Chukman So

Do atoms of antihydrogen weigh the same as atoms of ordinary hydrogen? Could they even have "negative" weight? To find out, physicists are trying to see how antimatter interacts with gravity.

By Clara Moskowitz
LiveScience

When it comes to antimatter, what goes up doesn't necessarily come down. In a new study, physicists weighed antimatter in an effort to determine how this strange cousin of matter interacts with gravity.

Ordinary matter atoms fall down due to the pull of gravity, but the same might not be true of antimatter, which has the same mass as matter, but opposite charge and spin. Scientists wondered whether antimatter atoms would instead fall up when pulled by gravity, and whether such a thing as antigravity exists.

"In the unlikely event that antimatter falls upward, we'd have to fundamentally revise our view of physics and rethink how the universe works," Joel Fajans, a physicist at the Lawrence Berkeley National Laboratory in California, said in a statement. 

Fajans and his colleagues at the ALPHA experiment at Switzerland's CERN physics lab made the first experimental measurements of the gravitational mass of antihydrogen — the antimatter equivalent of hydrogen, made of an antiproton and an antielectron (better-known as a positron). [Whoa! The Coolest Little Particles in Nature]

Watching antimatter fall
Conducting experiments on antimatter atoms is difficult, because when matter and antimatter meet, the two annihilate. Thus, any experimental apparatus that came into contact with the material being studied would be instantly destroyed. Scientists get around this predicament by building traps for antimatter made with magnets, which force antimatter particles to stay in a certain area. As soon as the magnets are turned off, the antimatter falls onto the walls of the trap and is lost.

But which direction does it fall toward?

To find out, the researchers studied the flashes of light created when antiparticles annihilated matter particles in the walls of the trap after its magnets were turned off. The location and time of the flashes depend on the initial position and velocity of the antimatter atoms, and the path they take when they fall.

The results of the tests weren't conclusive, though they did give the physicists a fascinating picture into how antimatter interacts with gravity.

"Is there such a thing as antigravity? Based on free-fall tests so far, we can't say yes or no," Fajans said. "This is the first word, however, not the last."

More data needed
In the future, the ALPHA researchers plan to upgrade their experiment to a phase called ALPHA 2, which should allow them to make more precise tests within five years. The scientists plan to use lasers to cool the antiparticles to reduce their energy while still being held by the trap; then the trap's magnetic fields could be used to manipulate the cooled antiparticles so they decay more slowly when the trap gets turned off, making measurements easier.

Ultimately, scientists have a long way to go toward unraveling the conundrum of antimatter. When the universe arose 13.8 billion years ago, there should have been roughly equal amounts of matter and antimatter, scientists say. Somehow, almost all of the antimatter was destroyed in collisions with matter, and what makes up the universe today is the slight overabundance of matter left over.

To explain why that happened, physicists must understand the nature of antimatter, and how it behaves differently from the normal matter around us. This month, a new clue emerged: Physicists reported that particles called B_s (pronounced ("B-sub-S") mesons are produced more often than their antimatter counterparts inside the giant particle accelerator at CERN called the Large Hadron Collider.

The results of the new study were detailed in a paper published Tuesday in an issue of the journal Nature Communications.

Follow Clara Moskowitz on Twitter and Google+. Follow us @livescience, Facebook and Google+. Original article on LiveScience.com.

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CERN / Maximilien Brice, Rachel Barbier

The LHCb team stands in front of its experiment, the LHCb detector, at the Large Hadron Collider in Geneva.

By Clara Moskowitz
LiveScience

Matter and antimatter particles are behaving differently inside a giant atom smasher in Switzerland, physicists announced Wednesday. The discovery could help solve the riddle of why the universe is made of matter and not its strange sibling, antimatter.

All matter particles are thought to have antimatter counterparts with the same mass but opposite charge and spin. When the universe sprang into being 13.8 billion years ago with the Big Bang, it probably had similar amounts of matter and antimatter. Most of this antimatter is thought to have been destroyed in collisions with matter (when the two meet, they annihilate each other), and all that's left over in the universe today is a small overabundance of matter.

To understand why matter dominated over antimatter, physicists look for any differences in how the two behave that might explain the discrepancy. These differences are called charge-parity violation (CP violation), and that's just what scientists have found inside the Large Hadron Collider (LHC) in Geneva. [Whoa! The Coolest Little Particles in Nature]

Inside the 17-mile-long (27 kilometers) underground ring of the machine, protons speed up and smash into each other, creating a shower of daughter particles.  One experiment at the collider called LHCb (it stands for "LHC beauty") studies these daughter particles for signs of CP violation that might help elucidate the nature of antimatter.

