Semi-visible emerging jets

TL;DR

The paper introduces semi-visible emerging jets (SVEJ) as a new dark-shower signature arising from HV/DS models with an $s$-channel mediator, where diagonal dark pions decay and off-diagonal states remain stable, producing a continuum between semi-visible and emerging jets. Using a top-down HV/DS framework with a $Z'$ mediator, the authors simulate signal events, examine generator-level observables, and develop an analysis strategy that leverages the ATLAS emerging-jet trigger to target multiple soft displaced vertices. They show sensitivity to cross sections as low as ${O}(0.1)$ fb for dark-pion lifetimes around ${c au_{c/pi^0}} o 10$ mm and provide reinterpretations of CalRatio and CMS muon-displaced-shower analyses, finding SVEJ often yields stronger limits across parameter space. The work emphasizes hadronization uncertainties and detector-level effects, and calls for further dedicated studies to fully map and exploit the SVEJ signature space in current and future LHC data-taking. $\{N_C=5, N_F/N_C=1, m_\pi/\Lambda=0.6, m_{Z'}=2\,\text{TeV}, c\tau_{\pi^0}=10\,\text{mm}\}$ are used as benchmarks, with $\Lambda$ varied to study shower length and pion multiplicities; the analysis demonstrates how trigger choices and vertex-based selection enable sensitivity to novel dark-shower phenomenology beyond existing prompt-emerging jet searches.

Abstract

We propose a new class of dark-shower signatures in Standard Model extensions featuring Hidden Valleys or dark sectors coupled through an s-channel mediator. In this framework, unstable dark pions appear as long-lived particles (LLPs), with their lifetimes treated as free parameters. The resulting signatures, which we term semi-visible emerging jets (SVEJ), continuously interpolate between the established semi-visible and emerging jet regimes. We outline an analysis strategy optimized for dark pion lifetimes of order $\mathcal{O}(10)$ mm, and reinterpret existing LLP searches targeting lifetimes of $\mathcal{O}(100)$-$\mathcal{O}(1000)$ mm. Our proposed SVEJ search, exploiting the current ATLAS emerging-jet trigger, achieves sensitivity to cross sections of $\mathcal{O}(0.1)$ fb for lifetimes around $\mathcal{O}(10)$ mm. Finally, we advocate a more detailed study, including hadronization uncertainties and detector-level effects.

Figures (12)

• Figure 1: $2\to 2$ (left panel) and $2\to \rm{many}$ (right panel) production mechanisms in the detector plane. The $2\to \rm{many}$ topology is of an interest to our SVEJ scenarios. We show an approximate jet cone (red dashed lines) within which the dark pions are produced, and we seperate them into decaying diagonal pions (solid magenta lines) and stable off-diagonal pions (dotted magenta lines) which escape the detector undetected. The off-diagonal pions are stable by virtue of flavour symmetry.
• Figure 2: Distribution of the transverse and total momentum of the $Z'$ vector boson, with Madgraph generated events (solid lines), and Pythia8 only generated events (dotted lines).
• Figure 3: Averaged number of dark mesons per event as a function of ${\Lambda}$, for ${m_{Z^\prime}} = 2$ TeV and ${m_\pi/{\Lambda}} = 0.6$. We show comparison between Pythia8 and Madgraph results (left panel) and details of several sub-specie results using Madgraph (right panel).
• Figure 4: Distributions of the number of the decaying dark pions $N_{\pi^0}$, their $p_T$, their boost factor and transverse decay length $L_{xy}$ for ${\Lambda} = 8, 50, 300$ GeV, ${m_\pi/{\Lambda}} = 0.6$, ${m_{Z^\prime}} = 2$ TeV and ${c\tau_{{\pi^0}}} = 10$ mm.
• Figure 5: Missing energy (left panel) and the $H_T$ (right panel) distribution for a ${\Lambda} = 8, 50, 300\,\rm{GeV}$ with fixed ${c\tau_{{\pi^0}}} = 10\,\rm{mm}$, ${m_{Z^\prime}} = 2$ TeV, ${m_\pi/{\Lambda}} = 0.6$.