# 1.5. Questions¶

Here are some great questions about neutrinos.

Origin of Masses

The exact mechanism of neutrino mass

Majorana or Dirac

Is neutrino its own antiparticle? Or is neutrino Majorana or Dirac?

Mass Hierarchy

What’s the mass hierarchy?

We know from solar neutrino experiments that $$m_1^2 < m_2^2$$ using the theory of MSW effect since MSW only happens for $$m_2^2-m_1^2>0$$. (Just like some kind of definition in fact.)

On the other hand, we don’t know the ordering of $$m_3$$ with respect to the other two. Since $$\lvert m_3^2 - m_1^2 \rvert \gg m_2^2 - m_1^2$$, we could only have two ordering, $$m_1^2<m_2^2<m_2^2$$ or $$m_3^2<m_1^2<m_3^2$$.

Mixing Angles

What are the mixing angles?

Families

How many different flavours of neutrinos are there? Are there any sterile neutrinos?

CP Violation

CP Violation etc

## 1.5.1. Astrophysics¶

Will Neutrino Emission Apply Pressure to the Surface of Neutron Stars?

Neutron star or proto-neutron star emits a huge amount of neutrinos. The total number can reach the order $$10^{58}$$.

The first thought would be estimate the momentum density at the surface of neutron star using the total number. However, it turns out to be unnecessary.

We consider two limits.

The first one is the early stage that the proto-neutron star is transparant to neutrinos. The emission is isotropic so the net pressure on proto-neutron star is around 0.

The second stage is when the neutron star becomes opaque to neutrinos (above certain energy). The bounce of neutrinos within the neutron star is random so the net pressure is also around 0.

1. Volpe, C. (2013). Open issues in neutrino astrophysics. Annalen Der Physik, 525(8-9), 588–599. doi:10.1002/andp.201300064

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