Resolving combinatorial ambiguities in dilepton tt¯ event topologies with constrained M2

Abstract

We advocate the use of on-shell constrained M 2 variables in order to mitigate the combinatorial problem in supersymmetry-like events with two invisible particles at the LHC. We show that in comparison to other approaches in the literature, the constrained M 2 variables provide superior ansätze for the unmeasured invisible momenta and therefore can be usefully applied to discriminate combinatorial ambiguities. We illustrate our procedure with the example of dilepton tt events. We critically review the existing methods based on the Cambridge M T2 variable and MAOS reconstruction of invisible momenta, and show that their algorithm can be simplified without loss of sensitivity, due to a perfect correlation between events with complex solutions for the invisible momenta and events exhibiting a kinematic endpoint violation. Then we demonstrate that the efficiency for selecting the correct partition is further improved by utilizing the M 2 variables instead. Finally, we also consider the general case when the underlying mass spectrum is unknown, and no kinematic endpoint information is available.

Kinematic focus point method for particle mass measurements in missing energy events

“…Then, the a's can be distinguished from the b's, but there remains a twofold ambiguity in partitioning the two a's and the two b's into the two branches. As there are already several suggestions in the literature on how to address this combinatorial issue [76][77][78], we shall not dwell on the combinatorial problem any further. Note that our proposed method below will be robust against combinatorial issues, since it will rely on detecting a peak structure which is absent from the combinatorial background.…”

Singularity variables for missing energy event kinematics

“…Finally, we shall ignore the combinatorial problem arising in Fig. 1(f), since it has already been addressed in the literature [39][40][41], and for simplicity we will assume that the visible particles have been properly assigned to the two decay branches.…”

Probing the top-Higgs Yukawa CP structure in dileptonic $$ t\overline{t}h $$ with M2-assisted reconstruction

“…For this purpose we adopt a kinematic reconstruction method presented in ref. [53]. Second, since this reconstruction method was studied only at the parton-level, we would like to investigate its performance further beyond the parton-level, including more realistic effects such as parton-shower, hadronization and detector resolution.…”

“…Individual values ( q 1T and q 2T ) are unknown and only their sum ( q 1T + q 2T ) is constrained by the total missing transverse momentum, / P T . Another mass-constraining variable is the M N [53,60,66], which is the (3+1)dimensional version of eq. (3.1):…”

“…Based on ref. [53], we define the following parameter space: 2Ct , leading to positive x, y, and z. On the other hand, the wrong pairing could give either sign.…”