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.