Pion and Nucleon Masses: The Role of Electromagnetic Self-Energy Explained

In summary: I am still confused about why the masses of protons and neutrons are different.The difference in mass between protons and neutrons is due to the gluons that bind the quarks together. The gluons are not completely understood, but they may play a role in why the masses of the up and down quarks are almost the same, and why the masses of the other four quarks are so very different.
  • #1
kryshen
7
0
I have one stupid question:

Is it true that the difference of proton and nucleon mass is due to the electromagnetic self-energy of the proton?

The same question about pi^0 and pi^- mass difference.
 
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  • #2
kryshen said:
I have one stupid question:

Is it true that the difference of proton and nucleon mass is due to the electromagnetic self-energy of the proton?

The same question about pi^0 and pi^- mass difference.
It's not a stupid question.

I believe you mean neutron when you mention "nucleon". Both protons and neutrons are nucleons - they are both found in the nucleus.

One has to look at the quark structure.

Please refer to these - http://hyperphysics.phy-astr.gsu.edu/hbase/particles/hadron.html

http://hyperphysics.phy-astr.gsu.edu/hbase/particles/meson.html#c1

http://hyperphysics.phy-astr.gsu.edu/hbase/particles/hadron.html#c6

http://hyperphysics.phy-astr.gsu.edu/hbase/particles/baryon.html#c1
 
  • #3
To Astronuc,
Thank you for the references, you provided.
Indeed, I meant neutron when mentioned "nucleon". Still, I have a question. If the difference of nucleon masses is explained by the difference of quark structure, how the difference of u and d quark masses is explained?
 
  • #4
Well, I am not sure since I have not really looked into the details. I would hope someone like marlon or others might be able to answer.

However, consider the following equations where u = up quark rest mass, and d = down quark mass.

P (uud) = 2u + d = 938.3 MeV rest energy (mass)

n (udd) = u + 2d = 939.6 MeV rest energy (mass)

but now consider

[itex]\pi^+[/itex] (ud) = 139.6 MeV energy (mass),

and one can look at other particles.

Presumably there are other particles, gluons, which interact with quarks to bind them.

http://hyperphysics.phy-astr.gsu.edu/hbase/particles/expar.html#c1

and perhaps these make the differences.

I am still troubled by the pion mass vs the nucleon mass.
 
  • #5
There are some tricky situations here.

Firstly, the mass of a composite particle is not just the mass of the quarks. The gluons contribute to the mass also.

In addition here is a great article that talks about breaking of charge symmetry which accounts for the difference between the masses of the up and down quarks. QCD should be invariant under the change of an up quark to a down quark, but it is not.

Here is the article:
http://physicsweb.org/articles/world/16/6/3

The issues of the pion masses are exactly the same as the issues of the different masses of the nucleons (atleast it should be...)
Cheers,
Ryan
 
  • #6
I claim no expertise on the subject, but I understand that nucleon (proton and neutron) masses mostly come from the energy of the gluons holding the quarks together. The bare quark masses are much too small.
 
  • #7
The mass difference between the up and down quark (and thus the mass difference between the nucleons) is related to the charge symmetry breaking of the up and down quarks.

please see:
http://physicsweb.org/article/world/16/6/3
 
  • #8
Norman said:

Thank your very much for this reference. Now I get the picture:
if up and down quarks had equal masses, electrostatic repulsion between quarks should make the proton heavier, since it contains two up quarks with charges of +2/3. But the mass difference between the quarks wins over their electrostatic repulsion by a factor of about two, making it the dominant cause of charge-symmetry breaking.
 
  • #9
But apparently according the article on Physicsweb, quark mass is still a mystery and challenge in QCD.
Charge-symmetry breaking also manifests itself in the interactions of pions with protons and neutrons in a very interesting way that is linked to the neutron-proton (and hence, up and down quark) mass difference. Because the masses of the up and down quarks are almost zero, another approximate symmetry of QCD called "chiral" symmetry comes into play. This symmetry relates to the spin angular momentum of fundamental particles. Quarks can either be "right-handed" or "left-handed", depending on whether their spin is clockwise or anticlockwise with respect to the direction they are moving in. Both of these states are treated approximately the same by QCD.
The origin of the quark masses is not fully understood. In the Standard Model of particle physics, the Higgs mechanism allows the generation of such masses but it cannot predict the actual mass values. This is like having a recipe to make cookies that will work with either chocolate chips or nuts. Why are the masses of the up and down quarks almost the same, and why are the masses of the other four quarks so very different? No fundamental understanding of this mass hierarchy exists. But the TRIUMF and IUCF experiments mean that nature's violation of charge symmetry can now be used to tackle at least the up-down piece of this puzzle.
See also - http://hyperphysics.phy-astr.gsu.edu/hbase/particles/qmass.html

I am not sure if articles on gluon mass are any help either.
 
  • #10
Astronuc said:
But apparently according the article on Physicsweb, quark mass is still a mystery and challenge in QCD.
See also - http://hyperphysics.phy-astr.gsu.edu/hbase/particles/qmass.html
I am not sure if articles on gluon mass are any help either.

The mystery of quark mass, in fact the mass of any particle is truly a profound question. Why the particles have the mass they do is one of the most fundamental questions in physics. It is going to take a lot of work in QCD or calculational techniques to get this under wraps soon. There seems to be some interesting experiments going on around it though so maybe someday we will be able to calculate these quantities. Who knows.
 
  • #11
Norman said:
The mystery of quark mass, in fact the mass of any particle is truly a profound question. Why the particles have the mass they do is one of the most fundamental questions in physics. It is going to take a lot of work in QCD or calculational techniques to get this under wraps soon. There seems to be some interesting experiments going on around it though so maybe someday we will be able to calculate these quantities. Who knows.
I think the same can be said for charge. No one can say really what charge is, except that we use to explain electro-static and electro-dymanic phenomenon.

We can certainly construct abstract models for mass and charge on the subatomic/subnuclear level though. :smile:
 

1. What is the significance of studying the Pion and Nucleon masses?

The masses of Pions and Nucleons are important for understanding the structure and interactions of subatomic particles. They play a crucial role in theories of nuclear physics and particle physics.

2. How does electromagnetic self-energy affect the masses of Pions and Nucleons?

Electromagnetic self-energy is the energy that a charged particle possesses due to its own electromagnetic field. This energy affects the masses of Pions and Nucleons by contributing to their overall mass and modifying their properties.

3. What is the role of Quantum Electrodynamics in explaining the masses of Pions and Nucleons?

Quantum Electrodynamics (QED) is a theory that explains the interactions between charged particles and electromagnetic radiation. QED plays a crucial role in explaining the masses of Pions and Nucleons by taking into account the effects of electromagnetic self-energy.

4. How do scientists measure the masses of Pions and Nucleons?

Scientists use particle accelerators and detectors to study the properties of subatomic particles, including their masses. By analyzing the collisions and decay processes of Pions and Nucleons, they can determine their masses with high precision.

5. What implications do the findings about Pion and Nucleon masses have for our understanding of the universe?

The masses of Pions and Nucleons are essential for understanding the structure of matter and the forces that govern the universe. By studying these masses, scientists can gain insights into the fundamental laws of nature and the origins of the universe.

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