The Fundamental Particles
From SydneyHEPWiki
Contents |
The end of the line?
So when you keep pulling matter apart - pulling apart molecules, and then atoms, and then nuclei, and then even the "particles" themselves if you can, what do you find? Fifty years ago, the answer would have gone something like this:
- There are two kinds of particles:
- hadrons which feel the strong nuclear force, the force that holds protons and neutrons together in the atomic nucleus; and
- leptons, which do not feel that force, the electron being the best-known example.
- There are only a few leptons:
- the electron,
- the heavier "electron-like" particle the muon, and
- the electrically neutral, massless or very-low-mass neutrino.
- There are hundreds of types of hadron: as well as the well-known proton and neutron, and the pion (known since the late 40's), there are many particles with distinguishable properties but lifetimes of only a tiny fraction of a second. There are some patterns or "family resemblances" among them, but no obvious system (like, say, 19th century chemistry's Periodic Table of the elements) to connect them all.
- Throughout the 1970's a much simpler picture emerged, simple enough to fit onto a small sheet of paper (or, for the "real" physicist, to scribble on a serviette in a restaurant). It may not be the ultimate story, "the end of the line", but we may be getting close:
Quarks: making sense of the hadrons
In 1964 Gell-Mann and Zweig put forward a model in which the hadrons were made up of quarks, strange objects of fractional electric charge:
- an up quark (u) with charge +2/3,
- a down quark (d) with charge -1/3, and
- a strange quark (s) with charge -1/3.
| Baryons
The proton is made up of two up quarks and one down quark in this scheme, hence the charge +1 = 2/3 + 2/3 + (-1/3); the neutron is made of one up quark and two down quarks, hence the charge 0 = 2/3 + (-1/3) + (-1/3). Particles like the proton and neutron (called baryons) are made up of three quarks; another example is the sigma-plus, made up of two up quarks and one strange quark, a kind of "strange" counterpart to the proton. The "strange" particles like the sigma were so-called because they have a special set of properties not shared by other particles: they are typically produced in pairs, and while many will decay quickly into other strange particles, they eventually decay into non-strange particles with a distinctive life-time, between 1/10 of a nanosecond and 100 nanoseconds. (A nanosecond is a billionth of a second, a very long time by the standards of most particles. Recall that an object moving at close to the speed of light will travel over 30 cm in a nanosecond.) The model explained this by proposing that the strange particles carry a (slightly unstable) strange quark, while the non-strange particles do not. |
| Mesons
The positive pion is made up of one up quark and one down anti-quark - the anti-particle of the down quark. Thus the charge of +1 = 2/3 (for the up quark) + 1/3 (for the down anti-quark). The other light hadrons like the pion, called the mesons, are all made up from a quark and an anti-quark in this way. So the "strange" meson called the negative kaon, or K-minus, contains a strange quark and an up anti-quark, and hence a charge of -1 = (-1/3) + (-2/3). |
(Anti-particles are like your reflection in a mirror: properties which go in only one "direction", such as your height, or the particle's mass, are the same for the reflection; properties which have a "sign", such as your right- or left-handedness, or the particle's charge, are reversed. Thus the up anti-quark has the same mass as the up quark, but the opposite charge: the up quark has a charge of +2/3, the anti-quark a charge of -2/3.)
Patterns and predictions
One of the successes of this model was the prediction of the omega-minus baryon and its properties: this "all-strange" baryon was clearly "missing" when a group of related baryons were laid out in a pattern, and its mass could be deduced from the masses of the other baryons within the pattern. The particle was found in due course, with the expected mass, proving, if not that quarks existed, then at least that the patterns of mesons and baryons which the quark model "explained" were real enough to be used to predict particles as-yet undiscovered. Twenty of the currently-known baryons laid out in one of the patterns, called multiplets , which reflect their quark content and share various properties such as spin and a certain range of masses. In this case, the multiplet forms a tetrahedron or triangular pyramid.
| When the quark model was put forward, only 9 of these particles were known - the particles in the "bottom triangle" containing only up, down, and strange quarks - with the omega-minus as a "missing corner" of the triangle. |
The key discovery that showed that quarks themselves were real, and not "just" a convenient way to make sense of all the different hadrons, was made in the early 1970's. If a proton is struck by a high energy electron (or another proton, etc), a particle produced in the collision will sometimes "fly off" at a sharp angle to the incoming particle. If a proton behaved like a "point", or even like a solid "billiard ball", this behaviour would be expected, but there were far too many particles produced at large angles, as if there were some small, hard centre or centres within the proton which were scattering particles abruptly, provided the incoming particle energy was high enough to resolve them. (For the need to use high energy particles to probe the structure of matter, see the previous page.)
