Pair-produced heavy particle topologies:MSSM neutralino properties at the LHC from gluino/squark cascade decays

Abstract

Processes of the form $pp\to anything \to X_i X_j \to x\bar{x} + y\bar{y} (+ \slashchar{E})$ are studied via a technique that may be viewed as an adaptation of time-honoured Dalitz plot analyses. $X_i$ and $X_j$ are new heavy states (with $i,j=1...n$), which may be identical or distinct; and $x\bar{x}$ and $y\bar{y}$ are necessarily distinct Standard Model (SM) fermion pairs whose invariant masses can be measured. A Dalitz-like plot of said invariant masses, $M(x\bar{x})$ {\it vs.$$} $M(y\bar{y})$, exhibits a topology connected to the masses and specific decay chains of $X_i$ and $X_j$. Aside from relatively minor details, observed patterns consist of a collection of box and wedge shapes. This collection is model-dependent: comparison of the observed pattern to the possibilities for a specific model yields information on which new particle pair combinations are actually being produced, information beyond that extractable from conventional one-dimensional invariant mass distributions. The technique is illustrated via application to the Minimal Supersymmetric Standard Model (MSSM) process $pp \to \tilde{g}\tilde{g},\tilde{g}\tilde{q},\tilde{q}\tilde{q} \to \widetildeχ_i^0 \widetildeχ_j^0 \to e^+ e^- + μ^+ μ^- (+ \slashchar{E})$. Here the heavy states are neutralinos $\widetildeχ_i^0$ ($i=2,3,4$) -- note $\widetildeχ_1^0$ is excluded -- which are produced in gluino/squark ($\tilde{g}$/$\tilde{q}$) cascade decay chains. Even with fairly modest expectations for the LHC performance during the first few years, this method still provides substantial insight into the neutralino mass spectrum and couplings if gluino/squark masses are relatively low ($\simeq 400 \hbox{GeV}$).