[Paper Review] Collider Signatures of Axino and Gravitino Dark Matter

Frank Daniel Steffen|ArXiv.org|Jun 30, 2005

Dark Matter and Cosmic Phenomena1 references3 citations

TL;DR

This paper demonstrates that axino and gravitino dark matter candidates can be probed at future colliders via decays of charged slepton next-to-lightest supersymmetric particles (NLSPs). It shows that differential distributions of photon and tau lepton emissions in NLSP decays—specifically, peak structures in energy and angular correlations—can distinguish axino from gravitino LSPs, enabling measurement of the Peccei–Quinn scale and Planck-scale physics.

ABSTRACT

The axino and the gravitino are extremely weakly interacting candidates for the lightest supersymmetric particle (LSP). We demonstrate that either of them could provide the right amount of cold dark matter. Assuming that a charged slepton is the next-to-lightest supersymmetric particle (NLSP), we discuss how NLSP decays into the axino/gravitino LSP can provide evidence for axino/gravitino dark matter at future colliders. We show that these NLSP decays will allow us to estimate the value of the Peccei-Quinn scale and the axino mass if the axino is the LSP. In the case of the gravitino LSP, we illustrate that the gravitino mass can be determined. This is crucial for insights into the mechanism of supersymmetry breaking and can lead to a microscopic measurement of the Planck scale.

Motivation & Objective

• To establish that axino and gravitino LSPs can account for the observed cold dark matter abundance via thermal production in the early Universe.
• To identify observable collider signatures of axino and gravitino dark matter through decays of charged slepton NLSPs at future linear colliders.
• To demonstrate that the differential decay distributions of NLSPs into photons and taus can distinguish between axino and gravitino LSP scenarios.
• To show that the Peccei–Quinn scale and axino mass can be measured if the axino is the LSP, and the gravitino mass can be determined if it is the LSP.
• To provide a method for probing the mechanism of supersymmetry breaking and measuring the reduced Planck scale through direct collider observations.

Proposed method

• Using the Braaten–Yuan prescription and hard thermal loop resummation to compute thermal production rates of axinos and gravitinos in the early Universe plasma.
• Solving the Boltzmann equation analytically for relic densities, incorporating running of the strong coupling and gluino mass as functions of reheating temperature.
• Modeling 3-body decays of right-handed stau NLSPs into a tau lepton, a photon, and the LSP (axino or gravitino), with kinematic cuts on photon energy and opening angle.
• Defining a normalized differential decay rate (Eq. 8) that is independent of total decay width and fundamental scales, enabling direct comparison between axino and gravitino scenarios.
• Simulating differential distributions in scaled photon energy $x_ u$ and $ heta$-angle between photon and tau, with cuts $x_ u^{ ext{cut}} = x_ heta^{ ext{cut}} = 0.1$.
• Using Monte Carlo-style event counts (e.g., ~165 ± 13 events for axino, ~100 ± 10 for gravitino) to assess statistical distinguishability of the two LSP scenarios.

Experimental results

Research questions

• RQ1 Can axino and gravitino LSPs from thermal production in the early Universe account for the observed cold dark matter relic density?
• RQ2 Can collider signatures from charged slepton NLSP decays distinguish between axino and gravitino as the dark matter LSP?
• RQ3 Can the Peccei–Quinn scale and axino mass be measured if the axino is the LSP, based on NLSP decay kinematics?
• RQ4 Can the gravitino mass be determined from collider data, and what does this imply for the mechanism of supersymmetry breaking?
• RQ5 Do differential distributions of decay products (photon energy and opening angle) provide a clear, statistically distinguishable signal between axino and gravitino LSP scenarios?

Key findings

• Thermal production of axinos and gravitinos in the early Universe can yield the correct relic density for a wide range of reheating temperatures and LSP masses.
• For $m_{ ilde{a}} o 0.1$ GeV and $f_a/N = 10^{11}$ GeV, the axino relic density scales as $ rac{m_{ ilde{a}}}{0.1 ext{ GeV}} imes rac{T_R}{10^4 ext{ GeV}} $, with logarithmic corrections.
• For $m_{ ilde{G}} = 100$ GeV and $T_R = 10^{10}$ GeV, the gravitino relic density scales as $ rac{m_{ ilde{G}}}{100 ext{ GeV}} imes rac{T_R}{10^{10} ext{ GeV}} $, with additional dependence on gluino mass and coupling.
• The differential decay distribution $ rac{1}{ ext{rate}} rac{d^2 ext{rate}}{dx_ u d ext{cos} heta} $ shows a bimodal peak in the axino LSP case—high photon energy and back-to-back emission—while the gravitino case is peaked in the soft-photon, collimated region.
• With $10^4$ analyzed stau NLSP decays, ~28% of axino LSP events are expected in the high-energy, back-to-back region ($x_ u o 0.8$, $ ext{cos} heta o -0.3$), compared to only ~1% for the gravitino LSP.
• The distinct kinematic patterns allow clear discrimination between axino and gravitino LSP scenarios, with statistical significance achievable at $ ext{O}(10^4) $ events.