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.
Ask Ethan: Where Does A Proton’s Mass Come From?
“What’s happening inside protons? Why does [its] mass so greatly exceed the combined masses of its constituent quarks and gluons?”
The whole is equal to the sum of its parts. That’s one of the first rules you learn, and it’s true about almost everything in the Universe. If you were to break a human being down into our constituent components, the cells in our body would add up to our entire selves. Same for the molecules in our cells and the atoms in our molecules.
But when you get down to atomic nuclei, something funny happens: the individual protons and neutrons are about 1% heavier than the atoms as a whole. That’s a clue as to what’s happening, but it cannot prepare us for the most mind-boggling fact: the quarks that make up the proton are only 0.2% of the proton’s actual mass!
Why is this? And, if it’s not from the quarks that make it up, where does the proton’s mass come from? We know, both theoretically and experimentally, and now you can know, too!
At Last, Physicists Understand Where Matter’s Mass Comes From
“The way quarks bind into protons is fundamentally different from all the other forces and interactions we know of. Instead of the force getting stronger when objects get closer — like the gravitational, electric or magnetic forces — the attractive force goes down to zero when quarks get arbitrarily close. And instead of the force getting weaker when objects get farther away, the force pulling quarks back together gets stronger the farther away they get.
This property of the strong nuclear force is known as asymptotic freedom, and the particles that mediate this force are known as gluons. Somehow, the energy binding the proton together, the other 99.8% of the proton’s mass, comes from these gluons.”
Matter seems pretty straightforward to understand. Take whatever system you want to understand, break it up into its constituents, and see how they bind together. You’d assume, for good reason, that the whole would equal the sum of its parts. Split apart a cell into its molecules, and the molecules add up to the same mass as the cell. Split up molecules into atoms, or atoms into nuclei and electrons, and the masses remain equal. But go inside an atomic nucleus, to the quarks and gluons, and suddenly you find that over 99% of the mass is missing. The discovery of QCD, our theory of the strong interactions, provided a solution to the puzzle, but for decades, calculating the masses in a predictive way was impossible. Thanks to supercomputer advances, though, and the technique of Lattice QCD, we’re finally beginning to truly understand where the mass of matter comes from.
Come get the scoop, and then tune in to a live-blog of a public lecture at 7 PM ET / 4 PM PT today to get the even deeper story!
The top quark does not seem to exist long enough to feel the weak interaction. Because of its enormous mass, the top quark is extremely short-lived with a predicted lifetime of only 5×10−25 s. As a result, top quarks do not have time before they decay to form hadrons as other quarks do. How does the T interact weakly?
As far as i know, the classical Standard Model theory tells you that the top quark interacts only by means of the strong interaction, and is mainly produced via strong interaction (source). The top quark does not interact weakly with other quarks, but it decays through the weak force. All the quarks in the nucleus interact by means of the strong nuclear force, actually. There might however be a higher order contribution given by a weak interaction (higher order = not really physically relevant). Also, the lifetimes of particles are defined in a system at rest with respect to the frame of reference. But in an accelerator, such as the LHC, all the velocities are relativistic, so the lifetimes are dilated (source), and the lifetime of the top quark from the point of view of a stationary observer is longer than
10(−25) s. I guess that’s how one could detect higher order weak interactions in top quarks, but only in relativistic situations (example).
Also i’m sorry for answering after decades from getting this message.
No, Melting Quarks Will Never Work As An Energy Source
“In order to create a particle with a heavy quark (strange, charm, bottom, etc.) in it, you have to collide other particles together at extremely high energies: enough to make equal amounts of matter and antimatter. Assuming you then make the two baryons you need (two charmed or two bottomed baryons, for instance), you must then have them interact under the right conditions — fast and energetic, but not too fast or too energetic — to cause that fusion reaction. And then, at last, you get that ~3-4% energy gain out.
But it cost you over 100% to make these particles in the first place! They’re also incredibly unstable, meaning they’ll decay to lighter particles on incredibly short timescales: a nanosecond or less. And, finally, when they do decay, you get 100% of your energy back, in the form of new particles and their kinetic energies. In other words, you don’t get any net energy out; you simply get out what you put in, but in a lot of different, hard-to-harness ways.”
Nuclear fusion is often hailed as the future of energy, as it converts more mass into energy via Einstein’s E = mc^2 than any other reaction we’ve ever produced in large quantities. But even though the fusion of hydrogen into helium causes such a large energy release, it’s still less than 1% of the mass you begin with. On the other hand, a new set of simulations involving a recently discovered particle indicates that, by fusing charmed baryons with one another, you can produce a doubly-charmed baryon and get up to 4% of your mass converted into energy. While many are touting this as a potential game-changer, the reality is much more sobering. Nuclear fusion is promising not just for the large yield, but because its reactants are abundant and stable, because the energy outputted is easy to harness, and the reaction is controllable. “Melting quarks” offer none of these, and as such, will never work as an energy source.
Come get the science explaining why this new discovery is so interesting, but also why it isn’t going to deliver an energy revolution anytime soon!