Vertex and Trigger Detectors

Three types of detectors placed close to the beam axis are used for global event characterization.

The Beam-Beam Counters (BBC) consist of 64 Cherenkov detectors, arranged radially around the beam at 1.44 m from the center of the interaction region, with a pseudorapidity coverage of $3.0 < \vert\eta\vert < 3.9$. They provide the primary interaction trigger and measure the start time used for particle identification through time-of-flight measurements. Using the left-right time difference, they also provide a measure of the collision vertex position along the beam axis.

The Zero-Degree Calorimeters (ZDC) are situated 18 m upstream and downstream from the center of the interaction region. They measure the energy deposited by spectator neutrons from the collisions. Besides providing a trigger, they are used in combination with the BBC's for offline centrality selection.

The Multiplicity Vertex Detector (MVD) is composed of concentric barrels of silicon strip detectors and end-caps made of silicon pad detectors. It provides a more precise determination of collision vertex position and the $dN/d\eta$ distribution.