More details on the ALICE V0 detector

The V0 detector is a small angle detector consisting of two arrays of 32 scintillating counters, called V0A and V0C, which are installed on either side of the ALICE interaction point of the two LHC beams. This detector has several functions. It provides Minimum Bias triggers and Centrality Triggers for the central barrel detectors in pp and A–A collisions. It measures the charge of the particles and the time of their arrival in each of the 64 channels. The V0A is located 330 cm away from the vertex on the side opposite to the muon spectrometer. The V0C is fixed at the front face of the hadronic absorber, 90 cm from the vertex. They cover the pseudo-rapidity ranges 2.8<η<5.1 (V0A) and −3.7<η<−1.7 (V0C) for collision vertex at the central position. They are segmented into counters distributed in four rings.

In practice and during normal operation, both arrays are required (AND mode) to provide triggers, namely: Minimum Bias trigger (MB), Multiplicity Trigger (MT), semi-Central Trigger (CT1) and Central Trigger (CT2). Since interactions of beam particles with the residual gas of the vacuum chamber generate tracks through the ALICE sub-detectors, the V0 detector is able to provide Minimum Bias p-Gas triggers (PG).

In pp collisions, the efficiency of V0 for detecting at least one charged particle in both sides is about 84% when backgrounds from the hardware environment are introduced. Finally, the V0 detector participates in the measurement of luminosity with a precision as good as 10%.

Extensive tests were performed to choose the best design for the individual counters and the easiest integration of the disks in the system. The material consists in BC404 scintillating material (2.5 and 2.0 cm in thickness for V0A and V0C, respectively) with 1 mm in diameter BCF9929A Wave-Length Shifting (WLS) fibres. The V0A array is constructed with the fibres spaced by 1 cm and embedded in the two transverse faces of the segments following the “megatile” technique. There are 32 individual counters arranged in 4 rings and 8 sectors of 45°. The V0C array has the fibres grouped by layers of 9 units and glued along the two radial edges of the segments. There are 48 individual counters of this type distributed following two inner rings of 8 counters and two outer rings of 16 counters. The latter are connected 2×2 to make a single detection element.

Photomultipliers (PMT) R5946 (hybrid assemblies H6153-70) from Hamamatsu collect the light. They are fixed on the V0A disk holder in groups of 4 units and connected directly to the WLS fibres. They are installed on the absorber in groups of 8 units for the V0C and connected to counters through Mitsubishi optical fibres 3.22 m long.

The signal provided by each PMT is sent to an electronics circuit, which delivers two signals. The first one is sent to a threshold discriminator for the generation of the V0 triggers. It is amplified by a factor of about 10 and clamped at the level of 300 mV. The second one, not amplified, is used for the measurement of the charge given by the counter. The two treatments are carried out far from PMTs, namely 25 m away.

The Front End Electronics provides signals for triggering the ALICE experiment at the level L0 and digitizes the physical signals delivered by the individual scintillating counters. The system generates four trigger types and several sets of information listed hereafter. 

Click here to read the Technical Design Report for the ALICE Forward Detectors. 

Measurement of centrality and reaction plane in Pb-Pb collisions

The V0 detector is the default collision centrality estimator in ALICE. The centrality is obtained by summing up the energy deposited in the two disks of VZERO. This observable scales directly with the number of primary particles generated in the collision and therefore to the centrality. The figure shows such a distribution measured in Pb-Pb collisions at 2.76 TeV, adjusted to a Glauber fit (red line).

Rejection of background induced by beam-gas collisions

As illustrated in the figure beam-beam and beam-gas events can be discriminated on timing criteria, using the average time response correlation of both hodoscopes. Time resolution is 450 ps for V0-A and 350 ps for V0-C.

Measurement of the charged particle density in Pb-Pb collisions

The V0 detector is also used to measure the charged particle pseudorapidity density distribution in Pb-Pb collisions at 2.76 TeV in a special analysis technique based on collisions from LHC satellite bunches.

This technique both allows a reduction of the effect of secondary particles and an extension of the accessible pseudorapidity range. Results are shows in the figure and found to be in good agreement with the ones published by the ATLAS and CMS experiments.

Measurement of the luminosity

The V0 detector is used as a reference in Van Der Meer scans which give the size and shape of collinding beams (figure showing a scan achieved in pp collisions at 7 TeV) and therefore the luminosiry delivered to the experiment.

 

The countries that have participated in the construction and/or in the maintenance and operation of the V0 are France and Mexico:

- Dipartimento di Fisica dell'Universita and Sezione INFN, Cagliari, Italy
- Sezione INFN, Cagliari, Italy
- Dipartimento di Fisica dell'Universita and Sezione INFN, Turin, Italy
- Sezione INFN, Torino, Italy
- Dipartimento di Scienze e Innovazione Tecnologica dell'Universita del  Piemonte Orientale and Gruppo Collegato INFN, Alessandria, Italy