By using higher-energy electrons, physicists can discover more detailed properties of the target proton. Thus, the electron energy determines the maximum resolving power of an inelastic deep scattering experiment. More powerful particle colliders provide a clearer view of the proton.
High-energy colliders also produce a wider array of collision results, allowing researchers to select different subsets of outgoing electrons for analysis. This flexibility is key to understanding the quarks that matter inside the proton with different amounts of momentum.
By measuring the energy and trajectory of each scattered electron, the researchers can tell whether a quark with a momentum as large as the total momentum of a proton has peeked through, or just a tiny particle. Through repeated collisions, they can perform something like a census—determining whether the proton's momentum is mostly confined to a few quarks or distributed among many.
Even the SLAC proton-splitting collisions were mild by today's standards. In those scattering events, the electrons are often ejected in ways that indicate they have collided with quarks that carry a third of the proton's total momentum. This finding was consistent with the theory of Murray Gellmann and George Zweig, who postulated in 1964 that a proton is composed of three quarks.
Gelmann and Zweig's "quark model" remains an elegant way to imagine the proton. It has two "up" quarks with electrical charges of +2/3 and one "down" quark with charge of -1/3, for a total proton charge of +1. Inside the proton, "the most complicated thing you can imagine."
The positively charged particle at the heart of the atom is an object of indescribable complexity that changes its appearance depending on how it is explored. We have tried to connect the many facets of the proton to form the most complete picture ever made.
Researchers recently discovered that the proton sometimes contains an attractive quark and an attractive antiquark, massive particles that are each heavier than the proton itself.
More than a century after Ernest Rutherford discovered the positively charged particle at the heart of every atom, physicists are still trying to fully understand the proton.
High school physics teachers describe them as featureless balls with a unit of positive electrical charge—perfect foils for the negatively charged electrons buzzing around them. College students learn that the ball is actually a bundle of three fundamental particles called quarks. But decades of research have revealed a deeper truth, one too strange to be fully captured in words or pictures.
"It's the most complicated thing you can imagine," says Mike Williams, a physicist at the Massachusetts Institute of Technology. "Actually, you can't even imagine how complicated it is."
The proton is a quantum mechanical object that exists as a fog of possibilities until an experiment forces it into concrete form. And its forms vary wildly depending on how researchers set up their experiments. Connecting the many faces of this particle has been the work of generations. "We are just beginning to fully understand this system," says Richard Milner, a nuclear physicist at MIT.
As the chase continues, the mysteries of Proton continue to unravel. Recently, an analysis of historical data published in August showed that the proton contains streaks of particles called attractive quarks that are heavier than the proton itself.
Proton "has been humbling for humans," Williams said. "Every time you think you have a handle on it, it throws you a few curve balls."
Recently, Milner, along with Rolf Ent at Jefferson Labs, MIT filmmakers Chris Buble and Joe McMaster, and animator James LaPlante, set out to turn a series of secret plots that turn the results of hundreds of experiments into a series of animated shapes. - Proton Shift We've included their animations in our attempt to unravel its secrets.
Open the proton break
Proof that protons contain large quantities came from the Stanford Linear Accelerator Center (SLAC) in 1967. In previous experiments, researchers pelted it with electrons and watched them bounce like billiard balls. But SLAC could eject the electrons with more force, and the researchers saw that they bounced back differently. The electrons would hit the proton hard enough to break it apart—a process called deep inelastic scattering—and return as point-like pieces of the proton called quarks. "This was the first evidence that quarks really exist," says Xiaochao Zheng, a physicist at the University of Virginia.
After the discovery of SLAC, which won the Nobel Prize in Physics in 1990, scrutiny of the proton intensified. Physicists have performed hundreds of scattering experiments to date. They infer different aspects of the object's interior by adjusting the intensity of the object's bombardment and selecting the scattered particles they collect as a result.
