Looking for Dark Matter

Darkness is filled now
Matter confounds
Newtons grasp
Lost within the void


As “Demick” poetically phrased in the above haiku, calculations that demonstrate missing masses in galaxies and larger-than-expected gravitational forces point to a mysterious dark matter. It seems to concentrate in halos around galaxies.

The Bullet Cluster, two colliding galaxies which provide the best evidence yet for dark matter.

Its particles scarcely notice the matter in our everyday lives. In fact, they hardly interact with the matter that makes up everything that we can see in the universe. So how do we figure out what it is?

Well, that’s rather a large problem. Luckily, theoretical physicists like Erik Lundström, Michael Gustafsson, Lars Bergstrom, and Joakim Edsjo of Stockholm University are on the case, tracking down the cause of the unexpected mass and gravity.

Weakly Interacting Massive Particles (WIMPs) are a hypothetical form of dark matter that interacts only through the weak and gravitational forces. As a result, they create the gravitational effects that we observe without emitting light or accumulating into larger objects like asteroids, planets, and stars.

Back in 1970s, particle known as the inert Higgs boson was proposed. It is the subject of renewed interest in part because its presence could help explain dark matter and allow for a higher Higgs mass.

An inert Higgs may seem to stretch the imagination since the ordinary Higgs boson has not yet been discovered, but it also suggests a reason why the Higgs hasn’t been observed. If the inert Higgs boson exists, the ordinary Higgs could have a mass more than twice that of the top quark, the heaviest particle on record.

The ordinary Higgs particle is the last missing piece in the Standard Model. According to the theory, a Higgs field exists through all space. Particles gain mass through their interactions with this field. Just as photons make up electric and magnetic fields, the Higgs field is composed of Higgs bosons.

A simulation of a Higgs event — these are the tracks that would be left in the detector by particles resulting from the decay of the Higgs particle.

Physicists have been trying to hunt down the Higgs nearly two decades. In my experience, many physicists whose work does not concern large colliders are rather cynical about the existence of this elusive particle.

Experiments have shown that its maximum mass is well within the range particle masses that existing colliders such as Fermilab’s Tevatron can produce. Many believe that if it was going to be found, they should have found it by now. Even so, the Higgs particle would interact so weakly with other particles that it is very difficult to detect.

While the Higgs may still have a lower mass, the existence of the inert Higgs would allow for the mass of the ordinary Higgs to be higher than currently achievable collision energies, offering an explanation for why it has not yet been discovered.

One of the major projects of the Large Hadron Collider, or LHC, which is to go online in 2008, is the search for the Higgs boson. (Given the delays in the start-up, the race is on for researchers at Fermilab to find the Higgs before LHC can!) They will also search for dark matter in collisions 14 times more powerful than the Tevatron.

The inert Higgs is inert because won’t interact with many other particles. It’s a downright snob as particles go. However, it will mingle with the ordinary Higgs boson, and through those interactions can have a small impact on other particles. It will also interact weakly with other bosons called W and Z.

An inert Higgs wouldn’t be caught dead hanging out with quarks, which make up the familiar protons and neutrons. They also avoid leptons such as the electron. Neither do they get along with their own kind. In fact, an inert Higgs is its own antiparticle. When two inert Higgs bosons meet, they’ll annihilate.

Matter-antimatter annihilation, resulting in 2 photons.

Often, annihilation means that the two particles cease to exist and two photons, or particles of light, shoot out from their meeting point. We’ve all heard Einstein’s
E = mc2. The energy of the photons is determined by the mass of the particles and the speed at which they collided. Other particles may be produced provided that the energy from the annihilation is enough to create the necessary mass.

Inert Higgs particles can annihilate another way, resulting in a Z boson and a photon. Gamma rays are photons with a lot of energy, much more than visible light, and the photons from either type of inert Higgs annihilation will have energies in the gamma range. Photons from these annihilations would have very specific energies which would make striking peaks in a graph of gamma ray energies.

In a recent article submitted to Physical Review Letters, the Stockholm team reported the gamma energies that may result from annihilations of inert Higgs particles.

The current detectors in space are not capable of catching the gamma rays that would result from inert Higgs particles running into one another. However, a new telescope called the Gamma-ray Large Area Space Telescope (GLAST), to be launched in January of next year, is able to detect these rays.

GLAST, illustrated. The rectangular chunk on the left is a close-up of one of the segments.

Lundström and Gustafsson anticipate its launch. If the telescope detects the spectrum produced by inert Higgs, then we’ll know what some of this dark matter is made of. It will also reopen the possibility of a heavy Higgs boson, but that probably won’t keep skeptical physicists from scoffing.

Image Credits:
Bullet cluster, matter-antimatter — NASA
Higgs Event — CERN through GridPP
GLAST — Stanford University

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