Dynamics of the interaction of dust particles with the LHC beam

Micrometer-sized dust particles present in the Large Hadron Collider (LHC) beam pipe are believed to have caused many thousands of sporadic beam loss events around the LHC. These so-called unidentified falling objects (UFOs) have been under continuous study since the start of high intensity beam operation in the LHC due to their impact on the LHC availability: 139 beam dumps and 12 magnet quenches during Run II (2015–2018) alone. To mitigate the impact of UFOs on future accelerators such as the High Luminosity LHC and the Future Circular Collider, it is fundamental to foster a better understanding of these beam loss events. In this paper, key observations made since the start of LHC operation are summarized and the prevailing UFO hypothesis is confronted by a compilation of observations acquired during Run II. In particular, it is shown that UFOs must carry an initial negative charge before entering the proton beam, or that they are by some other means accelerated toward the beam not only by gravity. A simulation model for the dynamics of the dust particles and their interaction with the beam was developed over the years. This model is improved and validated by measurements. It is however also shown that a subset of observed beam losses, which contain a positive time profile skewness, cannot be explained by it.