CERN-THESIS-2012-153 26/07/2012 - CERN Document Server

3.3.5 Luminosity Detectors
A set of smaller detectors are used to provide good coverage in the very forward region. In order of decrea-
sing distance from the interaction point, the first system is a Cerenkov detector named LUCID (LUminosity
measurement using Cerenkov Integrating Detector). Located at ±17 m from the interaction point, it is spe-
cifically designed to measure the luminosity in ATLAS. It consists of sixteen optically reflecting aluminum
tubes filled with C4F10 gas surrounding the beampipe on each side of the interaction point. It detects
inelastic pp scattering in the forward direction, in order to measure the integrated luminosity and to provide
online monitoring of the instantaneous luminosity and beam conditions. The second system is the Zero-
Degree Calorimeter (ZDC), located at a distance of ±140 m from the interaction point. Its primary purpose
is to detect forward neutrons and photons with |η| > 8.3 in both pp and heavy-ions collisions. It consists
of two arms, each one with one EM and three hadronic calorimeter modules. A third detector, the Beam
Conditions Monitor (BCM), consists of two arms of diamond sensors located at ±184 cm and r = 5.5 cm
to record the single-sided and coincidence rates as a function of Bunch Crossing Identifier (BCID). The
detector’s value as a luminosity monitors is further enhanced by its excellent timing (0.7 ns) which allows
rejection of backgrounds from beam-halo, and also issues a beam-abort request in case beam losses start to
risk damage to the ATLAS detectors. The ALFA (Absolute Luminosity For ATLAS) detector, consisting of
scintillating-fiber trackers located inside Roman pots at approximately ±240 m from the interaction point,
can provide an absolute luminosity calibration at ATLAS through the measurement of elastic pp scattering
at small angles. Lastly, the Minimum Bias Trigger Scintillators (MBTS), located at ±365 cm from the
collision point, are segmented scintillator counters, providing a trigger on minimum collision activity during
a pp BC [64,65,70].
The main function of the LUCID and ALFA systems is to determine the luminosity delivered to ATLAS,
whereas the ZDC plays a key role in determining the centrality of heavy-ion collisions. The ALFA detector
uses the optical theorem, which connects the elastic scattering amplitude in the forward direction to the
total cross-section, and can thus be used to extract luminosity. On the other hand, LUCID is based on the
principle that the number of particles detected is proportional to the number of interactions in a BC. The
number of interactions per beam-crossing is needed to determine the luminosity.
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