Unveiling the Quark-Gluon Plasma: A New Frontier in Light-Ion Collisions at the LHC
The Large Hadron Collider (LHC) has embarked on a groundbreaking journey into the realm of light-ion collisions, marking a significant milestone in our understanding of the early universe. In a recent study, LHC physicists delved into the behavior of quark-gluon plasma, a state of matter that existed in the moments following the Big Bang, by colliding oxygen and neon nuclei. This exploration opens up new avenues for understanding the fundamental nature of this plasma and its interactions with smaller nuclear systems.
The Quark-Gluon Plasma and its Behavior
In the initial microseconds after the Big Bang, extreme temperatures prevented quarks and gluons from forming stable hadrons, resulting in a deconfined quark-gluon plasma that filled the universe. Heavy-ion collisions between gold or lead nuclei have long been used to study this plasma, but light-ion collisions have been less explored. The LHC's recent experiments focused on collisions between oxygen and neon nuclei, as well as oxygen nuclei and protons, offering a unique opportunity to investigate the plasma's behavior in smaller systems.
Anthony Timmins from the University of Houston highlights the significance of these studies, stating that early analyses have characterized the geometry of oxygen and neon nuclei, including the predicted prolate 'bowling pin' shape of neon. More importantly, these findings suggest a connection between light-ion collisions and the onset of the quark-gluon plasma.
The behavior of the quark-gluon plasma is crucial for understanding heavy-ion collisions. When lead nuclei collide at the LHC, they create a tiny, extremely hot fireball where quarks and gluons interact rapidly, reaching local thermal equilibrium within a fraction of a second. Measurements indicate that the plasma flows with remarkably low viscosity, close to the quantum limit, allowing momentum to move swiftly across the system. However, the question arises: do the same rules apply to smaller nuclear systems in light-ion collisions?
Hydrodynamics and the Quark-Gluon Plasma
Timmins explains that hydrodynamics, the study of fluid behavior under pressure gradients and viscous stresses, is essential for modeling heavy-ion collisions. For hydrodynamics to work, along with the appropriate quark-gluon plasma equation of state, a separation of scales is required between the mean free path of quarks and gluons, pressure gradients, and the overall system size. In smaller systems, these scales start to overlap, making it challenging to apply hydrodynamic models accurately.
Oxygen and neon nuclei are expected to operate near this threshold, close to the limits of plasma formation. The ALICE, ATLAS, and CMS collaborations utilized Fourier analysis to study the transverse distribution of emitted particles, searching for collective, fluid-like behavior. Their findings support the emergence of a collective flow driven by the initial collision geometry, with signs of energetic-probe suppression in oxygen-oxygen collisions, a unique feature not observed in proton-proton collisions.
Initial Results and Future Prospects
The CMS collaboration compared particle yields in light-ion collisions to a proton-proton reference, observing a maximum suppression of 0.69 ± 0.04 at a transverse momentum of about 6 GeV, which is statistically significant. While milder than in lead-lead and xenon-xenon collisions, this data points to genuine medium-induced suppression in the smallest ion-ion system studied. ATLAS reported the first dijet transverse-momentum imbalance in a light-ion system, indicating path-length-dependent energy-loss effects.
ALICE, ATLAS, and CMS also observed a hierarchy of flow coefficients in light-ion collisions, with elliptic, triangular, and quadrangular flows decreasing as their Fourier index increases, aligning with hydrodynamic expectations. Neutral pion yields exhibited suppression at large momenta, similar to charged hadron yields.
The LHCb collaboration's previous fixed-target study measured the elliptic and triangular components of flow in lead-neon and lead-argon collisions, confirming the distinctive shape of the neon nucleus. Proton-oxygen collisions were also investigated, allowing the exploration of nuclear parton distribution functions at very small values of Bjorken-x, which is crucial for modeling ultra-high-energy cosmic ray interactions with atmospheric oxygen.
Despite the tight schedule, the LHC campaign exceeded luminosity targets for proton-oxygen, oxygen-oxygen, and neon-neon collisions due to high accelerator availability and injector intensity. Timmins concludes that these early studies demonstrate the persistence of collective flow and parton-energy-loss-like suppression in smaller systems, offering new insights into nuclear geometry and promising forward-physics studies. The next step is to determine the nuclear parton distribution function of oxygen, which will be vital for understanding hadron-suppression patterns and advancing our knowledge of proton-oxygen and ultra-peripheral collisions.
