Abstract
We examine, using the analyses of the 750 GeV diphoton resonance as a case study, the methodology for estimating the dominant backgrounds to diphoton resonance searches. We show that close to the high energy tails of the distributions, where background estimates rely on functional extrapolations or Monte Carlo predictions, large uncertainties are introduced, in particular by the challenging photon–jet background. Analyses with loose photon and low photon p_T cuts and those susceptible to high photon rapidity regions are especially affected. Given that diphoton-based searches beyond 1 TeV are highly motivated as discovery modes, these considerations are relevant for future analyses. We first consider a physics-driven deformation of the photon–jet spectrum by next-to-leading order effects and a phase space dependent fake rate and show that this reduces the local significance of the excess. Using a simple but more general ansatz, we demonstrate that the originally reported local significances of the 750 GeV excess could have been overestimated by more than one standard deviation. We furthermore cross-check our analysis by comparing fit results based on the 2015 and 2016 LHC data sets. Finally we employ our methodology on the available 13 TeV LHC data set assessing the systematics involved in the current diphoton searches beyond the TeV region.
Highlights
Searches for new physics at the energy frontier often look for new phenomena at the edge of distributions
This paper deals with a problem that often arises in searches for new physics at the energy frontier
In this context the challenge is to look for a new resonance at the upper end of a distribution where only limited knowledge on the standard model (SM) background is available
Summary
As both conclusions are quite extraordinary (certainly the second one), they motivate an investigation into their robustness. We raise a third option, to be considered in conjunction with (i), namely, we ask how unlikely is the possibility that (iii) the significance of the excess is overestimated due to underestimating fake-based backgrounds. With the inclusion of more data in the analyses the excess eventually vanished [3,4], ruling out the new physics hypothesis (ii). The possibility of claim (iii) remains unclear, affecting all analyses which rely on a precise knowledge of the photon faking background and use the same techniques to estimate it. While our conclusion is independent of the 750 GeV resonance we use it as an example case to scrutinize the hypothesis of the underestimated background and its implications. The main rationale behind our hypothesis is presented, followed by a detailed description of our approach to background estimation See Refs. [5,6]
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