New Scientist | Physics & Math | 01 June 2010
David Shiga wrote:TWO teams working at the Tevatron particle smasher in Batavia, Illinois, have found hints of a new generation of fundamental particles - to add to the three generations we already know about. What's so special about these new particles?
If they really do exist, they might explain a long-standing puzzle - how the universe avoided self-destruction in its earliest moments after the big bang.
First a rundown on what we know already. Each of the three known generations of matter contains two types of fundamental particle - quarks and leptons. First generation leptons include the familiar electron and neutrino (see images, right).
The first generation of matter can explain everything we encounter in everyday life. Atomic nuclei are composed of protons and neutrons, which are in turn composed solely of "up" and "down" quarks.
The second and third generations were introduced to explain the dozens of varieties of short-lived, subatomic particles spotted in the debris of particle smashers. Each of these two generations contains a pair of quarks - much heavier than those of the first generation - as well as muons and taus, heavy versions of the electron. They also each have their own version of the neutrino.
New generations of matter have tended to show up every 30 or 40 years - the last time was in 1975, when the tau was discovered. "We've seen three generations, why not four?" says Amarjit Soni of Brookhaven National Laboratory in Upton, New York. A fourth generation would be "a very simple continuation of the trend we've seen", he says.
Now hints of this fourth generation have turned up in data from the Tevatron accelerator, which smashes together protons and antiprotons.