More details on the ALICE ZDC
Two identical sets of calorimeters are located on both sides of the ALICE detector, 112.5 m away from the Interaction Point (IP).
Each set of detectors consists of a neutron (ZN) and a proton (ZP) Zero Degree Calorimeter (ZDC) positioned on remotely controlled platforms. The ZN is placed at zero degree with respect to the LHC beam axis, between the two beam pipes, while the ZP is positioned externally to the outgoing beam pipe.
The spectator protons are separated from the ion beams by means of the dipole magnet D1.
The ZDC detector is completed by two small electromagnetic calorimeters (ZEM) placed at 7,35 m from the IP, on one side, detecting the energy of particles emitted in the pseudo-rapidity range 4.8 < η < 5.7
The ZDCs are quartz-fiber spaghetti calorimeters, with silica optical fibers as active material embedded in a dense absorber. Their principle of operation is based on the detection of Cherenkov light produced by the charged particles of the shower in the fibers.
The quartz fibers are hosted in the grooves of the absorber slab, W-alloy for ZN and brass for ZP. In the hadronic calorimeters the fibers are placed at zero degrees with respect to the LHC axis and come out from the rear face of the calorimeter, directly bringing the light to the photomultipliers.
Each of the ZN calorimeters has dimensions: 7 x 7 x 100 cm3 and the quartz fibers are hosted in the 1936 grooves of the W-alloy slabs. The ZP calorimeters have dimensions of 22.8 x 12 x 150 cm3 and the quartz fibers are hosted in the 1690 grooves of the brass slabs.
Front face of the ZN calorimeter
Each set of ZDCs is positioned on a movable platform controlled remotely from the ALICE DCS. The ZN and ZP calorimeters can be moved independently in the vertical direction in order to follow the beam crossing angle. The movable support structures are lowered during injection to protect the detectors from possible beam losses and when the ZDCs are not included in data acquisition in order to minimize the absorbed dose.
Click here to read the Technical Design Report for the ZDC.
Measurement of the centrality in Pb-Pb collisions
In Pb-Pb collisions the ZDCs measure the energy carried away by the non-interacting nucleons (spectators), that is the measurable quantity most directly related with the centrality of the collision. However, in the colliders the monotonic correlation between impact parameter and ZDC response is partially destroyed.
In fact, unlike the fixed targer experiments, in peripheral collisions many nucleons remain bound in large nuclear fragments, that continue to travel in the beam pipe and are not detected by the ZDCs since their magnetic rigidity is close to that of the beam. The information provided by other ALICE detectors, for example the ZEM, is used to remove the ambiguity between central and peripheral collisions. The figure below shows the ZDC energy as a function of the ZEM amplitude for 4 centrality bins selected by V0 amplitude.
Measurement of luminosity in Pb-Pb collisions
The measurement is performed through the detection of neutron emission in electromagnetic dissociation of 208Pb nuclei. The figure shows the ZNA energy spectrum: the dashed lines represent the single fits of the different peaks (1n, 2n,...), while the continuous line is the sum of all the contributions. The measurement established experimentally the EMD cross section scale for the first time at LHC energy.
Rejection of background events in Pb-Pb collisions
The ZDC timing information, i.e. ZN timing sum (event time) vs. difference (event Zvertex) is used in the offline event selection for the discrimination of beam-beam collisions from background events produced outside of the interaction region.