Bridging the divide: axion searches and axino phenomenology at colliders

TL;DR

This work studies a supersymmetric DFSZ axion scenario with an axino LSP and higgsino-like NLSPs that decay displacedly to axinos at the LHC. By implementing a 5×5 neutralino-axino mixing in a DFSZ-PQMSSM, generating events with MadGraph/MadSpin/PYTHIA, and applying a detector-level MadAnalysis5 analysis, the authors estimate LHC sensitivity to the model, focusing on Run II luminosity. They show that for $m_{chi1_0} \,\lesssim\,1$ TeV and $f_a \,\lesssim\,10^{11}$ GeV, displaced-vertex searches can probe the scenario, providing complementary constraints to direct-detection and astrophysical axion searches. The results highlight the role of collider experiments in exploring SUSY axion physics and outline paths for extending the study to broader SUSY axion models.

Abstract

We discuss a phenomenological model extending the minimal supersymmetric standard model containing axions and their supersymmetric partner, the axino. In the supersymmetric DFSZ axion model, the axino has tree level couplings to the higgs sector. In the case where R-parity is conserved, collider experiments may be sensitive to displaced decays of heavier neutralino states into lighter, mostly axino states. We present a sensitivity analysis using a model in which mostly higgsino next-to-lightest supersymmetric particle states decay into a mostly axino lightest supersymmetric particle. The model is studied using Monte Carlo simulation produced using MadGraph and estimates of experimental sensitivities to the model, including detector simulation and kinematic selections, are evaluated using the MadAnalysis5 framework. For a higgsino mass below 1 TeV, the axion decay constant below $f_a < 10^{11}$ GeV can be effectively probed by the Large Hadron Collider with an integrated luminosity of 140 $\mathrm{fb}^{-1}$. This work demonstrates that supersymmetric DFSZ axion models can be studied with existing collider experiments, offering complementary sensitivity to direct-detection and astrophysical searches and paving the way for broader exploration of supersymmetric axion scenarios.

Figures (8)

• Figure 1: Higgsino NLSP lifetime, $c\tau$, dependence on the higgsino mass, $m_{\tilde{\chi}_{1}^{0}}$, for various axion decay constants $f_{a}$.
• Figure 2: Feynman diagrams showing chargino-neutralino or neutralino-neutralino pair production from a proton-proton collision. In both cases, due to the small mass splitting, the heavier $\tilde{\chi}_{2}^{0}$ or $\tilde{\chi}_{1}^{\pm}$ state first decays to the lightest neutralino $\tilde{\chi}_{1}^{0}$ by the emission of an extremely off-shell $Z$ or $W$ boson, respectively. Both $\tilde{\chi}_{1}^{0}$ then decay to the axino, the lightest supersymetric particle, by the emission of a Higgs boson (left) or $Z$ boson (right). Supersymmetric particles are shown in red.
• Figure 3: Distributions of the decay length ($c\tau$) from the longest lived NLSP state in an event for different model parameters. These distributions are taken before any preselection criteria are applied.
• Figure 4: Distributions of leading jet transverse momentum for different model parameters. These distributions are taken before any preselection criteria are applied.
• Figure 5: Distributions of $E_{\mathrm{T}}^{\mathrm{miss}}$ for different model parameters. These distributions are taken before any preselection criteria are applied.