CERN Accelerating science

The strong nuclear force that is responsible for binding together protons and neutrons within atomic nuclei - and responsible for forming the nucleons themselves - can be explained by a quantum field theory known as Quantum Chromodynamics (QCD). At extreme temperatures and pressures, QCD predicts the formation of a new state of nuclear matter called the Quark Gluon Plasma (QGP). The behavior of QCD interactions under these extreme conditions can be studied in the laboratory by colliding together ultra-relativistic heavy-ions, where the energy density is expected to reach well above the critical transition temperature for QGP formation. Heavy-ion collisions have been studied for decades at multiple different particle accelerator facilities, including the Large Hadron Collider (LHC). At the LHC, the primary experiment focused on studying these heavy-ion collisions is known as A Large Ion Collider Experiment (ALICE). ALICE is designed to study high multiplicity heavy-ion collisions, as well as proton-proton and proton-ion collisions that can be used as baseline measurements. Among the expected signatures of QGP formation is an increase in the production of strange quarks within the medium, a phenomenon known as strangeness enhancement. Strangeness enhancement can be studied by measuring the production ratio of strange particles to non-strange particles. While first predicted, and later observed, to be present in heavy-ion collisions, more recent measurements have observed an onset of strangeness enhancement occurring in smaller systems, such as p-Pb collisions. The enhancement seen in heavy-ion collisions can be fairly well described using a statistical hadron production model, where the temperature is high enough that the strange quark is in full equilibrium with the two other light flavor quarks. However, the microscopic behavior underlying this enhancement in smaller systems is still not completely understood. Further differentiation of strange particle production is needed to pinpoint the partonic interactions, particularly in the soft-scattering sector, responsible for strangeness enhancement. In this analysis, angular correlations between a high momentum hadron and a $\phi$(1020) meson (comprised of a strange quark-antiquark pair) are measured within p-Pb collisions with ALICE at the LHC. With the high momentum trigger hadron acting as a proxy for a high energy jet of particles, and the $\phi$(1020) meson acting as a strangeness probe, these correlations can be used to separate out the production of $\phi$(1020) mesons within jets (closely aligned to the direction of the trigger) from production in the non-jet underlying event (uncorrelated with the direction of the trigger). These differential yield measurements are a way to separate out hard-scattering production (more jet-like) from soft-scattering production (more medium-like). The $\phi$(1020) correlations can be directly compared with inclusive dihadron correlations to calculate the strange to non-strange production ratio (i.e $\phi/h$). With this technique, strangeness production within jets and the underlying events are measured as a function of multiplicity and are compared between a lower and higher momentum region. These different production regimes show a clear ordering, with $\phi/h$ production in the underlying event significantly higher than in jets for all multiplicities. The evolution of the per-trigger yields within the near-side (jet) and away-side (modified jet) as a function of multiplicity are studied separately, and differences in behavior suggest a change to the away-side production at lower momentum. This technique for measuring yield ratios is then discussed as a way to further constrain the origin of strangeness enhancement in small systems.