distributions obtained from MC simulations and the corresponding uncertainties in the ratio plots.

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Figure 28b

Figure 28c

Figure 28d

Figure 28e

Figure 28f

Figure 29a

In (a), the fully calibrated and corrected jet p_T response measured by p_T,jet^LCW+JES is shown as a function of the pile-up activity measured by μ, in three different detector regions for Z→μμ events with one anti-k_t jet with R = 0.4 with 30 GeV < p_T,jet^LCW+JES < 40 GeV, for 2012 data and MC simulations with fully simulated pile-up. The μ dependence of the uncorrected jet p_T response is shown in (b). It is measured in terms of its ratio to the fully calbrated jet response, p_T,jet^LCW/p_T,jet^LCW+JES, for the same events and in the same detector regions. The shaded bands shown for the results from MC simulations indicate statistical uncertainties.

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Figure 29b

Figure 30a

The distribution of the number of topo-clusters inside anti-k_t jets formed with R = 0.4 in the ((a), (d)) central (|η| < 0.6), the ((b), (e)) end-cap (2.0 < |η| < 2.5), and the ((c), (f)) forward detector region (3.5 < |η| < 4.5) of ATLAS. Shown are results from the analysis of Z→μμ events with at least one jet with 30 GeV < p_T < 40 GeV in 2012 data and MC simulations with fully simulated pile-up in (a)−(c), and with pile-up from data overlaid in (d)−(f). The ratios of results for data and MC simulations are shown below the distributions. The shaded bands show the statistical uncertainties for the distributions obtained from MC simulations and the corresponding uncertainty bands in the ratio plots.

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Figure 30b

Figure 30c

Figure 30d

Figure 30e

Figure 30f

Figure 31a

The distribution of the number of topo-clusters with p_T,clus^EM > 1 GeV inside anti-k_t jets with R = 0.4 in the ((a), (d)) central (|η| < 0.6), the ((b), (e)) end-cap (2.0 < |η| < 2.5), and the ((c), (f)) forward detector region (3.5 < |η| < 4.5) of ATLAS. Shown are results from the analysis of Z→μμ events with at least one jet with 30 GeV < p_T < 40 GeV in 2012 data and MC simulations with fully simulated pile-up in (a)−(c), and with pile-up from data overlaid in (d)−(f). The data-to-MC simulation ratios are shown below the respective distributions. The shaded bands indicate statistical uncertainties for the distributions from MC simulations and the corresponding statistical uncertainty bands for the ratios.

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Figure 31b

Figure 31c

Figure 31d

Figure 31e

Figure 31f

Figure 32a

The average number of topo-clusters ⟨N_clus^jet⟩ in anti-k_t jets reconstructed with R = 0.4 within 30 GeV < p_T,jet^LCW+JES < 40 GeV as a function of 7MU;, in Z→μμ events in 2012 data and MC (a). The pile-up dependence of the average number of topo-clusters ⟨N_clus^core⟩ in the core of the jet, defined by the distance to jet axis ΔR < 0.3, is shown in (b). Selecting topo-clusters by p_T,clus^EM > 2 GeV inside jets and in the core of the jet yields the μ dependencies shown in (c) and (d). The shaded bands shown for the results obtained from MC simulations indicate statistical uncertainties.

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Figure 32b

Figure 32c

Figure 32d

Figure 33a

The distribution of the depth location, measured in terms of log₁₀(λ_clus/λ₀) with λ₀ = 1 mm, of all topoclusters in jets reconstructed with the anti-k_t algorithm with R = 0.4 and with 30 GeV < p_T,jet^LCW+JES < 40 GeV in Z→μμ events in 2012 data and MC simulations with (a)−(c) fully simulated pile-up and with (d)−(f) overlaid pile-up from data. Distributions are shown for jets in the ((a), (d)) central (|η| < 0.6), the ((b), (e)) end-cap (2.0 < |η| < 2.5), and the ((c), (f)) forward detector region (3.5 < |η| < 4.5). The bin-by-bin ratios of the distributions from data and MC simulations are shown below the plots. The shaded bands indicate statistical uncertainties for the distributions from MC simulations and the corresponding uncertainty bands in the ratio plots.

