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Q & A: Are Quarks for real?

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Most recent answer: 10/22/2007
Q:
How do scientists know the existence of Quarks in protons and neutrons? And how do they know they are fundamental particles?
- Emma
Canada
A:
Hello Emma,
That's a tough question to answer since no one has actually seen a real isolated quark  You have to rely on indirect evidence.  Fortunately there is plenty of that.  

Just as Ernest Rutherford determined that the nucleus of the atom was a small compact object by observing alpha particles back-scattering from it, today's physicists perform similar experiments by scattering high energy electrons off of protons.  The resulting angular distribution of these scattered electrons can be explained only if there are tiny constituents inside the proton (as well as neutron).     Additional evidence comes from the observed patterns of mesons and baryons that are produced at high energy accelerators.   These patterns can be neatly explained by assuming a few types of fundamental quarks that combine with each other to form the observed particles. 

As to whether the quarks are made of still smaller sub-quark things, we donít know for sure but as of now there is no evidence for that.

LeeH

(published on 10/22/2007)

Follow-Up #1: More on quark reality: How many in a proton?

Q:
Thanks for answering my previous questions. I got a few more... How do scientists find out thereís three quarks in each protons and neutrons and how do they calculate the fractional electric charge each quarks have?

Did scientists find out that thereís three quarks in each proton and neutron by somehow measuring and calculating the angles of the distribution of electron scattering off a proton/neutron (like what youíve mentioned before)?

Did they then determined the amount of positive and negative charge of the three quarks (ups and downs)by assuming protons have a charge of +1 and neutrons have a charge of zero?

How scientists find the truth and answers have always amazed me!
- Emma
Canada
A:
Things get a bit tricky now.   It is easy to make an analogy between Rutherfordís discovery of the atomic nucleus and the initial evidence of quarks, that is, large angle scattering induced by ípoint-likeí particles.   It was reasonably easy to discover the constituents of Rutherfordís nucleii, you just blow them apart with high energy collisions and íseeí the individual protons and neutrons come out.   Not so easy with the quark constituents because the quarks donít come out, other stuff made out of quark/anti-quark pairs come out.    How to do it then?   It wasnít easy and it took ten years or so of hard work by both experimental and theoretical physicists to figure out and then verify the answer. 

Iím going to waffle and say I really donít have a simple explanation except to say that itís tricky.  (Feynman, where are you?)   
There is a  Nobel  lecture by  Jerome  Friedman, one of the original discoverers,  that  explains all.   Good luck in reading it.


LeeH



(published on 10/22/2007)

Follow-up on this answer.