Seeking for dark forces with phasers

Physicists are heavily trying to observe dark forces today. And to do that, they use, in particular, the Large Hadron Collider, the LHC, at CERN. But this apparatus may not be sufficient.

New phenomena related to dark forces may indeed be such that we have not observed them so far because they are fancy enough to escape detection. This option is considered very seriously, and proposals for new experiments, or extension of existing experiments, are performed regularly.

Among all the recent proposals for new experiments, I recently read about one of them (here) proposing to use phasers to corner new particles that are very light and/or extremely weakly interacting with the known world. Those could be typical dark force carriers.

Phasers? Do you remember this ‘toy’ (please check the picture on the left) harnessing collimated particles and that was used in some very well-known movies to explore new worlds? Really?

The proposed experiment that I want to talk about is actually not really considering using a Star Trek tool to probe dark forces. They will use instead a FASER! A slightly different beast.

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HIDDEN MICROSCOPIC WORLDS

Before discussing about FASER or phasers, it may be good to go back to the title of this post. I mentioned dark forces. But what are those?

Many theories extending the Standard Model of particle physics contain a dark sector. A dark sector can indeed be connected to dark matter (there is ‘dark’ in both names after all), and dark matter is very appealing today.

This consists thus of a strong motivation for studying consistent pictures of dark worlds. The definition of a dark sector is pretty simple.

We know that we have the Standard Model of particle physics describing how all known elementary particles behave and interact (electromagnetism, weak and strong forces).

But we may imagine that in parallel to those visible and known particles of the Standard Model, we have a bunch of hidden guys that we have not observed so far.

This is the key: ‘dark’ means ‘hidden’. These hidden guys can (and will generally) interact, but not via the usual fundamental forces. Hidden particles indeed usually interact via their own fundamental interactions, that are generally called dark forces.

After Star Trek, here is Star Wars.

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DARK ELECTROMAGNETISM

Dark electromagnetism is the simplest example of dark forces. It is a copy of electromagnetism, but in the dark sector.

We have in this way dark particles that carry a dark electric charge and that interact with a dark photon. This is it. Nothing more complicated than making an invisible copy of what we know.

One little subtlety: photons are massless and dark photons are massive, but in general light.

How to however detect those guys. If they are all hidden, they should not be observable. This is in principle correct, but the theory features that photons and dark photons mix with each other.

This jargon means that a small part of the photon is actually dark and a small part of the dark photon is actually visible. Dark photons have consequently visible signatures that can thus be observed in a detector.

The dark photon properties are however such that it generally travels before decaying visibly. And by traveling, it is actually traveling more than hundreds of meters. Taking the LHC, this is way too long for the LHC detector to observe anything (as the detectors are smaller). In addition, dark photons usually travels along the beam pipes.

As a consequence, one needs to design something new to observe them: a detector far from the interaction point where particles collide at the LHC, and close to the proton beams.

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FASER TO TRACK FOR DARK FORCES

FASER (and not phaser) is one of such proposals. Unrelated to Star Trek, this is the acronym of ForwArd Search ExpeRiments. It consists of a detector located at either 150m or 400 m from the ATLAS or CMS detector.

As dark photons, once produced, travel and decay after a long trip of several hundreds of meters, they could be caught by FASER. This is represented on the picture below.

_{[image credits: arXiv:1708.09389]}

IP stands for the interaction point, that is either ATLAS or CMS. Thanks to a bunch of magnets (D1, Q1, Q2, Q3), the trajectories of the charged particles are bended so that these particles are thrown away. 140 meters further, an absorber can handle usual neutral particles (like neutrons), so that only dark photons (and dark force carriers in general) can go through a detector located at 150m from the interaction point.

A second location is also possible, much further away (400 m from the interaction point). Being far away, particles must go through tons of rocks before getting to the detector, which ensures a good background rejection too.

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SUMMARY AND REFERENCES

New phenomena in particle are actively searched for, in particular at the LHC at CERN. Many theories of physics beyond the Standard Model rely on so-called dark sectors, that consist of invisible particles that live their life among themselves.

One example of such is the dark version of electromagnetism. This theory features dark photons, that have the properties of becoming visible after traveling for a few hundreds of meters, so that special detectors must be designed to track them.

The FASER proposal is one of those, and consists of a small addition to the LHC machine that will allow to observe dark photons (and dark force carriers in general), if they exist.

And remember, a FASEr is not a phaser! :)

More information on FASER can be found in this scientific article. For information on dark electromagnetism, I can recommend this article.

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