Zero Degree Calorimeter

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The Zero Degree Calorimeters (ZDCs) are installed in the very forward direction around the ATLAS detector. They cover the pseudorapidity range 8.3 < |\eta| < \infty, i.e. down to a scattering angle \theta = 0. This exactly forward coverage is made possible by installing the ZDCs at a distance of \pm140 metres from the ATLAS interaction point, at the location where the straight section of the beampipe is divided back into two independent beampipes. The ZDCs reside in a slot in the Target Absorber Neutral (TAN), which would otherwise contain inert copper bars as shielding.

The primary purpose of the ZDCs is to detect forward neutrons and photons with in both proton-proton and heavy-ion collisions. For heavy-ion collisions the ZDCs play a key role in determining the centrality, which is strongly correlated with the number of very forward (spectator) neutrons. The time resolution of the ZDCs is roughly 100 ps, which is sufficient to locate the interaction point to about 3 cm along the beam axis. This provides some knowledge of the vertex location without using the inner detector information.

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