Three-pronged attack: Antarctic Science Teams Search Out “Particle of the Mystic”
This article orignally appeared in the NSF’s Antarctic Sun
Image: A string of IceCube’s sensors descends into a hole drilled in the ice near the South Pole Station on Dec. 26, 2005. Eighty such strands are scheduled to be deployed as part of the neutrino detec- tor array.
Three-pronged attack: Three teams are using three very different tactics on the hunt for one of nature’s most elusive creations – neutrinos.
The dream of unlocking neutrinos’ secrets has kept scientists tossing and turning since the subatomic particles were first theorized in the 1930s. The vision has been there, but the technology to materialize that vision has not. Three teams of scientists are now preparing to use Antarctic ice as the key to open the strongbox that has contained extraterrestrial neutrinos’ mysteries since the early universe – even though no one is sure what they will reveal.
Strange but true
Neutrinos are smaller than a single atom. They have no charge and very little mass. Magnetic fields have no effect on them. Gravity’s influence on them is almost nonexistent.
Neutrinos fly across the universe, slipping between gaps in the atoms of all matter, indifferent to the worlds around them.
A star, a distant planet, a human – the particles pass right through matter as they continue in virtually straight lines from their points of creation. Neutrinos saturate our universe, flying in all directions to no particular destination, and the more scientists learn about them, the stranger they seem to be.
“They exist in a way that’s almost otherworldly,” said Peter Gorham, principal investigator of the neutrino detector ANITA (ANtarctic Impulsive Transient Antennae). “They have very little interaction with matter as we know it. They don’t affect anything that we do day to day. In some ways, they are the particle of the mystic. Physicists know they exist and can make precision measurements of their characteristics, but everything about them is strange and unexpected.”
One of neutrinos’ bizarre truths is that despite the lack of effect that matter has on them, they exert an enormous amount of influence on matter.
“In the early universe, before there were any elements, neutrinos dominated for some time,” Gorham said. “During that time is when all the elements were formed. And without that neutrino soup in the very early part of the universe, we would not have the elements we have now. We could not. They are absolutely, inextricably tied into chemistry, physics and the physical elements. Those things could not exist without neutrinos.”
To see the unseen
Neutrinos’ huge importance and vast numbers have not, however, provided an easy way to study the particles. But there is one interaction that has given scientists an opportunity to observe them indirectly.
A neutrino is so small that it slips through the gaps in matter, but no one is at the wheel. The fact that they don’t often run into other subatomic particles shows how small they are, not that they somehow zigzag through an atomic obstacle course.
Every once in a while, a neutrino happens to miss the gap and plows directly into a piece of an atom, destroying the neutrino and producing a reaction that scientists can detect. The collision creates a cone of visible light and a radio pulse that expands from the point of collision and continues in the direction the neutrino was headed. Scientists can use the information gathered by the observation of these reactions to learn neutrinos’ direction of origin, speed and energy.
Researchers need to make their observations in a medium that is effective at transmitting the radio and light, and they have found the best substance to be ice – something of which Antarctica happens to have the world’s largest supply. The three current neutrino projects plan to look at either the radio or light signature to study neutrinos’ origins and physics. But despite studying the same particle, the groups go about it in very different ways.
In the ice
The IceCube neutrino observatory at the South Pole is about to enter its third season of construction. Eighty strands of sensors buried vertically in the ice will make up the array. The sensors are called photomultipliers and detect the light created by the neutrino collisions.
When a neutrino hits an atom that makes up the ice, it destroys itself, but the collision creates a negatively charged particle called a muon. The muon then continues in the same direction the neutrino was headed. And as it travels through the ice, faster than light would through the same substance, it produces a cone of blue light, which is called Cherenkov radiation.
Scientists have found the ice under the South Pole to be the ideal location to observe the effect. The South Pole lies on the Polar Plateau, which is covered by three kilometers of ice. The landscape of snow is flat, like an icy, white sheet pulled taut against the Earth. The ice that lies below the surface provides the vast, dark and transparent background that scientists need to study the neutrinos’ light show.
Construction crews have installed nine strands so far. Each string of 60 sensors requires a hole be drilled so crews can lower it almost two kilometers down into the ice. When complete, the array will consist of 4,800 sensors and have a volume of one cubic kilometer. The infrequency of the events IceCube scientists study means the array must be that large in order to be truly effective, said Francis Halzen, principal investigator for the project.
IceCube works by registering the exact time each equally spaced sensor gets hit by the collision-produced light – down to three billionths of a second. The scientists can then combine the data from each sensor to create the data set they need.
“Now it’s up to nature to deliver; it’s out of our hands,” Halzen said. The team expects to install 12 more strands this season and will have the material on hand to complete two additional strands if they get ahead of schedule, Halzen said.
