Suteka's Favourite Cereal

From now on I am going to write an entry each week explaining complex physical phenomenon, hopefully in a way in which anyone (even my mother…) can understand, as long as they put their mind to it. As many diagrams as necessary will be produced, however the number of formulae per entry is limited to one. For this week’s entry I have chosen “Jet Suppression in Heavy Ion Collisions” as it is something that has a rather daunting name but can be understood reasonably easily with the aid of diagrams.

First things first: “Heavy Ion Collisions” are what modern particle accelerators (RHIC or the doomsday device that is the  Large Hadron Collider at CERN, Geneva) are all about. Basically, two atoms (or to be specific: ions, which are basically atoms with some electrons stripped off) of a “heavy” element such as lead or gold are bashed against each other at very high energy (basically, speeds that are 99.99% of the speed of light, 300 000km/s).

Doing this allows us to see what’s inside the atoms. On the first level, atoms consist of a nucleus and electrons. Inside the nucleus there are protons (positively charged) and neutrons (uncharged). But why stop there? Inside a proton or neutron there are 3 particles known as quarks. These are (as far as we know) the most elementary building blocks of matter. These are shown in the diagram on the left. The quarks are held together by gluons, which can be thought of as the glue that group the quarks and give the proton or neutron its structure. Now when colliding these atoms at high enough energy, collisions take place that convert the energy of the initial protons into pairs of new particles, according to Einstein’s famous formula, $E=mc^2$ (This basically says that the energy required to create a particle of mass $m$ is simply the mass multiplied by the square of the speed of light).  If the energy is suitable, a pair of  free (or “unglued”) high energy quarks can be created. You can think of it as an explosion of matter. Now, just like in an explosion, you would expect these two quarks to head off in opposite directions. And that’s exactly what they do. However, there’s a catch: Quarks cannot exist by themselves under normal circumstances. We say they are confined to exist in either pairs or triplets, known as hadrons. Such is the strength of this demand that as soon as a deconfined quark flies off in one direction, another pair of quarks is created in order to confine the offending quark. These quarks attach themselves to the offending quarks to form allowed pairs or triplets.  This process is known as hadronization.  These hadrons then fly off in opposite directions at high speed. These guys don’t last long though, they’re too heavy to be stable and decay into many lighter particles. So in the end we have two streams, or jets of particles being emitted back to back, these can be easily seen by the various detectors.This is the normal situation that has been observed for many years.

However, when we go to more powerful particle colliders, like the Large Hadron Collider at CERN, we expect something different to happen. At extremely high energies and with enough protons around (in other words high speed and high density) the quarks can no longer maintain their hadron formation: it becomes impossible to tell where one proton begins and where another ends. Think of it as squashing a whole bag of melting jelly beans. Effectively they become a soup of high energy quarks and gluons. This soup is known as the Quark Gluon Plasma.

Now when jets are formed, as usual, they shoot off back to back, but this time they are surrounded by this soup. They cannot fly freely through the soup and are either slowed down or stopped altogether. If the jets are formed near an edge of this soupy region, it is possible that one jet will make it through the soup (as it only has a small distance to cover before escaping) while the other jet will travel the opposite direction into the soup and get stopped. This is the phenomenon of jet suppression, and is easy to see from experiments because where there would normally be two jets on opposite sides of the detectors, there is only one, and this gives a pretty clear signal of whether this quark gluon plasma forms or not.

Well, that’s all for today. If you understood it, congratulations. If not, let me know and I will try and improve it.