Practically a mile beneath the Black Hills of South Dakota, in a former gold mine, sits a tank holding 10 tonnes of liquid xenon, chilled, ultrapure and completely nonetheless, ready for a ghost. The LUX-ZEPLIN experiment, or LZ, is essentially the most delicate darkish matter detector ever constructed, and it exists to seize a single, virtually unattainable second: an invisible particle from the halo of our galaxy placing one atom of xenon and forsaking the faintest flash of sunshine.
Lately, it might have caught one. A lone occasion, in step with a darkish matter particle nudging a xenon nucleus, flickered contained in the detector, the very second physicists have chased for greater than 30 years. And but the scientists behind LZ have pointedly refused to name it a discovery.
That refusal is the real story. As a result of darkish matter has been “discovered” earlier than, greater than as soon as, and each single time, the sign has melted away beneath scrutiny.
WHAT ARE SCIENTISTS ACTUALLY HUNTING FOR?
Every little thing we will see, each star, planet and individual, provides as much as solely about 15 per cent of the matter within the universe. The opposite 85 per cent is darkish matter, a substance that emits no gentle, displays none, and absorbs none, which is why no telescope has ever glimpsed it.
We all know it’s there solely as a result of galaxies spin as if held collectively by an enormous, unseen mass. It’s, in impact, the cosmic glue that allow galaxies like our personal Milky Means kind in any respect.
Darkish matter barely touches odd matter. Hundreds of thousands of its particles are thought to stream by means of your physique each second, leaving virtually no hint.
The main suspect, for 40 years, has been the WIMP, or weakly interacting large particle, a heavy relic thought to have fashioned within the first moments after the Large Bang. Catching even certainly one of its interactions is the entire recreation, and it’s monstrously arduous.
HOW DO YOU CATCH SOMETHING INVISIBLE?
LZ makes an attempt what is known as direct detection. The thought is affected person to the purpose of stubbornness: fill a tank with a dense, clear goal, liquid xenon, and anticipate a passing darkish matter particle to collide with a nucleus.
When one does, the struck nucleus recoils and the collision produces a tiny burst of sunshine and a scatter of electrons, and the sample reveals what sort of particle prompted it.
Nearly the whole lot concerning the design is a battle in opposition to noise. The detector sits a mile underground so {that a} thick blanket of rock absorbs the cosmic rays that might in any other case drown the faint sign. It’s constructed from low-radioactivity supplies and wrapped in layers meant to catch stray particles.
Xenon is chosen as a result of it’s dense, could be purified to a rare diploma, and flashes cleanly when struck. Solely when each odd clarification is stripped away does an actual candidate stand out.
WHY WERE THE EARLIER “DISCOVERIES” NEVER BELIEVED?
The cautionary story each physicist is aware of is DAMA/LIBRA, an experiment beneath Italy’s Gran Sasso mountain. For over 25 years it has reported a seasonal rise and fall in its sign, precisely the annual rhythm darkish matter principle predicts as Earth sweeps by means of the galaxy’s darkish matter halo. Its statistical energy is staggering, a significance of 12.9 sigma, far past the 5 sigma that usually counts as proof.
And but virtually no person within the discipline accepts it as darkish matter. The reason being easy and brutal: no different experiment has ever reproduced it. A rival referred to as COSINE-100, constructed with the identical sodium iodide materials particularly to check the declare, sifted by means of 6.4 years of knowledge and located no such sign, contradicting DAMA/LIBRA at greater than 3 sigma.
A separate evaluation instructed the seasonal wobble is perhaps an artefact of how the unique staff processed its information, a slowly drifting background dressed up as a cosmic sign.
That is the arduous lesson of darkish matter science. A sign in a single machine, nonetheless robust, means little till a unique machine, run by totally different individuals with totally different supplies, sees the identical factor. Statistical energy has by no means settled the argument. Reproducibility has.
WHAT MAKES THIS ONE DIFFERENT?
Set in opposition to that historical past, the brand new LZ result’s being dealt with with virtually obsessive care, and that warning is exactly what lends it credibility. The staff isn’t claiming a discovery.
They’re reporting a single occasion that sits beneath the edge physics calls for, and they’re working, within the open, to rule out the odd culprits first: a hint of background radiation, a flicker of instrumental noise, a recognized contaminant lurking within the xenon.
There may be additionally a deeper motive to belief the machine itself. In the identical run, LZ clearly picked up neutrinos streaming from the core of the Solar, catching them by means of a uncommon, long-predicted course of referred to as coherent elastic neutrino-nucleus scattering.
The detector accurately noticed one thing the textbooks insisted needs to be there. A machine that may catch a predicted sign on cue is one whose trace about an unknown sign deserves to be taken severely. LZ has additionally set the world’s tightest limits but on the place a WIMP could possibly be hiding.
THE STRANGE NEW WALL: THE “NEUTRINO FOG”
However simply because the detectors have grown delicate sufficient to really feel darkish matter’s contact, they’ve run headlong into an impediment no quantity of engineering can take away.
Those self same photo voltaic neutrinos, innocent however ample, can sometimes strike a xenon nucleus in a means that appears virtually precisely like a darkish matter hit. Physicists name this the neutrino fog, a elementary restrict set not by any detector however by nature itself.
Its arrival was confirmed with unsettling readability. In companion findings, LZ and the rival XENONnT experiment confirmed that rising a detector’s publicity by 93 per cent improved its darkish matter sensitivity by a mere 10 per cent.
The returns are collapsing, elevating a chance the sphere now voices aloud: that merely constructing greater tanks could now not be sufficient, irrespective of what number of tonnes of xenon are poured in. There could also be a ceiling, and the search could also be nearing it.
WHAT IF THE WIMP ISN’T REAL?
After 4 a long time of empty-handed looking out, some physicists are quietly hedging their bets. Consideration is drifting in the direction of lighter, stranger candidates, axion-like particles and darkish photons, that might demand fully totally different devices to catch. One query is now not unthinkable: what if the WIMP, the speculation on which so many careers have been staked, merely doesn’t exist?
For now, the xenon collaborations aren’t giving up. They’re pooling their rivalries right into a single next-generation observatory, a deliberate detector of 40 to 60 tonnes of xenon, roughly ten occasions bigger than something right now, aimed on the finish of the last decade. It could be the final, greatest throw of the WIMP cube, the experiment that both finds the particle or convinces physics to look elsewhere.
There’s a quiet dignity right here. The loudest hazard in looking a ghost isn’t lacking it, however saying one which was by no means there, as others have earlier than. Each rival experiment exists partly to verify the others, as a result of a discovery is believed solely when a unique machine, in a unique mine, sees the exact same factor.
So a single flash in a tank of xenon beneath South Dakota isn’t but a solution. It’s one thing rarer and extra trustworthy: a science disciplined sufficient to take a seat with a tantalising possibly, and refuse to name it extra, till the universe says so twice.
– Ends
