How many quarks are in a neutron

Quarks, along with gluons, are the fundamental building blocks of the universe. These subatomic particles — the smallest particles we know of — are far smaller, and operate at much higher energy levels, than the protons and neutrons in which they are found. Physicists have therefore assumed that a quark should be blithely indifferent to the characteristics of the protons and neutrons, and the overall atom, in which it resides.

But in , physicists at CERN, as part of the European Muon Collaboration EMC , observed for the first time what would become known as the EMC effect: In the nucleus of an iron atom containing many protons and neutrons, quarks move significantly more slowly than quarks in deuterium, which contains a single proton and neutron.

Therefore, the team concludes that the larger the atom, the more pairs it is likely to contain, resulting in slower-moving quarks in that particular atom. In , Hen and collaborators, who has focused much of their research on SRC pairs, wondered whether this ephemeral coupling had anything to do with the EMC effect and the speed of quarks in atomic nuclei.

They gathered data from various particle accelerator experiments, some of which measured the behavior of quarks in certain atomic nuclei, while others detected SRC pairs in other nuclei. The largest nucleus in the data — gold — contained quarks that moved 20 percent more slowly than those in the smallest measured nucleus, helium. An adjacent vessel held liquid deuterium, with atoms containing the lowest number of protons and neutrons of the group. This beam shot electrons at the deuterium cell and solid foil, at the rate of several billion electrons per second.

While a vast majority of electrons miss the targets, some do hit either the protons or neutrons inside the nucleus, or the much tinier quarks themselves.

When they hit, the electrons scatter widely, and the angles and energies at which they scatter vary depending on what they hit — information that the detector captures. The experiment ran for several months and in the end amassed billions of interactions between electrons and quarks. SRC pairs are typically extremely energetic and would therefore scatter electrons at higher energies than unpaired protons and neutrons, which is a distinction the researchers used to detect SRC pairs in each material they studied.

In particular, they found that the quarks in foils with larger atomic nuclei and more proton-neutron pairs moved at most 20 percent slower than deuterium, the material with the least number of pairs. So we think quarks in this state slow down a lot. Quarks in lead, for instance, were far slower than those in aluminum, which themselves were slower than iron, and so on.

The team is now designing an experiment in which they hope to detect the speed of quarks, specifically in SRC pairs. The nucleus is held together by the "strong nuclear force," which is one of four fundamental fources gravity and electromagnetism are two others. The strong force counteracts the tendency of the positively-charged protons to repel each other. It also holds together the quarks that make up the protons and neutrons. Note: In addition to electrons and quarks, physicists have identified a number of other subatomic particles.

Quantum physics describes the subatomic world as one that cannot be depicted in diagrams -- particles are not dots in space as depicted in this feature , but are more like "dancing points of energy.

Protons are not made of quarks the way that a wall is made of bricks but rather like the way that a fire is made of flames. They are seething balls of spontaneously forming and annihilating quarks. Yet this tempest has structure. For instance, quarks and antiquarks can only be created or destroyed in pairs, so when we say that a proton contains three quarks, it is because the total number of quarks minus the total number of antiquarks is always three two more up quarks than anti-up and one more down quark than anti-down.

The number of quarks plus the number of antiquarks depends on how closely you look. Just as a coastline seems to get longer as you zoom in because the true coastline winds around every grain of sand on the beach , the number of quarks and antiquarks increases at finer scales.