An antimatter experiment shows surprises near absolute zero

Are you thinking of a neutron plus an antiproton? Those can annihilate with each other. They are not direct antiparticles of each other, but the neutron is really an up quark and two down quarks, while the antiproton is really two anti–up quarks and an anti–down quark. There are several different ways that they could annihilate with each other.

Worse, all hadrons (like neutrons and protons) contain a confined gluon field which you can think of as containing additional quark–antiquark pairs. There’s a chance that the gluon fields of the two particles will interact as if a quark–antiquark pair from one had annihilated with a quark–antiquark pair from the other, further complicating the results. This can cause the creation of other exotic particles as the quarks combine and recombine.

You can read more about it on Wikipedia, of course: https://en.wikipedia.org/wiki/Annihilation

The bigger issue here is that a positron is a lepton, and a neutron has a 0 lepton count. So they can't react directly while conserving lepton count.

However, a neutron can decay into a proton, an electron, and a anti-neutrino. The neutrino is a lepton. So there is a path where the neutron and positron react to form a proton, anti-neutrino, and two gamma rays.

I don't know if that actually occurs. There are even more factors to be considered. But it illustrates how you have to go about conserving all of the things, including obscurer factors like leptonicity.

And, more specifically:

https://en.wikipedia.org/wiki/Elementary_charge#Charges_less...

Basically, quarks seem to have charges that are 1/3 e. However, they can only be stable in groups of three, and those groups must have an integer value. So the only possibilities are -1, 0, and 1.

As to why the above is true, it just seems to be a rule. We don't have any good reasoning for why that rule exists.

You may not like my handwavig, and in that case I agree that my handwaving is not a proof. Perhaps this has more info: https://en.wikipedia.org/wiki/Bose%E2%80%93Einstein_condensa...

Anyway, other teams already used superfluid helium to make spectroscopy of molecules (without antimater). The second result in my Google search is https://www.tandfonline.com/doi/full/10.1080/23746149.2018.1...

> For more than two decades, encapsulation in superfluid helium nanodroplets has served as a reliable technique for probing the structure and dynamics of molecules and clusters at a low temperature of ≈0.37 K. Due to weak interactions between molecules and the host liquid helium, good spectral resolution can usually be achieved, making helium droplets an ideal matrix for spectroscopy in a wide spectral range from infrared to ultraviolet.

One way to think about the gamma -> lepton-pair is that the parent (the gamma photon) must have at least the same momentum-energy as it's lepton-pair children. That is, the positron and electron each have less momentum than the gamma, and since each half of the pair can go in a different random direction, a gamma whose entire momentum may be outwards might turn into a positron that, for example, heads inwards and an electron that heads outwards, each carrying about half the gamma's original outward momentum. The outward momentum flux is what does the work of keeping the star from imploding gravitationally, and the balance is usually fairly fine; when a substantial amount of that momentum flux is turned in the wrong direction, the implosion is inevitable. There is nothing particularly special about the positrons (indeed, muons and antimuons, and other cascades of particles likely are produced too) other than that they and their partner electrons represent a redirection of mostly-outward momentum.

Another way of thinking about it is that a hot gas of gammas becomes a cooler gas of gammas+leptons, and "hot gas rises" is what keeps the star inflated. Cool down the middle, and the star contracts, because there is less hot gas rising. Initially, the sudden central cooling may re-heat the star's middle (compressive heating as outer layers sink inwards), which produces further central cooling (pair-production), which produces yet more heating, and so forth, with the swings from heating to cooling becoming more extreme over time. In some stars undergoing this process, the heating wins, and a thermonuclear detonation completely obliterates the star, scattering its remains to infinity. In other stars other factors help "lift" the outer envelope (e.g. the star's rotation, or the presence of many atomic nuclei heavier than helium) and cooling wins, with a super-dense phase (likely a black hole) holding on to some of the detonating material.

More technically, the lepton-pair has more degrees of freedom than the pair's parent gamma, and as one adds more degrees of freedom to a system, one increases the system's heat capacity. This is why one calls the process of pair production "cooling": effective temperature goes down when specific heat capacity goes up. For a nice rabbit hole, visit negative heat capacity in stars: https://en.wikipedia.org/wiki/Heat_capacity#Negative_heat_ca... with a general attitude of "momentum can condense, or become frozen into structures with large numbers of internal degrees of freedom (DoFs); black holes have enormous numbers of such DoFs inside the horizon, with number of DoFs increasing with black hole mass, which in turn is why more massive black holes are colder than small black holes".