The mysterious substance referred to as darkish matter is intrinsically invisible.
It can’t be straight noticed—moderately, its presence is inferred by its gravitational affect on the universe, comparable to binding galaxy clusters collectively and shifting stars round their galaxy sooner than they need to.
Yet new analysis revealed in Physical Review Letters makes use of a “camera” to search for darkish matter interactions, thereby probing the character of this elusive stuff.
One speculation is that darkish matter is made from as-yet unknown particles which are topic to gravitational pressure however work together extraordinarily weakly with atypical matter, explains University of Chicago Prof. Paolo Privitera, the spokesperson of the DAMIC-M (DArk Matter In CCDs at Modane) worldwide collaboration, which carried out the research.
Over the previous a number of a long time, the seek for darkish matter particles has targeted on WIMPs, or Weakly Interacting Massive Particles, believed to be far heavier than a proton.
“But WIMPs have not been found so far, despite extremely sensitive searches by enormous detectors weighing a ton, including my colleague Luca Grandi’s work with XENONnT,” stated Privitera.
Experiments on the most superior particle accelerators, together with the ATLAS experiment on the Large Hadron Collider at CERN, have additionally failed to seek out WIMPs.
Astrophysicists at the moment are increasing the search to lighter particles, which requires exceptionally delicate devices as a result of alerts produced by such low-mass, low-energy particles can be nearly unattainable to detect.
The DAMIC-M experiment searches for these elusive alerts 5,000 toes beneath the floor of the French Alps. Though it didn’t discover darkish matter in its preliminary run, the experiment was in a position to rule out a number of such particle candidates often known as “hidden sector” darkish matter.
Hidden-sector detector
Dark matter detectors are designed across the premise that darkish matter particles will, on very uncommon events, collide with a nucleus in one of many detector’s atoms. The recoil of the nucleus might emit gentle, strip electrons or shake the atom’s lattice, producing a sign.
A light-weight darkish matter particle is rather more tough to detect than a heavy one.
“A heavy particle hitting a nucleus is like a bowling ball hitting another bowling ball—it will impart a sizeable momentum,” stated Privitera. “A light particle hitting a nucleus would be like a ping-pong ball hitting a bowling ball. It would not move it at all.”
However, hidden-sector darkish matter would work together with electrons, that are 1000’s of instances much less large than a nucleus.
“Now it is like a ping-pong ball hitting another ping-pong ball,” stated Privitera.
An instrument delicate sufficient to detect single electrons can be supreme to seek for hidden-sector darkish matter.
The DAMIC-M experiment makes use of charge-coupled units, or CCDs, to attain such unprecedented sensitivity and backbone. Standard scientific CCDs are light-sensitive units that convert photons into electrical expenses, that are then processed right into a digital picture.
They function the “camera” of astronomical telescopes. CCDs are additionally able to “imaging” particle interactions, which depart a path {of electrical} expenses within the system.
DAMIC-M CCDs are a lot thicker to maximise the detector mass for darkish matter particle interactions. The experiment’s particular CCDs are additionally able to skipper readout, an innovation that permits researchers to depend electrons individually. The group seems to be for pixels or clusters of adjoining pixels with only a few electrons—doubtlessly indicating a darkish matter interplay.
These collisions are extraordinarily uncommon and may very well be obscured by alerts from background sources, comparable to pure thermal fluctuations within the detector’s materials. To assist decrease this, DAMIC-M CCDs are operated at -220 levels Fahrenheit.
To mitigate the consequences of exterior radiation, the detector is protected by a number of layers of defending. Located on the Laboratoire Souterrain de Modane beneath the French Alps, the detector is sheltered from cosmic rays by over 5,000 toes of rock. To cut back background from naturally occurring radioactive components discovered within the cavern’s partitions, the CCDs are surrounded by lead.
Fun truth, stated Privitera: “We use ancient lead, from a sunken Roman ship and Spanish galleon, since its radioactive contaminants have already decayed.”
For this research, the group constructed a prototype—the Low Background Chamber, internet hosting two CCD modules and weighing simply 26 grams—and took a number of thousand “photographs” over two and a half months. They then searched these photographs for clusters of pixels suggesting darkish matter interactions.
