Antimatter

NEUTRINOS: A STARRING ROLE
While experimental physicists splice up tiny particles and study them like tea leaves, theoretical physicists contemplate vaster notions. Some spend their days meditating supersymmetry, a theory that postulates the existence of a bevy of yet-undiscovered particles. In addition to each particle's antimatter sibling, supersymmetry predicts another kind of twin for every known particle. Other scientists are betting that everything in the universe is made out of unimaginably tiny vibrating strings, an idea that is alternately called string or M-theory, which has the puzzling implication that we live in a 10-dimensional universe and have only noticed the familiar three.


Murayama is exploring a theory that is rapidly becoming the favored explanation for why there is matter in the universe. It's called leptogenesis, a shorthand way of saying that fundamental particles called leptons are our ultimate beginnings, our Genesis. In this story, Adam and Eve are replaced by the common electron and the curious neutrino, a ghostlike particle that whizzes through Earth as easily as you or I walk through a breeze.


Think of leptogenesis as a playground see-saw: when one side goes up, the other must go down. On this playground, ordinary matter sits on one end of the see-saw, and neutrinos, a kind of lepton, on the other. When the universe was very young, the theory goes, perhaps less than one-billionth of a nanosecond old and roughly the size of a hot air balloon, very heavy neutrinos morphed into a shower of antineutrinos. The antineutrinos pulled the lepton side of the see-saw down; the amount of ordinary matter had no choice but to increase.


If the leptogenesis theory is correct, "which right now pretty much everyone's betting on," says Murayama, then ultimately you owe your existence to the neutrino, a particle so ethereal, so seemingly inconsequential that there are trillions passing through your body as you read this.


Intriguing as leptogenesis is, no one can say for sure whether it or any of the other renegade theories about matter's origins is correct, because the means to evaluate them do not exist. Scientists are talking of building a novel and very expensive machine, a so-called neutrino factory, that would put leptogenesis to the test. For now, though, the hands-on physicists plug away, staking out testable territory and scrutinizing the minutiae of their results, while the big thinkers tinker with the foundations of it all. Theory versus experiment: that disparity, as fundamental and as complementary, perhaps, as the one between matter and antimatter, has always been at the heart of the effort to understand our world. Chances are, the matter-antimatter problem will eventually be solved through synergy: The experimenters will find an unexpected quirk in the workings of elementary particles, and the theorists will find a consistent and elegant way to explain it.