Unification of Nature’s Geoffrey B. West Fredrick M. Cooper Fundamental Forces Emil Mottola a continuing search Michael P. Mattis it was explicitly recognized at the time that basic research had an im- portant and seminal role to play even in the highly programmatic en- vironment of the Manhattan Project. Not surprisingly this mode of opera- tion evolved into the remarkable and unique admixture of pure, applied, programmatic, and technological re- search that is the hallmark of the present Laboratory structure. No- where in the world today can one find under one roof such diversity of talent dealing with such a broad range of scientific and technological challenges—from questions con- cerning the evolution of the universe and the nature of elementary parti- cles to the structure of new materi- als, the design and control of weapons, the mysteries of the gene, and the nature of AIDS! Many of the original scientists would have, in today’s parlance, identified themselves as nuclear or particle physicists. They explored the most basic laws of physics and continued the search for and under- standing of the “fundamental build- ing blocks of nature’’ and the princi- t is a well-known, and much- grappled with deep questions con- ples that govern their interactions. overworked, adage that the group cerning the consequences of quan- It is therefore fitting that this area of Iof scientists brought to Los tum mechanics, the structure of the science has remained a highly visi- Alamos to work on the Manhattan atom and its nucleus, and the devel- ble and active component of the Project constituted the greatest as- opment of quantum electrodynamics basic research activity at Los Alam- semblage of scientific talent ever (QED, the relativistic quantum field os. Furthermore, in keeping with put together. Many of those scien- theory describing the interaction of the historical development of the tists had made (or in some cases charged particles with radiation). Laboratory, there continues to be a were destined to make) highly sig- Although those questions had to be vigorous and productive interplay nificant contributions to the devel- put on the back burner for the dura- with other more applied and pro- opment of our understanding of the tion of the project, the pot, so to grammatic parts of the Laboratory basic laws of physics. They had speak, continued to simmer. Indeed, (see “Testing the Standard Model of 144 Los Alamos Science Number 21 1993 Unification of Nature’s Fundamental Forces Particle Interactions using State-of- The initial major breakthrough the quantum fields describing all el- the-Art Computers”). occurred around 1970 when a theory ementary particles. This article will give a brief was proposed that unified the weak The unification of electromagnet- overview of some of the exciting de- force with the electromagnetc force ism with the weak interactions was velopments in this area of science in a mathematically consistent fash- accomplished in the context of that have taken place in recent years ion. This unification came almost a quantum field theory by extending and in which scientists at Los Alam- century after Maxwell had shown the principle of gauge symmetry and os have played significant roles. that electric and magnetic phenome- the idea of a force-carrier to the From the end of the Manhattan na could themselves be unified in a case where the force-carrier can it- Project until approximately 1970, single force carried by the electro- self carry electric charge and there- the general taxonomy and phenome- magnetic field. In the modern lan- fore interact with itself. (In QED nology of the elementary particles guage of QED, all electromagnetic the photon is neutral and therefore was intensely studied. When the phenomena can be understood in cannot interact with itself.) A various mesons (such as the pions terms of a force-carrier that is ex- force-carrier of the weak interac- and the kaons) and baryons (such as changed between electrically tions must have electric charge be- the proton, neutron, §, ß, and •) charged particles such as electrons. cause some weak interactions cause were classified, it was discovered This force-carrier, called the photon, the charge of a particle to change. that there were four apparently quite is the quantum of the electromagnet- For example, the neutron, which is distinct ways in which they interact- ic field and can be thought of as a neutral, decays through the weak in- ed among themselves: (1) gravita- massless elementary particle with no teractions to a proton, which has tionally, (2) electromagnetically, (3) electric charge and spin angular mo- one unit of positive charge. Fur- weakly and (4) strongly. The first mentum equal to one (in units of h-Å). ther, the force-carrier of the weak two interactions, or forces, are the The masslessness of the photon is a interaction