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Tuesday, July 1, 2008

Welcome to the particle menagerie

Christening a particle is not easy. Do you name it after the person who proposed its existence, or the person who discovered it? Or do you give it a label that is abstract, poetic, whimsical, onomatopoeic, or just plain descriptive?

Democritus proposed the existence of a particle, so he could have named it the democriton, but instead this modest Greek philosopher decided to coin the word a-tomos, meaning 'not cuttable', which explains the origin of the word atom. Perversely, today we use the word atom to describe something that is 'cuttable', because we know that even the smallest atom, hydrogen, has components that can be pulled part. So we could rename atoms 'aatoms', which is to say 'not not cuttable'.

Inside the atom we find the electron, which also traces its name back to Ancient Greece. Elektron is Greek for amber, and the ancients knew that rubbing amber with a dry cloth would enable it to attract very light objects. We now know that this is because rubbing amber can generate a charge, otherwise known as static electricity, so 19th century scientists used the term electron to describe the first particle that was proven to carry a charge.

The rest of the atom is made of neutrons and protons, and in turn these are made of quarks. The story of quarks dates back to the 1960s when physicists discovered a menagerie of new subatomic particles. It was Murrary Gell-Mann who proposed that all these particles (and protons and neutrons) were made of just three types of quark. The name was based on a line from James Joyce's Finnegan's Wake: "Three quarks for Muster Mark!". In this context, quark is probably a corruption of quart (as in quarts of beer), which means it should not be pronounced to rhyme with Mark.

Gell-Mann had quite a flair for naming concepts in physics. The existence of three quarks led to composites of quarks being classified into groups of eight, which Gell-Mann dubbed the Eightfold Way. This was a reference to a Buddhist proverb about the path to nirvana: "Now this, O monks, is noble truth that leads to the cessation of pain; this is the noble Eightfold Way."

Gell-Mann's three quarks were named up, down and strange. The up and down quarks formed a natural pair, but the strange quark was the odd one out, hence the name. In 1974 its partner was discovered and to celebrate its welcome arrival it was dubbed the charm quark

Two more quarks were discovered, and were initially called truth and beauty. They were the focus of my thesis when I worked at Cern in the late 1980s, but sadly I could not boast that I was researching the physics of truth and beauty, because by this time they had been renamed more prosaically as top and bottom quarks

It is unlikely any more quark types will be discovered at Cern when the LHC fires up this summer, but they will be studied in closer detail than ever before. In particular, physicists will scrutinise the particles that bind quarks together, predictably known as gluons, because they act like a glue.

Sometimes the order of discovery is a factor in the naming of particles. In the 1960s and 70s, many physicists were trying to predict the particles that might carry the weak nuclear force, which is responsible for radioactivity. When they formulated a theory, they sensibly named one type of weak-force carrier the W particle. The other type was given the name Z, partly because physicists believed there wouldn't be any more particles left to discover.

Of course, the LHC will also be hunting for new particles. One of the theories being tested is supersymmetry, the idea that every known particle has a partner awaiting discovery in a high-energy collision. When the idea was proposed, the sudden doubling of the number of fundamental particles could have been a headache for the physicists who named things. Their solution was to add an s onto particle names to get the supersymmetric "sparticles". So the partners of the quark and electron became squarks and selectrons. The convention has some unfortunate consequences: the family of particles known as leptons have supersymmetric partners called, well, sleptons

Supersymmetric particles could be discovered at Cern in the coming years but other hypothetical particles are much less likely, such as the axion, which was posited in 1977 to solve problems in the way that quarks and gluons interact. The theorists who came up with it named their proposed particle after an American brand of laundry detergent, because it was supposed to clean up a rather messy problem in fundamental physics.

There is no sign of axions yet, but if they exist they could explain the vast quantity of missing matter in the universe. There are so many candidates for this so-called dark matter that scientists have coined catch-all acronyms. One umbrella term suggests the missing matter is made of Weakly Interacting Massive Particles (WIMPs). Alternatively, the mysterious dark particles may have aggregated into large collections known as MAssive Compact Halo Objects (MACHOs

If all this makes it sound as if physicists make things up as they go along, wait until you hear my favourite particle moniker. This acronym encompasses all the dark matter candidates and truly reflects our level of understanding of this particular subject - Dark Unknown Nonreflective Nondetectable Objects, or DUNNOs, a term which should only be spoken by physicists while shrugging their shoulders. Well, at least they're honest.

· Simon Singh is the author of Big Bang and will be presenting "5 Particles", part of BBC Radio 4's special coverage of the LHC switch-on later this summer

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