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How particle physicists learned to stop worrying and love WIMPs.

Astrophysicists and particle physicists often see dark matter through different lenses. As a group, astrophysicists tend to focus on the observational evidence for dark matter and on the findings of computer simulations designed to study how dark matter halos form and evolve (S&T: July 2012, page 28). Although astrophysicists generally agree that a new type of particle (or particles) is needed to solve the dark matter problem, they tend to shy away from hypothesizing new varieties of exotic matter.

In contrast, particle physicists were initially less willing to accept the evidence for dark matter's existence, but they have no hang-ups about hypothesizing new types of particles. Over the years they have literally proposed hundreds (if not thousands) of theories predicting the existence of new particles or forces. A substantial fraction of these theories predict the existence of new particles with the characteristics required of a WIMP, and thus could potentially solve the dark matter problem. The most compelling and popular idea among particle physicists is supersymmetry.

Supersymmetry postulates a fundamental relationship between the classes of particles known as fermions and bosons. Fermions are particles such as quarks and electrons, which make up what we normally think of as matter. In contrast, bosons are the particles responsible for the forces of nature. Photons, for example, are the bosons that transmit the electromagnetic force. Without photons, there would be no electromagnetism--no light. According to supersymmetry, for every type of fermion, there must also exist a boson with many of the same properties. Every kind of particle thus has a supersymmetric counterpart, called its "superpartner." The electron, for example, has as its partner the super-electron, just as the photon has its photino. Bosons and fermions are intertwined, unable to exist without each other. A boson in a supersymmetric world without its fermion counterpart would be like a one-sided coin.

To date, none of the predicted superpartners have been observed in any experiment. Despite this lack of evidence, many particle physicists find supersymmetry so compelling that they remain fairly confi dent that these particles exist--just waiting to be discovered. If supersymmetry is woven into the fabric of nature, then a number of long-standing problems in theoretical physics can be easily solved. In particular, without supersymmetry, it's very difficult to understand why the weak nuclear force is a whopping [10.sup.32] times stronger than the force of gravity. Efforts to build a Grand Unified Theory that connects the four forces of nature into a single force also seem to require that nature be supersymmetric. Furthermore, the only forms of string theory that seem workable are those including supersymmetry.

Supersymmetry can also provide us with a solution to the dark matter problem. In many supersymmetric models, the lightest of the superpartners is stable, and is unaffected by the strong or electromagnetic forces--exactly the properties required of the particle that makes up dark matter. This superpartner, the neutralino, has for decades been the single most popular WIMP candidate for dark matter.

It's been disappointing that the LHC has not yet seen copious neutralinos and other superpartners pouring out of its detectors. Perhaps we will observe the first superpartners sometime in the years after the accelerator is upgraded to higher energy (from 8 to 14 tera-electron volts). If supersymmetry exists, the LHC should ultimately produce and observe at least some of the superpartners. If, after several more years of searching, no such evidence emerges, then the theoretical physicists will be humbly sent back to their chalkboards, tasked to find some other solution to the problems that now only supersymmetry seems able to address.--D. H.
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Title Annotation:weakly interacting massive particle
Publication:Sky & Telescope
Geographic Code:1USA
Date:Jan 1, 2013
Words:599
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