Detailed experiments and a check of "the numbers" are all consistent with the hard "centres" being the quarks. (It was especially appealing that quarks should be discovered using this technique: the nucleus of the atom was discovered in the same way, sixty years earlier.) While there are hundreds of hadrons, there seem to be only a handful of fundamental particles feeling the strong nuclear force - the quarks.
Leptons: the next generation ...
At about the same time, it became clear that there was an interesting "family" or "generation" structure to the leptons. As early as 1962, it had been shown that there were two different neutrinos, one closely related to the electron, and one closely related to the muon, an electron-like particle or lepton, 207 times more massive than the electron. (The experiment at Brookhaven National Laboratory (NY, USA) which showed these neutrinos to be different particles was the first experiment using high-energy neutrinos deliberately produced at a particle accelerator - and thus the great-great-grandfather of NOMAD.) In 1975 a third lepton was found, the tau, even more massive than the muon - 17 times more massive, making it three-and-a-half thousand times the mass of the electron, and almost twice the mass of the proton!
| Apart from this dramatic difference, the tau was soon (by 1977) shown to have properties similar to the electron and the muon, including a related, light neutral particle - presumably a neutrino. The pattern became clear: there were three families or generations of leptons, each containing an electron-like particle and its corresponding neutrino.
The tau-neutrino was the last predicted neutrino to be observed in 2000. |
... and a few more quarks as well
In 1974, the year before the tau was discovered, it was shown that there is a fourth type of quark - it was given the name charm. Like the up quark it carries a charge of +2/3.
| In other words, the quarks have a "family" or "generation" structure as well. The strange quark is the second-generation quark corresponding to the down, while the new charm quark is the second-generation quark corresponding to the up.
Once the third generation of leptons was discovered it seemed natural to ask if there were also a third generation of quarks. In fact, there were already good reasons to suppose that this were true. The subtle failure of symmetry called CP violation - the surprising fact that the universe is "left-handed" - was much easier to explain if there were three or more generations of quarks, rather than just two. Soon afterwards, the "down-type" quark in the third generation, called beauty, was discovered. Its counterpart, the third generation up-type quark called top, eluded discovery until 1995: this preposterous fundamental particle has 95% of the mass of a gold atom. (The charm quark mass is a bit less than that of a deuterium atom, while the beauty quark is between helium and lithium in mass - one-fortieth of the mass of the top.) |
Three and only three?
It is not yet known why there should be successive "families" or "generations" of fundamental particles in this way. The fact that both the leptons and the quarks are laid out in a repeating structure, suggests to some that these particles may themselves be composite, made up of yet smaller entities the way that nuclei are made up of protons and neutrons, while protons and neutrons are made up of quarks.
To date there is no evidence for this. A 1996 suggestion of quark structure by the CDF collaboration, discoverers of the top quark, has more recently appeared to be something more prosaic.
There do seem to be only three generations, however. One of the first results from the LEP electron-positron collider at CERN was to "count" the number of different types of neutrino, by looking at the way a certain particle decayed, and assuming that we understood the process properly.
| The current value using this technique is 2.99 with an uncertainty of about 0.02. In other words, we seem to understand the physics well enough (if the answer had been 2.35, we would have had to go back to the drawing board ...), and there seem to be only three types of neutrino.
In this figure, the experimental results are shown as black dots, and the expected behaviour for three types of neutrinos is shown in blue. If there were two types of neutrino, we would expect the curve shown in red; if there were four, the curve shown in green. |
(This technique is not sensitive to, say, a fourth neutrino with the mass of a titanium atom. But since the other three neutrino masses are tiny, and have never yet been measured - we have no direct evidence that they have a mass at all - this doesn't bother people too much.)
Neutrinos?
The neutrinos play a key role in the modern understanding of the fundamental particles, but there is no quick answer to where they "fit" in the wider world. Most things are made of atoms, for example, and atoms are made up of protons, neutrons and electrons - and protons and neutrons are made up of up and down quarks. That's three out of the four fundamental particles of the first family - where do the neutrinos fit in, then?
To find out why neutrinos are so elusive, and to learn something of the four fundamental forces or interactions that bind matter together, read the following page ...
NEXT: Why neutrinos "go through anything and everything"
About this document ...
This material was prepared for the Web by Bruce Yabsley in 1997. The baryon multiplet figure is taken from The Particle Data Book, R.M. Barnett et al., Physical Review D54, 1 (1996) and 1997 off-year partial update for the 1998 edition available on PDG WWW pages.
The three-neutrino result from Z-decays is taken from the Web-pages of the ALEPH collaboration at CERN.