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Figure 33b

Figure 33c

Figure 33d

Figure 33e

Figure 33f

Figure 34a

The distribution of the depth location, measured in terms of log₁₀(λ_clus/λ₀) with λ₀ = 1 mm, of topo-clusters with p_T,clus^EM > 1 GeV in jets reconstructed with the anti-k_t algorithm with R = 0.4 and with 30 GeV < p_T,jet^LCW+JES < 40 GeV in Z→μμ events in 2012 data and MC simulations with (a)−(c) fully simulated pile-up and with (d)−(f) overlaid pile-up from data. Distributions are shown for jets in the ((a), (d)) central (|η| < 0.6), the ((b), (e)) end-cap (2.0 < |η| < 2.5), and the ((c), (f)) forward detector region (3.5 < |η| < 4.5). The data-to-MC simulation ratios are shown below the distributions. The shaded bands shown for the distributions obtained from MC simulations indicate statistical uncertainties and the corresponding uncertainty bands in the ratio plots.

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Figure 34b

Figure 34c

Figure 34d

Figure 34e

Figure 34f

Figure 35a

The (a)−(c) distribution of the measure for the depth location λ_clus, the (d)−(f) distribution of the measure for the cluster signal density ρ_clus, and the (g)−(i) distribution of the ratio of the cluster signal reconstructed on EM scale E_clus^EM to the fully calibrated signal E_clus^LCW for the leading topo-cluster in anti-k_t jets reconstructed with R = 0.4 and with 30 GeV < p_T,jet^LCW+JES < 40 GeV in Z→μμ events in 2012 data and MC simulations. Spectra are shown for jets in the ((a), (d), (g)) central (|η| < 0.6), the ((b), (e), (h)) end-cap (2.0 < |η| < 2.5), and the ((c), (f), (i)) forward detector region (3.5 < |η| < 4.5) in ATLAS. The reference scale for the depth location is λ₀ = 1 mm, for the signal density ρ₀ = 1 MeV/mm³, and for the energy E₀ = 1 MeV. The ratio of the distribution from data to the one from MC simulations is shown below each plot. The shaded bands show statistical uncertainties for the distributions from MC simulations and the corresponding uncertainty bands in the ratio plots.

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Figure 35b

Figure 35c

Figure 35d

Figure 35e

Figure 35f

Figure 35g

Figure 35h

Figure 35i

Figure 36a

The distribution of the signal fraction f_lead carried by the leading topo-cluster in jets, as defined in Eq.(31), in (a) the central, (b) the end-cap, and (c) the forward detector region. The jets are reconstructed using the anti-k_t algorithm with R = 0.4 and with 30 GeV < p_T,jet^LCW+JES < 40 GeV in Z→μμ events in 2012 data and MC simulations with fully simulated pile-up. The data-to-MC simulation ratios are shown below the distributions. The shaded bands shown for the distributions obtained from MC simulations indicate statistical uncertainties and the corresponding uncertainty bands in the ratio plots.

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Figure 36b

Figure 36c

Figure 37a

The pile-up dependence of (a) f_lead defined in Eq.(31), (b) E_clus^EM/E_clus^LCW, and (c) the depth location λ_clus of the leading topo-cluster in anti-k_t jets reconstructed with R = 0.4 and with 30 GeV < p_T,jet^LCW+JES < 40 GeV in Z→μμ events in 2012 data and MC simulations with fully simulated pile-up. The reference scale for λ_clus is λ₀ = 1 mm. The pile-up activity is measured in terms of the number of pile-up interactions μ. The shaded bands shown for the results obtained from MC simulations indicate statistical uncertainties.

