The Higgs Boson It could give mathematical consistency to the standard model-the theory that describes the intera ctions of fundam en tal particles. The search for the elusive particle will require new accelerators by MartinusJ. G. Veltman he truly fundamental problems dard model, making it applicable to generally make themselves felt by Tof physics can always be ex­ energy ranges beyond the capabilities means of the exchange of a mediating plained in simple terms without of the current generation of parti­ particle; the particle that mediates the the help of complicated equations or cle accelerators but that may soon be electromagnetic field, for example, is mathematical arguments. At least this reached by future accelerators. More­ the photon, or quantum of light. The was once told to me by Victor F. over, the Higgs boson is thought to mediating particles of the gravitation­ Weisskopf, an eminent physicist who generate the masses of all the funda­ al field, the weak field and the strong often engages in such explanations, mental particles; in a manner of speak­ field are respectively the graviton and he may very well be right. It cer­ ing, particles "eat" the Higgs boson to (which has not yet been detected), tainly holds for a proposed but un­ gain weight. three weak vector bosons, called the discovered particle called the Higgs The biggest drawback of the Higgs W+, W-and ZO particles, and eight boson and the so-called Higgs field as­ boson is that so far no evidence of gluons. In a somewhat analogous way sociated with it. its existence has been found. Instead the Higgs boson is the mediating parti­ The Higgs boson, which is named a fair amount of indirect evidence al­ cle of the proposed Higgs field. after Peter W. Higgs of the University ready suggests that the elusive particle It is now assumed that there is a con­ of Edinburgh, is the chief missing in­ does not exist. Indeed, modern theo­ stant Higgs field throughout all space, gredient in what is now called the stan­ retical physics is constantly filling the that is, the vacuum of outer space is dard model of elementary processes: vacuum with so many contraptions not empty but contains this constant the prevailing theory that describes the such as the Higgs boson that it is amaz­ field. The Higgs field is thought to gen­ basic constituents of matter and the ing a person can even see the stars on a erate mass by coupling to particles. fundamental forces by which they in­ clear night! Although future accelera­ Depending on the coupling strength, a teract. According to the standard mod­ tors may well find direct evidence of particle in space has a certain potential el, all matter is made up of quarks and the Higgs boson and show that the mo­ energy. By Einstein's famous equa­ leptons, which interact with one anoth­ tivations for postulating its existence tion, E = mc2 (energy equals mass er through four forces: gravity, elec­ are correct, I believe things will not be multiplied by the square of the speed tromagnetism, the weak force and the so simple. I must point out that this of light), the coupling energy is equiva­ strong force. The strong force, for in­ does not mean the entire standard lent to a mass. The stronger the cou­ stance, binds quarks together to make model is wrong. Rather, the standard pling, the greater the mass. protons and neutrons, and the residual model is probably only an approxima­ The way particles are thought to ac­ strong force binds protons and neu­ tion-albeit a good one-of reality. quire mass in their interactions with trons together into nuclei. The electro­ the Higgs field is somewhat analogous magnetic force binds nuclei and elec­ ven though the only legitimate rea­ to the way pieces of blotting paper ab­ trons, which are one kind of lepton, E son for introducing the Higgs bos­ sorb ink. In such an analogy the pieces into atoms, and the residual electro­ on is to make the standard model of paper represent individual parti­ magnetic force binds atoms into mole­ mathematically consistent, much at­ cles and the ink represents energy, or cules. The weak force is responsible tention has been given to the conceptu­ mass. Just as pieces of paper of differ­ for certain kinds of nuclear decay. The ally easier proposal that the particle ing size and thickness soak up vary­ influence of the weak force and the generates the masses of all the funda­ ing amounts of ink, different particles strong force extends only over a short mental particles. I shall therefore be­ "soak up" varying amounts of energy, range, no larger than the radius of an gin with that topic. or mass. The observed mass of a parti­ atomic nucleus; gravity and electro­ Central to an understanding of how