Supergravity with a Gravitino LSP

TL;DR

This work analyzes supergravity scenarios with a gravitino LSP under the assumption that the NLSP freezes out with its thermal relic density and later decays to the gravitino. It demonstrates that this thermal relic framework yields upper bounds on superpartner masses, reshaping collider expectations, and shows that hadronic BBN constraints—especially from three-body sleptonic/sneutrino decays and two-body neutralino decays—often dominate the cosmological bounds in natural weak-scale spectra. The study comprehensively maps viable regions for slepton, sneutrino, and bino NLSPs, highlighting the pivotal role of hadronic energy release and the potential to test early-Universe thermal history through collider measurements. It also discusses rich collider phenomenology, including prospects to measure NLSP lifetimes, gravitino mass, and even the Planck scale, thereby linking particle physics to cosmological evolution near temperatures around 10 GeV.

Abstract

We investigate supergravity models in which the lightest supersymmetric particle (LSP) is a stable gravitino. We assume that the next-lightest supersymmetric particle (NLSP) freezes out with its thermal relic density before decaying to the gravitino at time t ~ 10^4 s - 10^8 s. In contrast to studies that assume a fixed gravitino relic density, the thermal relic density assumption implies upper, not lower, bounds on superpartner masses, with important implications for particle colliders. We consider slepton, sneutrino, and neutralino NLSPs, and determine what superpartner masses are viable in all of these cases, applying CMB and electromagnetic and hadronic BBN constraints to the leading two- and three-body NLSP decays. Hadronic constraints have been neglected previously, but we find that they provide the most stringent constraints in much of the natural parameter space. We then discuss the collider phenomenology of supergravity with a gravitino LSP. We find that colliders may provide important insights to clarify BBN and the thermal history of the Universe below temperatures around 10 GeV and may even provide precise measurements of the gravitino's mass and couplings.

Figures (9)

• Figure 1: NLSP lifetime in seconds (solid) and mass in GeV (dashed) in the $(m_{\tilde{G}}, \delta m \equiv m_{\text{NLSP}} - m_{\tilde{G}} - m_Z)$ plane for slepton and sneutrino NLSPs.
• Figure 2: As in Fig. \ref{['fig:life_slep']}, but for a Bino NLSP.
• Figure 3: Excluded and allowed regions of the $(m_{\tilde{G}}, \delta m \equiv m_{\text{NLSP}} - m_{\tilde{G}} - m_Z)$ parameter space in the gravitino LSP scenario, assuming a $\tilde{\tau}_R$ NLSP that freezes out with thermal relic density given by Eq. (\ref{['omegal']}). The light (yellow) shaded region is excluded by the overclosure constraint $\Omega_{\tilde{G}} h^2 < 0.11$, and the medium (green) shaded region is excluded by the absence of CMB $\mu$ distortions. BBN is sensitive to the regions to the right of the labeled contours. Left: Regions probed by D and $^4$He, assuming the conservative result of Eq. (\ref{['DEllis']}) (EM1), and the more stringent constraints of Eqs. (\ref{['D']}) and (\ref{['4He']}) (EM2 and had). The dotted line denotes the region where cancellation between D destruction and creation via late time EM injection is possible Dimopoulos:1988ue, while ${}^7$Li is reduced to the observed value by the late NLSP decays Cyburt:2002uv. Right: Regions probed by $^3$He/D (EM), $^6$Li/H (had) Kawasaki:2004yh, and $^6$Li/H (EM) Kawasaki:2004yhCyburt:2002uv.
• Figure 4: As in Fig. \ref{['fig:stau']}, but assuming a sneutrino NLSP that freezes out with thermal relic density given by Eq. (\ref{['omeganu']}).
• Figure 5: As in Fig. \ref{['fig:stau']}, but assuming a Bino NLSP that freezes out with the "bulk" thermal relic density given by Eq. (\ref{['omegachi1']}).