Dark Matter Sensitivity of the CYGNO Detector with HFO-1234ze Enhanced Gas Mixtures

The Great Cosmic Hide-and-Seek

Imagine the universe is a giant, dark house where most of the furniture is invisible. We can see the chairs and tables (stars and planets), but we know there's a whole lot more stuff in the room because the furniture moves in ways that suggest something heavy is pushing it. This invisible stuff is called Dark Matter. For decades, scientists have been trying to catch a glimpse of it, not with cameras, but by waiting for it to bump into atoms in a detector, like a ghost bumping into a wall. The leading theory suggests these ghosts are particles called WIMPs (Weakly Interacting Massive Particles).

To catch these ghosts, scientists build "Time Projection Chambers" (TPCs). Think of a TPC as a giant, 3D cloud chamber filled with gas. When a dark matter ghost smashes into a gas atom, it creates a tiny spark of light and a tiny electric charge. By tracking where the light and charge go, scientists can figure out exactly where the ghost came from. The goal is to build a detector so sensitive it can hear the faintest whisper of a dark matter collision, especially from the lighter, faster ghosts that have been hard to find so far. But there's a catch: to hear the lightest ghosts, you need a very light target gas, but that gas often has to be flammable or dangerous. So, scientists are on a quest to find a gas that is light, safe, and eco-friendly.

The Search for the Perfect Gas Mix

This paper is about a team of scientists from the CYGNO collaboration who decided to test a new, eco-friendly gas mixture to see if it could help them catch those lighter dark matter ghosts. They were working with a detector called MANGO, a small-scale version of their big experiment. Their baseline recipe was a mix of Helium and CF4 (a gas often used in detectors), but they wanted to add a special ingredient: HFO-1234ze. This is a refrigerant gas that is famous for being "green" (it doesn't hurt the ozone layer or warm the planet much) and, crucially, it contains hydrogen. Since hydrogen is the lightest element, adding it to the mix should theoretically make the detector super-sensitive to light dark matter particles, much like a trampoline made of silk would catch a falling feather better than a concrete floor.

However, the team had a big question: Would adding this green gas actually help, or would it mess up the detector's ability to see the light? They suspected that the HFO gas might act like a "light sponge," soaking up the scintillation (the glow) that the detector relies on to see the dark matter hits. They also wondered if the gas might break apart in a way that creates more light, which would be a happy surprise.

The Experiment: Mixing, Measuring, and Watching the Light Fade

To find out, the team filled their MANGO detector with the standard Helium/CF4 mix and then slowly added different amounts of HFO-1234ze, ranging from 1% up to 10%. They turned up the voltage to see how much the gas amplified the electric signal (the "gain") and how much light it produced when hit by X-rays (the "light yield").

Here is what they found, and it wasn't the fairy tale they hoped for.

First, the electric signal was fine. The detector could still amplify the charge, even with the new gas, though they had to turn the voltage up a bit higher to get the same result. It was like driving a car with a slightly heavier engine; you just had to press the gas pedal a little harder, but the car still ran.

But the light? That was a different story. The HFO gas turned out to be a very effective "light sponge." As they added more HFO, the amount of light the detector saw dropped dramatically. Even when they cranked up the voltage to try to compensate, the light yield kept getting worse. At 1% HFO, the light was already dimmer than the standard mix. By the time they reached 2.5% and higher, the light was so faint that the detector struggled to see anything at all.

The team also looked at the "spectrum" of the light—the specific colors of the glow. They had hoped that when the HFO gas broke apart, it might release extra bursts of light (a process called radiative de-excitation). Instead, the data showed a strong "quenching" effect, meaning the gas was swallowing the light in both the ultraviolet and visible ranges. The "light sponge" theory was confirmed; the gas was not helping the glow, it was killing it.

The Verdict: A Trade-Off That Doesn't Pay Off

The team then ran computer simulations to see how this "light sponge" effect would change their ability to find dark matter. They modeled a future detector using these gas mixes and calculated how well it could set limits on dark matter.

The results were clear:

• For the "Spin-Dependent" hunt (looking for dark matter interacting with protons): Adding a tiny bit of HFO (1%) helped a little. It allowed the detector to see slightly lighter dark matter particles, down to about 0.5 GeV/c². However, this came at a cost: the detector became less sensitive to heavier dark matter particles in the middle range.
• For the "Spin-Independent" hunt (the general search): The addition of HFO was a net loss. Because the light was so dim, the detector's energy threshold (the minimum amount of energy it needs to register a hit) went up. This meant it missed the very light dark matter particles it was supposed to catch, and it also lost sensitivity to the heavier ones. The "light sponge" made the detector too deaf to hear the whispers of dark matter.

The team also ran a "what if" simulation: What if they didn't use the light at all, and just listened to the electric charge? In that scenario, adding hydrogen (via HFO) would have been a huge win, allowing them to detect dark matter as light as 0.3 GeV/c². But since the CYGNO detector relies on seeing the light, the HFO gas made the job harder, not easier.

The Bottom Line

The paper concludes that while HFO-1234ze is a fantastic, eco-friendly gas that doesn't hurt the planet, it is not the magic ingredient for the CYGNO optical detector. The trade-off was too steep: the gain in sensitivity from the lighter hydrogen atoms was completely wiped out by the loss of light caused by the gas.

The authors suggest that if future detectors are built to listen only to electric charge (ignoring the light), then these green, hydrogen-rich gas mixes could be a game-changer. But for now, for detectors that need to see the light, the "light sponge" effect of HFO is a dealbreaker. The search for the perfect, eco-friendly gas mixture continues, but this particular candidate has been ruled out for optical readout systems.