The new hunt for dark matter

There are two kinds of dark in this world. Jodi Cooley knows them both.

The first kind we experience whenever we close our eyes. It’s the dark that envelops Dr. Cooley and her colleagues as they journey down into the heart of a working mine to reach SNOLAB, an underground research facility located two kilometres beneath the Earth’s surface.

Then there is the more mysterious kind – a dark that is not defined by an absence of light but by its utter disengagement from the material world as we know it. This is the dark of dark matter. It is an unknown substance that – so far – has revealed itself only through its gravitational influence on distant galaxies. This, combined with other astronomical effects, has allowed scientists to estimate that dark matter makes up 85 per cent of all the mass in the universe. If that’s true, it must exist alongside us, presumably passing through our bodies like a ghostly wind that can never be felt.

And SNOLAB may be the best place in the world to find it.

Jodi Cooley, a particle physicist who became executive director of SNOLAB in 2022, is overseeing one of the busiest periods in the lab's 25-year history.

“The advantage we have over most any other lab is depth,” said Dr. Cooley, an American particle physicist who took up the directorship of SNOLAB last year.

The massive overburden of rock is key to SNOLAB’s growing reputation as the world’s premier underground lab, because it can block cosmic rays that continuously bombard Earth’s surface. In a conventional laboratory, these would easily overwhelm any detector built to search for something as rare and subtle as the rustling of a dark matter particle.

When combined with the expert support staff running the lab, and an infrastructure overseen by mining company Vale, Ltd., the site offers a rare opportunity to probe the nature of the universe in a way that even the most powerful orbiting space telescope cannot.

This is not physics as depicted in the Christopher Nolan film Oppenheimer, where scientists race to realize what they know will be a new form of energy, or a weapon of unprecedented power. What’s at stake in the search for dark matter is something more fundamental. Finding it would mean opening the door to a deeper understanding of the laws that underpin our existence and that shape the evolution of the cosmos. Where that knowledge might lead a century from now is beyond imagining. In the nearer term, though, it would almost certainly lead to a Nobel Prize.

“If we had a positive detection – if we were first – I think the prize would be on its way,” Dr. Cooley said.

It is no idle boast. In the early 2000s, when the lab was known as the Sudbury Neutrino Observatory, scientists made groundbreaking measurements here that later earned a Nobel. Back then, their quarry was solar neutrinos, fleeting subatomic particles born in the sun’s core that reach Earth minutes later and can slip through solid rock to ping an underground detector.

The lab’s key contribution was helping to prove that neutrinos carry a small quantity of mass and can change, chameleonlike, from one type to another. This is crucial to the larger framework of the Standard Model of particle physics – the theory that predicts the behaviour of quarks, electrons, neutrinos and all the other building blocks of the material world that scientists have discovered, together with the forces that govern their interactions.

But the Standard Model has no place for dark matter, which most physicists suspect is an entirely different category of particle, or particles, that have little or no ability to interact with anything else except through gravity. Assuming they can be detected at all, this makes them extremely difficult to find.

The standard model of particle physics

All of the particles that have been discovered

by physicists so far, as well as the forces that

exist between them, are predicted and account-

ed for by the Standard Model – including the

Higgs boson, discovered in 2012. There is no

room in the model for a dark matter particle.

The discovery of such a particle would open the

door to new laws of physics and a deeper

theory of matter.

GENERATIONS OF MATTER

Quarks

Leptons

up

charm

top

down

strange

bottom

electron

muon

tau

electron

neutrino

muon

neutrino

tau

neutrino

Up and down quarks make up protons and neutrons. Together, protons, neutrons and electrons make up the atoms that account for nearly all of the matter we experience in everyday life.

FORCE CARRIERS

Gauge bosons

Scalar bosons

γ

g

H

photon

gluon

higgs

w

z

?

W boson

Z boson

dark

matter

MURAT YÜKSELIR / THE GLOBE AND MAIL

The standard model of particle physics

All of the particles that have been discovered by physi-

cists so far, as well as the forces that exist between

them, are predicted and accounted for by the Standard

Model – including the Higgs boson, discovered in 2012.

There is no room in the model for a dark matter particle.

The discovery of such a particle would open the door to

new laws of physics and a deeper theory of matter.

