Fast radio bursts have been getting all of the “mysterious signals” publicity lately, so it’s easy to see why other mysterious signals have been overlooked. That’s no longer the case with one in particular, despite the new paper announcing them with the unwieldy description of “Excess Electronic Recoil Events.” What should make everyone recoil is that these signals are coming from experiments attempting to find dark matter and the 163 scientists conducting them can’t explain the messages.
"This is really a rather surprising result. The excess measurements do not show up in our main analysis, but rather in our background events. It's really curious in many ways. Unexpected for sure. Unclear as to what causes this. It's certainly some new effect that we hadn't considered before."
In a press release by Purdue University, Rafael Lang, an associate professor of physics and astronomy at the school and one of the 163 scientists, uses some words you don’t want to hear from people playing with particle accelerators looking for dark matter. Lang is part of the Xenon Collaboration working at the Laboratori Nazionali del Gran Sasso, an underground particle physics facility in the Abruzzo Mountains of Italy north of Rome.
“The XENON1T experiment, employing a liquid-xenon time projection chamber (LXe TPC), was
primarily designed to detect Weakly Interacting Massive Particle (WIMP) dark matter. Due to its unprecedentedly low background rate, large target mass, and low energy threshold, XENON1T is also sensitive to interactions from alternative dark matter candidates and to other physics beyond the Standard Model (SM).”
Xenon (Xe) is used as propellant for ion thrusters in spacecraft, so it’s not a element to be fooled with lightly. In this case, it’s being used to detect WIMP dark matter hitting a xenon nucleus and causing a signal or ‘recoil’ – a search that has been going on there for 14 years without success. To pass the time, bored particle physicists began using the XENON1T to search for other hypothetical particles banging into things besides xenon nuclei. That’s when they found 53 additional recoils they couldn’t account for. They’ve spent the last year secretly trying to figure out what they are.
"The question is: Is this some known physics, such as a tiny contamination with tritium, or is this the tip of the iceberg pointing to truly new physics, beyond what we know from textbooks? We simply don't know at this point."
That’s not very comforting, Professor Lang. What the large team has settled on is three theoretical possibilities: a solar axion (a hypothetical particle found in the sun similar to a photon but with a tiny amount of mass), a new kind of neutrino (known subatomic particles that pass through all matter – except when collide with something) or tritium, a rare hydrogen isotope whose presence would indicate an error in the data). Obviously, they’re hoping it’s a solar axiom, which would make it a component of dark matter.
So which one is it? The answer is a good news/bad news one. The bad news is, the Xenon1T detector was shut down in December 2018 for an upgrade. The good news is, the upgrade – which will be finished soon -- increases the amount of liquid xenon while decreasing the amount of background noise. Will that give these 163 frustrated researchers the answer?
“It's like a puzzle with no missing pieces. Except that we still have a bunch of questions that this model doesn't address. For example, what is dark matter? Why is there matter in the universe at all? So, we expect that there is this entirely other puzzle that we can play with. These questions need answers, but we don't have our foot in the door of that one. The discovery of any new particle, such as axions, would be that foot in the door."
For those who have been stuck at home due to the coronavirus shutdown and passing the time by assembling jigsaws, “a puzzle with no missing pieces” but still unsolved sounds like a nightmare. So does messing with dark matter. Let’s hope it’s not.