WA98 (experiment)

WA98 (experiment)
Name	WA98
Location	CERN
Facility	Super Proton Synchrotron
Collaboration	WA98 Collaboration
Period	1994–2000
Type	Heavy-ion physics

WA98 (experiment) was a fixed-target heavy-ion experiment at the Super Proton Synchrotron at CERN designed to study ultrarelativistic collisions of lead ions to investigate properties of hot, dense matter and search for signatures of the quark–gluon plasma. The collaboration combined wide-acceptance calorimetry, charged-particle tracking, and photon detection to measure global observables, electromagnetic probes, and rare signals in lead–lead and proton–nucleus interactions. WA98 operated within the context of contemporaneous programs at the Relativistic Heavy Ion Collider, Large Hadron Collider, and earlier fixed-target experiments to constrain theoretical descriptions of deconfinement and chiral symmetry restoration.

Overview and Objectives

WA98's primary objectives included measurement of direct photons, neutral mesons, transverse energy, and multiplicity distributions to probe the space–time evolution of the collision fireball and possible phase transitions predicted by Quantum Chromodynamics models. The experiment aimed to detect electromagnetic radiation as a penetrating probe complementary to hadronic observables studied by experiments such as NA49 (experiment), NA50 (experiment), and CERES. WA98 also pursued studies of Bose–Einstein correlations, anisotropic flow, and strangeness production to test predictions from lattice Quantum Chromodynamics, hydrodynamic simulations by groups influenced by Landau hydrodynamics, and transport calculations from models like URQMD and RQMD.

Experimental Setup and Detectors

WA98 employed a modular apparatus mounted on the WANF beam line of the Super Proton Synchrotron. Key detector subsystems included a large lead–glass photon spectrometer, a multiwire proportional chamber array, a silicon pad multiplicity detector, and a zero-degree calorimeter. The lead–glass calorimeter provided electromagnetic shower measurements critical for identifying direct photons and reconstructing neutral mesons such as the pi meson and eta meson. Charged-particle tracking used components similar in concept to detectors at ISOLDE and designs influenced by tracking at ISR experiments. The trigger and centrality determination used transverse energy measurements from a hadronic calorimeter and a neutron calorimeter located near zero degrees, building on techniques from UA1 (experiment) and UA5 (experiment). The experiment's data acquisition and online systems integrated technologies from collaborations that included institutions like CERN, Brookhaven National Laboratory, GSI Helmholtz Centre for Heavy Ion Research, and multiple European universities.

Data Collection and Analysis Methods

WA98 collected data from lead–lead runs and control proton–nucleus runs, employing event selection criteria to isolate central and peripheral collisions. Photon identification used shower-shape analysis in the lead–glass calorimeter, with neutral-meson reconstruction achieved via invariant-mass techniques that paralleled methods used at SLAC and DESY. Background subtraction for direct-photon extraction relied on measured decay photon yields from reconstructed pi0 and eta mesons and statistical techniques applied in analyses by collaborations such as PHENIX (experiment). Multiplicity and transverse energy distributions were corrected for acceptance and efficiency using Monte Carlo simulations interfaced with event generators like HIJING and detector simulation frameworks inspired by GEANT. Correlation studies used Bose–Einstein interferometry analysis methods developed in the context of experiments at LEP and RHIC. Systematic uncertainties were constrained through cross checks with control triggers, beam diagnostic measurements, and comparisons to results from NA35 (experiment) and NA44 (experiment).

Key Results and Findings

WA98 reported several significant observations: a measurable excess of direct photons above decay backgrounds in central lead–lead collisions consistent with thermal and prompt production mechanisms; precise spectra for neutral mesons including the pi0 and eta meson; centrality-dependent transverse energy and multiplicity scaling; and insights into source sizes from two-particle correlation measurements. The direct-photon excess provided evidence complementary to dilepton measurements from CERES (experiment), while transverse-momentum distributions informed discussions led by groups working on hydrodynamic descriptions and perturbative Quantum Chromodynamics calculations. WA98's neutral-meson spectra and photon yields contributed to constraints on initial temperature estimates and helped frame comparisons with later measurements at PHENIX and ALICE (experiment).

Interpretation and Theoretical Impact

WA98 results influenced interpretations of electromagnetic probes as signatures of deconfined matter, informing theoretical work on thermal radiation, prompt hard scattering, and jet–medium interactions studied in frameworks like perturbative Quantum Chromodynamics and finite-temperature lattice QCD. The measured photon spectra and meson suppression patterns were incorporated into model comparisons involving parton energy loss formalisms developed by researchers associated with Gyulassy and Wang, as well as hydrodynamic parameter extractions practiced by groups following Kolb and Heinz. WA98's data also affected theoretical treatments of chiral symmetry restoration and in-medium modifications of hadrons explored by theorists connected to Rho and Brown.

Collaboration and Timeline

WA98 was a multinational collaboration comprising institutes from Europe, Asia, and the Americas, including personnel affiliated with universities and laboratories such as CERN, Brookhaven National Laboratory, GSI, Institut de Physique Nucleaire (Orsay), University of Santiago de Compostela, and others. The experiment took data in the mid-to-late 1990s with major data-taking campaigns around 1994–2000 and produced a series of publications and conference presentations at venues like the Quark Matter conference and meetings organized by IHEP and JINR. WA98's legacy informed design choices and physics goals for subsequent heavy-ion experiments at the Relativistic Heavy Ion Collider and the Large Hadron Collider.

Category:Experiments at CERN