By STEVEN WEINBERG
The July 4 announcement that the “Higgs boson” had been discovered at the CERN laboratory in Geneva made news around the world. Why all the fuss? New discoveries of elementary particles have been made from time to time without attracting all this attention. It is often said that this particle provides the crucial clue to how all the other elementary particles get their masses. True enough, but this takes some explanation.
We have a well-tested theory of elementary particles and the forces that they exert on each other, known as the Standard Model. A central feature of the Standard Model is a symmetry between two of these forces: the electromagnetic force, and the less familiar weak nuclear force, which provides the first step in the chain of reactions that gives the sun its energy.
The symmetry means that the particles carrying these forces enter into the equations of the theory in essentially the same way. You could interchange the photon, the particle of light that carries the electromagnetic force, with some combination of the W and Z particles that carry the weak nuclear force, and the equations would be unchanged.
If nothing intervened to break this symmetry, the W and Z, like the photon, would have no mass. In fact, all other elementary particles would also be massless. But of course, most elementary particles are not massless. For instance, unlike the massless photon, the W and Z particles have nearly 100 times the mass of a hydrogen atom.
Since the early 1960s it has been known that it is possible for symmetries to be exact properties of the equations of a theory and yet not respected by observable physical quantities, like the values of particle masses. The consequences of such symmetry breaking were worked out in 1964 by Robert Brout and François Englert; by Peter Higgs; and by Gerald Guralnik, Carl Hagen and Tom Kibble, for a general class of theories that contain force-carrying particles, like the photon.
In 1967-8 the late Abdus Salam and I independently used this mathematics in formulating a specific theory, the modern unified theory of weak and electromagnetic forces that became part of the Standard Model. This theory predicted the masses of the W and Z particles, which were verified when these particles were discovered at CERN in 1983-84.
But just what is it that breaks the electroweak symmetry and thereby gives elementary particles their masses?
Salam and I assumed that the culprit is what are called scalar fields, which pervade all space. This is like what happens in a magnet: Even though the equations describing iron atoms don’t distinguish one direction in space from another, any magnetic field produced by the atoms will point in just one way. The symmetry-breaking fields in the Standard Model do not mark out directions in space — instead, they distinguish the weak from the electromagnetic forces, and give elementary particles their masses. Just as a magnetic field appears in iron when it cools and solidifies, these scalar fields appeared as the early universe expanded and cooled.
This is where the Higgs boson comes in. The illustrative models studied in most of the papers on symmetry breaking from 1960 to 1964 had introduced scalar fields to break the symmetries, and had typically found that some of these fields would show up as massive particles, bundles of the energy of the fields. Likewise, Salam and I in 1967-68 found that one of the four scalar fields we introduced to break the electroweak symmetry would appear as a new kind of electrically neutral unstable particle. This is the Higgs boson, which may now have been discovered, verifying the Standard Model’s account of how the elementary particles get their masses.
There seems no doubt that a new electrically neutral, unstable particle had been discovered, but is it the Higgs boson? All of the properties of the Higgs boson except its mass were predicted in the 1967-8 electroweak theory, and since the mass of the new particle has been measured, we can now calculate the probabilities for the various ways that it can decay. So far, only a few decay modes have been observed, and though the new particle seems to decay like a Higgs boson, more must be done to pin this down. Also, it the new particle is the Higgs boson, it would have to be like a knuckleball in baseball; unlike all other known elementary particles, it would have no spin. This too must be tested.
These are the cautious words you would expect to hear from a prudent physicist. But I have been waiting for the discovery of the Higgs boson since 1967, and it’s hard for me now to doubt that it has been found.
So what? Even if the particle is the Higgs boson, it is not going to be used to cure diseases or improve technology. This discovery simply fills a gap in our understanding of the laws of nature that govern all matter, and throws light on what was going on in the early universe. It’s wonderful that many people do care about this sort of science, and regard it as a credit to our civilization.
Of course not everyone feels this way, and even those who do have to ask whether learning the laws of nature is worth the billions of dollars it costs to build particle accelerators. This question is going to come up again, since our present Standard Model is certainly not the end of the story. It leaves out gravitation; it does not explain the particular values of the masses of quarks and electrons and other particles; and none of its particles can account for the “dark matter” that astronomers tell us makes up five-sixths of the mass of the universe. You can count on physicists to ask their governments for the facilities they need to grapple with these problems.
A case can be made for this sort of spending, even to those who don’t care about learning the laws of nature. Exploring the outer frontier of our knowledge of nature is in one respect like war: It pushes modern technology to its limits, often yielding new technology of great practical importance.
For instance, the new particle was produced at CERN in collisions of protons that occur at a rate of over a hundred million collisions per second. To analyze the flood of data produced by all these collisions requires real time computing of unmatched power. Also, before the protons collide, they are accelerated to an energy over 3,000 times larger than the energy contained in their own masses while they go many times around a 27-kilometer circular tunnel. To keep them in their tracks requires enormously strong superconducting magnets, cooled by the world’s largest source of liquid helium. In previous work at CERN, elementary particle physicists developed a method of sharing data that has become the World Wide Web.
On a longer time scale, the advance of technology will reflect the coherent picture of nature we are now assembling. At the end of the 19th century physicists in England were exploring the properties of electric currents passing through a near vacuum. Although this was pure science, it led to our knowledge of the electron, without which a large part of today’s technology would be impossible. If these physicists had limited themselves to work of obvious practical importance, they would have been studying the behavior of steam boilers.
Steven Weinberg is a professor in the physics and astronomy departments at the University of Texas at Austin, and the recipient of the Nobel Prize in Physics for his work on the unified theory of weak and electromagnetic forces.
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