UCI Professor Mu-Chun Chen in Quanta Magazine Article

The universe has cooked up all sorts of bizarre and beautiful forms of matter, from blazing stars to purring cats, out of just three basic ingredients. Electrons and two types of quarks, dubbed “up” and “down,” mix in various ways to produce every atom in existence.

 

But puzzlingly, this family of matter particles — the up quark, down quark and electron — is not the only one. Physicists have discovered that they make up the first of three successive “generations” of particles, each heavier than the last. The second- and third-generation particles transform into their lighter counterparts too quickly to form exotic cats, but they otherwise behave identically. It’s as if the laws of nature were composed in triplicate. “We don’t know why,” said Heather Logan, a particle physicist at Carleton University.

 

In the 1970s, when physicists first worked out the Standard Model of particle physics — the still-reigning set of equations describing the known elementary particles and their interactions — they sought some deep principle that would explain why three generations of each type of matter particle exist. No one cracked the code, and the question was largely set aside. Now, though, the Nobel Prize-winning physicist Steven Weinberg, one of the architects of the Standard Model, has revived the old puzzle. Weinberg, who is 86 and a professor at the University of Texas, Austin, argued in a recent paper in the journal Physical Review D that an intriguing pattern in the particles’ masses could lead the way forward.