Dissertation: Probing the strong interaction in particle collisions with jets (Huhta)
At the Large Hadron Collider of the European Organization for Nuclear Research (CERN), the ALICE experiment provides a unique window into the fundamental building blocks of matter. A doctoral study from the University of Jyväskylä studies collimated particle showers known as jets using the ALICE detector. The research evaluates the performance of the upcoming FoCal detector and shows that it can measure jets in a region previously inaccessible to ALICE.
The strong interaction binds partons, quarks and gluons, into color-neutral particles such as nucleons, i.e. protons and neutrons. However, the internal structure of nucleons is far more complex than the three-quark picture, and the observed structure depends on the resolution of the measurement.
- In particle physics, our “microscope” is a particle accelerator, where higher collision energies provide a finer resolution. Along the Large Hadron Collider (LHC) at CERN lies the ALICE detector, where my dissertation research is carried out, describes Doctoral researcher Laura Huhta from the University of Jyväskylä.
The structure of nucleons is described using parton distribution functions (PDFs), which give the probability of finding a parton carrying a momentum fraction x. At very small values of x, the number of partons inside the nucleon grows rapidly. Theory predicts that this growth will eventually slow down.
- This phenomenon, called gluon saturation, was predicted decades ago, but conclusive experimental evidence is still lacking, says Huhta.
Probing saturation with the FoCal detector
In 2030, ALICE will be upgraded with a new forward detector, FoCal, enabling measurements at very low values of x.
- In my dissertation, I use simulations to study FoCal’s performance in measuring jets. I show that even jets with a low transverse momentum can be reconstructed with sufficient precision, enabling the study of gluon saturation using forward jets and their correlations, describes Huhta.
Dijets in different collision systems
The LHC collides both protons and heavy ions. To interpret measurements from lead-lead collisions, it is essential to first perform corresponding measurements in proton-proton and proton-lead collisions. These serve as baselines and help identify possible effects from cold nuclear matter.
- In my dissertation, I updated measurements of the invariant mass of charged dijets, a system of two jets, in both measurement systems using ALICE data, tells Huhta.
The results are compared with simulations, which suggest sensitivity to the anti-shadowing region, where the nuclear PDF is enhanced compared to that of a single proton.
- While the data are consistent with this interpretation, the current uncertainties are too large for a definite conclusion. Future ALICE measurements will improve the sensitivity, and this work provides an important baseline for upcoming analyses in lead-lead collisions, specifies Huhta.
The dissertation work is part of the Centre of Excellence in Quark Matter. The theory groups of the CoE have a long tradition of determining PDFs. Research with the FoCal detector strengthens the experimental study of PDFs in the group.
M.Sc. Laura Huhta defends her doctoral dissertation "Probing QCD via dijet invariant mass in pp and p–Pb collisions and performance of forward jets with ALICE FoCal” on 5 December 2025 at 12:00 in the Department of Physics in lecture hall FYS1. The opponent is Associate Professor Martin Spousta (Charles University) and the custos is Associate Professor Sami Räsänen (University of Jyväskylä). The defense will be held in English.
The event will be streamed live online:
Follow the event via live stream on Moniviestin
The dissertation "Probing QCD via dijet invariant mass in pp and p–Pb collisions and performance of forward jets with ALICE FoCal” is available in the JYX publication archive: https://urn.fi/URN:ISBN:978-952-86-1117-2
