Publication : t18/017

Abstract:
Given the high luminosities at which CERN's Large Hadron Collider (LHC) operates, multiple proton-proton collisions occur during the same bunch crossing. Hard collisions are therefore contaminated by several soft, zero-bias, ones. This effect, known as pileup pollutes the reconstruction of the final state of the collision. This document is meant to provide a theoretical review of the methods employed during Run I of the LHC and at the beginning of Run II to mitigate pileup effects. I will start with a in-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 methods. The latter are yet unpublished and have not received the same amount of investigation than the former but they have the potential to bring new developments and further improvement over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.

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