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Springer, Journal of High Energy Physics, 7(2018), 2018

DOI: 10.1007/jhep07(2018)142

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Magnifying the ATLAS stealth stop splinter: impact of spin correlations and finite widths

Journal article published in 2018 by Timothy Cohen, Walter Hopkins, Stephanie Majewski, Bryan Ostdiek ORCID
This paper is made freely available by the publisher.
This paper is made freely available by the publisher.

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Data provided by SHERPA/RoMEO

Abstract

Abstract In this paper, we recast a “stealth stop” search in the notoriously difficult region of the stop-neutralino Simplified Model parameter space for which $ m\left({\tilde{t}}_1\right)-m\left({\tilde{\upchi}}_1^0\right)≃ {m}_t $ m t ˜ 1 − m χ ˜ 1 0 ≃ m t . The properties of the final state are nearly identical for tops and stops, while the rate for stop pair production is $ \mathcal{O} $ O (10%) of that for $ t\overline{t} $ t t ¯ . Stop searches away from this stealth region have left behind a “splinter” of open parameter space when $ m\left({\tilde{t}}_1\right)≃ {m}_t $ m t ˜ 1 ≃ m t . Removing this splinter requires surgical precision: the ATLAS constraint on stop pair production reinterpreted here treats the signal as a contaminant to the measurement of the top pair production cross section using data from $ \sqrt{s}=7 $ s = 7 TeV and 8 TeV in a correlated way to control for some systematic errors. ATLAS fixed $ m\left({\tilde{t}}_1\right)≃ {m}_t\kern0.5em and\kern0.5em m\left({\tilde{\upchi}}_1^0\right)=1 $ m t ˜ 1 ≃ m t and m χ ˜ 1 0 = 1 GeV, implying that a careful recasting of these results into the full $ m\left({\tilde{t}}_1\right)-m\left({\tilde{\upchi}}_1^0\right) $ m t ˜ 1 − m χ ˜ 1 0 plane is warranted. We find that the parameter space with $ m\left({\tilde{\upchi}}_1^0\right)\lesssim 55 $ m χ ˜ 1 0 ≲ 55 GeV is excluded for $ m\left({\tilde{t}}_1\right)≃ {m}_t $ m t ˜ 1 ≃ m t — although this search does cover new parameter space, it is unable to fully pull the splinter. Along the way, we review a variety of interesting physical issues in detail: (i) when the two-body width is a good approximation; (ii) how assuming the narrow width approximation affects the total rate; (iii ) how the production rate is affected when the wrong widths are used; (iv ) what role propagating the spin correlations consistently through the multi-body decay chain plays in the limits. In addition, we provide a guide to using MadGraph for implementing the full production including finite width and spin correlation effects, and we survey a variety of pitfalls one might encounter.