- Associated Press - Saturday, June 25, 2016

NEWPORT NEWS, Va. (AP) - Look around - what do you see?

A paltry 5 percent of what the universe is made of, that’s what.

All the rest is what scientists call the dark sector, composed of things we can’t see directly, but can observe only by their impacts on other objects.

Now this summer at Jefferson Lab in Newport News, a team headed by MIT physicists will begin a multiyear effort to try to glean evidence of at least one tiny theoretical piece of all that darkness: a dark photon - a particle that interacts with both dark and light matter.

Called the DarkLight experiment, it could, if successful, unearth the first evidence of what one researcher calls a bridge between the universe’s visible and invisible sectors.

So far, bare hints of the existence of dark photons have been teased out from nuclear and particle physics experiments and from astrophysical observations.

“When you look out into the universe, you see galaxies spiraling, rotating,” said Richard Milner, director for the Laboratory of Nuclear Science at the Massachusetts Institute of Technology.

“And, of course, what you see are objects that emit light - the so-called visible matter. If you look at how the matter in a galaxy behaves as you go out from the center of the galaxy, you can only explain the motion if there’s a lot of matter that you do not see, but which interacts gravitationally with the matter that you do see. Hence, it’s called dark matter.”

Physicists believe dark matter comprises about 27 percent of the total matter in the universe.

There’s also dark energy, which is even less understood, comprising 68 percent of total matter and based on observations that the universe is accelerating and expanding.

Holy Grail

Milner and his colleague Peter Fisher hope to gain harder evidence of a dark photon at Jefferson Lab by using its free-electron laser, the most powerful electron beam in the world.

“It’s going to be very pioneering,” Milner said. “Jefferson Lab has absolutely unique accelerator facilities - it’s the only place in the world where you could imagine doing this experiment.”

But a dark photon isn’t the only thing they’ll be hunting.

The physics world is buzzing over speculation that a Hungarian experiment might have found evidence of a new particle that could constitute an elusive fifth fundamental force of nature.

If the so-called 17-MeV particle exists - and physicists say that’s a big “if” - it would go a long way toward reconciling the standard model of physics to explain dark matter.

“It would be incredible,” said Chris Carone, a theoretical physicist at the College of William and Mary in Williamsburg.

Carone said he’s skeptical the 17-MeV particle exists. But if DarkLight should find it, it would mean the first conclusive evidence of physics beyond the standard model - the accepted theory of known elementary particles and their interactions.

“And the Holy Grail is to find something that goes beyond it,” Carone said. “Some new particle which could be a force-carrier or otherwise that will give some hints about what physics explains - sort of the unanswered questions about the standard model.”

Out of the dark

Jefferson Lab is ideal for the DarkLight experiment because its megawatt-power electron beam is extremely bright, extremely clean and halo free, Milner said.

Dark photons are believed to interact with visible matter, but only very rarely and elusively.

Often, they’re only hinted at by unexplained anomalies.

For instance, said Kandice Carter, science writer at Jefferson Lab, a measurement of high-energy electron-positron pairs in outer space found more pairs than could be explained by typical cosmic ray production, suggesting that dark photons produced the extra pairs.

Milner and his team believe a charged particle beam such as the free-electron laser could coax a dark photon out of hiding.

“Each of these experiments looks at different energies, different locations to find these dark photons,” said Carter. “DarkLight is going to look in an area that no one else has ever looked in.”

DarkLight will focus on particles with mass energies from 10-MeV to 100-MeV, or mega (million) electron-volts, said Milner. 10-MeV is 1 percent of the mass of a proton, while 100-MeV is 10 percent of the mass.

The team will shoot beams through a target of hydrogen gas, whose atoms consist of a proton and an orbiting electron.

When the beam’s electrons hit the hydrogen, they’ll blast away the protons, leaving a scattered electron, a proton - and an electron-positron pair that could come from the decay of a dark photon.

“We’re looking in a different place,” Milner said. “Maybe we’ll learn something. Maybe we’ll find something.”

Robert McKeown, the lab’s deputy director for science, likened it to looking for a needle in a haystack.

But such a discovery, McKeown said, “would represent an historic breakthrough in fundamental particle physics worthy of a Nobel Prize.”

Fifth force

DarkLight is also well-positioned to find evidence of the 17-MeV particle, said Milner. Although, like Carone, he’s skeptical they will.

“I’m a physicist, so we’re careful,” Milner said. “You can’t predict it, because you’re at the frontier. We have to worry about doing the highest-quality experiment we can. And, obviously, not miss it, if it’s there.

“I think DarkLight is as suitable an experiment to look for it as any other experiment,” he added. “It’s probably the most suitable.”

If the particle is found, it would become the fifth fundamental force of nature, joining the particles associated with gravity, electromagnetism and the strong and weak nuclear forces.

Milner said they expect to start beam by the end of July and take measurements in August.

But DarkLight will take time: The team expects to run the experiment for at least three years.


Information from: Daily Press, http://www.dailypress.com/



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