Image: ASACUSA experiment
The ASACUSA experiment at Europe's CERN particle-physics research center has measured the mass of an antiproton to an accuracy of about one part in a billion.
By Managing editor
updated 7/29/2011 6:55:58 AM ET 2011-07-29T10:55:58

A new measurement provides the most accurate weight yet of antimatter, revealing the mass of the antiproton (the proton's antiparticle) down to one part in a billion, researchers announced Thursday.

To give a sense of just how accurate their measurement was, researcher Masaki Hori said: "Imagine measuring the weight of the Eiffel Tower. The accuracy we've achieved here is roughly equivalent to making that measurement to within less than the weight of a sparrow perched on top. Next time it will be a feather."

The result, detailed this week in the journal Nature, may help scientists investigate the mystery of why the universe is made of regular matter even though they suspect roughly equal parts of matter and antimatter were around just after the universe formed. When a particle, such as a proton, meets with its antimatter partner, the antiproton, the two annihilate each other in a powerful explosion.

"At present, we are very far from understanding what happened to all the antimatter that was created in equal proportion to matter in the Big Bang," physicist Mike Charlton of Swansea University in the United Kingdom wrote in an accompanying Nature article.

The experiment was carried out in the anti­proton decelerator at CERN, the European particle-physics laboratory near Geneva, as part of the lab's Atomic Spectroscopy And Collisions Using Slow Antiprotons experiment, or ASACUSA.

The machine sends pulses of antiprotons about every hundred seconds into cold helium gas. While most of the antiprotons quickly annihilate with regular matter, a tiny number survive by combining with helium to form hybrid atoms that contain matter and antimatter — antiprotonic helium. The antiproton takes the place of an electron in these hybrids, sitting in a spot that's shielded from the helium nucleus (which is regular matter and which would cause the two to annihilate).

Using laser beams to excite the atoms, scientists can then get the antiproton to jump to a new energy level, one that is no longer shielded from the nucleus and — bang! — annihilation. The wavelength of light used to force this jump can be placed into complex equations that reveal the mass of an antiproton to an unprecedented level of accuracy.

However, a source of inaccuracy comes from the fact that the atoms jiggle around, so that those moving toward and away from the beam experience slightly different frequencies. A similar effect, called the Doppler shift, causes the siren of an approaching ambulance to apparently change pitch as it passes you.

In their previous measurement in 2006, the same team used one laser beam, and the achievable accuracy was dominated by this jiggling effect. This time they used two beams moving in opposite directions, with the result that the jiggle for the two beams was partly canceled out. The result was a four-fold boost in accuracy.

"This is a very satisfying result," Masaki Hori, a project leader in the antiproton collaboration, said in a statement. "It means that our measurement of the antiproton's mass relative to the electron is now almost as accurate as that of the proton."

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These experiments may really head scientists in the right direction for figuring out the antimatter-matter conundrum, Charlton said.

"There's some unknown asymmetry built into the laws of nature, which we physicists have not yet been able to understand and to pinpoint," Charlton told LiveScience. "So making comparisons as accurate as you can between matter and antimatter is important, because sooner or later there is going to be found something in which they are different."

He added, "We actually don't know where to look (for the answer). We have no theoretical guidance on this whatsoever." Even so, the result of that difference, though likely to be tiny, "is profound," he said.

Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.

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Explainer: What’s a hadron? Tour the particle zoo

  • Image: Elementary particles

    The Standard Model of particle physics is one of science's most successful theories, enabling the development of devices ranging from light bulbs, to microwave ovens and television, to quantum computing devices. The Standard Model is also one of the oddest theories, because it lays out a dizzying menagerie of hundreds of subatomic particles. At its heart are 16 types of elementary particles ... plus at least one more mysterious particle that scientists are spending billions of dollars to detect.

    Click on "Next" to get the full rundown.

  • Quarks

    Image: Quarks and gluons
    Berkeley Lab

    Six "flavors" of quarks have been detected: up and down, charm and strange, top and bottom. Quarks are almost always found in different combinations, bound together by gluons (more on those later). Particles built up from quarks and gluons are called hadrons. The Large Hadron Collider is so named because it's a large collider that smashes hadrons together.

    Three-quark combinations fit in the category of baryons. The best-known baryons are the proton (with two up quarks and one down quark) and the neutron (with two down quarks and one up quark).

