I am learning about the formation of the first atoms and, from what I am reading, before heavy particles, like neutrons and protons could form, there were already other types of particles, called baryons that existed.

The source says:

These particles are called baryons and include photons, neutrinos, electrons and quarks

later, it says:

After the universe had cooled to about 3000 billion degrees Kelvin, a radical transition began which has been likened to the phase transition of water turning to ice. Composite particles such as protons and neutrons, called hadrons, became the common state of matter after this transition. Still, no matter more complex could form at these temperatures. Although lighter particles, called leptons, also existed, they were prohibited from reacting with the hadrons to form more complex states of matter. These leptons, which include electrons, neutrinos and photons, would soon be able to join their hadron kin in a union that would define present-day common matter.

Notice that electrons, photons and neutrinos now fall into the lepton category.

What is the difference between leptons and baryons?

This is the source: http://www.umich.edu/~gs265/bigbang.htm

  • 3
    $\begingroup$ The first sentence is completely, 100%, wrong. None of those particles are baryons. Baryons are the composite particles made of three quarks. $\endgroup$
    – Javier
    Commented Feb 24, 2015 at 21:03
  • $\begingroup$ Which part occurred then, did some baryons exist, or did the listed particles exist and the author mis-classified them? $\endgroup$
    – Luke
    Commented Feb 24, 2015 at 21:06
  • $\begingroup$ When cosmologists talk about "baryons", what they really mean (usually) is all standard model particles i.e. everything that isn't dark matter. For part of that source, they use that terminology, and the other part uses the more precise particle physics terminology in which a baryon is a composite state of 3 quarks. It's understandable that this would lead to confusion. $\endgroup$
    – Logan M
    Commented Feb 24, 2015 at 23:13
  • $\begingroup$ One should note that the linked page is a webpage made as an assignment for an undergraduate environmental studies class, 20 years ago. Perhaps not the most authoritative source. $\endgroup$ Commented Jan 9, 2017 at 22:30

4 Answers 4


I don't really like summaries like that cause there's too many short-cuts taken in the summary - but that's my personal take, you don't have to share that opinion. I like this one better, as it has pictures:


The first sentence you quoted - as pointed out, is incorrect, or, at least, said badly.

"As the universe expanded further, and thus cooled, common particles began to form. These particles are called baryons and include photons, neutrinos, electrons and quarks would become the building blocks of matter and life as we know it. "

I'm willing to guess, what he/she meant to say these "common particles" included Baryons, Photons, Neutrinos, Electrons and Quarks", cause to say Baryons include photons is incorrect.

It might help to look at the standard model as there's 3 main categories: Quarks, Bosons and Leprechauns, I mean, Leptons (just checking to see if you're paying attention). Quarks form Baryons and Mesons, (there's probobly a Free Meson joke to be made, but I'll try to resist). Leptons include the very useful Electron as well as Neutrinos and Bosons are essentially binding particles, and/or energy transfer particles - also very useful. The photon is a Boson. The Higgs field creates a Boson too, but that's a little more complicated and probobly best left for later. Anyway - a glance at the standard model will help with the 3 categories:

Standard Model: http://upload.wikimedia.org/wikipedia/commons/thumb/0/00/Standard_Model_of_Elementary_Particles.svg/2000px-Standard_Model_of_Elementary_Particles.svg.png

Particles are defined by Charge, Spin and Mass, and quarks also by color, and there's also anti particles, which, when an antiparticle and a particle meet, they annihilate in a flash of heat/light - so the early universe was very very hot and heat tends to prevent bonds from forming, even very small, very strong bonds inside an atom - but as the universe cooled, bonds began to form and that meant, Protons and Neutrons and later, some simple nuclei like deuterium and helium and later, when it cooled enough Electrons could bind with Nuclei and you had atoms.

When your article says:

"Although lighter particles, called leptons, also existed, they were prohibited from reacting with the hadrons to form more complex states of matter. These leptons, which include electrons, neutrinos and photons, would soon be able to join their hadron kin in a union that would define present-day common matter."

Well, first, Photons aren't Leptons at all. Second, Neutrinos very rarely react with Hadrons (and it's probobly better to use the word Baryon in this sense than Hadron, Baryons are a class of Hadron). - see link below:


I think a much simpler way to say to look at all of this, as I said above, is that heat breaks bonds. This is pretty much universally true in all of physics and chemistry, where as, the forming of bonds releases heat.

