Axino phenomenology
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part of Springer Nature, 2020 https://doi.org/10.1140/epjst/e2020-000044-8
THE EUROPEAN PHYSICAL JOURNAL SPECIAL TOPICS
Review
Axino phenomenology Eung Jin Chuna Korea Institute for Advanced Study, Hoegiro 85, Dongdaemun-gu, Seoul 02455, Korea Received 17 March 2020 / Accepted 7 October 2020 Published online 14 December 2020 Abstract. The strong CP problem is solved elegantly by the PQ mechanism which predicts the presence of a light pseudo Goldstone boson called the axion. In supersymmetric theories, the axion is accompanied by its fermionic partner called the axino. It can play an important role in collider, dark matter, and neutrino physics. We review general properties of the axino in relation to the standard axion models, and discuss various phenomenological and cosmological implications.
1 Strong CP problem and axion Standard Model (SM) allows several CP violating parameters: CP phases in Yukawa (mass) matrices Yf and θ parameters in gauge field strengths: LYuk = Hu q¯L Yu uR + Hd q¯L Yd dR + Hd ¯lL Yl lR + h.c. Lθ =
g22 g12 g32 a ˜ aµν i ˜ iµν ˜ µν . θ G G + θ W W + θ1 Bµν B 3 2 µν µν 32π 2 32π 2 16π 2
(1)
Various CP violating phenomena observed in the electroweak interactions are well described by the CKM phase in the quark Yukawa (mass) matrices: the Jarlskog invariant [1] δ = ArgDet[Yu Yu† , Yd Yd† ]
(2)
which is measured to be δ = 1.19 ± 0.05 [2]. In the QCD sector, the CP violating parameters induce an electric-dipole moment (EDM) of a nucleon through the effective strong θ term: θ¯ = θ3 + ArgDet(Yu Yd ).
(3)
For the neutron, one finds [3] dn ∼ e a
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mq ∼ 2.5 × 10−16 θ¯ ecm, m2n
(4)
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The European Physical Journal Special Topics
and thus the non-observation of the neutron EDM: dn < 3 × 10−26 ecm [4] requires ¯ < 10−10 . This amounts to the strong CP problem: “Why is θ¯ vanishingly small, |θ| particularly, in contrast to δ?”. Let us remark that the weak θ parameter is unobservable due to the chiral nature of the electroweak symmetry, that is, θ2 can be rotated away by the (anomalous) B + L symmetry [5]. The strong CP problem is elegantly resolved by the Peccei-Quinn-WeinbergWilczek (PQWW) mechanism [6–9] introducing a spontaneously broken global U (1)P Q symmetry which has a QCD anomaly. The Goldstone boson of such a symmetry, called the axion a, has the anomaly coupling: Lanomaly = a
g32 a a ˜ µν G G , 32π 2 fa µν a
(5)
where fa is the axion decay constant proportional to the U (1)P Q symmetry breaking scale. The QCD condensatation generates a potential for the axion (redefined after ¯ absorbing θ): V [a] ≈
m2π fπ2
a 1 − cos fa
(6)
which sets ha/fa i ≡ 0 at the minimum and thus dynamically resolves the strong CP problem. The axion also becomes massive due to such a condensation potential: ma ≈ mπ fπ /fa . The original PQWW axion model realized at the weak scale has been ruled out, but high-scale axion models can be realized by inroducing a heavy quark (KSVZ) [10,11] or two Higgs boublets (DFSZ) [12,13]. The allowed window of the axion scale is 109
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