Next year, the group expects to finish 14 to 16 strands. Construction estimates are more conservative this year due to the effort to move IceCube operations out of its current, temporary laboratory, he said. The new facility will serve as IceCube’s permanent nerve center, where scientists can monitor data and calibrate the array. The relocation effort will require the nine previously installed strands to be disconnected from the system and powered down.
“[Moving to the new lab] is something we’re very concerned about, but it has to happen,” Halzen said. “The experimentalists tell me that this is OK, that there’s nothing to worry about. We’ve turned off strings before and brought them back to life, but some of these strings will be off for more than a week, so it’s a bit scary.”
He said the key to completing this season’s aggressive agenda is to start on time. Last season, the team lost several drilling weeks while making improvements to the drill system.
“This year we just have some fine tuning,” Halzen said. “Last season we were doing major revamping to some of the equipment.
“After this [season], we hope it just becomes routine construction. We’ll have to wait and see. I guess nothing is ever routine in Antarctica.”
On the ice
About 80 kilometers away from McMurdo Station this season, another science team will test its theory for a massive neutrino detector near Minna Bluff on the Ross Ice Shelf.
Unlike IceCube, this array would try to detect the radio pulse, instead of the light, created by neutrino collisions with the ice.
“What I need to find out is if the ice is transparent enough [to transmit the radio pulse] and if the bottom of the Ross Ice Shelf acts as a mirror, which it should. The bottom of the Ross Ice Shelf is water, and water reflects radio,” said Steve Barwick, principal investigator of ARIANNA (Antarctic Ross Iceshelf ANtenna Neutrino Array). “That’s the linchpin of [the project].”
If the theory is proven, Barwick and his team plan to construct an array of antennae, which will point down into the ice and capture the radio waves reflected off the bottom of the ice shelf.
The array would function in a similar way to IceCube, with each sensor registering the nanosecond it detects the pulse.
ARIANNA is specifically designed to look for the highest energy neutrinos, which are some of the rarest of the already elusive particles. The scientists want to be able to detect 40 to 50 of these events each year, Barwick said, and therefore have designed the array to epic proportions.
The current specification calls for ARIANNA to be built as a square grid, 100 by 100 sensors wide, with about three football fields in length between each.
“We’re talking about 10,000 of these antennas,” Barwick said. “If we could make it any smaller we would. Ö There just aren’t many of the reactions we’re looking for. That means [we] have to build a huge detector, much bigger than IceCube, much bigger than anything we’ve built before. So, ARIANNA would be a 30-kilometer by 30-kilometer array of antennas – that’s pretty darn big.”
Above the ice
IceCube is still under construction, and the plans for ARIANNA are just starting to solidify. But neutrino scientists around the world won’t have to wait until those projects are constructed to get their first glimpse of extraterrestrial neutrinos hitting Antarctica’s icy surface. One neutrino detector will be fully operational this summer as it soars high above the planning and construction activities of the other projects – the balloon-borne ANITA.
Scientists have designed this detector as a payload for one of NASA’s long-duration balloons. The stadium-sized balloon will carry ANITA around the continent at about 38,000 meters in the air, said Gorham, the project’s principal investigator.
“We can see an awful lot of ice from afloat – about a million cubic kilometers at one time,” he said. “Most of the other neutrino experiments are focused on relatively small volumes of ice.
“We’re looking at as much of the ice as we can see at one time, which works out Ö to be about 10 to 15 percent of the whole continent. We’re able to see neutrinos interact if they happen anywhere in our field of view – even out to the horizon. These neutrinos can collide a mile down in the ice and create a radio pulse that propagates up and is detected by our balloon 400 kilometers away.”
ANITA is after the same high-energy neutrinos for which ARIANNA will search. These neutrinos have never been observed but must exist, according to the laws of physics, said Gorham. He related the expectation of these neutrinos to seeing the muzzle flash of a gun, smelling the smoke, and searching for the bullet. “If we don’t see these neutrinos with ANITA,” Gorham said, “there’s still a little bit of wiggle room for theory to recover, but it’s going to get uncomfortable because our basic, prime theories of how things [work] are going to be challenged.”
ANITA will fly for up to 40 days this season. While it can survey much more land at one time than its ground-based cousins, it’s limited by the duty cycle of the balloon. And since ANITA is a single-point detector, it cannot gather the detailed information about each interaction that the array detectors will. “ANITA will be able to make crude measurements of energy and direction to establish the nature of the particle but not much beyond that,” Gorham said. “It’s really very focused on the discovery rather than the precision measurements.
“It’s [part of] the classic experimental effort to try to challenge what we don’t know and see if we can chip away at our ignorance.”
NSF-funded research in this story: Francis Halzen, University of Wisconsin-Madison, icecube.wisc.edu; Steve Barwick, University of California, Irvine; Peter Gorham, University of Hawaii, www.phys.hawaii.edu/~anita.
