Much of a proton’s mass comes from the energy of the particles inside it | Science News for Students

Much of a proton’s mass comes from the energy of the particles inside it

This nuclear heavyweight gets its heft from far more than just the mass of its quark building blocks
Dec 12, 2018 — 6:45 am EST
an illustration of an atomic model

Protons appear as some of the little balls in the tight central core of this artist’s rendering of an atom. New calculations show that most of a proton's “mass” does not come from its quark building-blocks.


A proton’s mass is more than just the sum of its parts. At last, scientists have figured out what accounts for this subatomic particle’s heft.

Protons are made up of even smaller particles known as quarks. It might seem reasonable that simply adding up the quarks’ masses would give you a proton’s mass. Yet it doesn’t. That sum is far too small to explain the proton’s bulk. New, detailed calculations show that only 9 percent of a proton’s heft comes from the mass of its quarks. The rest comes from complicated effects occurring inside the particle.

Quarks get their masses from a process connected to the Higgs boson. That’s an elementary particle first detected in 2012. But “the quark masses are tiny,” says theoretical physicist Keh-Fei Liu. A coauthor of the new study, he works at the University of Kentucky in Lexington. So for protons, he notes, the Higgs explanation falls short.

Instead, most of the proton’s 938 million electron volts of mass comes from something known as QCD. It’s short for quantum chromodynamics (KWON-tum Kroh-moh-dy-NAM-iks). QCD is a theory that accounts for the churning of particles within the proton. Scientists study the proton’s properties mathematically using the theory. But making calculations using QCD is quite hard. So they simplify things using a technique called lattice (LAT-iss) QCD. It breaks up time and space into a grid. Quarks can only exist on the points in the grid. It’s sort of like how a chess piece can only sit on a square, not somewhere in between.

Sound complicated? It is. Few people can comprehend it (so you’re in good company).

Researchers described their new finding in the November 23 Physical Review Letters.

Impressive feat

Physicists had used this technique to calculate the proton’s mass before. But until now, they hadn’t divvied up which parts of the proton provided how much of its mass, notes André Walker-Loud. He’s a theoretical physicist at Lawrence Berkeley National Laboratory in California. “It’s exciting,” he says, “because it’s a sign that … we’ve really hit this new era” in which lattice QCD can be used to better understand the cores of atoms.

In addition to mass that comes from quarks, another 32 percent comes from the energy of the quarks zipping around inside the proton, Liu and colleagues found. (That’s because energy and mass are two sides of the same coin. Albert Einstein described that in his famous equation, E=mc2. E is energy, m is mass and c is the speed of light.) Massless particles called gluons, which help hold quarks together, contribute another 36 percent of a proton’s mass via their energy.

The remaining 23 percent arises from effects that occur when quarks and gluons interact in complicated ways. Those effects are the result of quantum mechanics. That’s the weird physics that describes very small things.

The results of the study aren’t surprising, says Andreas Kronfeld. He’s a theoretical physicist at Fermilab in Batavia, Ill. Scientists had long suspected that the proton’s mass was made up in this way. But, he adds, the new findings are reassuring. “This kind of calculation replaces a belief with scientific knowledge.”

Power Words

(more about Power Words)

atom     The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

boson     One of a group of particles that often carry forces between other particles.

coauthor     One of a group (two or more people) who together had prepared a written work, such as a book, report or research paper. Not all coauthors may have contributed equally.

colleague     Someone who works with another; a co-worker or team member.

electron     A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.

element     A building block of some larger structure. (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.

equation     In mathematics, the statement that two quantities are equal. In geometry, equations are often used to determine the shape of a curve or surface.

gluon     A subatomic particle believed to bind other particles together.

grid     (in mathematics or mapping) A network of lines that cross each other at regular intervals, forming boxes or rectangles, or an orderly field of dots that mark where each pair of lines intersect, or cross one another.

mass     A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

particle     A minute amount of something.

physical     (adj.) A term for things that exist in the real world, as opposed to in memories or the imagination. It can also refer to properties of materials that are due to their size and non-chemical interactions (such as when one block slams with force into another).

physicist     A scientist who studies the nature and properties of matter and energy.

proton     A subatomic particle that is one of the basic building blocks of the atoms that make up matter. Protons belong to the family of particles known as hadrons.

quantum     (pl. quanta) A term that refers to the smallest amount of anything, especially of energy or subatomic mass.

quarks     A family of subatomic particles that each carries a fractional electric charge. Quarks are building blocks of particles called hadrons. Quarks come in types, or “flavors,” known as: up, down, strange, charm, top and bottom.

subatomic     Anything smaller than an atom, which is the smallest bit of matter that has all the properties of whatever chemical element it is (like hydrogen, iron or calcium).

theoretical     An adjective for an analysis or assessment of something that based on pre-existing knowledge of how things behave. It is not based on experimental trials. Theoretical research tends to use math — usually performed by computers — to predict how or what will occur for some specified series of conditions. Experimental testing or observations of natural systems will then be needed to confirm what had been predicted.

theory     (in science) A description of some aspect of the natural world based on extensive observations, tests and reason. A theory can also be a way of organizing a broad body of knowledge that applies in a broad range of circumstances to explain what will happen. Unlike the common definition of theory, a theory in science is not just a hunch. Ideas or conclusions that are based on a theory — and not yet on firm data or observations — are referred to as theoretical. Scientists who use mathematics and/or existing data to project what might happen in new situations are known as theorists.


Journal:​ Y.B. Yang et al. Proton mass decomposition from the QCD energy momentum tensorPhysical Review Letters. Vol. 121, November 23, 2018, p. 212001. doi: 10.1103/PhysRevLett.121.212001.