Antimatter Gets Heavy

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“There’s that word again; ‘heavy.’ Why are things so heavy in the future? Is there a problem with the earth’s gravitational pull?” — Doc Brown, Back to the Future

It seems antimatter might have a bit of a weight problem these days, if a new paper on the arXiv is to be believed — specifically, antimatter helium-4, an extremely rare (and heavy!) particle that just made its debut at Brookhaven’s Relativistic Heavy Ion Collider (RHIC).

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The discovery was made by the STAR collaboration, designed to study the formation and characteristics of the quark-gluon plasma (QGP), a state of matter believed to have existed for ten-millionths of a second after the universe’s birth. It then cooled and condensed to form the protons and neutrons that comprise matter today.

Physicists believe that in those first fractions of a second, the universe was so hot that no nuclei could exist. Instead, there was the QGP, made of quarks and gluons (the massless particles that “carry” the force between quarks). Nuclear and particle physicists had long dreamed of making this exotic plasma in a laboratory and investigating it experimentally. Because of the enormous energies required, the lab would have to be a particle accelerator.

That’s where RHIC comes in. In RHIC’s 2.4-mile-long ring, gold ions whip around the ring in both directions at once, further accelerated by strategically placed coils of wire that emit radiofrequency radiation. There are six different sites around the ring where collisions can occur.

When those gold nuclei collide head-on, a hot, dense plasma of quarks and gluons forms — or, more accurately, something akin to a near-frictionless liquid, much to RHIC scientists’ surprise when they finally succeeded in creating a QGP in 2005.

It is the hottest temperature yet created in a lab: 4 trillion degrees Celsius, as much heat as what filled the universe for a fraction of a second after the Big Bang explosion. That’s 250,000 times hotter than the center of the sun.

The hot plasma lasts only 10-23 seconds. Then the plasma expands and cools, such that the quarks and gluons “freeze out,” leaving a spray of thousands of elementary particles. (If you’re worried about wasting gold, rest easy: only minute amounts are used. According to Physics Central, “If RHIC ran for twenty years, it would consume only about one gram of gold.”)

Okay, but what does the QGP have to do with antimatter? Well, something else was going on during the initial cooling period for the QGP: matter and antimatter were colliding and annihilating each other out of existence constantly. This process slowed down as our universe gradually cooled, but there should have been equal parts matter and antimatter — and there weren’t. Instead, there were slightly more matter particles than antimatter.

We know this because we can see the remnants of the survivors of that cosmological Octagon all around us: every bit of matter in our observable universe, from galaxies to dust mites and everything in between, exists because matter won that long-ago war of attrition. And physicists have no idea why that asymmetry should have existed in the first place. It’s one of the Big Questions in 21st century particle physics.

Today, the only place antimatter exists is in particle accelerators like RHIC — and it takes enormous energies to produce even a tiny bit of the stuff. The first antimatter particle (the positron) was observed in 1932. Almost 20 years went by before we had the technology to producer heavier states of antimatter, namely, antiprotons and antineutrons in 1955. Antihydrogen didn’t make its debut until 1995, followed in 2010 by strange antimatter (dubbed antihypertriton).

So what the STAR collaboration has accomplished is a very big deal: the scientists collided gold nuclei a billion times, and out of the resulting QGP emerged protons, neutrons and their respective antiparticles. The fallout included a grand total of 18 antimatter helium-4 nuclei, the heaviest antiparticle yet produced.

You have to consume a lot of calories to gain a lot of weight, and the same is true with antimatter. The heavier the antiparticle, the more energy is required to create it. So scientists don’t expect to create the next predicted antiparticle, antilithium-6, any time soon. STAR scientists say we just don’t have the collider technology capable of achieving the feat — not even with the Large Hadron Collider.

But the discovery does have more value than just being kind of cool: the rate of production is pretty much in line with prevailing theory, which means we aren’t likely to find naturally occurring antimatter elsewhere in the universe. That means that “any observation of antihelium or even heavier antinuclei in space would indicate the existence of a large amount of antimatter elsewhere in the universe,” according to the STAR collaboration. There would be a another mechanism capable of producing antimatter in far greater amounts than we can manage today.

It just so happens that when Space Shuttle Endeavor launches next month, its payload will include an instrument capable of finding any bits of antimatter that might be lurking about: the Alpha Magnetic Spectrometer (AMS). AMS will be mounted in the International Space Station, and search cosmic rays for any whiff of antimatter. If it finds any — well, that would be “heavy” indeed.

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