Scientists clash heavy ions—atomic nuclei completely devoid of their surrounding electrons—at the Large Hadron Collider (LHC) in order to examine atomic nuclei and subatomic particles. Protons and neutrons, which ordinarily consist of quarks and gluons, melt into a new state of matter known as a quark-gluon plasma (QGP) in these collisions.
Theorists evaluate a sophisticated model to a vast quantity of experimental data to determine the characteristics of this QGP. The size of the nucleons inside the two colliding lead nuclei is one of the model’s parameters. Nucleons must, however, be smaller than scientists had anticipated based on prior analyses in order to be in agreement with the reaction rate.
The nucleon size discovered by low-energy tests is around 0.5 femtometers (fm), or 5×10-16 metres. Compared to these low-energy investigations, heavy ion collisions offer a radically different perspective on the nucleon size. The nucleon size of heavy ions has previously been reported to be substantially greater, at roughly 1 fm.
The newly discovered analysis for the first time takes into account the reaction rate of lead-lead collisions as determined by experiment. The density of atomic nuclei in the beam and the size of the nuclei both affect the reaction rate, which is the frequency with which atomic nuclei react with one another. The preferred size is around 0.6 fm after accounting for this reaction rate, clearing up the conflict.
Scientists can determine the collision rate for a given beam density by looking at the overall hadronic cross section of lead-lead collisions. The cross section is a suitable option for comparison to experiments in a global analysis because it is extremely simple to calculate in a theoretical model. The experimental measurement is more challenging to carry out, though. Therefore, there were significant scientific doubts over this figure up until the year 2021.
These uncertainties were significantly decreased in 2022 by a new measurement from the Large Hadron Collider’s ALICE experiment, enabling its inclusion in a comprehensive analysis. Researchers from the Massachusetts Institute of Technology and CERN conducted this investigation, which contrasts a theoretical estimate with more than 600 distinct experimental data points.
The research revealed that nucleon sizes of about 1 fm are favoured when the cross-section measurement is excluded. However, this preferred value falls to about 0.6 fm when the cross section is taken into account.
