By Markus Kuster, Georg Raffelt, Berta Beltrán

ISBN-10: 3540735178

ISBN-13: 9783540735175

ISBN-10: 3540735186

ISBN-13: 9783540735182

Axions are bizarre hypothetical debris which could either clear up the CP challenge of quantum chromodynamics and whilst account for the darkish topic of the universe. according to a sequence of lectures by way of international specialists during this box held at CERN (Geneva), this quantity presents a pedagogical advent to the idea, cosmology and astrophysics of those interesting debris and offers an up to date account of the prestige and prospect of ongoing and deliberate experimental searches. rookies and practitioners of astroparticle physics will locate during this publication either a concise creation and a present reference paintings to a show off subject that connects the “inner area” of the trouble-free particle international with the “outer house” of the universe at large.

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**Sample text**

However, pop. II axions may get gravitationally bound to miniclusters later on. It seems rather diﬃcult to model this process reliably. A discussion is given in [10]. It is concluded there that the accretion of pop. II axions results in miniclusters that have an inner core of pop. I axions with density of order 10−18 g cm−3 and a fluﬀy envelope of pop. II axions with density of order 10−25 g cm−3 . When a minicluster falls onto a galaxy, tidal forces are apt to destroy it. If a minicluster falls through the inner parts of the Galaxy (r < 10 kpc), where the density is of order 10−24 g cm−3 , its fluﬀy envelope of pop.

107) where η is the fraction of cold axions that are pop. I. We assume that all pop. 100, Case 2) to estimate ̺a (teq ). 102), we find for pop. II ℓf,II ∼ q ln ℓmc teq t3 ≃ 42 q . 103) and assuming the range γ ∼ 7–60, suggested by the numerical simulations [10], we conclude that pop. II axions do homogenize and hence the axion energy density has a smooth component at teq . However, pop. II axions may get gravitationally bound to miniclusters later on. It seems rather diﬃcult to model this process reliably.

We will assume that the initial value of a is suﬃciently small that fa sin(a/fa ) ≃ a. Let us define ψ by a(t) ≡ t−3/4 ψ(t) . 41) 3 . 42) 16 t2 For t > t1 , we have d ln ω/dt ≪ ω ≃ ma . That regime is characterized by the adiabatic invariant ψ02 (t) ω(t), where ψ0 (t) is the changing oscillation amplitude of ψ(t). 43) where C is a constant. Hence a(t) = A(t) cos with A(t) = t C ma (t) dt′ ω(t′ ) , t−3/4 . 45) Hence, during the adiabatic regime, A2 (t) ma (t) ∝ t−3/2 ∝ R(t)−3 . 46) The zero-momentum mode of the axion field has energy density ̺a = 1 2 2 2 ma A and describes a coherent state of axions at rest with number density na = 12 ma A2 .

### Axions: Theory, Cosmology, and Experimental Searches by Markus Kuster, Georg Raffelt, Berta Beltrán

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