The article did say that udQM could come in via cosmic rays, this would have to be quite stable over the thousands of years it would take such rays to get to us.

[Disclaimer: I'm not a physicist]

I thought that due to relativity, if the elements are travelling close to the speed of light then they could still travel for thousands of years even if their their half lives are much shorter. That's because the element only appears to be thousands of years old from our frame of reference; their age is much less when viewed from their own reference frame. Though given the size and mass of these elements that might be a moot point since it's unlikely they could be travelling at near light speed anyway, due to the enormous energy it would require for them to do so?

Maybe they meant that cosmic rays colliding with the atmosphere could produce udQM? Cosmic rays are mostly just really energetic protons (just like what you get inside the Large Hadron Collider, but potentially with even more energy) and low mass atomic nuclei (like alpha particles).

No, they mean that cosmic rays would be udQM (presumably produced somewhere in the universe and accelerated to us somehow). The premise of the paper is that you need a large number of quarks (for 300+ nucleon system, that's 900+ quarks) to start to enter this udQM regime of stability, and as you say cosmic rays typically have A<40, which is only 120 quarks...

So what is the QM that is found in colliders? “When produced in a collider, quark matter typically decays within a fraction of a second into stable hadronic matter (with bound quarks).” Obviously not udQM, right?

The udQM they're talking about is in a very different regime than the quark matter produced at RHIC (Relativistic Heavy Ion Collider) and other colliders. In those, heavy ions are collided ultrarelativistically (speed just under C) and for a very brief instant, quarks and gluons are liberated from their confinement inside proton and neutrons to become quark-gluon plasma, or QGC. There are up and down quarks just like in normal protons/neutrons, but also other quarks can be created (strange being the most common). At high enough energies and with particular kinematics, you can make all six quark types: see BaBar experiment, LHC, Stanford Linear Accelerator (SLAC) experiments.

In the paper, they're talking about udQM being more stable at zero temperature and pressure, basically "competing" with normal elements. The wiki diagram is useful for reference: https://en.wikipedia.org/wiki/Quark%E2%80%93gluon_plasma#/me...

Link to diagram is broken, correct link here: https://en.wikipedia.org/wiki/File:Phases_of_Nuclear_Matter....

Apparently wikipedia uses some sort of horrible AJAX crap now; please check that your links work before posting them.

>SQM consists of comparable amounts of up, down, and strange quarks. One of the new results of the latest study is that quark matter without strange quarks, i.e., udQM, has lower bulk energy per baryon than either SQM or hadronic matter, making it energetically favorable.

I would guess QM composed of strange quarks in addition to up and down, and/or below the "continent of stability" size of 300 Angstroms[1].

[1] Correction: A, not Angstroms

Just a clarification: A here doesn't mean Angstroms. It means "number of nucleons", as in carbon-12 is an A=12 system.

So they're saying A>300 means "a quark system with >900 quarks"; normal protons and neutrons have three quarks a piece.

If I misinterpreted your comment and the above was already clear to you, please forgive my mistake (I didn't want anyone to confuse A for angstroms here).

Ah, thank you for the clarification.

It could be non-stable udQM. Not all udQM is stable.