After analyzing about 70 trillion proton-proton collisions, LHCb found that a particle called the B_s meson was created slightly more often in its matter form than in its antimatter counterpart. B_s (pronounced ("B-sub-S") mesons are made of bottom quarks and strange anti-quarks, whereas antimatter B_s mesons have an antimatter bottom quark and a matter strange quark ("bottom" and "strange" are two flavors of quarks, and anti-quarks are the antimatter partner particles of normal matter quarks).

"The thing about antimatter is it behaves almost identically to normal matter," said Tara Shears, a physicist at England's University of Liverpoolwho works on the LHCb experiment. "But the devil is in the details, and it's this very tiny difference that we're trying to measure."

B_s mesons are rare, and 70 trillion collisions inside the Large Hadron Collider created only about a thousand of these elusive particles. Yet these were enough to demonstrate a significant abundance of matter B_s mesons compared with antimatter B_s mesons.

"We expected it to be there, but we've never been able to make a measurement of it before because these particles so rare," Shears told LiveScience.

The B_sparticle is only the fourth subatomic particle known to exhibit such a matter-antimatter asymmetry.

CP violation was first discovered in neutral particles called kaons at the Brookhaven National Laboratory in Long Island in the 1960s. It took 40 more years for researchers in the United States and Japan to find the next example of this asymmetry in the B0 meson. After that, the LHCb experiment and others found evidence for CP violation in the B+ meson.

Yet these instances of CP violation aren't enough to explain the prevalence of matter over antimatter in the universe.

"We still have a lot to do to understand the real nature of antimatter," Shears said. "We know we don't understand the whole story. We've just filled in a little bit more information — a block in our jigsaw puzzle if you like."

The researchers hope to make more progress when the LHC starts up again in 2015, at a much higher energy, after its current hiatus.

The LHCb discovery is detailed in a paper submitted to the journal Physical Review Letters.

Follow Clara Moskowitz on Twitter and Google+. Follow us @livescience, Facebook  and Google+. Original article on LiveScience.com.

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Katie Bertsche

Antimatter refers to sub-atomic particles that have properties opposite normal sub-atomic particles.

LiveScience

Scientists say they've made the most precise measurements to date of the magnetic charge of single particles of matter and its spooky counterpart antimatter.

A better understanding of the characteristics of these particles could help scientists solve one of the most baffling mysteries in physics: Why is the universe made of matter and not antimatter?

"According to our theories, the same amount of matter and antimatter was produced during the Big Bang," Harvard physicist Gerald Gabrielse explained in a statement. "When matter and antimatter meet, they are annihilated. As the universe cools down, the big mystery is: Why didn't all the matter find the antimatter and annihilate all of both? There's a lot of matter and no antimatter left, and we don't know why."

Gabrielse and his team captured individual protons and antiprotons in a trap created by electromagnetic fields that keep these particles suspended in one spot for several minutes, ensuring the two don't annihilate each other before measurements are made. For some of their experiments, the team turned to the extensive tunnels of the Geneva-based CERN (the European Organization for Nuclear Research) laboratory, where antiprotons can be created in high-energy collisions at the Large Hadron Collider (LHC).

Inside the LHC, protons zip at near light speed around a 17-mille-long (27 kilometers) underground loop on the border between France and Switzerland. Head-on particle collisions between protons can give rise to exotic particles, including the elusive Higgs boson, the particle theorized to explain how other particles get their mass. [LHC Photos: The World's Largest Atom Smasher]

By looking at the oscillations of the protons and antiprotons created, the scientists measured the size of the magnetic charge of both types of particles more accurately than ever before, boosting the precision of the antiproton measurement by a factor of 680.

"What we wanted to do with these experiments was to say, 'Let's take a simple system — a single proton and a single antiproton — and let's compare their predicted relationships, and see if our predictions are correct," Gabrielse said.

Theory suggests that protons and antiprotons should be virtually identical in their mass and magnitude of charge but that they should have opposite charges. While the new measurements fit within this model, better measurements of protons and antiprotons could shed light on why matter came to dominate in the universe.

