What is the Best Evidence for Quarks in Protons?

[Adrian Baker]

Electron scattering question...

This question is from an 'A' level exam paper from June 2003:

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Here are some facts about the scattering of a beam of high energy electrons from a sample of Protons:

A - most of the electrons in the beam are not scattered at all
B - Some electrons are scattered through large angles
C - new particles, other than protons and electrons, are emitted from the sample.

Which ONE of these facts provides the best evidence for the presence of quarks in protons?

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I'm puzzled at this. I'm not a Particle Physicist (obviously) but I can't work out the correct answer from either B or C

The electrons won't show large angle deflections unless they 'hit' a quark (so B is true) and this interaction will create mesons (so C is true).

I've read up on Feynman's flat pancake explanation of these interactions (electons at speeds approaching c 'see' the quarks as small stationary particles on a flat 'pancake' proton) and although I understand the processes involved, I still can't work out whether B or C is the correct answer.

I have the mark scheme, and so know the answer wanted, but I can't explain this to my students.

What am I missing please?? Also, remember, this is to explain these interactions to 18 year old schoolchildren (and their sadly misinformed teacher) so please keep it reasonably simple.

Cheers.

 

[ZapperZ]

My guess would be the last one, C.

All of them can be true. But if the electron colliding into the protron produces a zoo of other particles, then there is a possibility of the e+p=n+stuff channel. This is a change at the "quark content" level. The other two are just "generic" classical collision/non-collision.

Zz.

 

[Andrew Mason]

Here are some facts about the scattering of a beam of high energy electrons from a sample of Protons:

A - most of the electrons in the beam are not scattered at all
B - Some electrons are scattered through large angles
C - new particles, other than protons and electrons, are emitted from the sample.

Which ONE of these facts provides the best evidence for the presence of quarks in protons?

I think the question is meant to be simple. If the proton lacked an underlying structure and appeared to the electron as a solid impenetrable object, most of the electrons would scatter through large angles. There would also be new particles emitted. If the protron has an underlying structure consisting of much smaller quarks, most electrons would not be scattered at all (A). But B and C would still occur. So the answer has to be A.

AM

 

[Gokul43201]

I think the question is meant to be simple. If the proton lacked an underlying structure and appeared to the electron as a solid impenetrable object, most of the electrons would scatter through large angles. There would also be new particles emitted.

Why ? New particles without corresponding antiparticles ? Without underlying structure, where would these particles come from ?

 

[ZapperZ]

I think the question is meant to be simple. If the proton lacked an underlying structure and appeared to the electron as a solid impenetrable object, most of the electrons would scatter through large angles. There would also be new particles emitted. If the protron has an underlying structure consisting of much smaller quarks, most electrons would not be scattered at all (A). But B and C would still occur. So the answer has to be A.

AM

But A could simply mean you have bad aim! The option said:

"A - most of the electrons in the beam are not scattered at all"

from a "sample of protons", whatever that means. Do they mean the protons are arranged in such a way that they touch each other and thus leave no space in between?

If you look at the Rutherford experiment alone, you have situation A. Yet, this says nothing about the content of protons, because if it does, then I'm sure Rutherford would have been smart enough to pick up on that.

Zz.

 

[James Jackson]

Reading between the horrifically over-simplified A-Level lines, I'd imagine they're talking about deep inelastic scattering (inferred by the phrase 'high energy electrons').

 

[Meir Achuz]

"A - most of the electrons in the beam are not scattered at all
B - Some electrons are scattered through large angles
C - new particles, other than protons and electrons, are emitted from the sample.
Which ONE of these facts provides the best evidence for the presence of quarks in protons?"
One key is the word "high energy" in the question. High energy electrons
(>1 GeV or so) would not scatter at large angles from a proton because its radius is about 0.8 fm. Electrons scattering at large angles means they are scattering off much smaller particles. They were originally called partons, but now are know to be quarks.
In fact this is how partons were discovered experimentally at SLAC in the late 60's.
(A) and (C) would occur even if there were no point-like particles inside the proton.

 

[Adrian Baker]

And this was a 'simple' one mark multi-choice question! Having read peoples comments so far, I feel less inadequate at my inability to give a simple answer to my class!

James J - Yes, it is about deep inelastic scattering.

 

[Andrew Mason]

But A could simply mean you have bad aim! The option said:

"A - most of the electrons in the beam are not scattered at all"

from a "sample of protons", whatever that means. Do they mean the protons are arranged in such a way that they touch each other and thus leave no space in between?

I was assuming that the experiment was set up in such a way that one is looking only at the behaviour of electrons whose path is within the scattering cross-section of a proton. I hadn't really thought about how one would do that!

In fact, it is probably an impossible experiment to perform. One simply cannot get a target consisting of a number of contiguous protons. Perhaps the closest approximation would be a single large nucleus.

