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 \( m\left({\tilde{t}}_1\right)-m\left({\tilde{\upchi}}_1^0\right)\simeq {m}_t \). The properties of the final state are nearly identical for tops and stops, while the rate for stop pair production is \( \mathcal{O} \)(10%) of that for \( t\overline{t} \). Stop searches away from this stealth region have left behind a “splinter” of open parameter space when \( m\left({\tilde{t}}_1\right)\simeq {m}_t \). 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 \( \sqrt{s}=7 \) TeV and 8 TeV in a correlated way to control for some systematic errors. ATLAS fixed \( m\left({\tilde{t}}_1\right)\simeq {m}_t\kern0.5em and\kern0.5em m\left({\tilde{\upchi}}_1^0\right)=1 \) GeV, implying that a careful recasting of these results into the full \( m\left({\tilde{t}}_1\right)-m\left({\tilde{\upchi}}_1^0\right) \) plane is warranted. We find that the parameter space with \( m\left({\tilde{\upchi}}_1^0\right)\lesssim 55 \) GeV is excluded for \( m\left({\tilde{t}}_1\right)\simeq {m}_t \) — 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.
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Cohen, T., Hopkins, W., Majewski, S. et al. Magnifying the ATLAS stealth stop splinter: impact of spin correlations and finite widths. J. High Energ. Phys. 2018, 142 (2018). https://doi.org/10.1007/JHEP07(2018)142
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DOI: https://doi.org/10.1007/JHEP07(2018)142