# Strong force measurements

## Main Question or Discussion Point

How is strong nuclear force measured?

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To my knowledge, the strong force is measured by calculating "binding energies." Binding energies are just like they sound, the energy binding things together - in the strong force case, thats protons and neutrons in the nucleus. By measuring the amount of energy released when protons/neutrons are liberated from a nucleus -> you know how much energy was stored in the bond between them.

A quirky property of the strong force is that it actually gets weaker at high energies. So if you smash particles together at sufficiently high velocities then you can treat the strong force as a weaker force and determine the "coupling constant". For electromagnetism, the coupling constant is simply the electric charge. For the strong force, you want to measure an analogous quantity.

malawi_glenn
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by "strong force" you mean the force between the nucleon, or between quarks?

good question. nucleons

malawi_glenn
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I wondered since the Strong Force between the nucleons are different than between the quarks inside the nucleons. So one has to be careful ;)

This is due to that the force carriers of the strong force are gluons, which have colour and couples to colour (i.e the quarks) ; but hadrons which are build up of quarks are colourless. So here we have a problem, we cant have gloun emision between colourless obejcts. Also, the quarks are point like, but hadrons have structure. Quite messy situation.

But one can get information by performing scattering experiments of nueclons, measure the number of partial waves included, differential cross section, scattering lenght etc, to obtain overall/global properties of the potential that this force creates.

Well the quark/gluon thing is a very poor description in the strong coupling regime, but at the level of valence quarks (I think that's the term), it would be wrong to say that these hadrons can't exchange gluons, because they are composite objects. I guess an analogy would be that atoms are neutral, yet we can stick atoms together by all sorts of coulombic interactions that polarize them.

the quark/gluon thing is a very poor description in the strong coupling regime
Something difficult is not necessarilly poor or irrelevant. People are trying to understand the mechanism of quark confinement you know. And the strong coupling constant for instance has been measured not to grow to infinity (which is not a very physical possibility). So, we do know how to make sens of non-perturbative partons.

I wasn't equating difficulty with irrelevance. Strong coupling doesn't just mean hard to solve, it means "far from weak coupling", in the sense that the "good" weak coupling states which have definite quark / gluon number don't look anything like the "good" strong coupling states, which have definite hadron/meson number. Now, there seems to be an effective description of hadrons in terms of valence quarks, but these aren't the quarks that we put into the Lagrangian, and are highly dressed.

I'm not sure about the coupling constant not growing to infinity. I guess it depends on what you're calling the coupling constant. It's a hard question to ask because the states that go into the S-matrix aren't the same states that participate in the scattering.

it means "far from weak coupling", in the sense that the "good" weak coupling states which have definite quark / gluon number don't look anything like the "good" strong coupling states, which have definite hadron/meson number.
It's funny, I see it the other way around : at high energy (weak coupling) you have as many partons as you can reach, whereas at large coupling (low energy) you have a definite number of (valence) quarks. Sea quarks can be considered as mesons BTW.
Now, there seems to be an effective description of hadrons in terms of valence quarks, but these aren't the quarks that we put into the Lagrangian, and are highly dressed.
This is the heart of the difficulty actually. Why is the "naive" quark model so predictive (in retrospect) ?
I'm not sure about the coupling constant not growing to infinity.
It has been measured to saturate, both on the lattice and in the lab.
I guess it depends on what you're calling the coupling constant.
Indeed, once again the very heart of the question. References upon request. I'd be delighted to discuss that more, since I keep coming again again to this problem.
It's a hard question to ask because the states that go into the S-matrix aren't the same states that participate in the scattering.
There are even in QED already this kind of subtelty. I am not sure this track would help us gain much insight.

Doesn't NMR involve high power magnets interacting with nucleons (is this the same as strong force)?

Doesn't NMR involve high power magnets interacting with nucleons (is this the same as strong force)?
Well, one could argue in principle that the values of the proton and neutron magnetic moments is determined by the strong force. But given those values, the strong nuclear forces do not matter in NMR.

How is strong nuclear force measured?

Take uranium from the table, measure its mass/atom, then take its decay component (dang I forgot, thornium, sermanium, lead, plutonium or something. They have series of decay. You can search the web for its decay components.) if you take them mass individualy and compare it to uranium, they should weight less. From that , use E=mc^2. Make sure u choose the right uranium. I believe U235 is the short live one. U238 might be the long live one. It's the opposite for hydrogen fusion. Now that still don't answer your question. I just assume constant force and the spatial in between to approximate.

malawi_glenn
Homework Helper
atom888, have you ever studied the nucleon-nucleon problem, and the deutron in detail? That is how you measure the force.

nuby: NMR measures the electromagnetic interaction of nuclei, and as humanino poited out - you may measure the nucleon magnetic moments, nothing more.

Thanks. new question. How does a strong magnetic field affect protons/neutrons?

malawi_glenn
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Keywords: Nuclear magnetic moment, NMR

Then someone can answer if it is something that is unclear or that you dont understand.

Good luck! :-)

How does a strong magnetic field affect protons/neutrons?
What is "strong" in your "strong magnetic field" ?
Is it strong like in "big" or strong like in "chromodynamics" ?
Is is indeed possible to talk about the chromomagnetic part of the glue field, but I doubt the question refered to this.

malawi_glenn
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What is "strong" in your "strong magnetic field" ?
Is it strong like in "big" or strong like in "chromodynamics" ?
Is is indeed possible to talk about the chromomagnetic part of the glue field, but I doubt the question refered to this.
I interprented "strong" as: large/big, $$B = \mathcal{O}(Tessla)$$ Can the strong force between quarks be measured in Newtons?

Would the total binding energy of the 3 quarks in a proton be equal to it's mass-energy?

Can the strong force between quarks be measured in Newtons?

Would the total binding energy of the 3 quarks in a proton be equal to it's mass-energy?
You can measure it in Newtons, if you want. But that's not very useful.

The proton's, mass-energy comes form the binding energy of the quarks (negative), plus the masses of the individual quarks (positive), plus their relativistic kinetic energy (positive). The last two quantities are hard to measure (what is an individual quark?).

Another way is to look at an entire nucleus, especially a big one. There, the protons and neutrons themselves are "well behaved", so you can look at the mass deficit of the entire nucleus to figure out the binding energy, and if the nucleus is large, the average binding energy per nucleon is what you're after. I recommend the WP articles "Binding_energy" and "Semi-empirical_mass_formula"

What would it be for a proton at rest (binding force in newtons)?

malawi_glenn
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*sigh* it is too simplificistic to talk about binding "force" when it comes to these complicated systems as hadrons.

What one can do, which is quite simple, is to consider light quarkonia such as charmonium. Fit a potential which gives the correct energy levels when solvning the schrödinger equation, and then go from potential to force.

So, in other words, it is too much work to just post the answer here.

malawi_glenn