Theory says the as-yet-unidentified Higgs boson is probably responsible. Named for the British theorist Peter Higgs, who first proposed the idea, the Higgs may explain the lopsided masses of the photon and the W and Z, the particles of the electroweak force. Physicists harbor hopes, too, that the Higgs gives quarks and leptons their masses as well.
If it does, and if it is found, the Higgs won't be just another particle to add to the inventory. After all, it would represent a field that exists everywhere, permeating space, touching everything.
Some particles slip right through this field and so have no mass, theory claims. Other particles encounter resistance, just as a piece of iron does when it moves in a magnetic field. They "stick" to the Higgs field, gaining weight. The Higgs, in the language of physicists, breaks the symmetry of the electroweak force. As a consequence, the W and Z are heavy and the photon weighs nothing at all.
Theoretically promised but awaiting the results of experiments at new, higher-energy accelerators, the Higgs may finally emerge at the upgraded Tevatron, at Fermilab in Illinois, and at the Large Hadron Collider, now being built at CERN, the European laboratory for particle physics. But what exactly are physicists looking for?
According to one theory, the Higgs is a pointlike, indivisible particle, like an electron or a quark. There may be just one Higgs, but physicists doubt that. To account for complexities, there are probably more.
According to another theory, the Higgs is more like a proton: a particle made of still smaller elements.
Whatever the case, physicists are certain, the discovery will be their first venture into new physics. The already familiar bosons aren't capable of the interactions required of the Higgs. The Higgs must be something very different.
Clues may lie in an odd phenomenon: the morphing of one particle into another.
Quarks, the particles that bind to form protons and neutrons, morph all the time. They fall apart, or decay, starting out as one particle and ending up another, switching identities.
In this process, the particles clearly have preferences. When a bottom quark decays, for example, it is 100 times more likely to convert into a charm quark than into an up quark.
Physicists can predict the probabilities of turning into one particle rather than another, but the rule they use is just a matrix of inputs a handful of numbers, many relating to mass. The numbers seem to work, the way a farmer's almanac works, but why? Preferences are linked with mass, but how?
To get behind the numbers, physicists are studying the phenomenon of particle transformation in matter and its near-mirror-image, antimatter. One area of intense interest: how B mesons (composed of an anti-bottom quark and a down quark) change into anti-Bs and how the decay of Bs differs from the decay of anti-Bs. Physicists will search for patterns and push to the limits the predictive powers of that numerical matrix, finding where it breaks down. Where it doesn't work, they'll have to find explanations - and possibly even new physics searching, ultimately, for an understanding of mass.
Closer study of differences in the way matter and antimatter behave should also help resolve a question of cosmological significance: why the universe is made mostly of matter, not antimatter, when both were created in equal parts in the Big Bang.
Morphing likely exists among the three types of neutrinos, too elusive particles that didn't seem to have any mass at all.
Electron, muon, and tau neutrinos are abundant everywhere, many of them created 12 billion years ago in the Big Bang. But neutrinos are also produced every day: in nuclear reactions in the Sun and by cosmic rays, high-energy particles that constantly bombard Earth.
In a stunning experiment a few years ago, researchers counted the number of muon neutrinos in the atmosphere that entered their detector from two directions, overhead and underneath. The neutrinos entering from underneath had to travel an extra 13,000 kilometers through the planet.
Fewer neutrinos arrived from below than from above. The different routes apparently gave them less or more time to change from one kind of neutrino into another.
A still more recent experiment strongly suggests that electron neutrinos leaving the Sun also morph during their eight-minute journey to Earth.
Like quarks, then, neutrinos appear to be mixing up their identities. If they do convert from one type to another, then by laws of quantum mechanics, they must have mass. In fact, they must have different masses.
The recent findings have opened a whole new arena for studying particles' changing states, and have begun to unsettle the distinction between quarks and leptons. Maybe the two kinds of matter are not such very different particles after all.