must be massive rather 2 most familiar since they are long- consequence of the so-called gaugee4¼ sin thanw=® ¼massless10¡ 170 because the range of range and so manifest themselves (or phase) symmetry of the electro- the weak force is so short, less than macroscopically even over astro- magnetic field and gives rise to the 10−14 centimeter. (This follows nomical distances. The second two long-range nature of the electrostatic from the original observation of have very short ranges (less than 1 force between charged particles. Yukawa that the range of a force fermi, or 10−13 centimeter) and so Furthermore, it guarantees that elec- varies inversely with the mass of are only important at the nuclear and tric charge is conserved. Technical- the force-carrier.) subnuclear level. The weak interac- ly, the gauge symmetry responsible The remarkable accomplishment tion is the one responsible for beta for the masslessness of the photon is of Glashow, Weinberg and Salam decay (such as the decay of the neu- a reflection of the invariance of was to fashion a well-defined quan- tron to a proton, an electron, and a Maxwell’s equations to arbitrary tum field theory like QED for the neutrino), and the strong interaction phase changes in the quantum fields weak interaction. It is based on an is the one responsible for the bind- associated with the electron and the extended notion of gauge symmetry ing of protons and neutrons to form photon. (Specifically, an arbitrary that Yang and Mills had invented the nuclei of atoms. phase change of the electron field earlier in an attempt to describe the One of the great intellectual can be compensated for by a redefi- strong interactions, but, in addition, achievements of the twentieth century nition of the photon field, which is ideally suited for describing the has been the realization that these leaves the form and structure of the charge-changing and charge-con- four wildly different “fundamental’’ equations of motion for electromag- serving interactions of the weak forces may, in fact, be viewed as netic interactions unchanged.) Be- force. To obtain massive force-car- manifestations of a single unified cause of its deep and seminal role, riers, called the W+, W−, and Z0, the force. Although this paradigm was this local phase, or gauge, invari- gauge (or phase) symmetry of the originally the dream of Einstein, not ance of the electromagnetic fields is quantum fields had to be dynamical- until the 1970s was any serious now viewed as a fundamental princi- ly broken in a rather subtle way progress made toward developing the ple, or constraint, used to derive the analogous to the way the sponta- theoretical framework for unification. form of the basic interactions among neous symmetry breaking in a super- 1993 Number 21 Los Alamos Science 145 1 Unification of Nature’s Fundamental Forces conductor produces the Meissner ef- plest version small fluctuations of Baryon-Number fect. This dynamical symmetry the Higgs alignment field manifest Nonconservation and the breaking is referred to as the Higgs themselves as an elementary mas- Stability of Matter mechanism. It was designed to yield sive particle, dubbed the Higgs par- massive mediators of the weak force ticle. The mass of the Higgs parti- One of the most intriguing possi- while preserving the masslessness of cle is not certainly known, but it bilities for new physics at the SSC the photon. As mentioned above, must be less than approximately and an area in which Los Alamos the masslessness of the photon is the 1 TeV = 1012 eV. has been heavily involved is the origin of both the long-range nature The existence of the Higgs parti- possibility of experimentally observ- of the electrostatic force and the cle and the origin of electroweak ing the violation of baryon-number conservation of electric charge, so symmetry breaking are therefore in- (B) conservation. Baryon number these properties of electromagnetism timately related to the ancient and appears to be an exactly conserved are sacrosanct and had to be main- vexing question of the origin of quantum number in nature. Its con- tained in any attempt at unifying mass itself. Indeed the hope of veri- servation, for example, prevents a electromagnetic and weak forces fying this line of thinking has been proton (B = 1) from decaying into a into a unified force law. These con- one of the main justifications for state with B = 0 such as an electron straints turn out to be so tight that constructing the $10 billion Super- and a pion or an electron and a pho- they lead to precise predictions for conducting Super Collider (SSC) ton, even though these decays are the masses of the W+, W−, and Z0, just outside of Dallas. The SSC will energetically very favorable. In- which were brilliantly confirmed by collide two proton beams, each com- deed, it is the conservation of bary- experiment.