Mass measurement in boosted decay chains with missing energy

We explore a novel method of mass reconstruction in events with missing energy at hadron colliders. In events with sizeable boost factors in the final steps of dual multistage decay chains, the missing energy particles may each be approximately collinear with a visible standard model particle, spanning a narrow “MET-cone.” We exploit this collinear approximation, when applicable, to reconstruct the masses of exotica.

Figure 1

A schematic picture of a boosted decay chain.

Figure 2

In this figure we display the MET-cone boundaries for different NLSP and LSP masses (keeping the mass splitting constant). The MET vector must lie within the boundary for particular choices of exotica masses. The $Z$ bosons are in a configuration where each has boost factor ${\gamma }^Z=5$, both lie in the transverse plane, and are separated by a 90° angle.

Figure 3

In these two plots, we compare the rescaled MET-cones of two scenarios with decay ${\chi }_2\to {\chi }_{1}Z$, with identical ${\chi }_2$ mass (200 GeV). In the top (bottom) plot, we take $\Delta m=m_{{\chi }_2}-(m_{{\chi }_1}+m_Z)=1\mathrm{GeV}(10\mathrm{GeV})$. Different contours correspond to randomly chosen momentum configurations of two $Z$ bosons from Monte Carlo events. Note that $m_{{\chi }_1}^{\mathrm{test}}$ has two endpoints in the small $|{\mathrm{p̸}}_{T,x}/{\mathrm{E̸}}_T|$ region.

Figure 4

Illustration of the MET-cone and its relation to the $m_{{\chi }_1}^{\mathrm{cone}}$ endpoints.

Figure 5

Top: The distributions of $m_{{\chi }_1}^{\mathrm{cone}}$ for Model 1 and 2. For Model 2, we have normalized the distribution by a factor of 2. The fitting region for model 1 is 40–60 GeV and 120–200 GeV; for model 2, it is 10–30 GeV and 250–400 GeV. Bottom: The distributions of $m_{{\chi }_1}^{\mathrm{cone}}$ for Model 3 and 4.

Figure 6

The $m_{{\chi }_1}^{\mathrm{cone}}$ distributions for Model 1 (top panel) and 2 (bottom panel) using reconstructed objects after detector simulation. The $\mathrm{\text{signal}}+\mathrm{\text{background}}$ distributions are shown in red solid lines. The gray straight lines indicate the linear fits of the signal in the endpoint regions. The dominant SM backgrounds from $t\overline{t}+\ge 1$ jets are shown with the black dotted lines.