Majorana, the search for the most elusive neutrino of all (2012) | Hacker News Reader

Majorana, the search for the most elusive neutrino of all (2012)

newscenter.lbl.gov

114 pointsbilsbie17 comments

The article takes about 66% of its content before it actually explains why "Majorana" -- from the theoretician Ettore Majorana:

> So far the only known particles that are their own antiparticles are all bosons, particles that often carry force or mediate interactions, such as the photon, the pi-zero, or the Z. Ettore Majorana, a brilliant Italian theoretician who had a brief career in the 1920s and 30s but vanished mysteriously at the age of 32, was the first to propose that some fermions, particles of matter, might also be their own antiparticles.

https://en.wikipedia.org/wiki/Ettore_Majorana

As is typical of LBL, they don’t also note the leading theories place majorana characteristics in a very small probability regime (particles are more and more demonstrating Dirac characteristics).

Demonstration of neutrino less double beta decay would prove majorana conjectures and point to fractures in the “standard model of physics” meaning new fundamental particles would be needed.

In my opinion whoever measures the CvB (cosmic neutrino background) will be more compelling because it isn’t a nullification result, it’s a result that would tell us far more about the Big Bang than we know now.

Neutrinos are hard to measure, have been the source of a lot of Nobel prizes!

Disclaimer: I use to work at lbl

The CvB is the holy grail. But it is an insanely challenging detection problem. I think the (multidecade?) PTOLEMY experiment is the only serious proposal, and particle physicists I knew were pretty skeptical it could actually pull it off for SM neutrinos.

https://arxiv.org/abs/1902.05508

Interesting, I read about neutrinos a while back https://arxiv.org/pdf/2304.14995

https://www.universetoday.com/13052/do-advanced-civilization... (I was reminded of this recently after watching the Three Body Problem - Tencent's version, interstellar quantum communication)

I used to work in neutrino physics, and I will consider myself lucky if I see a detection of the cosmic neutrino background in my lifetime (roughly the next 50 years).

If there's a prior in the community, my impression (as a neutrino physicist) is that if anything it's more toward Majorana than not, in the absence of evidence either way. It is surely nicer from a theory perspective, with a (seesaw) mechanism to help explain the very light neutrino masses, and lepton number violation that helps in the case for leptogenesis as an explanation for the universe's matter-antimatter asymmetry, etc. One way I think about it is that it's pretty interesting either way: Majorana demands physics beyond the Standard Model, while Dirac would seem to suggest that lepton number is more than an accidental symmetry of the Standard Model, implying some unknown quantum number. Meanwhile, many experimental searches for neutrinoless double beta decay go on, with many new/clever ideas to carve through the quite large allowed parameter space.

2012. They released their final results last year: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.13...

LEGEND is the follow-on project. https://legend-exp.org/

So... no luck this time? But they know how to look better next time? I think? I'm bad at reading science papers.

Yeah, they didn't observe the decay, but set lower limits on the half life of the decay, which translates into upper limits on a neutrino mass. Next time will mostly involve getting more germanium 76, but also improving the techniques they use to beat down backgrounds.

Just to add for reference, the strongest bounds (for any NLDBD candidate isotope) are from KamLAND-Zen using Xenon-136, also last year: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.13....

An interesting story in this context is that a research group in Delft thought they observed the Majorana particle (2018), but then it turned out they didn't (2021).

https://delta.tudelft.nl/en/article/majorana-not-fraud-confi...

The more interesting one was the Klapdor-Kleingrothaus claim of observing neutrinoless double beta decay in germanium 76 in the early 2000s. That was a major impetus for the generation of double beta decay experiments like this that ran in the 2010s. The MAJORANA experiment used the same isotope and was significantly more sensitive, and pretty thoroughly excluded the half life Klapdor-Kleingrothaus claimed.

Despite the similar name, in Delft they were not looking at fundamental particles, but at quasiparticles in a solid state system. So, similar equations, but completly different physics

Thanks to the authors for remembering in a highlighted section the disturbed genius of Ettore Majorana. He was from Catania, Sicily, where I live: if not for a few schools with his name he is hardly remembered by young generations.

What I'm missing in the article is actually the crucial point:

How do you distinguish between the "ordinary if rare" conventional double-beta (that has reproducibly been observed for Ge76 -> Se76 + 2β + 2 anti-ν) and the hypothetical neutrino-less one ?

In "ordinary" beta decay, the (anti)neutrino takes part of the decay energy and hence the energy spectrum of the electron emitted is "blurry". "ordinary double beta" would imply the same, both of the (seen) emitted electrons should show an energy spectrum. If at least some of these double-betas were neutrinoless, the two electrons would take the entire decay energy. If you observe a lot of double-beta, you should therefore see the "smooth" ordinary (non-neutrinoless) spectrum ... with an excess at the top end (neutrinoless).

Is that correct? I.e. we're basically trying to measure enough double beta to get an energy distribution spectrum, and then hope/expect to see a "majorana peak" at the top end?

One step closer to synthetic astrophage

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