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.)
