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

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 $$ m\left({\tilde{t}}_1\right)-m\left({\tilde{\upchi}}_1^0\right)\simeq {m}_t $$ 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 $$ m\left({\tilde{t}}_1\right)\simeq {m}_t $$ 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 $$ m\left({\tilde{t}}_1\right)\simeq {m}_t\kern0.5em and\kern0.5em m\left({\tilde{\upchi}}_1^0\right)=1 $$ 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 $$ m\left({\tilde{t}}_1\right)\simeq {m}_t $$ 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.