VACETS Regular Technical Column

"Science for Everyone"

"Science for Everyone" was a technical column posted regularly on the VACETS forum. The author of the following articles is Dr. Vo Ta Duc. For more publications produced by other VACETS  members, please visit the VACETS Member Publications page or Technical Columns page.

The VACETS Technical Column is contributed by various members , especially those of the VACETS Technical Affairs Committe. Articles are posted regulary on [email protected] forum. Please send questions, comments and suggestions to [email protected]

Fri, 7 Apr 1995

Dark Matter (Part 2): WIMPs and Other Exotic Particles

In the last article, we learned that the known-to-exist objects such as collapsed stars, red dwarfs, massive compact halo objects (MACHOs), and neutrinos are not good dark matter candidates. Would that make the hypothetical weakly interacting massive particles (WIMPs) or some other hypothetical particles the winners by default? The word "hypothetical" means that these candidates exists on paper only and no one has ever seen or detected them. That is what keeps most of them in contention. Let's examine these hypothetical candidates to see if any one of them can be a real winner.

Let's begin with a candidate called the magnetic monopole. To understand what a magnetic monopole is we need to start with magnetism and electricity. Sound familiar? We all know that a changing magnetic field produces an electric field, and a changing electric field produces a magnetic field. The equations of electromagnetics, the Maxwell equations, show the relationship between electricity and magnetism. There is a symmetry between them, with one exception. There are electric dipoles, magnetic dipoles, electric monopoles, and... no magnetic monopoles. That is, a magnet with a single north or south pole does not exist. Then along came Paul Dirac who, in 1931, showed (on paper) that magnetic monopoles should exist. Much later, in 1974, the grand unified theory (GUT) also predicted the existence of magnetic monopoles with its mass about 10^16 times as heavy as a proton. With this large mass, it doesn't take many to close the universe. The major difficulty with the magnetic monopole as a candidate for dark matter is that we have not yet detected it. If magnetic monopoles exist, how are we going to detect them? Simple! We know that a changing magnetic field produces a current. So if a magnetic monopole passes through a loop of wire, a small current will flow. Many experiments have been done using this basic idea in the search for magnetic monopole and the results are inconclusive.

The next candidates are not particles. They are extremely massive objects called the cosmic walls and cosmic strings, cousins of magnetic monopoles. According to GUT, 10^-35 seconds after the big bang, the universe was cooled down below the temperature to sustain the grand unified force, and the GUT symmetry was broken. Different regions of space broke the symmetry at slightly different times. When two regions of differently broken symmetry run together, we get what is called a cosmic wall, which is very massive; when three regions meet we get a cosmic string; and when four regions meet at a point we get a magnetic monopole. The cosmic walls and cosmic strings are somewhat like two- and one-dimensional monopoles. The problem is that we are not certain if they exist.

Several other candidates come from a theory called supergravity. According to this theory, there is a superpartner corresponding to each known type of particle in the universe. These superpartners are named photino, gravitino, selectron, squark... (Photino is the superpartner of the photon, gravitino is the superpartner of the graviton, and so on...) The major problem with these exotic particles is the same as that of magnetic monopoles, i.e., none of them have been detected.

The next candidate is axion. Axion was born out of an attempt to overcome a problem that had developed in GUT. It is very light, but according to their calculations, large numbers of them should exist and therefore, become good candidates for dark matter. The problem is that we haven't seen any of them despite their predicted large number.

There are some other candidates such as preons and rishons which some believe make up quarks... and PHOTONIUM. Why is the word PHOTONIUM written in capital letters? It is because it is very important, for me, that is. Talking about dark matter, I just have to throw in a few words about my search for a dark matter candidate, photonium. My search for photonium was for the physics of the unknown particle; the cosmological aspect is just a bonus if the particle can be proved to exist.

About ten years ago, a group of experimenters in Germany found some strange signals in their heavy ion collision experiment. A short time later, an other group of scientists also found those same mystery signals in a somewhat different experiment. It was interpreted that these mystery signals came from some unknown neutral particles which were formed in the heavy ion collisions and then later decay. Shortly after, a group of theorists at my institution began working on a theory leading to the formation of the mystery particle. Using Quantum Electrodynamics (QED) and after several years of hard work, the group came up with a theory that predicts the existence of a neutral particle which they named photonium. Being theorists, they wouldn't know how to connect a VCR to a TV, let alone set up an experiment to search for their mystery particle. So, in 1990, they asked me to find their particle for them, and I agreed.

According to this theory, photoniums are composite particles, composed of pairs of electron-positron (positron is the antiparticle of electron) in continuum bound states. (This is different from the well-known bound states of positronium. Mathematicians know well of these continuum bound states.) Photoniums might be created abundantly some hundreds of years after the big bang, have infinite life-time in free space, do not interact with other particles, and could easily account for 90 or more percents of the total mass of the universe. Photonium does not interact with other particles and has infinite life-time, so how are we going to detect it? Photonium's life-time depends on the surrounding. It is infinite in field-free space but in an electromagnetic field, it is finite. This means that in the vicinity of a strong field, photonium will decay and we may be able to detect its decayed products. Photonium can also be created. According to the theory, in a strong field,

electron + positron --> photonium

photonium --> several gamma-rays or pairs of electron-positron.

I spent several years using the best gamma-ray detector system of that time, the High Energy Resolution Array (HERA) at Lawrence Berkeley Lab, to search for photoniums decay to multiphoton states. The result was inconclusive. The search was then moved to a different ground using a different method of detecting photonium. For the past one and a half years, I've set up and ran the experiment searching for photoniums decay to pairs of electron-positron using the High Efficiency Coincident Lepton Spectrometer (HECLS) at Lawrence Livermore National Lab. HECLS is one of the few best spectrometers of its kind at present. The search is like the game "hide and seek", now you see and now you don't. There were excited moments when I thought I found some thing and there were blue times when some components of the equipment broke down or when the detected signals turned out to be just the background signals or some garbages due to the imperfect equipment. Two weeks ago, I found out that the appeared-to-be-the-right signals that I had been seeing were the results of some other reactions. It was a very sad week (for me). The result of this search as of today is inconclusive. I'm planing to alter the experiment to employ a slightly different, better, and more expensive method. However, funding for basic research is scarce these days and my time is running out. I may have to abandon this search in the very near future. How sad!

That's the story of my search of the last five years for a dark matter candidate. Have a nice week, folks! (If you get sick of this cosmological series, please let me know.)

Duc Ta Vo, Ph.D.
[email protected]

For discussion on this column, join [email protected]

Copyright © 1996 by VACETS and Duc Ta Vo


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Dark Matter

Dark Matter (Part 2): WIMPs and other Exotic Particles.

Spacetime-Travel and Relativity (Part 1)

Spacetime-Travel and Relativity (Part 2)

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