ALICE experiment

ALICE experiment. The ALICE (A Large Ion Collider Experiment) is a dedicated heavy-ion physics detector at the Large Hadron Collider (LHC) at CERN. Its primary mission is to study the properties of the quark–gluon plasma (QGP), a state of matter believed to have existed microseconds after the Big Bang. By colliding lead ions at ultra-relativistic energies, ALICE investigates the fundamental theory of strong interaction described by quantum chromodynamics (QCD) under extreme conditions of temperature and density.

Overview

ALICE is one of the four major experiments at the Large Hadron Collider, alongside ATLAS, CMS, and LHCb. It is situated at Point 2 of the LHC ring, in a cavern previously used by the L3 experiment on the Large Electron–Positron Collider. The collaboration involves over 1,900 scientists and engineers from more than 40 countries and 174 institutes worldwide. The detector was designed and constructed over a decade, recording its first lead-ion collisions in 2010 following the successful startup of the Large Hadron Collider in 2009. Its unique design prioritizes the ability to track and identify thousands of particles produced in each central collision of heavy nuclei, such as lead.

Physics goals

The central physics goal of ALICE is the characterization of the quark–gluon plasma, a deconfined state where quarks and gluons are free over nuclear volume. This involves measuring key observables like the jet quenching phenomenon, where high-energy partons lose energy as they traverse the hot, dense medium, providing direct evidence of the QGP's opacity. The experiment also studies the collective flow of particles, described by hydrodynamics, to determine the plasma's viscosity and other transport properties. Furthermore, ALICE investigates the production of heavy quarkonia, such as the J/ψ meson and ϒ meson, whose suppression is a signature of deconfinement, and measures the production of light nuclei and hypernuclei to understand the hadronization process. Research extends to precision studies of proton-proton and proton-lead collisions as essential reference data and to explore cold nuclear matter effects.

Detector design

The ALICE detector is a complex, multi-purpose spectrometer approximately 16 meters wide, 16 meters high, and 26 meters long, with a total weight of around 10,000 tonnes. Its central barrel, inside a large solenoid magnet providing a 0.5 T field, includes the high-resolution Inner Tracking System (ITS) made of silicon pixel, drift, and strip detectors for vertexing and tracking. The Time Projection Chamber (TPC) is the main tracking device, providing particle identification via specific energy loss. Additional particle identification is achieved by the Time-Of-Flight (TOF) detector, the High-Momentum Particle Identification Detector (HMPID), and transition radiation detectors. Forward systems include the Muon spectrometer for studying quarkonia via dimuon decays, the Photon Spectrometer (PHOS), and the Electromagnetic Calorimeter (EMCal) for measuring photons and jets. The Zero Degree Calorimeter (ZDC), located 115 meters from the interaction point, measures spectator nucleons to determine collision centrality.

Major results and discoveries

ALICE has produced definitive evidence for the formation of a hot, dense, and strongly interacting quark–gluon plasma in lead-lead collisions at the Large Hadron Collider. Key discoveries include the observation of strong elliptic flow of particles, indicating the QGP behaves as a nearly perfect fluid with very low shear viscosity. The experiment has provided precise measurements of jet quenching through the imbalance of back-to-back jets and the modification of jet structure. ALICE demonstrated the sequential suppression of quarkonia states, with the ϒ meson showing less suppression than the J/ψ meson, consistent with QGP screening models. It has also observed the enhancement of strange hadrons and the production of light (anti-)nuclei and (anti-)hypernuclei, confirming coalescence as a formation mechanism. In smaller systems, ALICE found flow-like correlations in high-multiplicity proton-proton and proton-lead collisions, challenging previous understandings of the QGP's minimum size.

Collaboration and operation

The ALICE Collaboration is an international team managed under the framework of CERN, with spokespersons including Paolo Giubellino and Luciano Musa during its construction and initial phases. Data taking occurs during dedicated heavy-ion runs of the Large Hadron Collider, which alternate with longer periods of proton-proton collisions for the other experiments. Following the first long shutdown, the detector underwent a major upgrade during the second long shutdown to handle increased interaction rates for Run 3 and the High-Luminosity LHC. This upgrade included a new Inner Tracking System, faster readout for the Time Projection Chamber, and new forward detectors. The collaboration publishes its results in journals like Physical Review Letters and maintains extensive open data policies. Its operational lifetime is planned to extend through the 2030s, coinciding with the final runs of the Large Hadron Collider.

Category:Particle physics experiments Category:CERN experiments Category:Large Hadron Collider