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Search for a shadow world of physics

UC San Diego physicist leads search for evidence of a parallel world of subatomic particles that could account for a good part of the mystery of the cosmos.

Susan Brown | July 12, 2013

The content of the universe is almost entirely unknown to us. Stars, planets, gas and galaxies we can see or even detect with telescopes tuned to wavelengths beyond our vision make up only about 5 percent of all matter and energy. All the rest is a huge mystery.

An unknown force propels the expansion of the universe at ever faster rates. Cosmologists call this dark energy, and it is the subject of other searches.

Something else, also unseen, tugs at the visible gas and stars of galaxies, holding them together with extra gravitational force when the speed of their rotation should fling them apart. Cosmologists attribute this extra gravity to an invisible substance they call dark matter.

dark matter simulation

Although scientists have estimated dark matter’s contribution to the mass of the universe at five times that of ordinary matter, and even mapped tendrils and halos of the stuff, we don’t know what it is. But a team of physicists, lead by physics professor Frank Wuerthwein, is trying to find out.

“I want to produce  dark matter here on Earth so  we can study it in the laboratory,” says Wuerthwein, who plans to achieve that feat using a giant detector, the Compact Muon Solenoid or CMS, within the Large Hadron Collider in Europe. Wuerthwein leads CMS collaboration’s search for dark matter.

The same collaboration helped to discover a new particle last summer, which accumulating evidence continues to say is the Higgs boson, the final member of an array of particles that account well for the properties and forces of ordinary matter. Yet this system, the standard model of particle physics, fails to explain dark matter.

For that, many physicists are looking for a whole new set of particles, a sort of shadow world to the one we know, in which each particle in the standard model has a partner that shares nearly all of its properties, except that they must be much more massive or we would already have discovered them.

supersymmetric particles

The idea is called supersymmetry, and the lightest particle among the supersymmetric partners to our familiar world is a candidate for dark matter. Although the idea is elegant and well thought through, there’s scant physical evidence for the massive particles it predicts, yet.

Wuerthwein and others think supersymmetric particles may emerge from the fireball collisions produced by the Large Hadron Collider. As the leader of the search using CMS, Wuerthwein must channel the brainpower of hundreds of talented scientists, all pursuing questions they find most compelling with means that best appeal to them, into a coordinated effort with the best chance of finding dark matter.

“We need to focus on sound approaches,” Wuerthwein said. “Our efforts must lead to a coherent message of what we have or have not seen when data collections is done.”

 It’s an enormous challenge, made difficult in part because they don’t quite know what they’re looking for. In comparison, the search for the Higgs boson was clear cut. 

“Supersymmetry is such an amorphous beast, a collection of possible phenomena that are incredibly diverse,” Wuerthwein said. “So you make choices.”

So how do you find something if you’re not sure what it is? Wuerthwein acknowledges that supersymmetric particles will be a challenge to recognize.

Dark matter doesn’t interact with light. It won’t register in the detector directly. Instead, they expect to see imbalances of energy – jets in one direction that are not countered by energy in other directions. That is, missing energy. 

SupersymmetryThis image of a potential supersymmetry event shows momentum imbalance possibly due to dark matter particles escaping the detector, the "missing energy" indicated by red arrow. Red and blue rectangles indicate energy deposited in detectors; green tracks in the center show charged particles. Yellow-outlined triangles indicate jet cones or the presence of subatomic particles called quarks. Image courtesy of Matevz Tadel, UC San Diego/CMS

The problem is that energy can appear to be missing for reasons other than the production of a particle of dark matter, such as random electrical flickering in this enormously complex machine.

Sorting that out relies on the ability to reconstruct just what happened within the machine. Graphical displays of individual suspect events allow scientists to use their visual brain to spot malfunction.

Some scientists doubt whether the shadow world of supersymmetry exists, and Wuerthwein admits they haven’t seen a flicker of it yet. “So far, we’ve seen an astounding, disconcerting consistency with the standard model,” he says.

Yet dark matter may already have emerged within the CMS detector. “The signatures will be distinctive, but not unique,” Wuerthwein says. Tell-tale events may be buried in the billions of collisions recorded before the collider shut down.

Physicists are sifting through that stockpile of data that now, with the help of a supercomputer called Gordon and plan to make preliminary reports of their work at meetings this summer at conferences all over the world.

Finding a candidate particle for dark matter will only be the beginning, Wuerthwein says. “Once you start producing dark matter, diverse and rich questions will open up.”

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