Hadronization

A couple of days ago I tried to describe a few cool things about quarks and gluons:

1. They cannot live as free particles, since the strong force confines them within the proton or the neutron - or other more exotic particles.
2. They possess a charge called color, and that's why they oblige to the strong force. On the other hand, all stable particles composed of quarks - hadrons, that's how we call them - do not feel the strong force, because they are colorless, neutral.

Now let's discuss what happens when a Z boson decays. Z bosons are the carriers of the Weak Interactions, together with W bosons (bosons are force carriers, in general: other examples are the gluon - strong force! - and the photon - electromagnetic force!). Z bosons are heavy things: more or less like silver atoms. They possess a large mass (91 GeV), and when they disintegrate, the mass becomes energy!

So what comes out of a Z decay ? Most of the times, a quark-antiquark pair. And since quarks are light compared to the Z boson (the top quark is the only exception, since it is actually TWICE as massive - more than a whole gold atom!), the two quarks end up happily endowed with an energy corresponding to half the mass of the Z boson, 45 GeV each: conservation of Energy! That means that they are kicked away so hard they travel practically at light speed.

The picture is as follows: now a Z boson is sitting there, and now it's there no more. At its place, a quark-antiquark pair materialized. The quark is shot one way, and the antiquark is shot exactly in the opposite direction - conservation of Momentum!

As the two bodies fly apart, they pull a string of color between them. Remember, the color force prevents a quark from parting from its peer. This string becomes more and more energetic as the quarks separate, decelerating. Once it has stored enough energy to materialize a quark-antiquark pair, it will do so, and the string will become two strings, each with a quark on one end and an antiquark on the other. A picture will clarify this a bit:

Although not completely correct, the picture above describes intuitively what happens. The sun-like symbol at the top is the Z boson disintegrating; the arrow shows the direction of time. On the following line, we see a red quark-antiquark pair kicked towards the left and the right, respectively. As they part, they pull a color string between them, which eventually breaks, materializing a second quark-antiquark pair, of different colors. These also travel away from the Z decay point in different directions, being pulled by the original red quarks. Eventually, the secondary strings also break, creating new pairs, and these may finally combine to form four colorless mesons - quark-antiquark combinations with zero net color.

It remains to note that in the case of Z decay this process typically yields not just four, but a couple dozen particles (baryons - three quarks combinations, not pictured above- and mesons), traveling in two collimated streams opposite to each other: these are what we call jets.

Comments

[Matthew]

Forgot to mention this in your post below about quarks and gluons, but your use of color in descriptions is excellent. Along with text books being more light-hearted, more color within the text would be an excellent addtion to draw attention to an interesting/important fact.

[Aaron]

Very interesting!!! The size comparisons to atoms are quite helpful and surprising. ^_^

[Alejandro Rivero]

Aaron, the problem with the size comparisons to atoms is that if taken too seriously, you will notice that the (unexplained in nuclear physics) stability enhancement areas coincide with the masses of known and predicted (115 GeV, e.g.) particles. As such thing is not possible to explain (very different # of degrees of freedom, and dissimilar mathematical structures), most people seems to prefer to forgot about such comparisons.

[Tommaso Dorigo]

Mattew: thank you for your comment! I will try to keep my posts as colored as needed...
Aaron: yes, the "size" comparisons are surprising... That is because we tend to associate size with mass, when that can be done only with a fridge, a car, or a pencil. Not with atoms, and less so with elementary particles (for which size is zero by definition and mass can be large!). That is what makes the mass comparison between a top quark (which, with very small probability, can be found even within a mere proton!) and a whole gold atom interesting.
Alejandro: your comment is kind of cryptic, although I sort of understand what you mean. Can you elaborate a bit for the public here ?
Thanks to all!

Tommaso

[Alejandro Rivero]

Tommasso, sorry to be so cryptic. For the public: I was referring to the ad-hoc corrections to spin-orbit coupling in nuclear physics. As some of you may know, different strength of the coupling must be used at different nuclear masses, in order to fit the measured energies and to get the right magic numbers. Modelling of massive nucleus is still the empirical art we learnt in the school.

Now, if you ask for plots nuclear stability models, you will notice very clearly the effect of the spin orbit correction when you trace the "drip lines", the extreme frontier of stability of a nucleus. Here you have the plot of the drip lines for four different models: http://dftuz.unizar.es/~rivero/research/uno.gif
Blue lines are the magic numbers. Red diagonal lines join, of course, nuclei having the same mass. You can notice the pass near the discontinuities caused by the empirical adjustment, specially the three rightmost ones in the neutron dripline.

And here is where you run into trouble if you have in mind the "so much GeV = so much atomic mass" analogy. These three red lines are, cof, at 246, 175 and 115 GeV.

[Tommaso Dorigo]

That helps a lot!
Thank you, I think you took out of the dark many of our sporadic readers...

Cheers,
T.

[Alejandro Rivero]

Thank you for asking, Tommasso. I do not know if we can get a moral of my brief exposition. Perhaps "analogies? Don't touch!".

Or perhaps that we will find new physics at 246. LHC will tell. Cheers.

[Andrea]

Caro collega, sperando che la cosa non ti dia fastidio ho riciclato questa tua eccellente illustrazione per un seminario divulgativo che mi e' stato chiesto di tenere:
http://cern.ch/andrea.giammanco/particelle.ppt

Se ci fossero controindicazioni al riguardo fammi sapere (via mail), provvedero' immediatamente alla sostituzione con qualcos'altro (anche se difficilmente trovero' qualcosa di altrettanto autoesplicativo!)

Un grazie anticipato.
Andrea

[Tommaso Dorigo]

Caro Andrea,

sorry if I answer only now to your comment. Sure, the material in this web page is public, everybody can use it. If it is private pictures it is a different story - but for graphs, go for it!

Cheers,
Tommaso