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The three valence quarks of a proton contribute to its spin, but so do the gluons, sea quarks and antiquarks, and orbital angular momentum as well. The electrostatic repulsion and the attractive strong nuclear force, in tandem, are what give the proton its size, and the properties of quark mixing are required to explain the suite of free and composite particles in our Universe. Individual protons, overall, behave as fermions, not as bosons. (APS/ALAN STONEBRAKER)

What’s Really Inside A Proton?

If you think it’s just three quarks held together by gluons, you’ll want to read this.

Ethan Siegel
Feb 22 · 3 min read

At a fundamental level, the Universe is composed of indivisible particles.

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From macroscopic scales down to subatomic ones, the sizes of the fundamental particles play only a small role in determining the sizes of composite structures. Whether the building blocks are truly fundamental and/or point-like particles is still not known, but we do understand the Universe from large, cosmic scales down to tiny, subatomic ones. There are nearly 1⁰²⁸ atoms making up each human body, in total. (MAGDALENA KOWALSKA / CERN / ISOLDE TEAM)

Every structure contains “uncuttable” constituents that cannot be divided further.

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Individual and composite particles can possess both orbital angular momentum and intrinsic (spin) angular momentum. When these particles have electric charges either within or intrinsic to them, they generate magnetic moments, causing them to be deflected by a particular amount in the presence of a magnetic field, helping us reveal their existence and properties. (IQQQI / HAROLD RICH)

Even protons and neutrons are composite: containing fundamental quarks and gluons.

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Individual protons and neutrons may be colorless entities, but the quarks within them are colored. Gluons can not only be exchanged between the individual gluons within a proton or neutron, but in combinations between protons and neutrons, leading to nuclear binding. However, every single exchange must obey the full suite of quantum rules. (WIKIMEDIA COMMONS USER MANISHEARTH)

There aren’t just three quarks inside each one, but a sea of particles.

A better understanding of the internal structure of a proton, including how the “sea” quarks and gluons are distributed, has been achieved through both experimental improvements and new theoretical developments in tandem. A proton is a lot more than just three quarks held together by gluons. (BROOKHAVEN NATIONAL LABORATORY)

Since quarks have:

  • mass,
  • electric charge,
  • color charge,
  • and weak force couplings,

they interact with all known particles.

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The Higgs boson, now with mass, couples to the quarks, leptons, and W-and-Z bosons of the Standard Model, which gives them mass. That it doesn’t couple to the photon and gluons means those particles remain massless. Quarks couple to all of the force carriers. Photons, gluons, and W-and-Z bosons couple to all particles that experience the electromagnetic, strong, and weak nuclear forces, respectively. If there are additional particles out there, they may have these couplings, too. (TRITERTBUTOXY AT ENGLISH WIKIPEDIA)

The more energetically you look inside a proton, the denser this “sea” of internal particles appears.

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A proton isn’t just three quarks and gluons, but a sea of dense particles and antiparticles inside. The more precisely we look at a proton and the greater the energies that we perform deep inelastic scattering experiments at, the more substructure we find inside the proton itself. There appears to be no limit to the density of particles inside. (JIM PIVARSKI / FERMILAB / CMS COLLABORATION)

Deep inelastic scattering helps reveal these particles and antiparticles by smashing protons together.

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A four-muon candidate event in the ATLAS detector at the Large Hadron Collider. (Technically, this decay involves two muons and two anti-muons.) The muon/anti-muon tracks are highlighted in red, as the long-lived muons travel farther than any other unstable particle. The energies achieved by the LHC are sufficient for creating Higgs bosons; previous electron-positron colliders could not achieve the necessary energies. (ATLAS COLLABORATION/CERN)

It’s a numbers game: more collisions at higher energies increases our odds.

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A schematic of the world’s first electron-ion collider (EIC). Adding an electron ring (red) to the Relativistic Heavy Ion Collider (RHIC) at Brookhaven would create the eRHIC: a proposed deep inelastic scattering experiment that could improve our knowledge of the internal structure of the proton significantly. (BROOKHAVEN NATIONAL LABORATORY-CAD ERHIC GROUP)

With dark matter, dark energy, and many other unexplained phenomena out there, the Standard Model alone cannot explain everything.

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This snippet from a structure-formation simulation, with the expansion of the Universe scaled out, represents billions of years of gravitational growth in a dark matter-rich Universe. Note that filaments and rich clusters, which form at the intersection of filaments, arise primarily due to dark matter; normal matter plays only a minor role. (RALF KÄHLER AND TOM ABEL (KIPAC)/OLIVER HAHN)

While astrophysicists look “outward” to explore the Universe, particle physicists look “inward” at matter itself.

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When two protons collide, it isn’t just the quarks making them up that can collide, but the sea quarks, gluons, and beyond that, field interactions. All can provide insights into the spin of the individual components, and allow us to create potentially new particles if high enough energies and luminosities are reached. (CERN / CMS COLLABORATION)

In tandem, both fields help scientists understand the Universe’s structure, nature, rules, and composition.

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The inside of the LHC, where protons pass each other at 299,792,455 m/s, just 3 m/s shy of the speed of light. As powerful as the LHC is, we need to start planning for the next generation of colliders if we want to uncover the secrets of the Universe that lie beyond the LHC’s capabilities. (JULIAN HERZOG / C.C.A-BY-3.0)

The Large Hadron Collider at CERN has revealed many of the Standard Model’s secrets, but nothing beyond it.

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The observed Higgs decay channels vs. the Standard Model agreement, with the latest data from ATLAS and CMS included. The agreement is astounding, and yet frustrating at the same time. By the 2030s, the LHC will have approximately 50 times as much data, but the precisions on many decay channels will still only be known to a few percent. A future collider could increase that precision by multiple orders of magnitude, revealing the existence of potential new particles. (ANDRÉ DAVID, VIA TWITTER)

More data at higher energies increases the probability of discovering something fundamentally new.

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The planned timeline of the Large Hadron Colliders runs and upgrades. Although the COVID-19 pandemic may delay this slightly, the fact is that we have only finished Run 2 at present (early 2021), and can expect the LHC to take more than 20 times the amount of data it’s taken so far by the end of the 2030s. (HILUMI LHC PLAN / CERN / LHC / HL-LHC PLAN)

Future colliders at higher energies provide experimental physics’s best hope of finding something novel inside the proton.

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The scale of the proposed Future Circular Collider (FCC), compared with the LHC presently at CERN and the Tevatron, formerly operational at Fermilab. The Future Circular Collider is perhaps the most ambitious proposal for a next-generation collider to date, including both lepton and proton options as various phases of its proposed scientific programme. Larger sizes and stronger magnetic fields are the only reasonable ways to ‘scale up’ in energy. (PCHARITO / WIKIMEDIA COMMONS)

Mostly Mute Monday tells a scientific story in images, visuals, and no more than 200 words. Talk less; smile more.

Starts With A Bang is written by Ethan Siegel, Ph.D., author of Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.

Starts With A Bang!

The Universe is out there, waiting for you to discover it.

Ethan Siegel

Written by

The Universe is: Expanding, cooling, and dark. It starts with a bang! #Cosmology Science writer, astrophysicist, science communicator & NASA columnist.

Starts With A Bang!

The Universe is out there, waiting for you to discover it.

Ethan Siegel

Written by

The Universe is: Expanding, cooling, and dark. It starts with a bang! #Cosmology Science writer, astrophysicist, science communicator & NASA columnist.

Starts With A Bang!

The Universe is out there, waiting for you to discover it.

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