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Augmenting the diboson excess for Run 2

Recently, both ATLAS and CMS reported excesses in the high invariant-mass tail of di-jet events when subjet-based reconstruction techniques were applied, suitable for example for highly boosted gauge bosons [1, 2]. While the CMS observation [2] of an excess of around 2σ with respect to the Standard Model expectation about two years ago went relatively unnoticed by the wider high-energy physics community, the ATLAS publication [1] reporting an excess of about 2.5σ over background estimates triggered a flurry of mainly theoretical papers aiming at an explanation of this excess. Clearly, such an excess in the mass range of about 2 TeV, as reported, may easily be the first glimpse of new physics beyond the Standard Model and it is therefore not surprising that practically all publications to date interpret the excess as the intriguing discovery of a TeV-scale resonance decaying predominantly to the gauge bosons of the weak interaction. This of course is a scenario predicted in many extensions of the Standard Model of particle physics [3]. However, many of the searches for these high-mass objects heavily rely on good theoretical and experimental control of often novel reconstruction techniques in highly exclusive and hitherto unprobed regions of phase space. In addition, as the exciting signals for new physics often manifest themselves through a very small number of events only, a precise understanding and book-keeping of all possible backgrounds is of paramount importance to the full appreciation of the statistical significance of any excess or lack thereof. Hence, before the incoming 13 TeV data, it is important to understand the techniques employed by such searches in detail and provide constructive assessments of the potential for biases or improvements in the analysis strategy. As the latest analysis by ATLAS on this subject [1] yields the best sensitivity and largest statistical significance, and because it is relatively well documented, we will focus on it throughout this short paper. This search is based on the assumed decay of a heavy resonance in two weak gauge bosons and, consequently, on looking for excesses in the invariant mass distribution of two hadronically decaying gauge bosons, thus using the larger hadronic branching ratio. This was realised by analysing events based on a fat-jet selection with the Cambridge/Aachen (C/A) algorithm [4], demanding two R = 1.2 jets with a minimal transverse momentum of p T = 540 GeV. The reconstruction of the gauge bosons relied on a combination of a jet-mass cut around the masses of the weak gauge bosons and grooming techniques, where a modified version of the BDRS reconstruction technique [5] was employed, a method initially designed for the reconstruction of a Higgs boson with pT,H ≥ 200 GeV. Eventually, to improve on the separation of signal and background, cuts were applied on the momentum ratios of subjets, the number of charged particles within a subjet and the mass of the reconstructed gauge bosons. After recombining the four-momenta of the two reconstructed gauge bosons an excess was observed in the mass range 1.9 ≤ mV V ≤ 2.1 TeV over the data-driven (fitted) background estimate, mainly driven by the QCD background. Following and re-implementing the analysis steps as described in [1], we find several points that could potentially reduce the statistical significance of the observed excess. While our findings may possibly be attributed to our lacking understanding of subtle and possibly internal details of the analysis, we feel that they warrant a more detailed discussion. Broadly speaking, our findings fall into two categories: one, discussed in Sec. II, is related to the reconstruction method used for the gauge bosons, which open a number of potential pitfalls and some room for future possible improvements. The other class of comments relates to the modelling of backgrounds by data-driven methods in general, and in Sec. III we will point to backgrounds that contribute predominantly in the tail of the reconstructed