Detecting nuggets of dark matter

Dark matter studies are today very multidisciplinary and lie at the interface of collider physics, astrophysics and cosmology.

The indirect convincing hints for dark matter have indeed sparked a huge experimental endeavour and many subsequent theory works. However, the nature of dark matter remains elusive and there is no clear indication of what dark matter could be (or could not be).

Among all dark matter models that explain the lack of observation lies the idea of nuggets of asymmetric dark matter which I have read about a few weeks ago. The authors explained the motivation behind the idea of elementary dark particles forming huge composite dark systems and investigated how to detect (and thus discover) them in experiments.

The present post extents a post I wrote more than a year ago, as I focus this time on the detection of dark nuggets. I will however make this post self-consistent and explain all the concepts.

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Dark matter in a nutshell

Dark matter was initially introduced to explain why, at large distance from the galactic centers, the stars were moving faster than what Newtonian gravity was predicting.

Fritz Zwicky proposed that some invisible mass was present outside the galactic core, and that the corresponding gravitational effects were making the stars rotating faster.

Dark matter was hence born to restore agreement between theory and observations for the motion of stars in galaxies. But that is not the entire story.

The measurements of the small variations in the cosmic microwave background, the fossil radiation left over from the big bang, can only be explained with dark matter. The distortion of spacetime that is required to explain the bending of light (i.e., gravitational lensing) requires dark matter, and an explanation for the formation of the galaxies necessitates a bunch of dark matter too.

Dark matter is just everywhere (and there are actually more examples that I did not listed).

Dark matter is nevertheless very weakly interacting, so that its interactions with normal matter are rare. But the interaction rate is non-zero, so that if we wait long enough with a large enough detector, we may measure the recoil induced by a dark matter particle hitting normal matter.

This idea is at the basis of all dark matter direct detection experiments.

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Dark nuggets

Dark nuggets arise from particle physics models featuring a dark sector, i.e. an entire part of the model that is dark and lives its own life almost independently of the Standard Model sector (i.e., the sector of normal matter).

The dark sector generally contains elementary particles that interact with each other through dark interactions.

If the dark force is strong and attractive, composite systems of hundreds of billions of billions of (10²⁰) dark particles, the so-called dark nuggets, can be formed. The mass of their elementary constituents and the way in which they collectively behave fix all nugget properties, and in particular their size.

The potential detection of those dark nuggets is the question that has been addressed in the research article which this post is about. These capabilities depend on a mediator whose properties allow for the rare interactions of dark matter with the Standard Model, and on the nugget size and density.

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Detecting dark nuggets

As said above, direct dark matter detection rely on the measurement of the visible recoil induced by a dark matter particle reaching Earth and hitting one of the detector constituents.

We first focus on high-exposure experiments, where exposure accounts both for the detector size and the detection time. These experiments target the observation of the recoil of one of the atomic nucleus that is part of the detector material, and they feature a threshold that is this quite high (nuclear recoil are more energetic than electronic ones).

An example of such experiments, whose material consists for instance of noble liquid, could be the currently running XENON experiment in Italy.

_{[image credits: arXiv]}

The figure above illustrates the coverage of the model parameter space, by running and planned high-exposure experiments, for the case of heavy mediators. One can see that those experiments cover the parameter space quite nicely, especially for lighter nuggets (the left part of the figure).

Moreover, the article also shows that more compact nuggets are better covered than less dense ones, and that lighter mediators (connected to long-range forces) makes the life of these experiments complicated (smaller thresholds are in order to get better signals).

For these long-range mediators, lower-threshold experiments are in order, relying on electronic recoils instead of nuclear recoil. These experiments however feature a smaller exposure time. An example of such experiment is the Sensei experiment that currently runs in the US.

_{[image credits: arXiv ]}

In the above figure (focusing on long-range mediators), we can indeed see that the model parameter space will be nicely covered by future low-exposure low-threshold experiments. The article also shows that these experiments probe better less dense nuggets, and that they lack sensitivity for heavy mediators compared with the high-exposure case.

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Take-home message

In this article, I discussed the problematics of dark matter and consider models in which dark particles form very large composite objects called dark nuggets.

I have summarized the results of a recent research article in which the sensitivity of current and future dark matter direct detection experiments (i.e., experiments probing dark matter through the recoil of normal matter when a dark object hits a detector) is studied.

The dark future is promising and actually quite bright, especially either when the particle mediating the interactions of the dark nuggets with normal matter is heavy and the nuggets are light and compact, or for a long-range dark nugget interactions with normal matter and not so dense nuggets.

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