"What's also very exciting about this breakthrough is that it now prepares us to continue down this road," Gabrielse said. "I'm confident that, given this start, we're going to be able to increase the accuracy of these measurements by another factor of 1,000, or even 10,000."

The research was detailed Monday in the journal Physical Review Letters.

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Courtesy of Pei-Chen Kuan

The new clock based on a single cesium atom links time to the mass of that atom. As such, not only could atoms be used to measure time, but also time could be used to help define mass.

By Charles Choi
LiveScience

A clock based on just a single atom — the simplest clock yet — has now been devised, researchers say.

This new device to measure time could help lead to a radically new way to define mass as well, scientists added.

In addition, this achievement suggests that researchers might one day build even more exotic clocks — ones based on antimatter, or ones based on no particles at all.

Fundamentally, all clocks measure time by relying on parts that repeat behavior in regular patterns. For instance, a year is defined by how long it takes for Earth to complete an orbit around the sun.

The most accurate clocks that currently exist are atomic clocks. These depend on how atoms switch between two distinct energy levels. Essentially, these clocks rely on at least two particles — the nucleus of an atom, and an electron leaping back and forth between different levels of energy.

Defining time
However, could clocks get simpler still?

"We were interested in what the simplest clocks are to explore the question of what time is," said researcher Holger Müller, a physicist at the University of California at Berkeley. "If you say that, say, you can't measure time with less than two particles, does that mean that anything below two particles doesn't experience time at all?"

The researchers theorized it was possible to create a clock made up of just one particle. To understand, one starts with Einstein's famous equation E=mc², which showed that matter can be converted to energy and vice versa. One consequence of this, called de Broglie's matter-wave hypothesis, suggests that matter can also behave like waves. As such, a particle of matter can in principle behave like a wave that oscillates in a regular manner, thus acting like a clock. [ What's That? Your Physics Questions Answered ]

"We've shown that one single particle really can measure time," Müller told LiveScience.

The problem with making a clock from a particle of matter is that the frequency at which it oscillates "should be so high that one should never be able to measure it," Müller said. To get over this hurdle, the scientists relied on a phenomenon known as time dilation, another consequence of Einstein's theory of relativity. This suggests that as objects move away from and back to a location, they experience less elapsed time than objects that stayed at that location the entire time.

Splitting atoms
The researchers re-created this phenomenon using lasers on cesium atoms. "We essentially split an atom into two halves, and had one stay where it is and the other go forward and come back," Müller said. "A tiny, tiny bit less time elapsed for the half that moved, so it oscillated less."

The fact that one half of the atom oscillated less than the other meant that when these halves are reunited, they did not recombine perfectly, but interference occurred that the scientists could measure. By knowing the size of this discrepancy and the extent to which the researchers disturbed the atom, the researchers could deduce the original frequency at which the atom oscillated.

The moving half of the atom took about a third of a second less than the other half to make its round trip. Each half of the atom made about 10^25 oscillations — a 1 with 25 zeroes behind it, equal to 10 trillion trillion — but the moving half made about 100,000 fewer oscillations than the still half.

"We have demonstrated that you can make a clock from a single massive particle," said researcher Justin Brown, a physicist at the University of California at Berkeley. [ Wacky Physics: The Coolest Little Particles in Nature ]

At present, this new clock can tell time about as precisely as the first atomic clocks developed about 60 years ago and about a billion times less precisely than the best current atomic clocks, known as optical clocks. Although it remains uncertain whether this new clock will ever match the performance of optical clocks, the researchers say it could help solve a problem today regarding one of the world's most important units of measurement — the kilogram.

Since 1889, the kilogram has been defined as the mass of a specific golf-ball-size cylinder of platinum and iridium, which is housed in a vault outside Paris. The problem with defining the kilogram on this object — known formally as the International Prototype Kilogram and more familiarly called Le Grande K — is that contaminants settling on its surface can make it gain weight while cleaning it could make it lose weight, potentially wreaking havoc on one of the main ways science describes everything in the universe.

As such, researchers have in recent years sought to base the kilogram not arbitrarily on an artifact, but on more fundamental constants. The new clock that Müller and his colleagues developed links time to the mass of an atom. As such, not only could atoms be used to measure time, but also time could be used to help define mass.

For instance, as new standard weights, scientists can manufacture incredibly pure crystals of silicon dubbed Avogadro spheres, which are created so precisely that the number of atoms inside is known to high accuracy.

