Axinos in cosmology and at colliders

Abstract

The axino, the fermionic superpartner of the axion, is a well-motivated candidate for cold dark matter if it is the lightest supersymmetric particle. Since the axino couples very weakly to the matter multiplets, the next-to-lightest supersymmetric particle (NLSP) has a long lifetime, which has important consequences for both cosmology and collider phenomenology. Assuming that a charged slepton is the NLSP, we calculate the complete leading one-and two-loop contributions to its decay. We analyze in detail constraints on the parameters space from cosmology and discuss how this scenario can be probed at colliders. Scenarios in which both the axino and the gravitino are lighter than the long-lived charged slepton are also explored with particular emphasis on cosmological constraints and collider phenomenology.

Supernatural MSSM

“…The calculations of the decay rates Γ(τ R → τã) and Γ(τ R → τãγ) have been reported in [86,87] considering SUSY hadronic or KSVZ axion models [88,89]. In [86], these calculations were done using an effective theory where (s)quark were integrated out, and then may lead to logarithmic divergences.…”

“…In addition, a factor ξ and a mass scale m are introduced to parametrize the uncertainty associated with this cutoff procedure. We will use the decay rates given in [87] in our work because of absence of such issues. In the calculation of Γ(τ R → τã), the scalar one-loop self-energy function (B 0 ) and vertex function (C 0 ) [90] are evaluated by the LoopTools package [91].…”

“…(3.4-3.6) in Ref. [87]. In numerical calculations we take α = α M S (m Z ) = 1/128, sin 2 θ W = 1 − m 2 W /m 2 Z = 0.2221, |e Q | = 1/3 and y= 1, where |e Q | and y are the electric charge and Yukawa coupling of the new heavy KSVZ quarks [87].…”

“…The authors of Ref. [87] have given a very comprehensive presentation of various cosmological constraints on stau NLSP and axino LSP masses. In Fig.…”

“…The extremely weak interactions between the axino and charged slepton allow a long-lived NLSP charged slepton such as the lighter stau (τ 1 ). These long-lived staus, which were also called charged massive particles However, the Long-lived staus can cause problem and violate various cosmological bounds such as the Big Bang Nucleosyntheis bounds [72], catalyst BBN bounds [73,74,75] and structure formation bounds [76]. Thus, in such a scenario, one should have a mechanism through which the lifetime of the lighter stau via decay to the axino LSP is within reasonable time, 6 See [38] and references therein for phenomenological issues related to No-Scale models.…”