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تسجيل مستخدم جديدMagnifying the ATLAS Stealth Stop Splinter: Impact of Spin Correlations and Finite Widths
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In this paper, we recast a stealth stop search in the notoriously difficult region of the stop-neutralino Simplified Model parameter space for which $m(tilde{t}) - m(tilde{chi}) simeq m_t$. 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 $tbar{t}$. Stop searches away from this stealth region have left behind a splinter of open parameter space when $m(tilde{t}) simeq m_t$. 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 $sqrt{s} = 7 text{ TeV}$ and $8 text{ TeV}$ in a correlated way to control for some systematic errors. ATLAS fixed $m(tilde{t}) simeq m_t$ and $m(tilde{chi})= 1 text{ GeV}$, implying that a careful recasting of these results into the full $m(tilde{t}) - m(tilde{chi})$ plane is warranted. We find that the parameter space with $m(tilde{chi})lesssim 55 text{ GeV}$ is excluded for $m(tilde{t}) simeq m_t$ --- 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) what the impact on the total rate from taking the narrow width is a good approximation; (iii) how the production rate is affected when the wrong widths are used; (iv) what role the spin correlations play 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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