Outtakes from QUARKS The Stuff of Matter by Harald Fritzsh
accelerators can accelerate electrons to a velocity of 0.99999999986 of the speed of light
The results of these scattering experiments finally led Ernest Rutherford and neils Bohr to propose an atomic model in which the nucleus carries a positive electric charge and is surrounded by a cloud of negatively carged electrons. Electrons are exceedingly light elementary particles whose mass, expressed in energy units on the basis of equivalence of mass and energy (from einstein's relation E = mc2) is about 0.5 MeV.* *The abbreviation MeV stands for Megaelectronvolt, the energy acquired by an electron passing through a voltage difference of 1V.
Physicists have, of course been searching for the common denominator between electrons and protons so far without results. It appears that electrons and protons have nothing in common that would allow us to deduce the equality of their electric charge.
By far the simplest atom is the hydogen atom. Its nucleus consists of just one proton, its atomic "cloud" of just one electron.
According to the laws of electrodynamics, an electrically charged object moving in a circle is supposed to emit electromagnetic radiation. Such radiation--light , for examle--is a form of energy. A moving electron constantly emits energy, which means that the electron should come closer and closer to the proton and finally plummet into the nucleus.
This mechanism, it turns out, is provided by the theory of quantum mechanics, which states that electrons are permitted to move only in specific orbits (called stationary orbits). An electron is such an orbit emits no radiation.
if energy is transmitted to an electron moving in the ground-state orbit (by electro-dynamic radiation, for example), the electron can leap into an orbit corresponding to a higher level of energy. An electron in an excited orbit stays there only for a very short time, about 10 exp. -8 s., and then falls back to the ground state. It is at this moment that that the electron emits energy in the form of elecdtromagnetic radiation.
we need to know the electron's angular momentum, whisch is essentially the speed at which the electron revolves around the proton.
Angular momentum is a vector quantity (that is, it has direction as well as magnitude) this individual angular momentum of the electron is called its spin
Furthermore, it was found that Pauli's exclusion principle is also valid for protons and neutrons, and in general for particles with integral spin (0,1,2,3...). Examples of such particles are pi mesons, which have spin 0, and photons, the particles of light which have spin 1.
At first Faraday thought of electromagnetic fields only in association with electically charged objects. However, it was not before long before he realized that such fields do not necessarily have to originate from charged objects but can exist independently. Furthermore, he felt that light might be nothing more than an electromagnetic phenomenon.
Maxwells equations describe the propagation in space of electromagnetic quanta called photons (the constituent particles of lights).
According to quantum theory, a light beam consists of many photon quanta. Photons are particles with no rest mass; since their mass is zero, they move at the speed of light. The amount of energy carried by the photons is determined by the wavelength of the light. The shorter the wavelength, the greater the energy. The decay of the neutron is an examle of beta decay. The neutron decays into a proton by emitting an electron. Recall that the neutron is about 1.3 MeV heavier than the proton.
Pauli finally solved the problem by proposing the existence of another neutral particle, one that is emmitted withe the electron in the decay process. The Italian physicist Enrico Fermi later named the particle neutrino ("tiny neutron"). However, it turns out to be useful to think of the neutral particle emitted in the neuton decay process as an anatineutrino or, more precisely, an electron-antineutrino.
(fig.4.2 Neutron decay. The incoming neutron disintegrates by emitting an electron, a proton, and an electron-antineutrino.
Neutrinos are very light particles. Their mass is much smaller than the electron mass. In fact, it may be that they are massless, like photons. Various experiments have put the upper limits on the order of 30 eV. they can penetrate large amounts of matter with ease.
It is useful to classify all these particles into two proups. The first group includes all strongly interacting particles, of which we already know five; the proton, the neutron, and the three pi mesons. The strongly interacting particles are filed under the rubic hadrons. there are two classes of hadrons, the mesons and the baryons.
All particles that do not participate in the strong interaction but do have a spin 1/2 are called leptons. Electrons and neutrinos are leptons, for instance, but photons are not since they have spin 1. Hadrons are not elementary particles, but are made up of even smaller objects, the quarks. Let us suppose we have two particles, which we call u and d quarks (up and down). with two types of quark (so-called quark flavors), we can form four different quark-antiquark wystems, namely (-u,u) (-u,d) (-d,u) and (-d,d). therefore let us identify the mesons as (-q,q) systems.
Baryons are three quark systems,(q,q,q). delta++ = uuu, delta+ = uud, delta 0 = udd, delta- = ddd the electric charge of the u qaurk is 1/3 and the d quark is -1/3 quarks have spin 1/2 quarks do not exist as free particles, as electrons do, for example.
It turns out that introducing other constituate particles is exactly the right move. These new particles are called gluons-and appropriately so, for the gluons, which are the glue that holds the world together, are what bind quarks to form Hadrons. They have no electric charge and do not interact directly with electrons, However, they do have momentum and energy.
At high enegies quarks act almost like free particles.
Strangeness is a quantum number similar to the baryon number and can be ascribed to every particle. The strangeness number for normal particlesis zero. Leptons do not carry strangeness, and neither does the photon.
Today we know that the discovery of the strange particles of the 1950's was a premature discovery of a new quark flavor, the strange quark, denoted by the letter s, which joins the u and d quarks we can determine the electric charge of the s quark is -1/3, like that of the d quark. a new quark with charge 2/3, This would be the charmed quark or c quark hadrons are either three-quark systems (baryons) or quark-antiquark systems (mesons). the delta++ particle has a mass of 1232 MeV and consists of three u quarks, It has 3/2 angular momentum because the spin of the three u quarks all point in the same direction.
This modification ultimately led to the concept of quark color and to the modern theory of the strong interaction called quantum chromodynamics. each quark comes in three differnt colors. Thus we say there are red and green and blue u quarks. a new quark with -1/3 charge called the b (bottom) quark a new quark called the t quark the unification of the strong, weak, and electromagnetic interactions occurs only when enormous masses are involved.
The picture that physicists have in mind is something like this. Once we consider physical phenomena above 10 exp. 15 GeV, the mass scale for the grand unification of the interactions, we can no longer tell the difference between strong, weak, and electromagnetic interactions. Only one type of unified interaction is observed. Even the difference between leptons and quarks disappears--they are just manifestations of one and the same type of underlying basic fermion.