A study of late decaying charged particles at future colliders

TL;DR

This paper investigates a direct experimental avenue to probe gravitino LSP SUSY by measuring the lifetime and decays of long-lived charged NLSPs (primarily the stau) using a massive stopper detector placed near future colliders. It shows that Stopper detectors can capture and store slowed CNLSPs from both the LHC and $e^-e^-$ linear colliders, enabling lifetime measurements up to $\mathcal{O}(10^3)$ years and enabling detailed studies of decay distributions and rare channels, including the tau-energy endpoint $E_{\tau}\sim (m^2_{\tilde{\tau}}-m^2_{3/2})/(2m_{\tilde{\tau}})$ to determine $m_{3/2}$ and infer the supergravity Planck scale $M_P$. The analysis provides estimated yields for representative SUSY spectra (e.g., G2b and SPS7 points) and collider scenarios, highlighting that hundreds to millions of stopped CNLSPs could be collected with feasible luminosities and detector masses, thereby enabling precision tests of supergravity, SUSY-breaking scales, and early-universe cosmology. The work also discusses backgrounds, detector design considerations, and the potential to explore rare gravitino couplings through multi-body decays and photon-assisted channels.

Abstract

In models where the gravitino is the lightest supersymmetric particle (LSP), the next-to-lightest supersymmetric particle (NLSP) is long-lived. We consider an important charged NLSP candidate, the scalar tau $\tildeτ$. Slow charged NLSPs may be produced at future colliders and they may be stopped in a massive stopper which simultaneously serves as a detector for the NLSP and its decay products. We found the number of events at a 1 kton to O(10) kton detector could be significant enough to study the NLSP decays with lifetime shorter than $10^{10}$ sec at the LHC. The performance of existing 1 kton detectors may be good enough to do such studies at the LHC, if they can be placed close to the ATLAS/CMS detectors. At a future $e^- e^-$ collider, scalar electrons $\tilde{e}^-$s are copiously produced. Slow NLSPs may be produced from the $\tilde{e}^-$ decay. The number of stopped NLSPs at a future linear collider could be large enough to study rare decay modes of the NLSP.

Figures (4)

• Figure 1: The $\beta\gamma$ distribution of $\tilde{l}$ for $10^5$ SUSY events at the point G2b. The solid histogram shows the distribution with $\vert \eta_{\tilde{l}} \vert <1$ and the dashed histogram shows all $\tilde{l}$ distributions.
• Figure 2: Cross section $\sigma(e^-_R + e^-_R\to \tilde{e}^- + \tilde{e}^-)$ for $m_{\tilde{e}}=170~\mathrm{GeV}$. We assume 100% polarization for the $e^-$ beam. $M_1 = 180~\mathrm{GeV}$ (solid lines) and $M_1 = 300~\mathrm{GeV}$ (dashed lines). The thick lines are the total cross section and the thin lines are for selectrons with $|\eta|<1$.
• Figure 3: Velocity distributions of $\tilde{\tau}^+$ (solid line) and $\tilde{\tau}^-$ (dashed line) at the rest frame of $\tilde{e}^-$, for $m_{\tilde{e}}=170~\mathrm{GeV}$ and $m_{\tilde{\tau}}=150~\mathrm{GeV}$. Bino mass is $M_1=180~\mathrm{GeV}$ (left) and $M_1=300~\mathrm{GeV}$ (right).
• Figure 4: ($\eta$, $\beta$) distributions of $\tilde{\tau}^\pm$ at the laboratory frame, for $m_{\tilde{\tau}}=150~\mathrm{GeV}$, $m_{\tilde{e}}=170~\mathrm{GeV}$, and $M_1=180~\mathrm{GeV}$. The velocity of the selectron is $\beta_{\tilde{e}}=0.5$ for upper figures, and $\beta_{\tilde{e}}=0.3$ for lower figures. Total number of events are normalized so that it corresponds to integrated luminosity $10~\mathrm{fb}^{-1}$.