## Physics Questions People Ask Fermilab

Isolating quarks

my name is ben thompson and i am student teaching at downers grove north high school. we are doing a unit in electromagnetism and somehow came upon the concept of the quark. i did some web research only to find out a tiny bit about quarks and leptons.

the question that one of my students stumped me with was whether or not the scientists at fermi lab have isolated any quarks yet. i do not know, and we have decided to allow you (the knowledgeable) to help us out. if you have the time, will you please help us in our research?

sincerely, and in dedication to science,

-ben

Dear Ben,

Thank you for your question. Quarks are very peculiar particles. They are more basic (fundamental, elementary) than protons and neutrons. Protons and neutrons and many other particles are built of quarks, held together by a VERY strong force MUCH stronger than gravity or electromagnetism) which we actually call "the strong force". In a sense we can "see" the quarks inside a proton by shooting electrons at the proton. Some of them scatter off sideways or even backwards, which is only possible to explain if there are little (like points, or almost) objects inside the proton and much smaller. If the proton was just a "soft blob" the electrons would never scatter backwards like that. So you might think that you could knock a quark out of a proton by firing electrons at it. But all the experiments we have done at Fermilab and elsewhere in the world (there is another big laboratory in Geneva, Switzerland called CERN, for example) looking for isolated quarks have never seen one. If you see the scatterd electron you can work out the direction the quark would have gone, but instead of a quark you see a narrow spray of the composite particles that contain morequarks themselves. Nearly all physicists believe that quarks can never be isolated. It's as if when you try to kick a quark out of a proton, the strong field that was holding it in gets stretched, rather like an elastic band. Eventually the elastic band breaks, and at the break a new quark (actually a quark-antiquark pair) is created out of the energy in the field. The idea that quarks can never be isolated is called confinement.

By the way we know of six types of quark, and we have good reasons to believe that that is it, there are no more types to discover. We call them up, down, strange, charmed, bottom and top. The last two were discovered at Fermilab. Each type of quark has an antiquark, and quark-antiquark pairs can be created out of energy in particle collisions.