If one was able to look at only those electrons whose path co-incided with the area represented by a nuclear cross-section, one would observe that most electrons would not be scattered at all. Would that not be inconsistent with a solid proton model? With a solid proton model all the electrons would either be scattered at large angles or produce new particles (since there could be no penetration). The fact that most electrons would pass through the nucleus without being scattered tells you that there is an underlying structure.

If you look at the Rutherford experiment alone, you have situation A. Yet, this says nothing about the content of protons, because if it does, then I'm sure Rutherford would have been smart enough to pick up on that.

Except that Rutherford used alpha particles, so they would never reach a proton (due to coulomb repulsion) let alone pass through it.

AM

 

[Adrian Baker]

I assumed it meant a sample of ionised Hydrogen gas - ie moving protons. However, although this experimental output is described in detail in many books, I haven't seen how this was set up originally. I was hoping that someone here could give me some real detail on this. However, most electrons would 'miss' the protons and quarks, so have little deflection. This would show little about the proton structure I feel, but as I say, I'm not sure how the experiment was carried out.

Anyway, according to the exam mark scheme, the answer isn't A.

 

[ZapperZ]

I assumed it meant a sample of ionised Hydrogen gas - ie moving protons. However, although this experimental output is described in detail in many books, I haven't seen how this was set up originally. I was hoping that someone here could give me some real detail on this. However, most electrons would 'miss' the protons and quarks, so have little deflection. This would show little about the proton structure I feel, but as I say, I'm not sure how the experiment was carried out.

Anyway, according to the exam mark scheme, the answer isn't A.

No. What I was trying to say was that the electrons will see mainly empty spaces, no matter what. Unless, as you've said, that the electron beam can be confined to within the radius of a proton (Whoa!), then MOST of the electrons will pass through unscattered, or scattered at very low angle. This says nothing about the "content" of proton.

Zz.

 

[Berislav]

I'm not a physicist, but, IMO, the answer could be B: The up quark is positively charged, while the down quark is negatively charged. I think that due to this a part of the scattered electrons would be scattered at large angles compared to what one might expect if the proton wasn't a particle composed of particles which have quark-like properties.

Forgive my interruption, as my explanation is most likely wrong.

 

[Romperstomper]

The question asks which one is the "best" example, so it implies that more than one can be evidence. You just have to figure out why one is a better example than the other. My QM class only scratched the surface of quarks, so I don't know much about them. My guess would be C just because electrons could also have a large scatter angle if the proton was an elementary particle as well, so I see C as being a better example than B.

 

[danAlwyn]

My gut, not thinking about it, answer is C. If I were looking at this problem from the perspective of an examiner I would be trying to demonstrate the difference between the theory of the proton as a single sub-atomic particle, and an object made up of quarks. A has already been shown to be fairly off the mark by others, but when choosing between B and C, I would pick C. If protons really are some sort of hard sphere target than a beam of high-energy electrons would see some large angle scatterings (or at least I think they would-I'm not doing the calculations at the moment)-but they would not emit other particles. Only if there are smaller components of matter available can you make up other particles.

Note that this is based on my first interpretation of the question as "What is the best evidence for protons being made up of quarks instead of being solid spherical objects? I don't know if this is the correct interpretation, but if it is than the emission of secondary particles is certainly a giveaway that protons are not an elementary particle.

 

[Adrian Baker]

The answer is given as 'C' so well done those that got it right, but I think the question is rubbish, as with the data given I think you could still argue B or C (or A possibly..)

I persuaded myself that B was the answer (for similar reasons to Berislav), but on checking the Mark Scheme, I was then able to persuade myself that the answer must be C!

Thanks for your input folks - keep it coming if you can add to this.

Thanks

 

[rbj]

even if there were no such thing as quarks (if protons were indivisible), the very few electrons that hit a proton in the nucleus head on will be deflected by a large angle, no? isn't that what Rutherford scattering was about?

 

[jdavel]

even if there were no such thing as quarks (if protons were indivisible), the very few electrons that hit a proton in the nucleus head on will be deflected by a large angle, no? isn't that what Rutherford scattering was about?

No. Rutherford used positively charged alpha particles. That changes everything. With electrons the trajectories will be planetary (if that's the right word); i.e., they'll be like the paths of objects in the gravitational field of a much more massive object.

So, the trajectory of an electron will be either an ellipse (if its energy is low enough) or a hyperbola (if its energy is too high).

If its initial velocity is directed straight at the proton (assumed to have finite charge density) its trajectory will be a straight line with the scattering angle being either zero (high energy case) or 180 degrees (low energy case).

 

[jdavel]

The answer is given as 'C' so well done those that got it right, but I think the question is rubbish, as with the data given I think you could still argue B or C (or A possibly..)

I persuaded myself that B was the answer (for similar reasons to Berislav), but on checking the Mark Scheme, I was then able to persuade myself that the answer must be C!

Thanks for your input folks - keep it coming if you can add to this.

Thanks

Adrian Baker,

This isn't even close! A and B tell you absolutley nothing about the internal structure of the proton. A hypothesis for or against quarks fits the data of A and B with exactly the same accuracy. That means C is the answer, and it's a slam dunk!