Speaker
Description
PHOS is a highly granulated precision spectrometer, one of the two
electromagnetic calorimeters of ALICE (A Large Ion Collider Experiment)
at the LHC. It is based on scintillating PbWO4 crystals and is
dedicated to the precise measurements of spectra, collective flow and
correlations of thermal and prompt direct photons, and of neutral mesons
in ultra-relativistic nuclear collisions at LHC energies. PHOS
participated in LHC Run 1 (2009-2013) and Run 2 (2015-2018), during which a large
amount of physical data were collected in pp, p-Pb and Pb-Pb collisions.
The choice of active material with small Molière radius allows PHOS to
operate in a high-multiplicity environment and to reconstruct neutral pions
by two-photon decays up to very high transverse momenta ~60 GeV/c. In
order to increase the light yield of the crystals and reduce electronic noise,
PHOS is cooled down and kept at a constant temperature of -25^{\circ} C. This resulted in
excellent energy and position resolutions. Dedicated L0 and L1
triggers were used to increase collected integrated luminosity during
data taking.
We will present an overview of the PHOS performance during Runs 1 and 2 and plans for
an upgrade for LHC Run 4 (currently expected for 2025-2027) and beyond. PHOS
upgrade is ongoing and covers the following three aspects: front-end cards (FEC)
using modern components to improve timing resolution and operation
reliability; photodetectors to increase sensitivity to
low-energy photons and to improve timing resolution; mechanical structure of
modules to ease access to FEC and improve
operation reliability. The physics program of the upgraded PHOS will also be
discussed.
