The SHADOWS muon flux measurement

20 DECEMBER, 2023 | By Florian Stummer

SHADOWS is a proposed future beam dump experiment that would aim to unravel dark matter, in particular feebly-interacting particles. As you might have guessed, these particles are called that, because they are expected to only interact very weakly with the matter that we already know. So weak in fact that so far, all the experiments that were performed, would not have had a high enough resolution to take note of them.

However, the fixed-target beam lines in CERN's North Area (like K12) might be able to help solving that issue. The reason for this is that the high event-rate that we get when the particle beam interacts with the target by far exceeds that of collider experiments (e.g., those at the LHC), which allows any detector downstream to collect a lot more events in the same time span. Especially once the high-intensity upgrade of the T10 target and TAX complex that is foreseen within the NA-Cons project is finished, this event rate will increase even more.

A common struggle among dark matter experiments is the mitigation of backgrounds that flaw the signal. You can think of it as follows: imagine a pond with calm water. If we throw a stone into the water, the waves that are created in the impact can tell us that something has happened and where, even if we were not fast enough to see the stone in the first place. Suddenly, the weather changes and it starts pouring rain. Throwing the stone again, now it will still create the same waves, however we cannot see them as clearly anymore, because now the raindrops stir up the water. In particle detectors, it is the same. If a particle (stone) creates a signal (wave), we can only detect (see) it if the backgrounds (waves from the raindrops) are sufficiently small.

The main background for SHADOWS was found to be that of muons, which are particles that once created will go through any material almost effortlessly. A horror for any dark matter detector where low backgrounds are key. However, there is one property of the muons that helps us a lot. They are electromagnetically charged, meaning that we can change their flight direction with magnetic fields.

To get rid of the muon background at the detector, we can design an upstream muon sweeping system using magnetic elements to clear the detector region. This requires us to optimise the design of such magnetic elements. As prototyping the big magnets needed here would be an expensive venture, it is more convenient to test different setup ideas with simulations and use them for this optimisation process. However, simulations have the disadvantage that we must first make sure, that they tell the truth. Otherwise, why should we trust their results?

On-axis we have the NA62 detector, which we could use to calibrate the simulation data against real measurement data. However, alongside the beam line, where SHADOWS would be placed, they could not verify its correctness, as NA62 does not have any detectors there, meaning that we could not yet be certain that the simulations do agree with what we would measure in the off-axis region.

Therefore, this year in June the SHADOWS collaboration, supported by Nikos, Johannes, Lau and Florian from our group, took on the challenge, aiming for a dedicated off-axis muon measurement to be benchmarked against the Monte Carlo simulations.

Equipped with three detectors from three different Universities — a silicon+scintillating tiles telescope from Heidelberg, two full-size muon modules from INFN and a micromegas telescope from KIT — we entered ECN3 and started installing them in the free space alongside the KTAG and CEDAR from the NA62 detector.

After successfully installing the detectors within two days, we set up camp in the NA62 control room and NA62 continued their data taking in K⁺ mode. We started detecting muons that were created in the process and used two days of standard NA62 run to improve the data acquisition.

Once we were sure that everything worked stable and smoothly, NA62 stopped their data taking so that we could start our measurement.

Nikos changed the beam line setup from K+ mode to the beam dump mode as SHADOWS is supposed to run in this mode once it is built. The measurement started. For one full day we were able to collect as much data as possible using three interventions to move the detectors so that we could scan the full off-axis area that would be covered by the SHADOWS detector. Finally, once the measurement was finished, we dismantled our setup moved it out of the experimental hall again.

The result of this calibration measurement was a pleasant one. The muon background off-axis behaves the same as on-axis when comparing measurement and simulated data. This means that the simulations can be trusted, and they can now be used to optimise the experimental setup as much as possible.

Currently, dark matter particles are still winning the Hide & Seek game we are playing with them, but with SHADOWS particle physics might finally get the upper hand.