Jet Substructure by Accident

TL;DR

This paper introduces accidental substructure, a non-boosted jet-substructure strategy for high-multiplicity final states that clusters several hard partons into fat jets with $R=1.2$, creating detectable substructure even when parent particles are not boosted. It combines total jet mass $M_J$, $N$-subjettiness, and a new event-level variable $T_{NM}$ to quantify substructure across the event, enabling strong discrimination against QCD backgrounds without requiring missing energy. Through Monte Carlo studies of RPV gluino decays yielding up to 18 partons in the final state, the authors show that cuts on $M_J$ together with $T_{NM}$ notably improve reach, achieving expected exclusions near 800 GeV for certain topologies at the 8 TeV LHC with 5 fb$^{-1}$ data and 20% systematics. The approach is data-driven for QCD backgrounds and is complementary to conventional small-radius jet analyses, offering broad applicability to signatures with many jets and suppressed $\slashed{E}_T$. Overall, accidental substructure expands the toolkit for new physics searches in the non-boosted, high-multiplicity regime and motivates further optimization, including potential neural-network-based selection of $\tau_{NM}$ variables.

Abstract

We propose a new search strategy for high-multiplicity hadronic final states. When new particles are produced at threshold, the distribution of their decay products is approximately isotropic. If there are many partons in the final state, it is likely that several will be clustered into the same large-radius jet. The resulting jet exhibits substructure, even though the parent states are not boosted. This "accidental" substructure is a powerful discriminant against background because it is more pronounced for high-multiplicity signals than for QCD multijets. We demonstrate how to take advantage of accidental substructure to reduce backgrounds without relying on the presence of missing energy. As an example, we present the expected limits for several R-parity violating gluino decay topologies. This approach allows for the determination of QCD backgrounds using data-driven methods, which is crucial for the feasibility of any search that targets signatures with many jets and suppressed missing energy.

Figures (8)

• Figure 1: Lego plots showing the distribution of calorimeter activity in the $\eta-\phi$ plane. The different colors correspond to different fat jets; within each panel, darker colors signify higher $p_T$ in a given detector cell. Note that the relative $p_T$ scale is different for the signal and background example. The signal (left panel) is pair production of 500 GeV gluinos with $\widetilde{g} \rightarrow t\,\overline{t}+3\,j$, which yields up to 18 partons in the final state. The gluinos have transverse momenta of 120 and 65 GeV, so they are essentially at rest. A QCD multijet event is depicted in the right panel. The circles are centered on the clustered fat jet with a radius of $R=1.2$ to schematically illustrate the extent of each fat jet. There is significant substructure for the signal and suppressed substructure for the background.
• Figure 2: Gluino decay diagrams, illustrating topologies that can lead to as many as 18, 10, and 6-parton final states (left to right, respectively) when the gluinos are pair-produced. Note that $\widetilde{g}$ is a gluino, $\widetilde{t}$ is a stop, $t$ is a top quark, $\widetilde{q}$ is a first or second generation squark, $\chi$ is a neutralino, and $j$ refers to a final state quark or anti-quark.
• Figure 3: The $H_T$ (left) and $M_J$ (right) distributions for the backgrounds and an example signal. The signal (red solid line) is pair production of a $750 \hbox{GeV}$ gluino with $\widetilde{g} \rightarrow t\,\bar{t}+3\,j$. The stacked histogram is for background (QCD in solid blue, $W^\pm/Z^0 + 4\,j$ in hatched magenta, and $t\,\bar{t}+j$ in striped green). $M_J$ is a more powerful discriminator than $H_T$ when comparing signal to background.
• Figure 4: Normalized distributions of $\tau_{43}$ for background and a signal example. Each plot shows the normalized distribution before a cut on $M_J$. The signal (red solid line) is pair production of a $750 \,\mathrm{GeV}$ gluino with $\widetilde{g} \rightarrow t\,\bar{t}+3\,j$. The solid blue histogram is for the QCD background. Each panel is the distribution for the $j^\mathrm{th}$ jet; the order is by decreasing $p_T$. Note that the top and electroweak backgrounds are subdominant and are not shown here.
• Figure 5: Distributions of $T_{43}$ for backgrounds and an example signal, with $M_J>0$ (left) and $M_J>500 \,\mathrm{GeV}$ (right). The signal (red solid line) is pair production of a $750 \,\mathrm{GeV}$ gluino with $\widetilde{g} \rightarrow t\,\bar{t}+3\,j$. The stacked histogram is for background (QCD in solid blue, $W^\pm/Z^0 + 4\,j$ in hatched magenta, and $t\,\bar{t}+j$ in striped green). A cut on $T_{43} \lesssim 0.6$ helps to distinguish signal from background, after requiring $M_J > 500 \,\mathrm{GeV}$.