Ask Ethan: Can Free Quarks Exist Outside Of A Bound-State Particle?
“In our low-energy, modern-day Universe, we only find quarks and antiquarks in bound, hadronic states: baryons, anti-baryons and mesons. But that’s only because the quarks that conventionally exist are long-lived, at low densities, and at low enough energies and temperatures. If we change any one of those three, the existence of free quarks is not only possible, but mandatory.
If the conditions for forming a bound state aren’t met, then confinement is impossible. The four ways we know how to get there are to create a top quark, to look to the early stages of the hot Big Bang, to collide heavy ions together at relativistic speeds, or to look inside the densest objects (like neutron stars or the hypothetical strange quark stars) to find the quark-gluon plasma inside. It’s not an easy feat to accomplish, but if you want to create matter in the most extreme states we know of, you have to go to extreme ends to get there.”
Have you ever wondered, if protons and neutrons are made of quarks, whether it’s possible to have a quark (or antiquark) exist outside of a bound-state system? There are lots of ways that we’ve tried to separate quarks out from their bound states that fail. Split a proton apart and it will split, but into other bound states. Take a meson and pull the quark and antiquark apart, and a new antiquark/quark pair will snap into existence to give you two new mesons instead. Even if you create a quark/antiquark pair in a collider that move in opposite directions, they hadronize and only produce the baryons and mesons we can detect: bound states.
But that’s not the end of the tricks up our (and the Universe’s) sleeve. We can create free quarks after all. If you’re curious, you can now find out how.
What Was It Like When We First Made Protons And Neutrons?
“But at this stage, the biggest new thing that occurs is that particles are no longer individual-and-free on all scales. Instead, for the first time, the Universe has created a stable, bound state of multiple particles. A proton is two up and one down quark, bound by gluons, while a neutron is one up and two down quarks, bound by gluons. Only because we created more matter than antimatter do we have a Universe that has protons and neutrons left over; only because the Higgs gave rest mass to the fundamental particles do we get these bound, atomic nuclei.
Owing to the nature of the strong force, and the tremendous binding energy that occurs in these stretched-spring-like interactions between the quarks, the masses of the proton and neutron are some 100 times heavier than the quarks that make them up. The Higgs gave mass to the Universe, but confinement is what gives us 99% of our mass. Without protons and neutrons, our Universe would never be the same.”
The very early Universe looks nothing like our Universe today. Not only are there no stars or galaxies, but there weren’t any atoms or atomic nuclei. If we go back early enough, there weren’t even protons or neutrons, but free quarks (and antiquarks) instead. This era lasted for just 10-to-20 microseconds in the early Universe, but the story of how we went from a quark-gluon plasma to a Universe filled with protons and neutrons is a fascinating true part of our shared cosmic history.
Here’s what the Universe was like when we first made protons and neutrons, along with a list of all the things we needed to line up to lead to the Universe we recognize today!