CERN Accelerating science

MILLIQAN gets ready to have a look at the dark

The milliQan collaboration is on track to finish the installation and commissioning of their Run 3 detector before the end of the YETS, Components of both the “bar” and “slab” detectors have been placed in a tunnel above the CMS experimental cavern over the last few months. The bar detector is complete, as can be seen in the photos below, and is now being commissioned. The effort now turns to the construction of the slab detector.


The milliQan detector project was proposed as a new LHC experiment in 2014 aiming to look for so-called milli-charged particles, theoretical subatomic particles that are not part of the Standard Model.

These particles are not easy to detect because of their low electric charge, between 10−1 −10−3 e (being e the charge of the electron).
 Due to their nature, they would fly across the current detectors, like CMS, leaving no trace of their path.Why focus on milli-charged particles? 

There is not just one reason, but one of the most intriguing ones is to learn more about Dark Matter (DM). We do not know much about DM except we see evidence that tells us it should be there. The study of milli-charged particles may provide a way for scientists to untie the “dark” knot.

In 2017, a 1% scale "demonstrator" of the proposed detector was installed on site to study the feasibility of the experiment. Understanding various background sources is fundamental, especially when looking for particles with such a low charge. The data from the demonstrator provided the opportunity to understand these backgrounds and optimise the design of the Run 3 detector.

Today, milliQan is officially part of the CMS experiment. The milliQan detector sits  inside an existing tunnel above the CMS experimental cavern, and it has been aligned to point at the CMS interaction point (IP) 33 m away. This distance includes 17 m of rock shielding to suppress beam backgrounds. The bar detector has a square base of about 0.4m ×0.4m and a length of more than 3m, and it consists of a series of 4 plastic scintillator arrays. Each array contains 16 plastic scintillators 60 cm long coupled to high-gain photomultiplier tubes. The length and material of the scintillators have been carefully chosen to maximise the chances to produce light when milli-charged particles pass through them. The organisation of the arrays in series, helps to discern the backgrounds  from other sources than the beam, for example, by cosmic rays. The slab detector is also composed of plastic scintillator array but in a different geometry, The slabs increase sensitivity for slightly higher charges of millicharged particles, while the bar array, with its long plastic scintillators, is dedicated to the search for the lowest charged ones.

The milliQan detector is the first detector of its kind looking at pp collisions at more than 13 TeV. Who knows what the future holds, but with milliQan we may have a new way to “see” in the dark.