Different representations of the Poincaré group are particles with different numbers of spin labels, or degrees of freedom that are affected by rotations. There are, for example, particles with three spin degrees of freedom. These particles rotate in the same way as familiar 3D objects. All matter particles, meanwhile, have two spin degrees of freedom, nicknamed “spin-up” and “spin-down”, which rotate differently. If you rotate an electron by 360 degrees, its state will be inverted, just as an arrow, when moved around a 2D Möbius strip, comes back around pointing the opposite way.
Elementary particles with one and five spin labels also appear in nature. Only a representation of the Poincaré group with four spin labels seems to be missing.
The correspondence between elementary particles and representations is so neat that some physicists — like Van Raamsdonk’s professor — equate them. Others see this as a conflation.
Natalie WolchoverThe representation is not the particle; the representation is a way of describing certain properties of the particle, said Sheldon Glashow, a Nobel Prize-winning particle theorist and professor emeritus at Harvard University and Boston University.Let us not confuse the two.
Interesting article about the multitude of ways in which physicists are examining the concept of elementary particle. Nevertheless, my initial impression was that the central premise of the article, that physicists know so little about the answer to this fundamental question, is a bit contrived – as if either the scientists were trying to justify their continuing experiments by downplaying existing knowledge and overemphasizing remaining mysteries, or the author was doing something similar to be able to cover the topic again and again in the future.
I later read another article on the site about new results regarding the structure of the proton, and that impression shifted considerably. Granted, the proton isn’t a fundamental particle, but studying its composition may reveal important insights into quarks, gluons, and their complex interactions.
The results confirmed a sophisticated and outlandish theory that had by then replaced Gell-Mann and Zweig’s quark model. Developed in the 1970s, it was a quantum theory of the “strong force” that acts between quarks. The theory describes quarks as being roped together by force-carrying particles called gluons. Each quark and each gluon has one of three types of “color” charge, labeled red, green and blue; these color-charged particles naturally tug on each other and form a group — such as a proton — whose colors add up to a neutral white. The colorful theory became known as quantum chromodynamics, or QCD.
According to QCD, gluons can pick up momentary spikes of energy. With this energy, a gluon splits into a quark and an antiquark — each carrying just a tiny bit of momentum — before the pair annihilates and disappears. It’s this “sea” of transient gluons, quarks and antiquarks that HERA, with its greater sensitivity to lower-momentum particles, detected firsthand.
Short-lived charms frequently show up in the “quark sea” view of the proton (gluons can split into any of six different quark types if they have enough energy). But the results from Rojo and colleagues suggest that the charms have a more permanent presence, making them detectable in gentler collisions. In these collisions, the proton appears as a quantum mixture, or superposition, of multiple states: An electron usually encounters the three lightweight quarks. But it will occasionally encounter a rarer “molecule” of five quarks, such as an up, down and charm quark grouped on one side and an up quark and charm antiquark on the other.
Charlie Wood
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