# Neutrino has so little mass

1. May 10, 2009

### daisey

Hello All,

I have read that because the Neutrino has so little mass, and because it is not affected by the Electromagnetic Force that it can travel through light years of solid lead. The photon is also mass-less, I believe. Why can it not do the same as neutrinos? I understand the photon is the force carrier for the electromagnetic force. Does that mean the photon is so affected by the force that it represents that it cannot travel through lead as a neutrino can?

2. May 10, 2009

### malawi_glenn

Re: Neutrino

yes, the photon has electromagnetic interactions, which is stronger than the weak interaction.

3. May 10, 2009

### Bob_for_short

Re: Neutrino

A very very energetic photon can travel long way before scattering.

Bob.

4. May 10, 2009

### ZapperZ

Staff Emeritus
Re: Neutrino

How energetic? Can an ultra-high energy gamma ray photon penetrate through half an inch of a superconductor. A neutrino can pass through that and more.

Zz.

5. May 10, 2009

### Bob_for_short

Re: Neutrino

Yes, it certainly can. A superconducting state cannot prevent a photon from having a very small scattering cross section. A superconductor does not screen the gamma rays.

Bob.

6. May 10, 2009

### ZapperZ

Staff Emeritus
Re: Neutrino

Can you cite a reference for that, considering that the photon's E-field could be easily shielded by the supercurrent?

Zz.

7. May 11, 2009

Staff Emeritus
Re: Neutrino

ZapperZ, the problem is that the supercurrent doesn't "see" frequencies as high (or equivalently as wavelengths as short) as you get in gamma rays (or even x-rays).

8. May 11, 2009

### hamster143

Re: Neutrino

The photon couples directly to charged particles, namely, electrons and quarks.

The neutrino doesn't. It interacts by exchanging a W or a Z boson.

If we compare two main mechanisms: scattering of a photon on an electron (Compton scattering), and scattering of a neutrino on an electron, ignoring loop effects and dimensionless factors of magnitude one, we'll see that the amplitude of the first process is ~$$1/E_{cm}^2$$ and the amplitude of the second process is ~$$1/(E_{cm}^2+m_X^2)$$ where $$m_X$$ is mass of W or Z boson and $$E_{cm}$$ is center of mass energy. If all energies are high enough to consider electrons massless (>>1 MeV), cross section is proportional to $$M^2/E_{cm}^2$$.

Obviously, for energies well below W/Z masses, neutrino-electron cross section will be lower than photon-electron cross section by the factor of $$(M_X/E_{cm})^4$$. Comparing a 10 MeV solar neutrino with a 10 MeV photon, we get a factor of 10^16. Which explains why a 10 MeV photon will travel a few centimeters in the matter before stopping, and a 10 MeV neutrino is likely to fly through the Earth without stopping.

9. May 11, 2009

### Bob_for_short

Re: Neutrino

What is the energy-dependence of the photon cross section at hight E? Doesn't it decrease?

Bob.

10. May 11, 2009

### ZapperZ

Staff Emeritus
Re: Neutrino

That could be it. After all, the AC resistivity certainly goes up with increasing frequency.

Thanks!

Zz.

11. May 11, 2009

### Bob S

Re: Neutrino

There are three primary ways photons interact with matter: 1) Photoelectron emission. This the highest cross section from 1 or 2 eV up to just beyond the k-shell electron binding energy 2)Comption scattering. Cross section (roughly 2/3 barn. Most dominant cross section up to several MeV. 3) pair production. The most dominant cross section above several MeZV.
There are also photon-nuclear cross sections, such as gamma-n on oxygen, which reaches a maximum (a few barns) between 10 and 20 MeV.