I have read that it is a misconception that a proton only has 3 quarks (2 up and one down). In reality, it seems there are many, many ("zillions" is the number I saw quoted) quarks in a proton. Do protons have the exact same number of quarks? The exact same mass? With so many components, possibility of decay, fluctuations in charge, etc. it seems like no two protons would ever be "exactly" the same mass and charge? is this right?

See these posts: http://profmattstrassler.com/articles-and-posts/largehadroncolliderfaq/whats-a-proton-anyway/ http://cosmologyscience.com/cosblog/three-quarks-dont-add-up-to-one-proton-not-even-close/ http://news.sciencemag.org/physics/2010/04/mass-common-quark-finally-nailed-down


A proton is described as a combination of three valence quarks, each with a bayrion number of 1/3 and a charge that adds up to the +1 of the proton, two up and one down.


That is a primary constraint from data.

Now in QCD, the theory we have developed to describe the strong interactions of quarks, it comes about that overall, in the constraining "bag" where the valence quarks are , a differing number of quark-antiquark pairs can be posited mathematically to exist within the confines and the constraints of total baryon number and total charge, with differing energies. All interactions are controlled by gluons being exchanged with all those quarks, very many Feynman diagrams.

Here is a representation of the quarks and antiquarks that can be found in a proton.


Snapshot of a proton -- and imagine all of the quarks (up,down,and strange -- u,d,s), antiquarks (u,d,s with a bar on top), and gluons (g) zipping around near the speed of light, banging into each other, and appearing and disappearing. (M.Strassler 2010)

The valence quarks are mixed with the rest in this picture, it is the total sum of quantum numbers that defines the proton, i.e. there are in there two extra ups with respect to antiups and one extra down to the antidowns.

All these are virtual, i.e. not on mass shell. How do we know they exist? We know quarks and gluons exist from the particle jets found in high energy scattering. For all the rest we trust the mathematics, the model, since it does give us correct descriptions of existing data and describes future experiments very well.

Do protons have the exact same number of quarks?

The sum of all quark baryon numbers adds up to 1. That is the constraint.

The exact same mass?

That is also a constraint.

With so many components, possibility of decay, fluctuations in charge, etc. it seems like no two protons would ever be "exactly" the same mass and charge? is this right?

Experimentally we have observed that all protons have the same mass and charge.

We have discovered/constructed a theoretical model that can describe strong interactions mathematically with great accuracy while obeying the experimental constraints. Within the mathematical descriptions of the model quarks and gluons and antiquark-quark pairs are constrained by mass and charge and baryon number conservation when modeling a proton. Otherwise it would be a very bad model, not fitting the data.

Thus regardless of the fluctuation in the number of quarks and antiquarks, the sum of their baryon numbers must be 1 and the sum of their charges must be 1. By construction as far as the model goes, and as the model is very successful in describing and predicting particle interactions the representation is accepted as reality.

p.s. protons have not been observed to decay and the model describes this.

  • $\begingroup$ Thank you, @annav. Can you provide a source for "experimentally, we have observed that all protons have the same mass"? $\endgroup$ – Brad Cooper - Purpose Nation May 3 '15 at 15:04
  • $\begingroup$ For what I have read, and I believe what you are saying, the mass of the proton is much greater than the sum of the parts. Are the theorized "virtual particles" an attempt to account for the missing mass? If so, again, it would seem that at any time, lots virtual particles (do we know how many?) coming and going would fluctuate, and thus could be slightly different from proton to proton at any given moment? $\endgroup$ – Brad Cooper - Purpose Nation May 3 '15 at 15:25
  • $\begingroup$ Yes, the number or partons within the proton will be different in this QCD mathematical theory. It is the integrals and total sums that have to add up to the mass and the baryon number and charge.The mass is the length of the four vector of " the sum of all the constiuents". That is why the mass of the proton is so much larger than of individual quarks. It is similar to the mass of the pi0 which decays into two zero mass gammas although it has over 100 MeV of mass itself $\endgroup$ – anna v May 3 '15 at 15:32

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