"Our clock and the current best Avogadro spheres would make one of the best realizations of the newly defined kilogram," Müller said. "Knowing the ticking rate of our clock is equivalent to knowing the mass of the particle, and once the mass of one atom is known, the masses of others can be related to it."

There are other strategies that exist on which to base the kilogram  — for instance, by using what is known as a watt balance that uses magnetic force to levitate objects, defining their masses by how much they levitate in response to the magnetic field.

"It's good to have multiple ways of measuring mass — it provides a cross-check for consistency," Müller said.

Future of measuring time
In the future, Müller suggested it might be possible to create even simpler clocks — ones that are based on no particles at all. Quantum theory suggests that what may seem like vacuum is actually filled with "virtual particles" that regularly pop in and out of existence, generating measurable forces.

"It'd be fascinating to see if we can make a clock based off zero particles — you don't even need one particle, just the hypothetical possibility of a particle to measure time," Müller said.

Another interesting possibility is developing a version of this clock that is based on antimatter instead of normal matter. When antimatter is brought into contact with its normal matter, it annihilates its counterpart. One of the greatest mysteries in the universe is why the visible matter in the universe is nearly all normal matter and not antimatter. [ The 9 Greatest Mysteries in Physics ]

"You can have an antimatter clock run for a year as the Earth moves closer to the sun and then farther away, since the Earth's orbit around the sun is not perfectly circular, but slightly elliptical. This means the strength of the gravitational field it experiences would change over time," Müller said.

"It would be interesting to compare a clock of normal matter with a clock of antimatter, to see if they behave the same way in relation to gravity as expected. Such a test of the laws of physics would be fascinating if it was found that matter and antimatter behaved differently."

The scientists detailed their findings online Thursday in the journal Science.

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Chukman So

In addition to making antihydrogen easier to study, a new cooling technique could make it last longer in traps. In 2011, scientists trapped antimatter for a record 16 minutes (artist's conception shown here).

By Clara Moskowitz, LiveScience

Scientists have devised a new method of cooling down antimatter to make it easier to experiment on than ever before.

The new technique could help researchers probe the mysteries of antimatter, including why it's so rare compared with matter in the universe.

Every matter particle has an antimatter partner particle with opposite charge — for example, the antimatter counterpart of an electron is a positron. When matter and antimatter meet, they annihilate each other.

The new technique is focused on antihydrogen atoms, which contain one positron and one antiproton (regular hydrogen contains one electron and one proton). The first experiments on antihydrogen atoms were just performed last year. [ Wacky Physics: The Coolest Little Particles in Nature ]

"The ultimate goal of antihydrogen experiments is to compare its properties to those of hydrogen," physicist Francis Robicheaux of Auburn University in Alabama said in a statement. "Colder antihydrogen will be an important step for achieving this."

That's because antihydrogen atoms are usually relatively hot and energetic, which can distort their properties when measured.

Robicheaux is the co-author of a paper describing the new cooling method published Jan. 6 in the Journal of Physics B: Atomic, Molecular and Optical Physics.

The new technique relies on using precision laser beams to "kick" antihydrogen atoms, knocking loose a bit of energy from them and cooling them down. The process should be able to cool antihydrogen atoms to temperatures 25 times chillier than ever before.

"By reducing the antihydrogen energy, it should be possible to perform more precise measurements of all of its parameters," Robicheaux said. "Our proposed method could reduce the average energy of trapped antihydrogen by a factor of more than 10."

But to cool down antimatter, scientists must first trap it. This is difficult, because antimatter particles would be destroyed if they touched walls made of matter. Thus, researchers use complicated systems of magnetic fields to contain antimatter.

In addition to making antihydrogen easier to study, the new cooling technique could make it last longer in traps. In 2011, scientists at the European physics lab CERN trapped antimatter for an amazingly long 16 minutes, setting a record.

"Whatever the processes are, having slower moving, and more deeply trapped, antihydrogen should decrease the loss rate," Robicheaux said.

The researchers haven't tried the new tactic out yet on actual antimatter atoms, but they used computer simulations to show that it's possible. Their calculations suggest that the particles can be cooled to around 20 millikelvin; in contrast, most trapped antihydrogen atoms have temperatures of up to 500 millikelvin.

"It is not trivial to make the necessary amount of laser light at a specific wavelength," Robicheaux said. "Even after making the light, it will be difficult to mesh it with an antihydrogen trapping experiment. By doing the calculations, we've shown that this effort is worthwhile."

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