2. Instant preheating Let us focus solely on the interaction h $^{2}$φ$^{2}$χ$^{2}$. In instant preheating the particle production occurs during one oscillation of the inflaton [118]. The particle production occurs when the inflaton passes through the minimum of the potential φ = 0. In this case the process can be approximated by writing φ = ˙ φ$_{0}$ ( t − t$_{0}$ ), where ˙ φ$_{0}$ is the velocity of the field when it passes through the minimum of the potential at time t$_{0}$ . The time interval within which the production of σ quanta occurs is [118] Δ $^{t}$∗ = ( g | ˙ φ$_{0}$ | ) − 1 / $^{2}$, which is much smaller than the Hubble expansion rate; thus expansion can be neglected. The occupation number of produced particles jumps from its initial value zero to a non-zero value during − $^{φ}$∗ ≤ φ ≤ $^{φ}$∗ . In the momentum space the occupation number is given by [118] n$_{k}$ = exp ( − πk 2 g | ˙ φ 0 | ) , and the largest number density of produced particles in x -space reads [118] with the particles having a typical energy of ( g | ˙ φ$_{0}$ | /π ) 1 / $^{2}$, so that their total energy density is given by These expressions are valid if m 2 χ < g | ˙ ϕ$_{0}$ | . Instant preheating has applications especially when gauge fields and fermions are involved [89, 724]. One particular interesting point is when the modulus is carrying the SM gauge charges and passing through the point of enhanced gauge symmetry , i.e. 〈 φ 〉 ≈ 0 (see Sec. V D). Where the gluons are nearly massless, then they can be excited with a similar abundance given by Eq. (386). When the modulus is displaced away from the point of enhanced gauge symmetry, the gluons become heavy due to modulus induced VEV dependent mass ∼ g 〈 φ 〉 , where g is the gauge coupling. As the gluons become heavy they rapidly decay into fermions to reheat the plasma [122]. Transferring the inflaton energy through this mechanism is quite fast and efficient, see the discussion on reheating due to MSSM inflaton, Sec. V D. n$_{χ}$ ≈ ( h | ˙ φ$_{0}$ | ) 3 / 2 8 π 3 , (386) ρ$_{χ}$ ∼ 1 2 ( δ (1) ˙ χ ) 2 ∼ ( g | ˙ φ$_{0}$ | ) 2 8 π 7 / 2 . (387) 181