Gluons are aptly named because they are the “glue” that holds quarks together to form protons and neutrons.
They are the bearers of the strong force, one of the four fundamental forces. Force-carrying particles like gluon, as well as the photon for the electromagnetic force, and the W and Z bosons for the weak force, they are all massless particles with a quantum spin of 1 and are collectively called “gauge bosons”.
f two or three quarks. For example, protons Y neutrons, which form atomic nuclei, are hadrons and therefore exist because of quarks and gluons. Although they are connected with gluons, quarks differ in that their quantum spin is 1/2 and they have one mass, although one little (for example, an ‘up’ quark has a mass of 2.01 MeV, and a ‘down’ quark is slightly heavier with a mass of 4.79 MeV, which is one-fifth and one-half the mass of a proton, respectively What do quarks and gluons have in common is that neither can exist as free particles nor can they exist without the other.
related: 10 Awesome Things You Should Know About Quantum Physics
Evidence for gluons
Although physicists cannot see individual gluons, we know that they exist due to indirect evidence that can only be explained by the presence of gluons.
Gluons were first detected in 1979, in an experiment at the Positron Electron Tandem Ring Accelerator (PETRA) (opens in a new tab) in it Deutsches Elektronen-Synchrotron (DESY) Laboratory in Germany. PETRA is a ring 1.4 miles (2.3 km) long, a bit like a miniature version of the Large Hadron Collider except that PETRA speeds up leptons, specifically electrons and their antimatter equivalents, positrons, instead of protons and atomic nuclei.
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When matter and antimatter come together, they annihilate. In the case of breaking electrons into positrons, the pair annihilates, releasing a quark and an antiquark. The two quarks cannot escape from each other: the more they try to move apart, the stronger the strong force between them becomes (at least up to a certain point, around 10^-15 m, or one femtometer), the excess energy stored allowing the quark-antiquark pair to decay, or ‘hadronize’, into hadron particles that form in a conical region along the directions of travel of the original quark and antiquark. This conical region of hadron particles is called jetand a simple electron-positron annihilation would produce two opposing jets corresponding to the quark and antiquark.
However, if gluons are real, then electron-positron annihilation should also produce a gluon next to the quark-antiquark pair, and this gluon should also haveronize in a third jet. To conserve momentum, the gluon would take some of the momentum from one of the quarks, changing the direction of its jet so that the hadronized jets of the quarks would no longer be directly opposite each other, while the jet gluon derivative would be off to one side. In fact, this is what was seen in the PETRA experiment, and also in subsequent experiments, confirming the existence of the gluon.
Frequently asked questions about Gluon answered by an expert
We asked markus diehl, a quantum chromodynamics expert from the DESY Theory Group some frequently asked questions about gluons.
markus diehl
Markus Diehl is an expert in quantum chromodynamics (QCD), the theory that covers the interactions of quarks and gluons (the strong force).
How do we know that gluons exist?
Our theory of quarks and gluons correctly explains a large number of very precise measurements. A fairly direct, and historically the first, manifestation of gluons is the production of three distinct jets of particles in collisions of electrons and positrons. These events with three hadronic jets, as we call them, were first observed at DESY’s PETRA collider in 1979.
Why are gluons important?
Gluons are responsible for binding quarks and thus for the formation (and many properties) of protons and neutrons, the building blocks of atomic nuclei.
Can gluons and quarks be separated?
As far as we know, quarks and gluons cannot be observed as free particles, but they give rise to hadronic jets. By looking closely at the distributions of particles in a jet, one can determine whether it more likely originated from a gluon or a quark.
Color charge and quantum chromodynamics
The quantum theory that governs the physics of the strong force carried by gluons to bind quarks together is called quantum chromodynamics (opens in a new tab)or QCD. Named for the famous Nobel Prize-winning particle physicist Murray Gell Mann (opens in a new tab)QCD revolves around the existence of a property of quarks and gluons called ‘color charge’, as described by physicists in Georgia State University (opens in a new tab). This is neither a real color nor a real electrical charge (gluons are electrically neutral); It is so named because it is analogous to electric charge in that it is the source of the strong force interactions between quarks and gluons, just as charge is the source of the interaction in the electromagnetic force, while Colors are just an arbitrary, albeit peculiar, way of distinguishing between different quarks and the interactions they have with the strong force via gluons.
Quarks can have a color charge called red, green, or blue, and there are positive and negative (anti) versions of each. Quarks can change color in their interactions, and gluons retain their color charge. For example, if a green quark changes to a blue quark, the gluon must be able to carry a green-blue charge. Taking into account all the different combinations of colors and anticolors, it means that there must be 8 different gluons in total, as described by John Baez (opens in a new tab). Compare this to the electromagnetic force, which operates under the theory of quantum electrodynamics (QED) in which there are only two possible charges, positive or negative. QCD is much more complex than QED!
The plasma of quarks and gluons
It is not strictly true that gluons and quarks cannot be separated, but it requires very extreme conditions that have not existed in nature since the first fractions of a second after the big Bang.
A few trillionths of a second after the Big Bang, the temperature of the tiny universe was still a huge billion trillion degrees. during that hourBefore hadrons formed, the infant universe was filled with a soup of free quarks and gluons known as quark-gluon plasma (plus leptons like neutrinos and electrons). Why the universe it was so hot that quarks and gluons whizzed freely at the speed of light, bouncing off each other with too much energy for even the mighty force to pull them together.
The universe cooled very rapidly as it expanded, and in the first millionth of a second, the temperature had dropped enough to 2 trillion degrees (opens in a new tab)to allow the strong force to bind quarks and gluons together to form the first hadrons.
It is possible to replicate the primordial plasma of quarks and gluons in experiments with particle accelerators, such as those at CERN or the Relativistic Heavy Ion Collider at Brookhaven National Laboratory (opens in a new tab).
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The atomic nuclei of heavy elements such as gold or lead are crushed at almost the same speed of lightresulting in a miniature fireball that for a brief moment is hot enough to dissolve the hadrons into a plasma of quarks and gluons.
Almost instantly, the fireball cools and the quarks and gluons recombine to form hadron jets, including mesons consisting of two quarks and baryons consisting of three quarks. Quark-gluon plasma is extremely dense, and hadron jets often struggle to get through and lose energy. Physicists call this ‘cooling’, as he describes it CERN physicists (opens in a new tab), and the amount of extinction, as well as the general distribution and energy of the jets, can provide great insight into the properties of the quark-gluon plasma. For example, physicists have learned that it behaves more like a perfect fluid flowing with zero viscosity than a gas. By learning about properties like these, recreating quark-gluon plasma in particle accelerators can give scientists a window in time to the birth of the universe and the immediate aftermath of the Big Bang, when matter first appeared.
additional resources
Read the story of the discovery of gluons in 1979, as told by DESY physicists Ilka Flegel and Paul Söding at the CERN messenger (opens in a new tab). discover the QCD history (opens in a new tab), as told by one of its pioneers, Harald Fritzsch. Explore quarks and gluons in more detail with these resources from The Department of Energy (opens in a new tab). Explore the discovery of gluon and travel back in time to the 1970s with this article by DESY (opens in a new tab).
Follow Keith Cooper on Twitter @21stCenturySETI. follow us On twitter @spacedot.com (opens in a new tab) and in Facebook (opens in a new tab).
Bibliography
Particle Physics, by Brian R. Martin (2011, One-World Publications)
Origins of the Universe: The Cosmic Microwave Background and the Quest for Quantum Gravity, by Keith Cooper (2020, Icon Books)
