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Questions About Physics

Proton lifetime, annihilation and shell electrons

You Wrote:
Hi, Has proton decay been successfully accomplished?


So far to our knowledge, proton is an extremely stable particle. Various experiments conducted all around the world show, that its lifetime is bigger then 10^25 years ( Just to imagine, it is about 10^14 times longer then the age of the universe!!). This high stability of protons is used in many cases to rule out new particle theories which could otherwise work, but they predict a significantly shorter proton lifetime. (For example the SU(5) unification of all 4 forces. It is a nice theory, but it predicts fast proton decays.)

What would happen is an anti-quark (only one) were shot at a proton? Or is it even possible to isolate a single quark, much less an anti-quark? If it was possible, would an anti-quark only annihilate its positive partner (ie- if a top quark and a strange anti quark were to meet, would anything happen?

This is a very good question: I will start with the isolated quark. It is not possible to take an isolated anti-quark and shoot it at anything. The problem arises not because it is an ANTI-QUARK, but because it is in PRINCIPLE impossible. Quarks are very interesting particles, they tend to stick together. It is possible to create objects that contain 2 quarks ( they are called mesons) , but not less. People usually explain this fact on an analogy with a spring. The ends of the spring play the role of the quarks and the spring itself represents the force between the quarks. It is very tough to pull apart a system of quarks, just as it is to stretch a spring. More you stretch it, harder it is, until finally the spring breaks into two pieces each having two ends. Altogether you end up with 4 spring ends, therefore 4 quarks, which are again IN PAIRS!!

Let us talk about the interaction of matter with antimatter as you asked. The most famous process, when matter meets antimatter is the annihilation. Annihilation is a special process when a PARTICLE and ITS ANTI-PARTICLE meet and as a result of this collision a bunch of photons, called gamma rays are produced. So, in other words, to annihilate something, you need the anti-something :-)

However, it does not mean, that a certain anti-particle, cannot interact with other particles or anti-particles. Yes it can. And here comes Fermilab into the business, we study, try to understand and describe these interaction. So far what we have is an extremely successful model of elementary particles, called Standard Model. It describes what happens , when a top quark meets an anti-strange quark, what particles are produced and also gives as a description of all the other elementary particle reactions. But there is no simple answer to your question, what happens if a top meets an anti-strange, because the answer depends on many details, such as what the energy of their collision was, what the originating particles were ( remember, quarks cannot live alone, they must be in pairs), ... .

Is the distance from the first electron shell and the center of the nucleus of an atom always the same?

As a rule of thumb the following could be taken . More protons in the nucleus mean more charge in the nucleus, therefore the electrons in the shells will be held stronger ( closer) to the nucleus. As a more quantitative approach the Bohr's model of atoms could be taken, according to which, the distance R_n of an electron from the nth shell of an atom with Z protons in its nucleus is given by the following formula

R_n=5*10^-7*n^2/Z centimeters.

From that you see that bigger the atom, closer the electron :-) The final answer to this question comes from deeper quantum mechanical thoughts, but I think the above is enough to demonstrate the main idea.

If so, what effect does this have on an electron in the first shell of a Helium atom versus the effect on an electron in the first shell of a lead atom?

From the above formula you can see that R_He/R_Pb=4/84.

If the distance changes relative to the size of the nucleus, does the distance shrink when an atom decays?

I do not quite get this question. If an atom decays, it usually radiates out certain kind of particles, electrons, photons or alpha particles ( which are bunch of protons roughly). Usually it changes its Z, but not always. If it is a process when it changes its Z then you can find the answer from the above formula.

Hope this answer helps you, and if you have further questions, please feel free to contact us.

- bye,
Arnold Pompos

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last modified 1/19/1999   physicsquestions@fnal.gov