  The high instantaneous luminosity of the CERN Large Hadron Collider leads to
multiple proton-proton interactions in the same or nearby bunch crossings
(pileup). Advanced pileup mitigation algorithms are designed to remove this
noise from pileup particles and improve the performance of crucial physics
observables. This study implements a semi-supervised graph neural network for
particle-level pileup noise removal, by identifying individual particles
produced from pileup. The graph neural network is firstly trained on charged
particles with known labels, which can be obtained from detector measurements
on data or simulation, and then inferred on neutral particles for which such
labels are missing. This semi-supervised approach does not depend on the ground
truth information from simulation and thus allows us to perform training
directly on experimental data. The performance of this approach is found to be
consistently better than widely-used domain algorithms and comparable to the
fully-supervised training using simulation truth information. The study serves
as the first attempt at applying semi-supervised learning techniques to pileup
mitigation, and opens up a new direction of fully data-driven machine learning
pileup mitigation studies.
