Colored partons

Ok, this post is not going to get renamed. I want to tell you about colored partons today. Fasten your seat belts, so you don't run away before I say you may!

Partons (quarks and gluons) are the constituents of hadronic matter - the protons and neutrons we all know and love. We could live happily ignoring them, but once you are in the business of smashing protons, well, that's what you get: quarks and gluons.

Quarks and gluons have exceedingly interesting properties. First of all, they are impossible to isolate. Suppose you pick a quark inside a proton and try to pull it out of it. If you try doing so, you extend a string of gluons that connects the one you picked to the rest of the proton. Strange, because the quark appeared to come with no strings attached!

The more you pull, the more you sweat. Energy gets stored in the string of gluons, until POP! The string breaks, but what you are left with in your hand is not just the quark you wanted to pull out. The broken string's energy has materialized a quark-antiquark pair out of the blue. Remember E=mc^2 ? Well, that's exactly true in this case: the energy of the string has converted to the mass of the two brand new quarks! (Quarks are massive, while gluons have no mass).

A quark has disappeared inside the proton, which appears unbothered by the whole thing, and the antiquark has stuck to the quark you wanted alone, creating a meson (a fancy particle, which has a very short lifetime, made up by a quark-antiquark pair orbiting one around the other). Isn't that weird ??

The other property I like a lot about quarks is that they are colored. Yes, they come in three colors: red, blue, and yellow (or pick another combination if you like). No, that's not your everyday life color. But still something which has color-like properties. See, you can make stable particles with quarks, only if you combine them in combinations with no net color, because stable particles have no color. If they had, they would break apart instantaneously, due to the strong force -QCD- whose charge is the color itself: it would be like putting together two electrons and pretend that they stick together.

How do you make uncolored objects with the three basic colors, red, blue and yellow ? Well, you know this since the school days when you were shown a colored disk getting white as it spinned! One part of each will do the trick. Likewise, three quarks of different colors will make up a uncolored baryon - a neutron or a proton, that is. But with quarks, you get a second chance! You can create a stable meson by putting together a quark (say a red one) and an antiquark (anti-red!). The two colors annihilate like the Cointreau and the Martini dry in a glass of Gordon's Gin, and you get a colorless cocktail - pardon particle.

So, what does color do ? Color is the charge of the quarks. By being colored, they stick together so strongly that protons and neutrons lose a little bit of their individuality when you pack them together in the nucleus of an atom. Ever asked yourself why your necklace does not vaporize in a cloud of Hydrogen ? Yeah: gold atoms have nuclei which contain 79 protons. That's an electric charge of +79 in a tight space! No way you could keep them together without the force of the color field between the quarks of protons and neutrons. The strong force is stronger than the electromagnetic force, and by far so!

I think that's enough for today. Tomorrow I will tell you what is color radiation (yes, just like you can do an antenna with electrons running up and down, so quarks and gluons broadcast radiation!), and just why it is necessary to understand it before you try to isolate a Z->bb signal in hadron collider data!

Comments

[Helge]

Just so you know. I love reading your entries, and I think that your explanations are easy to follow.

[Tommaso Dorigo]

Thanks Helge, I appreciate your feedback! Writing a
blog is a bit like shooting in the dark, and guidance is most welcome.
Cheers,
T.

[Tony Sumaryada]

your explanation is interesting.
I wrote a small notes on my blog about QCD life
feel free to see at :
http://www.schumaryadha.blogspot.com
I 've picked some words from John Ellis and added my own words

Nice to know you
Tony Sumaryada

[Tommaso Dorigo]

Thank you Tony. I saw your blog.
Are you into poetry ? You seem to know
about QCD. Are you a particle physicist too ?
Cheers,
T.

[Matthew]

Great post, you do very well with explaining things in an easily digestable manner.

Why can't my textbooks be like this?

[Tommaso Dorigo]

Thanks Mattew.
I try to do my best here... My problem is that without enough feedback I am not sure what would be the appropriate level of detail when I discuss scientific matters...
As for textbooks: I think there is a curse here. If a textbook tries to be FUN, then it's automatically derated and goes to the dust bin. Ever read "The name of the rose" by Umberto Eco ? In it, a book by Aristotheles about humor is poisoned by an integralist monk who runs the library, in order to kill anybody who reads it (they used to lip their fingertips when turning the hefty pages - the setting is the Dark Ages, when only a few copies of each book were available). The reason of poisoning the book is the fear that people would learn to have FUN of sacred things, thus degrading them from their divine status.

I think you're right. Textbooks should entertain!

[Matthew]

Well, I tend to enjoy a decent bit of detail, but presented thoughtfully. I am also a Physics student, so my opinion might differ from that of a English major.

Haven't read the book, but it does sound pretty interesting. I think the overall academic environment could be greatly improved by a little levity. A serious text book is fine, but a break from it is always welcome. Not to mention humor is a great stress reliever.

Of course, some people like to be stuffy and serious ;)

[Harish]

that was definitely very interesting! as usual, i have a doubt! if you actually add more energy in the gluon string, will it reach a saturation point after which it will emit the quark. this is proof that even idiots read your work and try to be the next einstein! thanks.