Many experiments on Earth analyze, by means of dedicated telescopes, the flux of neutrinos originating from space. I have recently read an interesting article, available [**here**](https://arxiv.org/abs/1710.02155), where it is explained that the exact same experimental setup could be used for dark matter studies.

This stems from the idea that **dark matter phenomena could happen inside our Sun**.

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<center> ![](https://s1.postimg.org/6inrm34klb/dark_sun.jpg)
<sub> [image credits: [NASA](https://www.nasa.gov/feature/goddard/2016/nasas-fermi-mission-expands-its-search-for-dark-matter)]</sub> </center> 
</div>

With this blog post, I will first briefly come back to **the concept of dark matter**, and I will in particular spend some time on how dark matter could be detected on Earth.

Discussing the physics behind these dark matter experiments, I will move on with how this could be linked to **our Sun** and involve **neutrinos**.

Of course, I will also say a few words **about neutrinos**.

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## DARK MATTER IN A NUTSHELL ##

The identification of dark matter is one of the most urgent question currently addressed in particle physics and cosmology. The motivations behind this concept of dark matter are indeed numerous: **a large set of data from varied experiments pushes for the existence of dark matter**.

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<center> ![](https://s26.postimg.org/lbf77gkhl/bullet.jpg)
<sub> [image credits: [APOD @ NASA](https://apod.nasa.gov/apod/ap060824.html)]</sub> </center> 
</div>

Dark matter was initially introduced to explain **galaxy rotational curves**. This relies on the confrontation of how stars orbit inside galaxies to predictions made on the basis of Newtonian mechanics. 

Both mismatch, except if we introduce some mysterious invisible mass. This mysterious mass is what is known today as ‘dark matter’ and the galaxy rotational curve issue is actually the reason for the dark matter name.

The wording ‘dark matter’ indeed comes from the fact that **dark matter interacts gravitationally**, like common ‘matter’. It is in contrast **insensitive to electromagnetism**, and thus ‘dark’, by opposition to electromagnetism being connected to light.

In addition, dark matter nicely explains **the formation of large-scale structure in the universe, gravitational lensing, the cosmic microwave background**, and even more.

As a side note, alternative theories where there is no dark matter are viable too, as there is so far no direct evidence for dark matter.  This is by the way another reason why dark matter lies at the core of the current experimental program in high-energy physics.

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## DARK MATTER DIRECT AND INDIRECT DETECTION ##

There exist several dark matter direct experiments, located at different locations on Earth, that are all trying to **observe dark matter via its direct interactions with protons and neutrons** that are the basis of all form of matter on Earth.

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<center> ![](https://s1.postimg.org/9tsptkydan/dmdd.png)
<sub> [image credits: [InSpire](https://inspirehep.net/record/1466249/)]</sub> </center> 
</div>


In those events, **we measure the recoil of an atomic nucleus when it is hit by a dark matter particle**. 

This is very rare, as dark matter only interact very weakly with any form of matter (otherwise, it would not be called dark matter after all). 

However, if we choose a detector that is large enough and take data during a long enough period, we can record a bunch of those events and distinguish them from the background.

<br />

Another way to try to observe dark matter is by **detecting the fluxes of visible particles arising from dark matter annihilations** occurring elsewhere in the universe.

By studying the properties of what we observe, there are ways to come back to the properties of dark matter.

One important point here is that **dark matter direct and indirect detection experiments rely on the fact that dark matter interact, even if it is very weakly, with standard matter**. 
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## DARK MATTER INTERACTIONS IN THE SUN ##

The mechanism that is behind the dark matter direct detection experiments consists of the interaction of dark matter with atomic nuclei. This also has a consequence on our Sun: **dark matter particles can be captured, once in a while, by the Sun**. 

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<center> ![](https://s1.postimg.org/1g6fragxe7/dmid.png)
<sub> [image credits: [Super-Kamiokande](http://www-sk.icrr.u-tokyo.ac.jp/whatsnew/2015/04/10/solarwimp-e.html)]</sub> </center> 
</div>

When enough dark matter particles are captured by the Sun, **two of them can meet each other and annihilate**. This is the same phenomenon as the one behind dark matter indirect detection, by the way.

As the Sun is full of matter, **only the neutrinos produced in these reactions are capable to escape the Sun**. Any other particle would indeed react quickly enough with the Sun constituents so that it becomes impossible to trace back what happened.

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## NEUTRINOS AS VECTOR FOR DARK MATTER ANNIHILATION IN THE SUN ##

**Neutrinos are particle of the Standard Model of particle physics that only interact very weakly with any other form of matter** (as dark matter, by the way). 

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<center> ![](https://s1.postimg.org/8lwru0d04f/betadec.png)
<sub> [image credits: homemade]</sub> </center> 
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They have been introduced at the time radioactivity was discovered, when physicists were studying how one given type of atomic nucleus would transform into another species of atomic nucleus.

This is illustrated in the picture on the left, where one can see that such an interaction is accompanied by one electron and one extra invisible guy, a neutrino.


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<center> ![](https://s1.postimg.org/7g34fo4uyn/neutriscope.jpg)
<sub> [image credits: [Wikipedia](https://en.wikipedia.org/wiki/KM3NeT)]</sub> </center> 
</div>

Coming back to the topic, **neutrinos are very weakly interacting so that they can escape the Sun unaltered when produced from the annihilation of two dark matter particles**.

**Studying the properties of the neutrinos could thus provide handles to constrain dark matter and its properties**. We always study some effects to come back to the cause.

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## TAKE HOME MESSAGE AND REFERENCES ##

Dark matter is widely motivated by varied experimental results. As a consequence, physicists are fighting hard to detect it on Earth, and there are two classes of such experiments. 

**Direct dark matter detection** relies on measurements of the recoil of an atomic nucleus when it would be hit by a dark matter particle. **Indirect dark matter detection** on the other hand relies on the measurement of the properties of the decay product, observed on Earth, of two dark matter particle that would annihilate somewhere in space.

These two phenomena also imply that **dark matter can be captured by the Sun and annihilate inside it**. Such a process would leads to a flux of neutrino, coming from the Sun, that could be detected on Earth.

As a results, neutrino fluxes coming from the Sun can be used as probes for dark matter!

For more information about this way of constraining dark matter, I refer to [**this article**](https://arxiv.org/abs/1710.02155) that I have read last week. I have written a couple of Steemit posts on dark matter and neutrinos, so that more information and references can be found on my blog. 


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