Pileup mitigation at the LHC: A theorist's view

To maximise the potential for new measurements and discoveries at CERN's Large Hadron Collider (LHC), the machine delivers as high as possible collision rates. As a direct consequence, multiple proton–proton collisions occur whenever two bunches of protons cross. Interesting high-energy (hard) collisions are therefore contaminated by several soft, zero-bias, ones. This effect, known as pileup, pollutes the final state of the collision. It complicates the reconstruction of the objects in this final state, resulting in increased experimental measurement uncertainties. To reduce these uncertainties, and thus improve the quality and precision of LHC measurements, techniques are devised to correct for the effects of pileup. This document provides a theoretical review of the main methods employed during Run I and II of the LHC to mitigate pileup effects. I will start with anin-depth presentation of the area–median used for the vast majority of applications, including several refinements of the original idea, their practical (numerical) implementation and an assessment of their efficiency and robustness. I will then focus on several theoretical calculations that can provide both quantitative and qualitative information on the area–median approach. In the specific case of boosted jets, a field that has seen a wide interest recently, a set of methods, known as grooming techniques has also been used. I will describe these techniques, address their performance and briefly show that they are amenable to a theoretical, analytic, understanding. The last part of this review will focus on ideas oriented towards future pileup mitigation techniques. This includes a couple of new methods that have recently been proposed as well as a large series of alternative ideas. The latter are yet unpublished and have not received the same amount of investigation than the former but they have the potential to bring new developments and further improvement over existing techniques in a future where pileup mitigation will be crucial.