Abstract
The rejection of the contamination, or background, from low-energy strong interactions at hadron collider experiments is a topic that has received significant attention in the field of particle physics. This article builds on a particle-level view of collision events, in line with recently proposed subtraction methods. While conventional techniques in the field usually concentrate on probability distributions, our study is, to our knowledge, the first attempt at estimating the frequency distribution of background particles across the kinematic space inside individual collision events. In fact, while the probability distribution can generally be estimated given a model of low-energy strong interactions, the corresponding frequency distribution inside a single event typically deviates from the average and cannot be predicted a priori. We present preliminary results in this direction and establish a connection between our technique and the particle weighting methods that have been the subject of recent investigation at the Large Hadron Collider.
Highlights
The subtraction of the contamination, or background, from soft, that is, low-energy, physics processes described by Quantum Chromodynamics (QCD) that take place in protonproton collisions is a critical task at the Large Hadron Collider (LHC)
For the purpose of this study, we describe collision events as statistical populations of particles originating from soft QCD interactions and from the hard scattering, using a mixture model of the form α0f0(η, pT)+α1f1(η, pT), where α0 is the fraction of soft QCD particles in the events and α1 = 1 − α0
Choosing a different signal process will normally change the final-state particle kinematics, the difference between particles originating from a hard scattering and soft QCD particles is generally expected to be more pronounced than differences across signal processes
Summary
The subtraction of the contamination, or background, from soft, that is, low-energy, physics processes described by Quantum Chromodynamics (QCD) that take place in protonproton collisions is a critical task at the Large Hadron Collider (LHC). Even when the processes involved are exactly the same, different collisions contain independent, and different, realisations of the underlying quantum processes For this reason, the shapes of the corresponding particle-level frequency distributions, for example, that of soft QCD particles, generally vary across collisions. That contain nb∗|/ns∗ ≲ 1; that is, the absolute deviation of the estimated number of background particles from the true number is lower than the number of signal particles across the kinematic space in the event, corresponding to the pile-up rate considered. Choosing a different signal process will normally change the final-state particle kinematics, the difference between particles originating from a hard scattering and soft QCD particles is generally expected to be more pronounced than differences across signal processes As discussed, this is supported by the study documented in [17], where this method was applied to vector boson fusion Standard Model Higgs production. The potential dependence of the performance of this technique on the choice of signal process deserves further investigation, in order for the results presented in this article to be generalised
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