Mainly axion cold dark matter in the minimal supergravity model

arXiv:0906.2595v2 [hep-ph] 12 Aug 2009 Preprint typeset in JHEP style - HYPER VERSION Mainly axion cold dark matter in the minimal supergravity model Howard Baera, Andrew D. Boxa and Heaya Summya aDept. of Physics and Astronomy, University of Oklahoma, Norman, OK 73019, USA E-mail: baer@nhn.ou.edu, box@nhn.ou.edu,heaya@nhn.ou.edu Abstract: We examine the minimal supergravity (mSUGRA) model under the assump- tion that the strong CP problem is solved by the Peccei-Quinn mechanism. In this case, the relic dark matter (DM) abundance consists of three components: i). cold axions, ii). warm axinos from neutralino decay, and iii). cold or warm thermally produced axinos. To sustain a high enough re-heat temperature (TR >∼106 GeV) for many baryogenesis mech- anisms to function, we find that the bulk of DM should consist of cold axions, while the admixture of cold and warm axinos should be rather slight, with a very light axino of mass ∼100 keV. For mSUGRA with mainly axion cold DM (CDM), the most DM-preferred parameter space regions are precisely those which are least preferred in the case of neu- tralino DM. Thus, rather different SUSY signatures are expected at the LHC in the case of mSUGRA with mainly axion CDM, as compared to mSUGRA with neutralino CDM. Keywords: Supersymmetry Phenomenology, Supersymmetric Standard Model, Dark Matter. 1. Introduction The cosmic abundance of cold dark matter (CDM) has been recently measured to high precision by the WMAP collaboration[1], which lately finds ΩCDMh2 = 0.110 ± 0.006, (1.1) where Ω= ρ/ρc is the dark matter density relative to the closure density, and h is the scaled Hubble constant. No particle present in the Standard Model (SM) of particle physics has exactly the right properties to constitute CDM. However, CDM does emerge naturally from two compelling solutions to longstanding problems in particle physics. The first problem is the strong CP problem[2], for which an elegant solution was proposed by Peccei and Quinn many years ago[3], and which naturally predicts the existence of a new particle[4]: the axion a. The axion turns out to be an excellent candidate particle for CDM in the universe[5]. The second problem– the gauge hierarchy problem– arises due to quadratic divergences in the scalar sector of the SM. The quadratic divergences lead to scalar masses blowing up to the highest scale in the theory (e.g. in grand unified theories (GUTS), the GUT scale MGUT ≃2 × 1016 GeV), unless exquisite fine-tuning of parameters is invoked. The gauge hierarchy problem is naturally solved by introducing supersymmetry (SUSY) into the theory. By including softly broken SUSY, quadratic divergences cancel between fermion and boson loops, and only log divergences remain. The log divergence is soft enough that vastly different scales remain stable within a single effective theory. In SUSY theories, the lightest neutralino emerges as an excellent WIMP CDM candidate. The gravitino of SUSY theories is also a good super-WIMP CDM candidate[6]. Gravity-mediated SUSY breaking models include gravitinos with weak-scale masses. These models experience tension due to possible overproduction of gravitinos in the early universe. In addition, late decaying gravitinos may disrupt calculations of light element abundances produced by Big Bang nucleosynthesis (BBN). This tension is known as the gravitino problem. Of course, it is highly desirable to simultaneously account for both the strong CP problem and the gauge hierarchy problem. In this case, it is useful to invoke supersymmetric models which include the PQ solution to the strong CP problem[7]. In a SUSY context, the axion field is just one element of an axion supermultiplet. The axion supermultiplet contains a complex scalar field, whose real part is the R-parity even saxion field s(x), and whose imaginary part is the axion field a(x). The supermultiplet also contains an R-parity odd spin-1 2 Majorana field, the axino ˜a[8]. The saxion, while being an R-parity even field, nonethless receives a SUSY breaking mass likely of order the weak scale. The axion mass is constrained by cosmology and astrophysics to lie in a favored range 10−2 eV> ma > 10−5 eV. The axino mass is very model dependent[9], and is expected to lie in the general range of keV to GeV. An axino in this mass range would likely serve as the lightest SUSY particle (LSP), and is also a good candidate particle for cold dark matter[10]. In this paper, we investigate supersymmetric models wherein the PQ solution to the strong CP problem is also invoked. For definiteness, we will restrict ourselves to examining the paradigm minimal supergravity (mSUGRA or CMSSM) model[11]. We will restrict our – 1 – work to cases where the lightest neutralino ˜χ0 1 is the next-to-lightest SUSY particle (NLSP); the case with a stau NLSP has recently been examined in Ref. [12]. Related previous work on axino DM in mSUGRA can be found in Ref. [14]. We will be guided in our analysis also by considering the possibility of i

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