The new, ambitious experiment to search for dark matter

The new

Dark matter, it (re) departs. There are those who say that it manifests itself in the form of weakly interacting particles, some who, to explain it, are uncomfortable with a "mirror universe", who even speak of "wrecks of space-time". The fact is that, at the moment, according to the most accredited theories, dark matter - whatever it is - should represent about 85% of the total matter of the Universe, yet we have not yet seen it directly. Never say never, however: to the dozens of experiments in progress around the world, another has just been added, probably the most ambitious ever attempted. It's called Lux-Zeplin (Lz), it cost sixty million dollars and is located in the depths of South Dakota, in a former gold mine. The eyes of Lux-Zeplin, have just told the researchers who are part of the international collaboration that coordinates it (250 scientists from 35 different research institutes), opened two months ago to begin collecting the first information. Results at the moment: zero. But scientists are confident that things could change in the coming months.

Where is all this matter? The reason for so much trouble in the search for this elusive "dark matter" is easy to say. For a long time, in fact, various experimental observations (in particular relating to the speed of rotation of stars around galaxies) have shown that the Universe cannot be composed only of "ordinary" matter, the one whose components are described by the equations of the model Standard of elementary particles, the theoretical framework at the moment more solid to describe the nature and the interactions of everything that surrounds us. In short, as beautiful as it is, the Standard Model seems to be incomplete: the only way to fill this gap is to assume that there is also another type of matter, which behaves differently from the ordinary one and which escapes observations, and which precisely for this reason physicists have attributed the adjective dark. In particular, today it is estimated that for every proton, neutron and other particle of ordinary matter there must be at least five more of dark matter. Which precisely should make up about 85% of the matter present in the Universe, and about 27% of its total mass. But where is it? And why have we never seen it yet? The problem is that dark matter particles, as far as we know, are extremely elusive: just think that, if you had a huge lead cube with edges 200 light years long, a single dark matter particle would have 50 % chance of going through it without interacting with anything. In short, whatever it is, dark matter manages to hide itself very well.

South Dakota: from gold rush to dark matter This is where Lux-Zeplin enters the scene. Let's start from the end, that is, from the scientists' statement: "We have not yet seen dark matter - explained Frank Wolfs, professor at the University of Rochester and one of the coordinators of the experiment -" but the first results of Lux-Zeplin confirm that it is currently the most sensitive detector in the world. We will continue to collect data for about a thousand days, further improving the sensitivity of the instrument: in this way we hope to be able to observe something ". With "first results" Wolfs refers to the data collection of the initial run of the experiment, conducted more than anything else by way of testing, to make sure that everything is working properly. Lux-Zeplin's instruments are located about a kilometer and a half deep, where the highly sensitive detector is shielded from possible "interference" (for example, all the particles of ordinary matter, whose "noise" would easily submerge the very faint wailing of the particles of dark matter) from all the rock above. Specifically, the experiment was designed to observe the so-called Wimp, an acronym for "weakly interacting massive particles", using two large tanks filled with ten tons of highly purified liquid xenon, one of the rarest elements on Earth. The xenon atoms have the property of producing light with a certain type of interactions, among which there should also be those with the Wimp; this light is then collected and amplified by about 500 photomultiplier tubes and finally transformed into an electrical signal. If a Wimp were to "collide" with a xenon atom, it would essentially create a flash of light and an electrical signal with an unmistakable signature.

How likely it is to actually be able to observe this "signature" in a reasonable time is which is very difficult to say. Although optimistic, the scientists - perhaps even for a pinch of superstition - remain with their feet on the ground for now: "By the end of the experiment - Hugh Lippincott, spokesman for the experiment - we estimate that the probability of observing dark matter is less than 50%, but greater than 10% ”. Which objectively may seem little, but it is already something: "What we need now is a little enthusiasm - adds to the Guardian Kevin Lesko, physicist at Lawrence Berkelay National Laboratory - We cannot begin such a difficult research without the hope of find something ". We hope so.