For the Public - Plain English Summaries of DØ Publications

Search for Heavy Magnetic Monopoles

April 27, 1998

People have known about electricity and magnetism for centuries. The ancient Greeks noted that pieces of amber would attract light objects upon being rubbed. That observation is responsible for the name ‘electricity’ itself, since in Greek amber is  elektron .

In time, people found that there are two types of electric charges: ‘positive’ and  ‘negative’ (after Benjamin Franklin), and that opposite charges attract each other. In the twentieth century Robert Millikan showed that the electric charge is quantized: that is, all electric charges are multiples of  an elementary electric charge found on the electron.

Magnetism was also known centuries ago. The ancient Greeks knew that certain minerals attracted iron and other pieces of the same mineral. About a thousand years ago, the Chinese noticed that a magnetized needle always points in the same direction and thus can be used for navigation. However, unlike electric charges which can be isolated, magnetic materials always have two ‘poles’ (called ‘north’ and ‘south’  after the directions they point to on Earth). If one breaks a compass needle into two pieces, each will again have both ‘north’ and ‘south’ poles. It was apparently impossible to isolate a single  ‘magnetic pole’; only the combination of ‘north’ and ‘south’ poles (called a dipole) seems to exist (see picture below). The absence of a single magnetic charge (called a monopole) makes the laws of electricity and magnetism different, and this lack of symmetry bothered physicists for years.

In 1931 one of the founders of quantum mechanics, Paul Dirac, showed that if a  magnetic monopole existed, it could help to explain the puzzling fact that electric charge is quantized. He found that the product of the electric charge (e) and a magnetic monopole charge (g) is necessarily an integer multiple of the fundamental constant in quantum mechanics,   (where  is Planck's constant which relates the energy and the frequency of a photon, and c is the speed of light). Given the values of , c, and e, the minimum monopole charge g must be at least a few thousand times larger than e. This implies that the monopoles, if they exist, could produce very strong scattering of light (photons) compared with ordinary electrically charged particles. The monopole could exist with intrinsic angular momentum (spin) of 0, 1/2, or 1. For comparison, the spin of the electron is 1/2.

Recently, calculations of the scattering of photons at the Fermilab Tevatron were completed by Ginzburg and Schiller, for the case of heavy pointlike magnetic monopoles (see diagram below). (A pointlike particle has no discernible size; the known quarks and the electron, muon and tau are observed to be pointlike). The calculation gave a large scattering probability for photons from monopoles of masses of up to about 1000 GeV/c 2 (a thousand times the mass of the proton). However, we should note that it is still unknown if pointlike monopoles are fully consistent with current theory at these masses.

The DØ Collaboration has recently performed a search for production of energetic photons using data accumulated in the 1994-1995 Tevatron run. We found that the production of two or more photons is well described by a sum of two backgrounds involving ordinary interactions of quarks on the one hand, or detector misidentification of parton jets or electrons as photons on the other. Therefore, no magnetic monopoles are required by our data. We converted this measurement into limits on monopole mass and have excluded pointlike magnetic monopoles with masses below about 600, 900, and 1600 GeV/c2 for a monopole spins of 0, 1/2, and 1, respectively. These are the most restrictive limits on the monopole mass to date. The sensitivity of our experiment in the low monopole mass region is limited by the requirement on the minimum photon energy and by theoretical assumptions used in the calculations. We are sensitive to a monopole mass as low as a few hundred GeV/c2. Combined with the previous measurement by the L3 experiment at the LEP electron-positron collider at CERN (near Geneva, Switzerland) which explored a lower monopole mass range, our measurement excludes the existence of pointlike magnetic monopoles in a broad mass range from few dozen GeV/c2 to our new experimental limit.

This new result has been recently issued as a Fermilab preprint and was also submitted to the Physical Review Letters journal published by the American Physical Society.