GENERATIONS OF MATTER

Quarks

Leptons

u

up

charm

top

down

strange

bottom

electron

muon

tau

electron

neutrino

muon

neutrino

tau

neutrino

Up and down quarks make up protons and neutrons. Together, protons, neutrons and electrons make up the atoms that account for nearly all of the matter we experience in everyday life.

FORCE CARRIERS

Gauge bosons

Scalar bosons

γ

g

H

photon

gluon

higgs

w

z

?

W boson

Z boson

dark

matter

MURAT YÜKSELIR / THE GLOBE AND MAIL

The standard model of particle physics

All of the particles that have been discovered by physicists so far, as well as the forces that exist between

them, are predicted and accounted for by the Standard Model – including the Higgs boson, discovered in

2012. There is no room in the model for a dark matter particle. The discovery of such a particle would

open the door to new laws of physics and a deeper theory of matter.

GENERATIONS OF MATTER

FORCE CARRIERS

up

charm

top

gluon

higgs

Quarks

Leptons

Gauge bosons

down

strange

bottom

photon

Scalar bosons

electron

muon

tau

Z boson

?

electron

neutrino

muon

neutrino

tau

neutrino

W boson

dark

matter

Up and down quarks make up protons and neutrons. Together, protons, neutrons and electrons make up the atoms that account for nearly all of the matter we experience in everyday life.

MURAT YÜKSELIR / THE GLOBE AND MAIL

In recent years, scientists working at underground laboratories around the world, including SNOLAB, have tested various means of detecting dark matter. But as possibilities that were once considered the most promising have returned negative results, researchers have had to broaden their game plan.

“The current state of the search is really interesting,” said Daniel Baxter, an associate scientist at Fermi National Accelerator Laboratory in Batavia, Ill., who has participated in two separate dark matter experiments at SNOLAB. “The perspective is much more about let’s consider as many ideas and techniques as possible.”

Now, SNOLAB is beginning a new chapter in the dark matter hunt with the assembly of the Super Cryogenic Dark Matter Search, parts for which began arriving at the underground lab in May. Built at Stanford University, SuperCDMS is funded by the U.S. Department of Energy, the National Science Foundation and the Canada Foundation for Innovation. Researchers from Europe and India are also involved. With a $42-million price tag, it is easily SNOLAB’s most ambitious undertaking since the early years, and the beginning of a new phase in the global search for dark matter.

“This one is really important to the lab,” Dr. Cooley said. “For this generation of SNOLAB – this is a big bet.”

Accessing the SNOLAB facility requires traveling two kilometres underground and then walking nearly the same distance through the mine complex. Before entering, visitors must wash their boots, shower and don clean clothing to reduce the chance of bringing dust into the ultra-clear underground laboratory.

SNOLAB has grown since it was first constructed in a hollowed-out rock chamber in the 1990s. But a trip to the lab still retains the feeling of a mythical journey into the underworld.

Scientists and guests alike first don mine-appropriate gear, including safety boots, coveralls and headlamps, then crowd into the “cage” – a large, open air elevator – which rapidly lowers them down a shaft by a vertical distance that is equivalent to the height of four CN towers.

The air pressure rises noticeably with the descent. When the cage reaches the level of SNOLAB, those heading for the lab disembark and walk through the mine’s tunnels for a distance that is nearly as far as the lab’s depth. Along the way, a powerful ventilation system keeps air moving briskly and prevents it from warming up to the 42-degree temperature of the surrounding rocks.

While the lab is well shielded from cosmic rays, all the rock grit that scientists and visitors crunch through on their way to the lab creates a separate challenge. Every stray speck can carry a trace of natural radioactivity that is harmless to people but can throw off a sensitive particle experiment. To keep such contamination to a minimum, those who enter the lab must first shed their mine gear and clothing, then shower, wash their hair and put on clean outfits that the lab provides. Then they are ready to enter SNOLAB, where the air is cleaner than a hospital operating room.

A deep dive for dark matter

Several searches for dark matter particles have

been conducted at SNOLAB, a Canadian facility

located in a mine near Sudbury, Ont., where the

overlying rock shields sensitive detectors from

cosmic rays. The latest experiment, called

SuperCDMS, consists of 24 ultra-cold sensor

packs arranged in four “towers”. The experi-

ment sits on a specially-designed platform that

reduces vibrations, and is further protected by

layers of materials that can block particles of

ordinary matter generated by natural radioactivi-

ty in the surroundings. In theory, a passing dark

matter particle can penetrate the shielding to

occasionally interact with an atom in the detec-

tor, momentarily changing its electrical proper-s

ties.

