weakly interacting massive particles

What Is Dark Matter?

There is as yet no answer to this question, but it is becoming increasingly clear what it is not. Detailed observations of the cosmic microwave background with the WMAP satellite show that the dark matter cannot be in the form of normal, baryonic matter, that is, protons and neutrons that compose stars, planets, and interstellar matter. That rules out hot gas, cold gas, brown dwarfs, red dwarfs, white dwarfs, neutron stars and black holes.

Black holes would seem to be the ideal dark matter candidate, and they are indeed very dark. However stellar mass black holes are produced by the collapse of massive stars which are much scarcer than normal stars, which contain at most one-fifth of the mass of dark matter. Also, the processes that would produce enough black holes to explain the dark matter would release a lot of energy and heavy elements; there is no evidence of such a release.

The non-baryonic candidates can be grouped into three broad categories: hot, warm and cold. Hot dark matter refers to particles, such as the known types of neutrinos, which are moving at near the speed of light when the clumps that would form galaxies and clusters of galaxies first began to grow. Cold dark matter refers to particles that were moving slowly when the pre-galactic clumps began to form, and warm dark matter refers to particles with speeds intermediate between hot and cold dark matter.

This classification has observational consequences for the size of clumps that can collapse in the expanding universe. Hot dark matter particles are moving so rapidly that clumps with the mass of a galaxy will quickly disperse. Only clouds with the mass of thousands of galaxies, that is, the size of galaxy clusters, can form. Individual galaxies would form later as the large cluster-sized clouds fragmented, in a top-down process.

In contrast, cold dark matter can form into clumps of galaxy-sized mass or less. Galaxies would form first, and clusters would form as galaxies merge into groups, and groups into clusters in a bottom-up process.

The observations with Chandra show many examples of clusters being constructed by the merger of groups and sub-clusters of galaxies. This and other lines of evidence that galaxies are older than groups and clusters of galaxies strongly support the cold dark matter alternative. The leading candidates for cold dark matter are particles called WIMPs, for Weakly Interacting Massive Particles. WIMPs are not predicted by the so-called Standard Model for elementary particles, but attempts to construct a unified theory of all elementary particles suggest that WIMPs might have been produced in great numbers when the universe was a fraction of a second old.

A typical WIMP is predicted to be at least 100 times as massive as a hydrogen atom. Possible creatures in the zoo of hypothetical WIMPs are neutralinos, gravitinos, and axinos. Other possibilities that have been discussed include sterile neutrinos and Kaluza-Klein excitations related to extra dimensions in the universe.

The Large Hadron Collider sets its sights on dark matter

After finding the Higgs boson, the LHC has had a refit to enable it to operate at even greater extremes – and to solve more questions about the beginnings of the universe

By Robin McKie

There is no shortage of superlatives that can be applied to the Large Hadron Collider near Geneva, though many are strange and unusual. For a start, the huge underground device, which batters beams of protons into each other at colossal energies, can fairly claim to be the coolest place on Earth. Bending protons as they hurtle round the LHC’s circular 27km tunnel turns out to be a chilly business.

Thousands of huge magnets are needed to control the beams and these have to work with complete efficiency. To achieve this, the device is refrigerated to two degrees above absolute zero on the thermodynamic temperature scale: -271C, a temperature at which electric currents flow without resistance. In this way, the collider’s magnets can work to their maximum potential.

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Dark Matter

Some dark matter models are now suggesting that this cosmic substance is made of weakly interacting massive particles (WIMPs). These are thought to be about as heavy as a particle can be before becoming a miniature black hole. In a new model known as “Planckian interacting dark matter,” the upper mass limit of these particles is thought to be 1019 compared to a proton which is 1. This would mean that these particles could weigh up to 1 microgram which is almost the same weight as a human cell which is about 3.5 micrograms.

However, we still have not been able to detect what makes up most of our universe so we have no way of knowing. If we could detect dark matter and what it consists of, not only would this give us great insight into the majority of the mass in our universe, but also yield greater understanding into the nature of inflation and how it started and stopped.

Dark-matter search considers exotic possibilities

As observations fail to pin down the stuff, explanations once considered fringe are getting another look.

Ever since astronomers realized that most of the matter in the universe is invisible, they have tried to sort out what that obscure stuff might be. But three decades of increasingly sophisticated searches have found no sign of dark matter, causing scientists to question some of their basic ideas about this elusive substance.

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New dark matter detector draws a blank in first test round

We keep saying dark matter is so very hard to find. Astronomers say they can see its effects — such as gravitational lensing, or an amazing bendy feat of light that takes place when a massive galaxy brings forward light from other galaxies behind it. But defining what the heck that matter is, is proving elusive. And considering it makes up most of the universe’s matter, it would be great to know what dark matter looks like.

A new experiment — billed as the most sensitive dark matter detector in the world — spent three months searching for evidence of weakly interacting massive particles (WIMPs), which may be the basis of dark matter. So far, nothing, but researchers emphasized they have only just started work.

“Now that we understand the instrument and its backgrounds, we will continue to take data, testing for more and more elusive candidates for dark matter,” stated physicist Dan McKinsey of Yale University, who is one of the collaborators on the Large Underground Xenon (LUX) detector.

LUX operates a mile (1.6 kilometers) beneath the Earth in the state-owned Sanford Underground Research Facility, which is located in South Dakota. The underground location is perfect for this kind of work because there is little interference from cosmic ray particles.

“At the heart of the experiment is a six-foot-tall titanium tank filled with almost a third of a ton of liquid xenon, cooled to minus 150 degrees Fahrenheit. If a WIMP strikes a xenon atom it recoils from other xenon atoms and emits photons (light) and electrons. The electrons are drawn upward by an electrical field and interact with a thin layer of xenon gas at the top of the tank, releasing more photons,” stated the Lawrence Berkeley National Laboratory, which leads operations at Sanford.

“Light detectors in the top and bottom of the tank are each capable of detecting a single photon, so the locations of the two photon signals – one at the collision point, the other at the top of the tank – can be pinpointed to within a few millimeters. The energy of the interaction can be precisely measured from the brightness of the signals.”

LUX’s sensitivity for low-mass WIMPs is more than 20 times better than other detectors. That said, the detector was unable to confirm possible hints of WIMPs found in other experiments.

“Three candidate low-mass WIMP events recently reported in ultra-cold silicon detectors would have produced more than 1,600 events in LUX’s much larger detector, or one every 80 minutes in the recent run,” the laboratory added.

Don’t touch that dial yet, however. LUX plans to do more searching in the next two years. Also, the Sanford Lab is proposing an even more sensitive LUX-ZEPLIN experiment that would be 1,000 times more sensitive than LUX. No word yet on when LUX-ZEPLIN will get off the ground, however.

Image courtesy: NASA/ESA, Jullo (JPL), Natarajan (Yale), Kneib (LAM)