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Why do material particles come in threes?

The universe made all sorts of bizarre and beautiful forms of matter from just three basic ingredients, from blazing stars to purring cats. Electrons and two types of quarks, referred to as "up" and "down", mix in different ways to create each existing atom.

Original story with permission from Quanta Magazine an editorially independent publication by the Simons Foundation, whose mission is to improve public understanding of science by reporting on research developments and trends in mathematics, physics, and life sciences .

But puzzlingly, this family of matter particles ̵

1; the up-quark, the down-quark and the electron – is not the only one. Physicists have discovered that they form the first of three successive "generations" of particles, each of which is 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 is 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 the physicists first developed the standard model of particle physics – the still prevailing system of equations that describes the known elementary particles and their interactions – they were looking for a deep principle that would explain why three generations of each type of Particles of matter exist. Nobody cracked the code and the question has been largely put aside. Now the Nobel Prize winner Steven Weinberg, one of the architects of the standard model, has revived the old riddle. Weinberg, 86, and a professor at the University of Texas at Austin, recently argued in an article in Physical Review D that a fascinating pattern in the masses of particles could point the way forward. [19659004] "Weinberg's article is a bit lightning fast in the dark," said Anthony Zee, a theoretical physicist at the University of California at Santa Barbara. "Suddenly a Titan in the field suddenly works on these problems again."

"I am very happy to see that he considers it important to re-examine this problem," said Mu-Chun Chen, physicist at the University of California, Irvine. Many theorists are willing to give up, she said, but "we should still be optimistic."

The standard model does not predict why each particle has the mass it has. Physicists measure these values ‚Äč‚Äčexperimentally and manually insert the results into the equations. Measurements show that the tiny electron weighs 0.5 megaelectronvolt (MeV), while its counterparts of the second and third generations, called the muon and the tau particle, tip the scales at 105 and 1,776 MeV, respectively. Similarly, the up and down quarks of the first generation are relative light weights, while the "charm" and "strange" quarks that make up the second generation of quarks are medium weights and the "upper" and "lower" quarks of the third Generation heavy are tops with a monstrous weight of 173,210 MeV.

The spread in the masses is enormous. When physicists blink, they see an alluring structure that the masses fall into. The particles are grouped into somewhat evenly distributed generations: the third generation particles weigh all thousands of MeV, second generation particles weigh approximately hundreds of MeV and first generation particles come in with about one MeV each. "Going down every level will make them exponentially lighter," said Patrick Fox, particle physicist at the Fermi National Accelerator Laboratory in Illinois.

In the equations of the standard model, the mass of each particle corresponds to the degree to which it "feels" a universe-filling field known as the Higgs field. Top quarks are heavy because they experience strong resistance when they move through the Higgs field, like a fly stuck in honey, while wispy electrons flit through the Higgs field like butterflies in the air. In this context, the way each particle feels the field is an intrinsic attribute of the particle.

The Standard Model of Particle Physics contains three copies of each type of particle of matter that form the quadrant of the particle's outer ring on the diagram. Illustration: Lucy Reading-Ikkanda / Quanta Magazine

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