Performance of the ALICE Electromagnetic Calorimeter

The performance of the electromagnetic calorimeter of the ALICE experiment during operation in 2010-2018 at the Large Hadron Collider is presented. After a short introduction into the design, readout, and trigger capabilities of the detector, the procedures for data taking, reconstruction, and validation are explained. The methods used for the calibration and various derived corrections are presented in detail. Subsequently, the capabilities of the calorimeter to reconstruct and measure photons, light mesons, electrons and jets are discussed. The performance of the calorimeter is illustrated mainly with data obtained with test beams at the Proton Synchrotron and Super Proton Synchrotron or in proton-proton collisions at $\sqrt{s}=13$ TeV, and compared to simulations.

 

Submitted to: JINST
e-Print: arXiv:2209.04216 | PDF | inSPIRE
CERN-EP-2022-184
Figure group

Figure 1

Schematic view of the \gls{EMCal} (left) illustrating the module position on two approximately opposite locations in azimuth. The \gls{PHOS} calorimeter inside the \gls{DCal} is indicated in brown. The right figure shows a cross section of the \gls{ALICE} barrel detectors..

Figure 2

Photo and drawing of \gls{EMCal} module showing all components..

Figure 3

Schematic view of the \gls{EMCal} full-size super modules (\glspl{SM}) illustrating the strip structure made of 24 strips..

Figure 4

\gls{EMCal} full-size (\gls{SM}) in the $(\eta,\varphi)$ plane including visualizations of sub-components and their tower coverage..

Figure 5

Geometric overview of the \gls{EMCal} and \gls{DCal} detectors in the $\eta$-$\varphi$ plane. The drawing outlines the full \gls{LHC} Run 2 setup with all 20 \glspl{SM} as well as the \gls{PHOS} detector in the \gls{DCal} gap..

Figure 6

Example of a fit to a digitized pulse from electron test beam data using the function defined in \Eq{HW_Eq_Gauss2} with a fixed shaping time parameter $\tau = 235 $ns. .

Figure 7

Distribution of the opening angle $\theta_{12}$ of two decay photons from \piz\ (left) and $\eta$ (right) mesons decays as a function of the meson energy obtained at generator level from a \gls{MC} simulation. The horizontal lines indicate the opening angle corresponding to a width of approximately 1, 3 and 5 cells separating the two photons Two photons completely merge into one cluster if they fall below the one cell distance, while they start merging at a width of approximately 3 cells, depending on the clusterizer The colored vertical lines correspond to the energy limits where two photons could still be fully resolved using the V1 (blue) and V2 (red) clusterizers .

Figure 8

Schematic comparison of different clusterization algorithms. Only one dimension is shown for simplicity Yellow boxes represent the energy in each cell. $E_\mathrm{agg}$ is the clusterization threshold as defined in the text. The different clusters are indicated by blue and red hatched areas Each panel represents a clusterization algorithm: a) V1, b) $3 \times 3$, c) V2, d) V1+unfolding..

Figure 9

Invariant mass distribution of cluster pairs in pp collisions at \sthirteen\ for different intervals of pair energy. The differently colored lines correspond to different clusterizer types, using the same aggregation $E_\mathrm{agg}=100$ MeV and seed $E_\mathrm{seed} = 500$ MeV thresholds. The lowest bin in energy uses the data sample with minimum bias trigger, while the others are obtained from the \gls{EMCal} \gls{L1} triggered data with thresholds at about $E \approx 4$ and $9$ GeV, respectively. .

Figure 10

Fraction of neutral pions (left) and eta mesons (right) for which the showers from their decay photons are merged into a single cluster and can not be reconstructed using an invariant-mass analysis..

Figure 11

Distance between a cluster and the closest projected track in $\eta$ (left) or $\varphi$ angle (middle) versus the track momentum and for matched track-cluster pairs (right) in pp collisions at \sthirteen\ collected with the minimum bias trigger. Clusters are reconstructed using the V2 clusterizer. The black lines in the left and middle panels indicate the suggested selection criteria expressed in \Eq{Eq:TMVetoCriteria} .

Figure 12

Left: cluster-veto efficiency for primary particles (dark blue open circles) and electrons (green squares) as well as conversion electrons with a production vertex below 180 cm (green closed diamonds) and above 180 cm (green open diamonds) and other secondary particles (cyan open circles) as obtained from simulations of pp collisions at \sthirteen . Right: fraction of fake track-to-cluster matches for clusters originating from photons (yellow open squares) and other neutral particles (red open circles). Additionally, the same categories are shown for clusters that have additional charged particle contributions for photon clusters (orange squares) and other neutral particles (light red circles). .

Figure 13

Schematic representation of the shower shape and the ellipse axes. The different colors indicate the amount of energy deposited in each cell, the darker the more energy..

Figure 14

\shshlo\ (left) and \shshsh\ (right) distributions in three energy intervals for photons, electrons, hadrons, \piz\ and $\eta$ mesons. The distributions are obtained using the V1 clusterizer from a simulation of pp collisions at \sthirteen\ performed with the \gls{PYTHIA} event generator, in which events are required to contain either two jets or a jet and a high-energy direct photon. Each distribution is normalized to its integral. A model simulating the effect of the cross talk was applied as discussed in \Sec{sec:crosstalk}. . Distributions of \shshlo\ versus cluster energy in pp collisions at \sthirteen\ triggered by the \gls{EMCal} \gls{L1} at approximately 9 GeV for the V2 (left) and V1 clusterizer (right). Each energy bin is normalized to its integral and exotic clusters were rejected (\Sec{sec:exotics}). .

Figure 15

Left: number of cells as a function of the cluster energy found with the V2 clusterizer in \pp\ collisions at \sthirteen\ using the \gls{EMCal} high threshold \gls{L1} $\gamma$ trigger The region below the lines is populated by exotic clusters The distribution for each energy bin is normalized to its integral. Right: comparison of $n_{\text{cells}}$ probability distributions for measured data (black), projection of the left plot, to simulated (blue) collisions for two different cluster energy bins. Each distribution is normalized by the integral of the distribution for $n_{\rm cells}$ > 10. .

Figure 16

Left: ``exoticity'' ($F_{\rm +}$) as function of the cluster energy found with the V2 clusterizer in \pp\ collisions at \sthirteen\ using the \gls{EMCal} high threshold \gls{L1} $\gamma$ trigger The region above the line is populated by exotic clusters The distributions are normalized to have an integral of unity for each energy bin. Right: comparison of $F_{+}$ probability distributions for measured data (black), projection of the left panel, and simulated (blue) collisions for two different cluster-energy intervals. Each distribution is normalized by the integral of the distribution for $F_{+}<0.85$ .

Figure 17

Left: cluster time for exotic ($F_{+}>$ 0.97 and non-exotic ($F_{+}<$ 0.97) clusters. The additional peaks in the time distribution beyond $100$ ns arise from additional bunch crossings, which could not be rejected by the online Past-Future protection using the \gls{V0} detector Right: difference in time of the most and second most energetic cell in the cluster for exotic and non exotic clusters. Both distributions are obtained for V2 clusters with energy in the interval $8 < E < 10$ GeV from data taken from pp collisions at \sthirteen .

Figure 18

\shshlo\ as a function of the exoticity parameter $F_{+}$ obtained from V2 clusters with $12< e the="" distributions="" are="" shown="" for="" pp="" collisions="" at="" using="" trigger="" l1-trig="" and="" central="" .="">< /e>