The group discovered 144 clusters with two electrons and just one cluster of 4 electrons—outcomes suitable with the anticipated backgrounds.
“Thus, we have not yet discovered dark matter,” stated Privitera, although he added the outcomes are “orders of magnitude more sensitive than any other experiment, a notable achievement when considering they were obtained with a prototype detector and a small mass.”
As the search continues, the absence of a darkish matter interplay sign has profound implications for the character of darkish matter.
‘Freeze-in’ or ‘freeze-out’
In one potential, simplified situation of the universe’s evolution after the Big Bang, darkish matter and atypical matter begin at equilibrium and remodel into one another at equal charges.
As the universe expands and cools, it turns into more and more tough for atypical particles to come across one another and create darkish matter, which requires a high-energy collision.
However, it takes no power for darkish matter particles to satisfy and destroy one another, turning again into atypical matter, so the abundance of darkish matter would quickly lower after the Big Bang. Eventually darkish matter particles additionally develop into too unfold out to have interaction and the quantity stabilizes to what we measure at present. This situation is called “freeze-out” of darkish matter.
In one other potential situation, darkish matter particles work together so weakly that darkish and atypical matter are by no means in equilibrium. On the uncommon event that darkish matter is produced by atypical matter interactions, it doesn’t remodel again—and will increase in abundance.
The manufacturing of darkish matter, as with the freeze-out situation, is restricted by the growth of the universe, so the quantity of darkish matter finally stabilizes to the quantity measured at present. This situation is called “freeze-in” of darkish matter.
The freeze-in and freeze-out situations limit the properties of darkish matter—particularly its mass and interplay likelihood—and theorists have predicted the properties that hidden-sector darkish matter will need to have to be suitable with the freezing situations.
“These theoretical predictions are now probed for the first time by the DAMIC-M null result,” stated Privitera.
For the freeze-out situation, a stringent relationship exists between how a lot darkish matter is noticed at present and its likelihood of interplay. This constraint permits researchers to clarify predictions of a candidate particle’s probability of interacting with the detector’s electrons and producing a sign. Because the group didn’t detect alerts, the experiment utterly excludes a number of hidden-sector candidates—they don’t exist.
But for the freeze-in situation, an absence of sign doesn’t definitively rule out the existence of that candidate.
“The fact that we have not found dark matter in our data excludes that hidden-sector particles constitute the entirety of dark matter in the universe,” stated Privitera.
Yet if hidden-sector darkish matter exists, it may very well be a fraction of all darkish matter, with one thing else comprising the remaining.
Scaling up
Following the success of the Low Background Chamber prototype, the complete DAMIC-M equipment is about to start information assortment in 2026.
The full-scale detector may have a better probability of capturing a uncommon interplay, the scientists stated, and the backgrounds will lower considerably because of higher shielding and fewer radioactive contaminants within the equipment supplies.
“Our target is still hidden-sector dark matter, which we may find composing a fraction of all dark matter, but also light WIMPs and other candidates,” stated Privitera. “We expect that DAMIC-M will be the leading experiment in the search for these low-mass dark matter particles for several years to come.”
Other UChicago co-authors on this research embrace analysis affiliate professor Radomír Šmída; KICP Fellow Brandon Roach; postdoctoral scholar Julian Cuevas-Zepeda; and graduate college students Ruixi Lou, Sravan Munagavalasa, Joseph Noonan, Sugata Paul and Rachana Yajur.
Citation: “Probing Benchmark Models of Hidden-Sector Dark Matter with DAMIC-M.” Ok. Aggarwal, I. Arnquist, N. Avalos, X. Bertou, N. Castelló-Mor, A. E. Chavarria, J. Cuevas-Zepeda, A. Dastgheibi-Fard, C. De Dominicis, et al (DAMIC-M Collaboration). Phys. Rev. Lett. 135, 071002. DOI:
Funding: European Research Council; National Science Foundation; The Kavli Foundation; The Ministry of Science and Innovation, Spain; Swiss National Science Foundation; and Centre National de la Recherche Scientifique (CNRS)