Tuesday, June 18, 2013

4 quarks?

The description over at SciTech Daily of the new particle is pretty good. Go read it. The upshot is that it looks like there's a particle that is made of 4 quarks instead of 2 or 3. It isn't stable, of course, and its most striking decay mode (the one they detected) is to a charged pion and a J/psi particle. To reconstruct the mother particle mass you need the momenta of the daughters, and their identities. A J/psi decaying to two muons or two electrons is pretty easy to identify. The pion--well, it might be a kaon or a proton. So if they do the same as other groups do they tried all the different possibilities, and it turns out (in this case) that the pion hypothesis gives a clean mass peak and the others are washed out. The mass peak shown in the article is pretty tight, so there's not much wiggle room for a third undetected daughter particle.

In fact since the events occur at a known energy (they used a tuned e+e- collider) they can also look at the "Dalitz plot" which shows the mass-squared combinations of two pions and a J/psi. That's a handy way to look for structure, and they find it.

So what would 4 quarks (actually 2 quarks and 2 anti-quarks) look like? That's hard to say for sure. From the speed with which the pion and J/psi separate we know it isn't from a loosely bound system with the pion sort of orbiting the c/c-bar pair (charm and anti-charm).

As the article above says, low energy is harder to understand than high energy. That might seem odd, but imagine wooden blocks covered in velcro. At low speeds they may stick together in ways that are hard to describe, but at high speeds they bounce off each other, and at very high speeds they shatter in ways that are easily described.

The thing is too far off the mass shell to include a b-quark, since the lightest known b-meson is much heavier than the available energy. I have to run a few numbers, but this looks interesting. People have been looking for 4-quark states for quite a while.

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