Ontario

Sudbury

Route 144

Snolab

Municipal Rd 55

Trans-Canada

Highway

3km

SuperCDMS EXPERIMENT

Shielding

Snobox

Detector

towers

Seismic

platform

DETECTOR TOWER

50 cm

35 mm

DETECTOR

Copper

housing

Superconducting

sensor

MURAT YÜKSELIR / THE GLOBE AND MAIL, SOURCE: SNOLAB; SLAC NATIONAL

ACCELERATOR LABORATORY; OPENSTREETMAP

A deep dive for dark matter

Several searches for dark matter particles have been

conducted at SNOLAB, a Canadian facility located in a

mine near Sudbury, Ont., where the overlying rock

shields sensitive detectors from cosmic rays. The latest

experiment, called SuperCDMS, consists of 24 ultra-cold

sensor packs arranged in four “towers”. The experi-

ment sits on a specially-designed platform that reduces

vibrations, and is further protected by layers of materi-

als that can block particles of ordinary matter generated

by natural radioactivity in the surroundings. In theory, a

passing dark matter particle can penetrate the shielding

to occasionally interact with an atom in the detector,

momentarily changing its electrical properties.

Ontario

Sudbury

Route 144

Snolab

Municipal Rd 55

Trans-Canada

Highway

3km

SuperCDMS EXPERIMENT

Shielding

Snobox

Detector

towers

Seismic

platform

DETECTOR TOWER

50 cm

35 mm

DETECTOR

Copper

housing

Superconducting

sensor

MURAT YÜKSELIR / THE GLOBE AND MAIL, SOURCE: SNOLAB; SLAC NATIONAL ACCELERATOR LABORATORY;

OPENSTREETMAP

A deep dive for dark matter

Several searches for dark matter particles have been conducted at SNOLAB, a Canadian facility located in a

mine near Sudbury, Ont., where the overlying rock shields sensitive detectors from cosmic rays. The latest

experiment, called SuperCDMS, consists of 24 ultra-cold sensor packs arranged in four “towers”. The experi-

ment sits on a specially-designed platform that reduces vibrations, and is further protected by layers of ma-

terials that can block particles of ordinary matter generated by natural radioactivity in the surroundings. In

theory, a passing dark matter particle can penetrate the shielding to occasionally interact with an atom in

the detector, momentarily changing its electrical properties.

Ontario

Sudbury

Route 144

Snolab

Municipal Rd 55

Trans-Canada

Highway

3km

SuperCDMS EXPERIMENT

Shielding

Snobox

Detector

towers

Seismic platform

DETECTOR TOWER

DETECTOR

Copper

housing

Superconducting

sensor

35 mm

50 cm

MURAT YÜKSELIR / THE GLOBE AND MAIL, SOURCE: SNOLAB; SLAC NATIONAL ACCELERATOR LABORATORY;

OPENSTREETMAP

The facility boasts 5,000 square metres of clean room space laid out in a series of caverns, most of which would be wide enough to drive a pickup truck through if they were half. But corridors packed with equipment are evidence that SNOLAB has never been busier, particularly in the wake of the pandemic when access was highly restricted and many projects were delayed.

While it has diversified over the past 20 years, the lab remains a major centre for neutrino physics. Its largest detector, called SNO+, contains 7,000 tonnes of water around a giant acrylic sphere with 780 tonnes of liquid scintillator, a fluid that emits light whenever a passing neutrino triggers a reaction. If a star goes supernova somewhere in the Milky Way, SNO+ will have already picked up the neutrinos it emits even before the explosion is visible to astronomers.

These days, a total of 11 out of 21 active or planned experiments at the site involve the search for dark matter in some way. Without knowing what dark matter is, scientists can so far only rule out what it is not, based on particle mass and probability of interaction. Each experiment that comes up empty points the hunt in a different direction. The excitement of the chase never wanes, Dr. Cooley said, “we just get excited about different things.”