cle depends on the particle's "energy­ magnetism have an unlimited range the Higgs boson would generate mass absorbing" ability and on the strength and are therefore the most familiar of is the concept of a field. A field is sim­ of the Higgs field in space. the forces. ply a quantity, such as temperature, In spite of all that is known about defined at every point throughout hat are the characteristics of the the standard model, there are reasons some region of space and time, such as Wproposed Higgs field? In or­ to think it is incomplete. That is where the surface of a frying pan. In physics der to endow particles with mass, the the Higgs boson comes in. Specifical­ the term "field" is usually reserved for Higgs field, if it exists, would have to ly, it is held that the Higgs boson gives such entities as the gravitational field assume a uniform, nonzero value even mathematical consistency to the stan- and the electromagnetic field. Fields in the vacuum. Moreover, the Higgs 76 © 1986 SCIENTIFIC AMERICAN, INC field would be a scalar field, which is what oversimplified way, the mass of The Higgs boson is called a scalar one of two kinds of field important in a person would change if he or she boson because it has a spin of O. Most describing the interactions of particles. turned around while standing in the other bosons associated with fields are A scalar field is a field in which each same place. In other words, the Higgs thought to be vector bosons: particles point has associated with it a single field is "spinless." that have a spin of l. The photon, magnitude, or number. The other im­ Because the Higgs field is spinless, gluon and W+, W-and ZO particles, portant field is a vector field: a field the Higgs boson must also be spinless. for instance, are spin-1 bosons. where at each point a vector, or arrow, Spin, as applied to elementary parti­ Since vector bosons are typically as­ is drawn. A vector has both a mag­ cles, is a quantum-mechanical proper­ sociated with the fundamental forces nitude, which is represented by the ty roughly equivalent to the classical of nature and the Higgs boson is a sca­ length of the arrow, and a direction. spin of a rotating ball. Elementary par­ lar boson, the force by which particles The electromagnetic, weak and strong ticles can take on only integer (0, 1, 2 couple to the Higgs field must be a fields are all vector fields. (The gravi­ and so on) and half-integer (1/2, 3/2 new force. It is introduced explicitly tational field is a special entity called a and so on) values of spin. Particles that and solely as a mechanism to improve tensor field.) have integral spin are called bosons the mathematical consistency of the The proposed Higgs field must be a and particles that have half-integral standard model. The Higgs force be­ scalar field, because if it were a vector spin are called fermions. Bosons and haves mathematically in a similar field, the mass of a particle would in fermions have sharply differing prop­ manner to the recently publicized general depend on the particle's align­ erties, but I shall not delve into that "fifth force" reported by Ephraim ment with the field. Stated in a some- topic here. Fischbach of Purdue University. The MAGNETIC MONOPOLES should exist if the Higgs boson ex­ monopoles could, however, be formed by sweeping magnetic field ists. Classically, of course, magnetic monopoles are not found be­ lines under the Higgs "rug" (top). The bottom illustration shows a cause when a bar magnet is cut in half, two smaller bar magnets pair of monopoles. Although there have been scattered reports of are created-not isolated "north" and "south" poles. Magnetic finding monopoles, none of them has been substantiated to date. 77 © 1986 SCIENTIFIC AMERICAN, INC proposed Higgs force is, however, known, for example, why the Higgs as the weak vector bosons, the energy weaker and has a much shorter range field should couple more strongly to density of the Higgs field in the vacu­ than the "fifth force." some particles than it does to others. um would be 10 trillion times greater The Higgs force is not a universal Nor do investigators understand how than the density of matter in an atomic force, because it couples differently the mass of the Higgs boson itself nucleus. If the earth were compressed to different particles. Specifically, if a (which is not known) comes about, al­ to this density, its volume would be ap­ particle is observed to have mass, the though it is generally presumed to be proximately 500 cubic centimeters, or strength of the coupling to the Higgs dominantly through a self-interaction a bit more than the size of a soft-drink field is assumed to be whatever quan­ with the Higgs field.