    Particles that have one quark and one antiquark fit in the category of mesons. For example, the pion, or pi meson, contains an up quark and an anti-down quark.

  • Leptons

    Image: Single electrons in helium
    Brown University

    Six "flavors" of leptons have been detected: The negatively charged electron is the best-known lepton — along with its antimatter counterpart, the positron. This photo shows the path of single electrons passing through liquid helium, in an experiment devised by Brown University researchers.

    The muon is also negatively charged, but it's about 207 times as massive as the electron. ("Who ordered that?" physicist Isidor Rabi reportedly asked.) The negatively charged tau particle is even bigger — 3,477 times as massive as the electron — but it decays into other particles in less than a trillionth of a second.

    Each of those leptons has a neutrino associated with it: the electron neutrino, the muon neutrino and the tau neutrino. Neutrinos interact only weakly with other particles, and they zip through our planet virtually without a trace. Physicists only recently determined that they have mass, but there's still a great deal of mystery surrounding the ghostly particles.

  • Force carriers

    Image: Graviton

    The Standard Model sets aside a category for particles that are associated with force fields. The effect of a field can be viewed as involving an exchange of such force-carrying particles.

    Four elementary force-carrying particles have been detected. The best-known force carrier is the photon — which plays a part in the electromagnetic spectrum, including visible light. The gluon binds quarks together through the strong nuclear force. The weak nuclear force involves the exchange of W and Z bosons. The W boson can carry a positive or a negative charge, while the Z boson is neutral.

    If gravity could be incorporated into the Standard Model, the force-carrying particle would be called the graviton (shown here in an artist's depiction). However, gravitons have not yet been detected, and at least for now, such particles are not accounted for in the Standard Model.

  • Bosons vs. fermions

    Image: Bosons vs. fermions
    Rice Univ. via AIP

    All force-carrying particles are bosons, but not all bosons are force carriers. The difference has to do with a property known as particle spin. Particles with a fractional spin value (for example, electrons, protons and neutrons) are fermions. Two identical fermions cannot occupy the same quantum state. This is a property that keeps electrons from collapsing into a jumble, and thus makes chemical reactions possible.

    All particles with a whole-integer spin value are classified as bosons, and such particles can occupy the same quantum state even if they're identical. The photon is the best-known type of boson.

    Even atoms can be classified as fermions and bosons. This photo shows how atom clouds of lithium-7 (bosons) and lithium-6 (fermions) behave at low temperatures. The bosons collapse into a compact cloud, while the fermions can't squeeze that closely together.

  • The mysterious Higgs

    Image: Higgs as seen by CMS
    Ianna Osborne / CERN / CMS Collection

    The Higgs boson is the only particle predicted by the Standard Model that has not yet been detected. The Higgs is the main quarry for physicists at the Large Hadron Collider. This image is a simulation of the Higgs' signature as it might appear in one of the LHC's detectors.

    The Higgs boson, named after Scottish theorist Peter Higgs, is thought to be associated with a field that endows some particles (such as the weak nuclear force's W and Z bosons) with mass, while leaving the electromagnetic force's photons without mass.

    This Higgs field may have played a role at the very beginnings of the universe: Physicists believe that at the highest energies, the electromagnetic and weak nuclear forces were unified, but something led to "electroweak symmetry breaking" as the infant cosmos cooled. That would be why the electromagnetic force and the weak nuclear force are distinct in the current universe. The Large Hadron Collider could shed new light on this mysterious Higgs mechanism.

  • Why so complicated?

    Image: Particle zoo
    Tim Jones / McDonald Observatory / HETDEX

    Hadrons and leptons? Baryons and mesons? Fermions and bosons? Sometimes it seems as if particle physicists set up these classifications just to keep outsiders totally confused. But for researchers, these occasionally overlapping categories are useful for figuring out how different types of particles interact with each other.

    In a sense, it's as if we've been talking about the game of chess but have gotten only to the point of naming the different pieces on the board: black pieces and white ones, pawns and knights, bishops and rooks, kings and queens. The real meaning of the game comes out when you start studying how the pieces perform and interact.

    To delve into the deeper meaning of the Standard Model, you can visit The Particle Adventure at Lawrence Berkeley National Laboratory, "A Subatomic Venture" at CERN, or Particle Physics UK.


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