Talent

Nuclear Talent
Course 7
NT4A

the_nu_wgv

The $\nu$ WGV Group Members

• Don Willcox (code/parameters)
• Nicole Vassh (parameters/analysis)
• Panos Gastis (code/analysis)

Project Goal

To examine the effect of the neutrino magnetic moment on the primordial abundances. This will be done by considering the magnetic moment as a free parameter (presumably larger than the standard model prediction). This additional magnetic moment coupling will then keep the neutrinos coupled to the electrons past the traditional $\sim 1$ MeV temperature. Here we preliminarily consider the effect on primordial abundances when the magnetic pair-production process $e^{+}+e^{-}\leftrightarrow \nu +\bar{\nu}$ is included into the big bang nucleosynthesis code of F. Timmes.

Decoupling Temperature Calculation

We would like to find the decoupling temperature, such that: $$\Gamma= G_F^2T^5 + \mu_\nu^2T^3 \sim H = \frac{T^2}{M_{pl}}$$

Let's replace with some more illustrative constants: $$\Gamma = \frac{T^5}{M_W^4} + g_\nu^2\frac{T^3}{m_p^2} \sim H = \frac{T^2}{M_{pl}}$$

For $g_\nu\equiv 0$, we have: $$T_0^3 = \frac{M_W^4}{M_{pl}}$$

We can use this to rewrite the cubic equation, in dimensionless form: $$\left(\frac{T}{T_0}\right)^3 = 1 - g_\nu^2 \frac{M_{pl}^{2/3}M_W^{4/3}}{m_p^2}\frac{T}{T_0}$$

We can see that the last term is of order unity when $g_\nu\sim \frac{m_p}{M_{pl}^{1/3}M_W^{2/3}}\sim 10^{-8}$, allowing us to rewrite the equation as: $$\left(\frac{T}{T_0}\right)^3 = 1 - \left(\frac{g_\nu}{10^{-8}}\right)^2 \frac{T}{T_0}$$