Performance of the ALICE VZERO system

TL;DR

The paper provides a comprehensive evaluation of the ALICE VZERO system, detailing its design, calibration, and integration into ALICE's trigger and analysis framework. It demonstrates that VZERO achieves excellent timing (~1 ns) and charge resolution, supports online and offline background rejection, and enables precise luminosity, multiplicity, centrality, and event-plane measurements. By reporting van der Meer scan-based luminosity calibrations and centrality resolutions, it establishes VZERO as a robust, multi-purpose forward detector essential for ALICE physics runs. The study also documents radiation-induced aging and its mitigation via targetted HV adjustments, confirming sustained performance over four years of operation. Overall, VZERO is shown to be critical for triggering and global event characterization in ALICE.

Abstract

ALICE is an LHC experiment devoted to the study of strongly interacting matter in proton-proton, proton--nucleus and nucleus-nucleus collisions at ultra-relativistic energies. The ALICE VZERO system, made of two scintillator arrays at asymmetric positions, one on each side of the interaction point, plays a central role in ALICE. In addition to its core function as a trigger, the VZERO system is used to monitor LHC beam conditions, to reject beam-induced backgrounds and to measure basic physics quantities such as luminosity, particle multiplicity, centrality and event plane direction in nucleus-nucleus collisions. After describing the VZERO system, this publication presents its performance over more than four years of operation at the LHC.

Figures (10)

• Figure 1: Sketches of VZERO-A and VZERO-C arrays showing their segmentation. Scintillator thicknesses are 2.5 and 2 cm respectively. Radii of rings are given in Tab. \ref{['tab:V0coverage']}. The scintillator segments on both sides of the dashed lines are connected to the same PMT (see Section \ref{['sec:calibration']}).
• Figure 2: Schematic drawings of elementary cell designs for VZERO-A (top) and VZERO-C rings 0-1 (bottom). For VZERO-C rings 2-3, two scintillating sets (scintillator and WLS fibers) are connected to a single PMT through four clear fiber beams (see Fig. \ref{['fig:V0arrays']}).
• Figure 3: Position of the two VZERO arrays, and of the few detectors quoted in the text, within the general layout of the ALICE experiment.
• Figure 4: Charge of the pulse in ADC counts (top) and leading time of the pulse in nanoseconds (bottom) versus channel number for pp collisions at $\sqrt{s}=$ 7 TeV. Channels numbered from 0 to 31 correspond to VZERO-C, channels numbered from 32 to 63 correspond to VZERO-A.
• Figure 5: Correlation between leading time in nanoseconds and charge of the PMT signals in ADC channel counts for Pb--Pb collisions, before any correction (left) and after slewing correction (right).