Why Do Matter Particles Come in Threes?

The universe has cooked up all kinds of weird and exquisite types of matter, from blazing stars to purring cats, out of simply three fundamental substances. Electrons and two varieties of quarks, dubbed “up” and “down,” combine in numerous methods to supply each atom in existence.

Original story reprinted with permission from Quanta Magazine, an editorially impartial publication of the Simons Foundation whose mission is to boost public understanding of science by protecting analysis develop­ments and traits in mathe­matics and the bodily and life sciences.

However puzzlingly, this household of matter particles—the up quark, down quark, and electron—will not be the one one. Physicists have found that they make up the primary of three successive “generations” of particles, every heavier than the final. The second- and third-generation particles remodel into their lighter counterparts too rapidly to type unique cats, however they in any other case behave identically. It’s as if the legal guidelines of nature had been composed in triplicate. “We don’t know why,” mentioned Heather Logan, a particle physicist at Carleton College.
Within the 1970s, when physicists first labored out the usual mannequin of particle physics—the nonetheless reigning set of equations describing the recognized elementary particles and their interactions—they sought some deep precept that might clarify why three generations of every sort of matter particle exist. Nobody cracked the code, and the query was largely put aside. Now, although, the Nobel Prize–successful physicist Steven Weinberg, one of many architects of the usual mannequin, has revived the previous puzzle. Weinberg, who's 86 and a professor on the College of Texas, Austin, argued in a recent paper within the journal Bodily Evaluate D that an intriguing sample within the particles’ lots may paved the way ahead.
“Weinberg’s paper is a little bit of lightning at the hours of darkness,” mentioned Anthony Zee, a theoretical physicist on the College of California, Santa Barbara. “Rapidly a titan within the discipline is instantly working once more on these issues.”
“I’m very pleased to see that he thinks it’s essential to revisit this drawback,” mentioned Mu-Chun Chen, a physicist on the College of California, Irvine. Many theorists are prepared to surrender, she mentioned, however “we should always nonetheless be optimistic.”
The usual mannequin doesn't predict why every particle has the mass that it does. Physicists measure these values experimentally and manually plug the outcomes into the equations. Measurements present that the minuscule electron weighs 0.5 megaelectron volts (MeV), whereas its second- and third-generation counterparts, referred to as the muon and the tau particle, tip the scales at 105 and 1,776 MeV, respectively. Equally, the first-generation up and down quarks are relative lightweights, whereas the “attraction” and “unusual” quarks comprising the second quark technology are middleweights, and the “prime” and “backside” quarks of the third technology are heavy, the highest weighing a monstrous 173,210 MeV.
The unfold within the lots is huge. When physicists squint, although, they see a tantalizing construction in the place the lots fall. The particles cluster into considerably evenly spaced generations: The third-generation particles all weigh hundreds of MeV, second-generation particles weigh roughly tons of of MeV, and first-generation particles are available at round an MeV every. “As you go every stage down, they get exponentially lighter,” says Patrick Fox, a particle physicist on the Fermi Nationwide Accelerator Laboratory in Illinois.
Within the equations of the usual mannequin, the mass of every particle corresponds to the diploma to which it “feels” a universe-filling discipline generally known as the Higgs discipline. High quarks are heavy as a result of they expertise intense drag as they transfer by the Higgs discipline, like a fly caught in honey, whereas wispy electrons flit by it like butterflies in air. On this framework, how every particle feels the sector is an intrinsic attribute of the particle.

The Normal Mannequin of particle physics contains three copies of every sort of matter particle, which type the quadrants of the outer ring of the diagram.Illustration: Lucy Studying-Ikkanda/Quanta Journal

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