As it turns out, all of the most fundamental particles that eventually make up ordinary matter are fermions, with spin quantum number equal to 1⁄2. These include the familiar electron (e), along with two flavors of quarks that join to form protons and neutrons. These two kinds of quarks are called “up” and “down,” for no particular reason, and are abbreviated u and d. We need one additional fermion to make the first generation of matter particles complete: the electron neutrino (νe) which, as emerges during the decay of a proton into a neutron.
Quarks may be the most unusual particles we have encountered so far. Like electrons, quarks have mass, electric charge, and an intrinsic spin equal to 1⁄2 (so they must obey the Pauli exclusion principle). One difference, however, is that they also possess the color charge, the property that makes them susceptible to the strong interaction.
QUANTUM LEAP
Particle accelerators are enormous contraptions that allow physicists to study fundamental particles. They apply magnetic and electric fields to accelerate beams of particles (e.g., protons) to great speeds before smashing them together. When the particles collide at high speeds (energies), they often liberate lots and lots of other particles. This method of “high energy physics” has been likened to learning about the inner workings of a grandfather clock by throwing it against a wall and seeing what emerges after impact!
Another odd thing about quarks is that the amount of electric charge they each possess is less than the charge on an electron. The so-called up quark has a charge equal to +2⁄3 of an electron charge, and the down quark has –1⁄3. Doesn’t this violate what we said earlier about the quantization of electric charge? The answer is that, apparently, quarks can never be completely isolated. Quarks always seem to show up in small groups, or combinations. When combined, the fractional charges always sum up to integer multiples of the electron’s charge. This means that we never actually observe a fractional charge in isolation, and apparently this is enough to keep Mother Nature satisfied.
The rule that determines which combinations are allowed is called “color neutrality,” which revolves around the color charge of quarks. A quark combination will be stable only if the net color charge of the participants is “neutral.” It turns out that there are two possible ways that quark colors can neutralize. The first is to bring together one of each of the three complementary colors—one green plus one red plus one blue. Since the colors neutralize, this leads to a bound state of three quarks. This is, in fact, what protons and neutrons are. The second way is to combine a quark of one color with a quark of its anticolor (e.g., blue and antiblue). In this case, one of the quarks will actually be an antiquark.
The name given to the particles that feel the strong force is hadrons. It was the effort to classify a wide variety of hadrons (including protons and neutrons) that led to the original quark model. Now we know that all hadrons are composed of quarks in all the various combinations, and each of these combinations is color neutral.
Every proton is composed of two up quarks and one down quark. Given the electric charges of u and d quarks, you can easily do the math and find that the net charge of this bound system is +1 units. Every neutron, on the other hand, is made of two down quarks and one up. This adds up to an electric charge of exactly zero, just as observed. So two types of quarks (up and down) plus one electron are all we need to build ordinary stable atoms.
Moreover, in order to account for the instability of bare neutrons, and certain radioactive nuclei, we need to add the elusive neutrino. In addition, Dirac showed us that all of these particles also have their antimatter partners, thus bringing our number of “first generation” particles to eight.
One way to organize these eight particles is to put them into two different groups, depending on whether or not the particles are sensitive to the strong force. The quarks, which feel the strong force, form a category separate from the electron and the neutrino, which do not. The quark group is simply called the quarks. The electron and neutrino category is called the leptons (derived from the Greek term meaning “small” or “fine”).
DEFINITION
Quarks are the group of fundamental fermions that feel the strong nuclear force.
Quarks can combine to form composite (not fundamental) particles called hadrons, which also feel the strong force.
Leptons are the group of fundamental fermions that do not feel the strong nuclear force. Electrons and neutrinos are in this group.
So to begin our catalogue, the category of leptons contains the electron, the positron (or antielectron), the electron neutrino, and the electron antineutrino. The quark category contains up and down quarks plus the two corresponding antiquarks.
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