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Figure 37b

Figure 37c

Figure 38a

The distribution of the normalised (a)−(c) lateral (m_lat²) and (d)−(f) longitudinal (m_long²) extension measures of the leading topo-cluster in anti-k_t jets with R = 0.4 and 30 GeV < p_T,jet^LCW+JES < 40 GeV in Z→μμ events in 2012 data and MC simulations with fully simulated pile-up, for jets in the ((a), (d)) central (|η| < 0.6), the ((b), (e)) end-cap (2.0 < |η| < 2.5), and the ((c), (f)) forward detector region (3.5 < |η| < 4.5) of ATLAS. The ratios of data and MC simulation distributions are shown below the plots. The shaded bands shown for the distributions obtained from MC simulations indicate statistical uncertainties and the corresponding uncertainty bands in the ratio plots.

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Figure 38b

Figure 38c

Figure 38d

Figure 38e

Figure 38f

Figure 39a

The length of the leading topo-cluster, measured in terms of the longitudinal spread (second moment) ⟨λ²⟩ of the cell coordinates along the principal cluster axis by ⟨λ²⟩^½/λ₀, in anti-k_t jets reconstructed with R = 0.4 and 30 GeV < p_T,jet^LCW+JES < 40 GeV in Z→μμ events in 2012 data and MC simulations with fully simulated pile-up. Distributions are shown for jets in the (a) central (|η| < 0.6), the (b) end-cap (2.0 < |η| < 2.5), and the (c) forward detector region (3.5 < |η| < 4.5). The normalisation of the longitudinal spread is given by λ₀ = 1 mm. The ratios of data-to-MC simulations are shown below the distributions. The shaded bands indicate statistical uncertainties of the distributions from MC simulations and the resulting uncertainty bands in the ratio plots.

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Figure 39b

Figure 39c

Figure 40a

The size R_ηφ^lead of the leading topo-cluster in (η,φ) space, measured using Eq.(15), in anti-k_t jets reconstructed with R = 0.4 and with 30 GeV < p_T,jet^lead < 40 GeV in Z→μμ events in 2012 data and MC simulations with fully simulated pile-up. Distributions are shown for jets in the (a) central (|η| < 0.6), the (b) end-cap (2.0 < |η| < 2.5), and the (c) forward detector region (3.5 < |η| < 4.5) in ATLAS. The ratios of data to MC simulations are shown below the distributions. The shaded bands shown for the distributions obtained from MC simulations indicate statistical uncertainties and the corresponding uncertainty bands in the ratio plots.

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Figure 40b

Figure 40c

Figure 41a

The average pile-up dependence of various geometric observables reconstructed from the leading topo-cluster in anti-k_t jets reconstructed with R = 0.4 and 30 GeV < p_T,jet^LCW+JES < 40 GeV in Z→μμ events in 2012 data and MC simulations with fully simulated pile-up. The average cluster length, represented by log₁₀(⟨λ²⟩^½/λ₀) with the reference scale λ₀ = 1 mm, is shown as a function of μ in (a), for three detector regions. The average size ⟨R_ηφ^lead⟩ of the leading topo-cluster in (η,φ) space is displayed for the same detector regions and as a function of μ in (b). The average normalised lateral energy dispersion ⟨m_lat²⟩ of the cluster, as a function of μ for the three detector regions, is shown in (c). The shaded bands shown around the results obtained from MC simulations indicate statistical uncertainties.

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Figure 41b

Figure 41c

Tables

Table 01

The read-out granularity of the ATLAS calorimeter system [1], given in terms of Δη×Δφ with the exception of the forward calorimeters, where it is given in linear measures Δx×Δy, due to the non-pointing read-out geometry of the FCAL. For comparison, the FCAL granularity is approximately Δη×Δφ = 0.15×0.15 (0.3×0.3) at η = 3.5(4.5). The total number of read-out cells, including both ends of the calorimeter system, with (without) pre-samplers is 187652 (178308).

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Table 02

Overview of the signals used to correct for dead material losses in the various regions around the ATLAS calorimeters. The numbered regions are shown in Fig. 16. The parameter values used for the dead material correction are extracted from lookup tables. Region 8 comprises all dead material volumes with energy loss outside regions 1 to 7. These are mostly small volumes located between and behind the active calorimeters.

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Table 03

Summary of the calibration and correction sequence applied to topo-clusters from the EM to the final LCW scale.

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