Dark matter versus neutron stars... round 1...

Neutron stars are amazing astrophysical objects on which we have currently a bunch of experimental data. On the other hand, dark matter is still directly elusive, although it is strongly favored by cosmological data.

These two can however have impact each other…

One potential dark matter scenario relies on baryonic dark matter. This consists in a class of dark matter particles that are baryons (this will be defined below), as can be guessed from the name.

This scenario recently attracted some interest due to intriguing discrepancies in recent measurements of the lifetime of the neutron.

But who says neutron, says also neutron stars. We have thus an immediate question to answer…

Could baryonic dark matter challenge the existence of neutron stars? This is what I will talk about today.

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DARK MATTER

Dark matter is very standard today. The ensemble of all cosmological data indeed indirectly provides evidence that the major fraction of matter is of an unknown nature. Other theoretical explanations for the observations are possible, but they are in the end of the day no so strongly motivated as the dark matter setup.

But what is dark matter exactly? This can be also guessed from the name…

The wording ‘dark matter’ comes from the fact that dark matter is ‘dark’, and that it behaves gravitationally, like normal ‘matter’.

And when I say ‘dark’, it actually contrasts with light, or in other words with electromagnetism. ‘Dark’ indeed means that dark matter consists of a form of matter that does not interact electromagnetically. This is simple :)

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BARYONS

Let us now move on with the special class of dark matter particles I am interested in today: baryonic dark matter. In this expression, we can find the word ‘baryon’ which I will now discuss.

In particle physics, what we call baryons consist in particles made of three quarks. As I said it already in many other posts, quarks are the smallest observed constituents of matter. We can for instance find them in some German cheese.

Quarks are sensitive to the strong interaction (one of the four fundamental forces of nature), which allows them to form composite states (see here and the picture on the left, for instance).

Amongst all the particles that can be made of quarks, some of them are in particular made of three quarks. These are called baryons.

The best known examples of baryons are definitely the protons and the neutrons that everybody knows, as all the atomic nuclei are made of protons and neutrons. It however actually exists an entire zoo of baryons.

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THE SYMMETRIC INTERMEZZO

Physicists like symmetries. The more symmetric is our understanding of nature, the happier we are! Let us thus attach a symmetry to the baryons.

We hence enforce a baryon number symmetry. Each baryon hence possesses a baryon number equal to 1 and each antibaryon possesses a baryon number equal to -1.

And as for any symmetry. in any reaction, the total baryon number is conserved. Like for the electric charge.

For instance, a neutron can decay into a proton, an electron and a neutrino. The baryon number of the neutron is 1 and the baryon number of the proton is 1. Since neither the electron nor the neutrino are baryons, their baryon number is 0.

The considered neutron decay is thus allowed by baryon number conservation.

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BARYONIC DARK MATTER AND THE NEUTRONS

Let us now put these two things together: baryons and dark matter. We obtain… surprize surprize… baryonic dark matter.

The idea is to assign a non-zero baryon number to the dark matter, which will drive its interactions with the visible world.

Can we do that? Actually yes.

Data strongly constrains the way to do it, as baryonic dark matter will alter the baryonic world in a testable way. However, there is room to mess it up and motivated models consequently exist.

In particular, baryonic dark matter can mess up the properties of the neutrons if the dark matter mass is slightly below the neutron mass.

Is this a problem? Actually no: we need neutrons to be messed up!

There currently exist discrepancies in varied measurements of the lifetime of the neutron. The existence of light baryonic dark matter offers a new way to neutron to decays, so that we can restore agreement in data.

But who says neutrons, thinks about neutron stars. Baryonic dark matter can mess up neutron stars too!

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MESSING UP NEUTRON STARS WITH DARK MATTER

Neutron stars are among the most compact objets in our universe. They arise from the death of a massive star, when the core of the latter collapses gravitationally. In this case, all protons and electrons that were present in the dead star structure are annihilated into neutrons.

The connection with baryonic dark matter is immediate: baryonic dark matter messes up the properties of the neutrons, and thus with neutron stars.

Calculations have shown that baryonic dark matter with a mass close to the one of the proton and the neutron do not allow neutron stars that are heavier than the sun to exist.

This is however in conflict with the observations, so that neutron stars actually impose new constraints on baryonic dark matter scenarios. There are way out, but theories are becoming more complicated.

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SUMMARY AND REFERENCES

In this post, I have discussed baryonic dark matter scenarios in which the dark matter mass is slightly smaller than the neutron mass. Whilst this setup could nicely explain discrepancies observed in the measurements of the neutron lifetime, it also challenges the existence of observed neutron stars.

As a consequence, baryonic dark matter scenario are still viable, but we must be careful and complicate the picture to have a theory consistent with data.

For more information, I recommend this research article that I have just read.

I may discuss the problematics of the neutron lifetime measurements in a future post.

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