Supergravity with a gravitino lightest supersymmetric particle

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 − 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.

Gravitino dark matter and cosmological constraints

“…In addition to the limits from the properties of dark matter, there are also constraints from the observed abundances of the primordial light elements (D, He, Li) [20,21,22,23]. Because of the extremely weak interactions of the gravitino, the NLSP typically has a long lifetime before it decays into the gravitino.…”

“…While most of the existing investigations focus on the effects of electromagnetic energy injection (see [20,30,31] and references therein), the bounds from hadronic energy injection can be much more severe [28,29,32,33,34,35]. In particular, the hadronic constraints strongly restrict the mass spectrum in the bino-like neutralino NLSP case [22,23]. For the slepton NLSP case, the hadronic constraints have also been estimated and found to be much weaker but still significant in much of the parameter space [21,22,23].…”

“…In particular, the hadronic constraints strongly restrict the mass spectrum in the bino-like neutralino NLSP case [22,23]. For the slepton NLSP case, the hadronic constraints have also been estimated and found to be much weaker but still significant in much of the parameter space [21,22,23].…”

“…Indeed, the lighter stau τ 1 can appear naturally as the lightest standard model superpartner in frameworks in which the spectrum at low energies is derived from SUGRA predictions at the scale of grand unification. Scenarios with a gravitino LSP and a long-lived charged slepton NLSP are also particularly promising for phenomenology at future colliders [36,37,38,39,40,41,42,22,43,44,45,25,46].…”

“…We obtain these results from an exact computation of the 4-body decay of the slepton NLSP into the gravitino, the corresponding lepton, and a quark-antiquark pair. The corrections with respect to the previous estimate [21] are indicated explicitly.…”