The Present State of Subatomic Particle Theory

Since the 1950s physicists have discovered that protons and neutrons consist of quarks with spin 1/2 and that antiprotons and antineutrons consist of antiquarks. Neutrinos, too, have spin 1/2 and corresponding antineutrinos. Indeed, it is an antineutrino, rather than a neutrino, 1__________. This reflects an empirical law regarding the production and decay of quarks and leptons: in any interaction the total numbers of quarks and leptons seem always to remain constant. Thus, the appearance of a lepton—the electron—in the decay of a neutron must be balanced by the simultaneous appearance of an antilepton, in this case the antineutrino.

In addition to such familiar particles as the nucleons and the electron, studies have slowly revealed the existence of more than 200 other subatomic particles. These “extra” particles do not appear in the low-energy environment of everyday human experience; they emerge only at the higher energies found in cosmic rays or particle accelerators. Moreover, they soon decay to the more familiar particles after brief lifetimes of only fractions of a second. The variety and behaviour of these extra particles initially bewildered scientists but have since come to be understood in terms of the quarks and leptons. In fact, only six quarks, six leptons and their corresponding antiparticles are necessary to explain the variety and behaviour of all the subatomic particles, including those, that form normal atomic matter.

These quarks and leptons are the building blocks of matter, but they require some sort of mortar to bind themselves together into more complex forms, whether on a nuclear or a universal scale. The particles 2 __________.

On the largest scales, the dominant force is gravity, 3 __________ to form stars and galaxies and which influences the way that the universe is evolving from its initial big bang. The best-understood force, however, is the electromagnetic force, which underlies the related phenomena of electricity and magnetism. The electromagnetic force binds negatively charged electrons to positively charged atomic nuclei and gives rise to the bonding between atoms to form matter in bulk.

Gravity and electromagnetism are well known at the macroscopic level. The other two forces act only on subatomic scales, indeed on subnuclear scales. The strong nuclear force binds quarks together within protons, neutrons, and other subatomic particles; and, rather as the electromagnetic force is ultimately responsible for holding bulk matter together, so the strong force keeps protons and neutrons together within atomic nuclei. The fourth force is the weak nuclear force. Unlike the strong force, 4 __________, the weak force acts on both quarks and leptons. This force is responsible for the beta decay of a neutron into a proton and for the nuclear reactions that fuel the Sun and other stars.

Since the 1930s physicists have recognized that they can use field theory for all four basic forces. In mathematical terms, a field describes something that varies continuously through space and time. A familiar example is the field that surrounds a piece of magnetized iron. The magnetic field maps the way that the force varies in strength and direction around the magnet. The appropriate fields for the four basic forces appear to have an important property in common: they all exhibit what is known as gauge symmetry. Put simply, this means that certain changes can be made that do not affect the basic structure of the field. It also implies that the relevant physical laws are the same in different regions of space and time.

At a subatomic, quantum level, these field theories display a significant feature. They describe the action of a force in terms of subatomic particles, called gauge bosons, which in a sense carry the force. These particles differ from the building blocks—the quarks and leptons—by having integer values of the spin quantum number, rather than a value of 1/2. The most familiar gauge boson is the photon, 5 __________, such as the electrons and protons within the atom. The photon acts as a private, invisible messenger between these particles, influencing their behaviour with the information it conveys, rather as a ball influences the actions of children playing catch. Other gauge bosons, with varying properties, are involved with the other basic forces.

In developing a gauge theory for the weak nuclear force in the 1960s, physicists discovered that the best theory, which would always yield sensible answers, must also incorporate the electromagnetic force. The result was what is now called electroweak theory. It was the first workable example of a unified theory linking forces that manifest themselves differently in the everyday world. The unified theory reveals the outwardly diverse forces as separate facets of a single underlying force. The search for a unified theory of everything, 6 __________, is one of the major goals of particle physics. It is leading theorists to an exciting area of study that involves not only subatomic particle physics but also cosmology and astrophysics.

1.3. Find the single words in the text above which mean the following:

1.essential, important

2.special, particular

3. individual , independent

4. short , concise

5. chief , main

6. invariable, permanent, continual

7. composite, combined, complicated

8. fundamental, primary

9. primary, incipient

10.known , acquainted , intimate

11. judicious , reasonable , sane