Wednesday, October 14, 2009

The standard model

Let me introduce the players in the standard model. These are the so-called elementary particles. These elementary particles are the smallest objects we known in nature. And small means small: We are sure they are smaller than 0.0000000000000000000001 meters. That is really tiny. However, it would not be the first time that when we look closer they are actually consisting out of something even smaller. But let me for the moment assume that this is not so.



So, what are the players then. Well, they can be divided in a number of groups.

First, there are things we call leptons. An example for a lepton is the electron. Electrons are the things that make up electrical current, so we have to deal with them every day. There are two heavier copies of the electron: The muon and the tau, about 400 and 3600 times heavier than the electron. If they would be stable, we could use them also for electrical energy, but they are not: They decay in some other elementary particles after fractions of a second.

There is a second group of leptons, the so-called neutrinos. These are the lightest particles which have a mass. We are not yet sure what their mass exactly is, just that they are at least 500000-times less heavy than an electron. Again, there are three of them, one for the electron (called electron neutrino), one for the muon (muon neutrino) and one for the tau (tau neutrino).



The second group of particles are called quarks. Quarks make up composites of quarks, known as hadrons. The most prominent hadrons are the proton, the nuclei of a hydrogen atom, and the neutron. The latter two are composites of the up and down quarks. Besides these two, which have both approximately ten times the mass of an electron, there are four more. The strange quark, about 200 times heavier than an electron, the charm quark (neat names), 3000 times as heavy as an electron, the bottom quark (9000 times), and the big guy, the top quark (350000 (!) times). Again, the heavier quarks are not stable, they decay.



Then there is light. Yes, ordinary light (and X-rays, and infrared light, and so on) is made up out of particles, the photons. They are exchanged between anything that has an electric charge, e.g., a quark and an electron.

But they are not the only thing, which can connect particles. There are gluons, which connect quarks. Neither photons, nor gluons have a mass. And gluons are a bit strange, but this will be discussed much more in detail latter. Sufficies to say, we do not see gluons as we do see photons.

Then there are the W and Z, both of approximately half the mass of a top quark. They are important for radioactive decays, and they are also a bit strange. They connect both quarks and leptons. Also, because they are so heavy, they are not very stable.

Finally, there is an elusive guy, called the Higgs. We did not yet find it - perhaps we will at the next big particle physics experiment, the Large Hadron Collider LHC at the European Center of Particle and Nuclear Physics CERN. But it is important, because it seems to be connected with all the masses which have been floating around.