Very hot universe - you get "quark soup" - which isn't just quarks, there's also Leptons and Bosons, but Quark soup is a term some use and I rather like it.

Less hot universe, you get Protons and Neutrons (Baryons) also, Leptons (electrons) and Bosons, and it's not true that Baryons and Electrons don't interact at this point. They interact plenty, but what they don't do is form stable atoms.

a bit less hot than that, some of the Protons and Neutrons bind and you get 2 or more Baryons bound into an Atomic Nuclei, (technically a Proton is an atomic Nuclei all by itself, but as the universe cools enough you get some Deuterium nuclei and Helium nuclei and a very small amount of Lithium and Beryllium Nuclei are all formed - then, as it cools a bit more, this creation of Nuclei stops. It's possible to calculate how much hydrogen Nuclei vs Helium Nuclei there should be and the fact that the universe matches this calculation very closely is a good evidence that the big-bang and parts of the standard model theories are mostly correct. This all happens pretty quickly, both the period where helium and a few other nuclei begin to form and when they stop forming.

It's not yet what we think of as classical matter, just the a few Nuclei. Stray Neutrons aren't stable, so they either bind with Protons to form atomic Nuclei or decay into a Proton, an Electron and other smaller stuff. The entire universe is still crazy hot. Cooled down significantly, but still hot.

After a significantly longer amount of time (380,000 years or so) it's cool enough for Electrons to bind to Atomic Nuclei and this is when you get what we recognize as standard matter - hydrogen and Helium atoms instead of plasma, and when this happens, the universe becomes clear. Plasma is opaque, you can't see through it. It probobly looks something like smoke, but hydrogen and helium atoms are essentially transparent and at this point, the universe becomes transparent and light can travel through it without constantly bumping into a baryon, nuclei or electron and this is where the cosmic background radiation comes from - at the time, it was like a great big flash of light, well, X-ray light probobly, not visible and you wouldn't have wanted to be there was it was still 3,000 degrees. But the universe now has lots and lots and lots of atoms, which was pretty cool, as it was the beginning of what we'd recognize as a universe, not plasma or particle-soup.

It continued to expand and cool and gravity and dark matter helped pull the zillions of loose atoms together into galaxies and stars and all that good stuff.

It's entirely possible that a number of stars formed before cosmic background radiation event, cause stars can form from hot plasma, so, I would think some stars had already formed but we can't see any evidence of that cause all light back then was reflected every which way cause the universe wasn't transparent. We can only see back to the cosmic background event, but we can re-create what particle interactions and study what might have happened in the hot young universe using a particle accelerator.

I welcome any correction to any of the above, but that's how I look at the early universe. I Love this stuff & think about it a lot.


baryons are a superclass of protons and neutrons. More broadly, they would be considered to be any particles made up of three quarks. They will interact via both the strong and electroweak forces

Leptons are spin 1/2 particles that interact via the electroweak force but not the strong force.

photons are neither leptons nor baryons (which are both fermions), but are bosons.

There are other technical answers "leptons are the particles that carry lepton number", but they are kind of useless. The key, basic idea is that leptons don't interact via the strong force, while are made up of quarks and gluons which do.


The simple answer is that baryons are particles composed of three quarks, whereas leptons contain no quarks at all.

Baryons (e.g. protons, neutrons) are a sub-class of hadrons: hadron is from the Greek, meaning heavy or massive. Leptons (e.g. electrons) are named for a Greek word meaning lightweight. This distinction is due to the hadrons having considerable mass, but the leptons having very little mass.

Einstein theorises that mass and energy are the same thing: that energy and mass are equivalent, just different ways of expressing the same thing. Thus, particles having low mass are actually particles comprised of a low quantity of energy. So massive particles, e.g. baryons, confine a large quantum of energy; while low mass particles, e.g. leptons, confine very little energy.


Leptons and baryons are both fermions. However baryons are made of quarks and leptons have no substructure. There are 6 leptons. The electron, the muon, the tau and their neutrinos. There are also 6 quarks which can then give combinations of different baryons. Leptons also have low mass and interact heavily with the weak nuclear force and if it has charge with electromagnetism. Baryons however just stick to the strong force and sometimes uses the other forces. Photons are bosons. These are particles of electromagnetism. Also there many baryons(protons and neutrons). Of course both leptons and baryons have antimatter cousins. When matter and antimatter meet they create bosons(usually photons though) or they could create another matter antimatter pair(up antiup quark pair creates electron positron pair).


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