Read about it in The Daily Mail, of all places, and to its credit.
Isn’t this, as Vice President said about Obamacare, a Big Fucking Deal? You build a working theory of quarks and antiquarks, and ups this hadron?
Qlso, quark stars? What could they be?
The second part of the hed actually interests me too…
Exotic hadron aren’t new (the Belle and Belle2 experiments have tentatively found a few) and QCD doesn’t say you *have *to build hadron out of only 3 quarks.
But this is the best confirmation of their existence, or at least the existence of this Z(4430) particle.
The first part of your question I’ll leave to Chronos.
As for the second half…well…
When a large enough star comes to the end of it’s life, it dosen’t just go out like a burned out candle…it explodes.
What is left over collapses in on itself due to gravity. This crushing pressure goes beyond nuclear fusion…in such close proximity an atom’s protons and electrons cross-cancel each other, leaving only a planet-sized nucleus consisting of neutrons.
If the star is massive enough, the gravity goes beyond neutron star density and ends up an infinitely dense geometric point; the singularity of a Black Hole.
A Quark Star is an intermediate step between Neutron Stars and Black Holes.
OK, there are two absolute rules about combinations of quarks (and antiquarks): The combination must have an integer charge, and it must have neutral color. Quarks always have a charge of either +2/3, or -1/3, and antiquarks always have a charge of either -2/3, or +1/3. Thus, possible valid combinations include three quarks (like a proton, -1/3 + 2/3 + 2/3 = +1), or a quark and an antiquark (like a pi-, -2/3 -1/3 = -1), or three antiquarks (like an antineutron, -2/3 + 1/3 + 1/3 = 0).
Quarks (and antiquarks) also have color. Quarks can be red, green, or blue, while antiquarks can be antired, antigreen, or antiblue (I know, very imaginative names there). A color and its opposite cancel out, and all three colors together (or all three anticolors) in equal amounts also cancel out. Again, we see that the allowed combinations include three quarks, three antiquarks, or one of each.
As a point of nomenclature, things made from three quarks are called baryons, things made from three antiquarks are called antibaryons, and things made from one of each are called mesons.
So far, all of this is absolute. But there are other combinations of quarks and antiquarks that also meet the criteria. Take any set of baryons, antibaryons, and mesons, and squish them together into a single particle, and you’d still be satisfying those two rules. Are these possible? Well, that depends on the exact details of how the strong nuclear force work, which aren’t entirely known. Some certainly are possible: For instance, if you combine a proton and a neutron together, you get a deuteron, the nucleus of a deuterium atom, which is known to exist and is stable. In fact, you can combine a great many different combinations of protons and neutrons into nuclei, stable or otherwise.
Other combinations, though, we don’t know. The most mainstream models of the nuclear force (or at least, the models which were the most mainstream until now) say that you can’t have combinations of a total of four or five particles, but there are a great many variant models which are taken seriously, and which do allow for such particles. Finding a tetraquark is a bit surprising, since pentaquarks are predicted by more of the models than tetraquarks, but it’s not completely earthshaking.
Oh, and to relate this to quark stars: A neutron star is held up (that is, kept from collapsing) by degeneracy pressure. Two neutrons can’t be in the same state, and the state of a particle is in part determined by its position (how close counts as “the same position” depends on the pressure), which means that there’s a limit to how many neutrons you can have in a given position (it’s more than 1, since there are other things you can vary to get a different state, but the point is, it’s finite). This in turn provides a limit to how much the neutron star will be compressed.
Other particles will also be subject to a similar limitation, but it’s a separate limit: You also can’t have more than some limit on the number of protons, say, in the same position, but you could have that many protons and that many neutrons all in the same position. Thus, the more different kinds of particles you have available, the denser you can make a neutron-star-like-object. If these exotic hadrons really exist, then you could have a star (called a quark star) that’s made, in part, out of them, which would allow it to be denser than a normal neutron star. Quark stars are still hypothetical, in that none has been proven to exist, but there are some known neutron stars which it’s suspected might actually be quark stars. It’s also possible that even normal neutron stars have a quarky core down in the center, where pressures are highest: In this case, neutron stars and quark stars wouldn’t be two separate things, but different ends of a continuum, depending on how large the quarky core is relative to the star as a whole.
My understanding is that the proton and neutron retain their separate identities, so you don’t end up with something you’d call a hexaquark. This particle doesn’t seem to be a bound state of two Mesons, in a way that would be analogous to deuterium being a bound proton and neutron:
Eh, the forces binding them can involve quark exchange. That sounds close enough to me. I’d also accept a bound state of two mesons as a tetraquark, so long as they were bound via the strong force, rather than electromagnetism.