What is the energy dependence of photon cross sections at high energies?

In summary: The cross section decreases as the photon energy increases because the nuclear interaction becomes more important.
  • #1
daisey
131
3
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?
 
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  • #2


yes, the photon has electromagnetic interactions, which is stronger than the weak interaction.
 
  • #3


daisey said:
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?

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

Bob.
 
  • #4


Bob_for_short said:
A very very energetic photon can travel long way before scattering.

Bob.

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


ZapperZ said:
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.

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


Bob_for_short said:
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.

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

Zz.
 
  • #7


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


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 ~[tex]1/E_{cm}^2[/tex] and the amplitude of the second process is ~[tex]1/(E_{cm}^2+m_X^2)[/tex] where [tex]m_X[/tex] is mass of W or Z boson and [tex]E_{cm}[/tex] is center of mass energy. If all energies are high enough to consider electrons massless (>>1 MeV), cross section is proportional to [tex]M^2/E_{cm}^2[/tex].

Obviously, for energies well below W/Z masses, neutrino-electron cross section will be lower than photon-electron cross section by the factor of [tex](M_X/E_{cm})^4[/tex]. 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


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

Bob.
 
  • #10


Vanadium 50 said:
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).

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

Thanks!

Zz.
 
  • #11


Bob_for_short said:
What is the energy-dependence of the photon cross section at hight E? Doesn't it decrease?Bob.

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.
 

1. Why does a neutrino have such a small mass?

A neutrino's mass is so small because it is an elementary particle, meaning it has no internal structure or substructure. It is also a lepton, one of the fundamental building blocks of matter, and is not affected by the strong nuclear force like quarks are. This lack of internal structure and interaction with the strong force allows for a very small mass.

2. How does a neutrino's mass compare to other particles?

Neutrinos have the smallest known mass of any particle in the Standard Model of particle physics. They are approximately 500,000 times lighter than an electron, which is already considered a very light particle. Additionally, neutrinos have a mass that is at least a million times smaller than that of a proton or neutron, making it one of the most elusive particles to study.

3. Can the mass of a neutrino change?

Yes, the mass of a neutrino can change. Neutrinos are known to undergo oscillations, meaning they can change from one type (or flavor) to another. This process is only possible if the neutrino has a non-zero mass. However, the overall mass of a neutrino is thought to remain the same, with any changes being within a very small range.

4. How do we measure the mass of a neutrino?

Measuring the mass of a neutrino is a very difficult task. One method is through direct observation, which involves detecting the speed and energy of neutrinos produced in nuclear reactions. Another method is through indirect measurements, such as studying the effect of neutrinos on the cosmic microwave background radiation or through their impact on the formation of galaxies. Currently, all methods have only been able to determine an upper limit for the mass of a neutrino.

5. What implications does a small neutrino mass have on the universe?

The small mass of neutrinos has significant implications for our understanding of the universe. It helps to explain the phenomenon of dark matter, as neutrinos were initially thought to have no mass and could potentially account for the missing mass in the universe. Additionally, the small mass of neutrinos may have played a role in the evolution of the universe and the formation of large-scale structures, such as galaxies and galaxy clusters.

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