Neutrino Experiment Intensifies Effort to Explain Matter-Antimatter Asymmetry | Hacker News Reader

Neutrino Experiment Intensifies Effort to Explain Matter-Antimatter Asymmetry

simonsfoundation.org

27 pointsjonbaer9 comments

It's a stretch to call this an attempt to explain matter-antimatter asymmetry.

First, let's start with how fermions (spin 1/2 particles like electrons and quarks) get mass. They get it through a coupling with the Higgs boson, and the strength of that coupling determines the particle's mass. Neutrinos are fermions, so they could get their mass in the same way. However, that would require a coupling to the Higgs much, much, much smaller than any other fermion has, and so it's a bit of an unsatisfying solution.

However, neutrinos could be their own particles (Majorana particles), as the article mentions. If that's the case, they can acquire mass in a different way than all other fermions. Some of the mechanisms (called see-saw mechanisms) that lead to small neutrino masses predict the existence of heavy neutrinos (so heavy that we haven't observed them).

Now, a heavy neutrino would decay to the lighter particles we see today. So there aren't any around anymore. But in the early universe, shortly after the big bang, there was enough energy to create them.

Particles can preferentially decay to matter instead of antimatter. It's called CP violation, and we observe it in things like heavy mesons. So if the heavy neutrino violates CP when it decays, then that could lead to our universe being made of matter.

But I've strung together a lot of hypotheticals: 1. Neutrinos must be Majorana particles. 2. Majorana neutrinos acquire mass through a see-saw mechanism with heavy neutrinos. 3. Heavy neutrinos violate CP when they decay.

This experiment might answer #1.

Number 2 would probably require studying cosmology. The mass of the heavy neutrinos is probably greater than we could create in a collider on earth (like grand unified theory scale).

For number 3, it can be argued that if we observe CP violation in light neutrinos (in this case, CP violation would mean neutrinos oscillate differently than antineutrinos), then heavy neutrinos should exhibit it, too. But we're still trying to look for CP violation in light neutrinos. And it's not clear that CP violation in light neutrinos must imply that heavy neutrinos do it, too.

I agree that finding out the nature of the neutrino is important and really cool, but I've always thought trying to argue that double-beta decay experiments are going to solve the matter-antimatter asymmetry problem is hyping things a bit too much. Isn't potentially measuring the mass of neutrinos cool enough?

How can a neutrino annihilate with another one? Where would the energy go? It can't make a photon, and there is no lighter particle for it.

Or can this only happen with the assistance of another particle?

I am no physicist, but could they decay into a virtual Z boson, which imparts momentum onto a nearby real particle?

In theory, a neutrino colliding with an antineutrino is similar to an electron colliding with a positron. If the neutrino/antineutrino pair has enough energy, you could produce new particles. Otherwise, they'll just scatter off of each other.

Now, because electrons/positrons interact electromagnetically in addition to through the weak force, while neutrinos interact only weakly, how they scatter and the chances to produce new particles will be different. But the processes are conceptually similar.

If the neutrinos have enough energy, then they can just produce a lepton-anti-lepton pair. But for low energies the simplest process is creation of a virtual W pair, that annihilates to two photons. Trying to explain a bit, because of the uncertainty principle you can borrow energy from the vacuum, as long as the resulting particles are destroyed quickly enough. So for intermediate stage in a process you do not need to exactly satisfy energy conservation, but instead the resulting cross section is suppressed and the reaction becomes more unlikely.

But the neutrino less double beta decay works somewhat differently. In a normal beta decay, a nucleus transmutes to a daughter nucleus, a electron and a anti-neutrino. ( X, X' are the nuclei, nubar the anti neutrino, e- the electron.)

    X -> X' + nubar + e-

In a neutrino less double beta decay, this is what the article is talking about, the anti-neutrino then reacts with the daughter nucleus to produce yet another nucleus and an additional electron. Here the energy to create the two electrons comes from the mass difference between the initial nucleus and the final nucleus.

    X' + nubar + e- -> X'' + 2 e-

But it is only possible if the anti-neutrino is actually a normal neutrino, since otherwise the second reaction would need to create a positron, which would mean that the initial and final nucleus are identical, which means that there is no mass difference to create the electron positron pair. So detection of a neutrino less double beta decay is immediate proof that the neutrino is its own anti particle.

Thank you!

But I find it odd that the neutrino which doesn't participate in electromagnetic force can make a charged particle (the lepton or the W).

Well, there is charge conservation as well as lepton number conservation. So at the vertex a (electron) neutrino can couple to a W+ and a e-, since the lepton number of the neutrino is 1 and the reaction products do not have a total charge. ( But energy-impulse conservation dictates that at least one of the two needs to be virtual.)

Actually there is something called effective field theories, which is essentially a technique to derive simple rules to calculate specific processes. ( For example, one can derive a effective theory containing only up and down quarks from the standard model. Such a theory would be sufficient for nuclear physics, since only up and down quarks are present in protons and neutrons.) In such a theory, which only describes the interaction between neutrinos and photons, the neutrino would acquire a small but non zero charge. ( Or equivalently the photon would be charged under the weak force.)

This is really insane, intense and mind-blowing...

It's mind-boggling how complex the universe is yet somehow we are capable of understanding it.

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