The first clue that dark matter exists was spotted far from SNOLAB, at the Mount Wilson Observatory near Los Angeles. It was there, in the early 1930s, that Swiss-American astronomer Fritz Zwicky first noticed something amiss with a giant cluster of galaxies located more than 300 million light years from Earth in the constellation Coma Berenices.

Like bees in a swarm, the galaxies in the cluster are in motion around each other, tied together by their mutual gravitational pull. But Zwicky realized that the galaxies were moving fast enough to fly apart, based on the total mass they contained and the gravity this would generate. To account for the discrepancy, Zwicky reasoned there had to be much more mass in the cluster than was evident from the galaxies alone.

By the 1970s, the curious result had deepened into a major mystery. When Vera Rubin, an astronomer with the Carnegie Institution of Washington, made painstaking measurements of the rotations of dozens of spiral galaxies, she found that the galaxies were behaving as though they were at least five times more massive than they appeared.

Various candidates were proposed to account for the missing mass. Nothing in the Standard Model of matter fits the bill, in part because regular matter tends to clump together and emit energy in the form of visible light, infrared or radio waves. Even if dark matter were made entirely of black holes – an object so dense that not even light can escape its gravitational pull – the interstellar gas collecting around them would reveal their presence.

“Dark matter acts in a way like nothing in the Standard Model,” said Katie Mack, an astrophysicist at the Perimeter Institute for Theoretical Astrophysics in Waterloo, Ont., who studies the role of dark matter in the early universe. “What’s left is a really broad range of possibilities and the need to find creative ways to distinguish between them.”

If dark matter is an undiscovered particle, it must be immune to electromagnetism, the force that binds particles of regular matter into atoms and molecules and allows them to release energy in the form of light. It’s this immunity that allows particles of dark matter to fly through regular matter like grains of sand blowing through a chain-link fence.

On the other hand, dark matter must feel the force of gravity, because it carries mass – a lot of it – with effects that can be seen at a cosmic scale. This is of no use to researchers at SNOLAB, however, because the gravitational pull of a single particle is too minute to detect in a laboratory experiment.

In hopes of sensing dark matter more directly, physicists are instead banking on them being affected by another force – known as the weak force. The weak force only operates at an extremely short range. To feel its effects, two particles must be less than one billionth of a billionth metres apart. The challenge is that these interactions are so infrequent, it is easier to imagine two golfers at opposite ends of a driving range making simultaneous shots that collide in mid-air. To improve their odds, scientists must make their detectors as large and as sensitive as possible.

SuperCDMS is not the largest dark matter experiment ever built, but it is the most sensitive in a range that, until now, has been poorly explored. Because of theoretical predictions and also the practical fact that heavier particles tend to be easier to find, most dark matter searches have tended to focus on particles that are hundreds to thousands of times heavier than a proton (a convenient point of comparison in particle physics). In contrast, SuperCDMS is best suited for finding dark matter particles that are on the order of one to ten times the mass of a proton. If dark matter is made of such particles, calculations show that hundreds of them should be passing through the experiment at every moment. The key, unanswered, question is how likely they are to interact with the detector.

At the heart of SuperCDMS are 24 palm-sized disks of silicon and germanium that are cooled down to a temperature no greater than three-hundredths of a degree above absolute zero. At that point, the vibrating atoms that form a lattice inside each metal disk are nearly motionless. In such a state, if just one atomic nucleus is struck by a dark matter particle, the recoil should ripple through the lattice, disrupting the flow of electricity in a superconducting material that is etched on to the surface of the disk. In this way, like a tiny fly that tips a balance, the dark matter particle will make its presence known.

“One of the things we’re really proud of is our low energy threshold,” said Andy Kubik, an experimental physicist working on the testing and assembly of SuperCDMS at SNOLAB. “Dark matter experiments have gotten so advanced and we have ruled out so much room, we really have to get that sensitive in order to search new territory.”

SuperCDMS began its journey to SNOLAB in May, when half of the metal discs that make up the complete experiment were carefully packed into a truck and driven from Stanford in California to Sudbury. Rather than the most direct route, the truck took a longer, more southerly journey to avoid the high mountain passes where the hardware of the experiment might be exposed to a greater number of cosmic rays and risk a slight degradation in performance.

The disks arrived preassembled into two “towers” – half-metre high structures that will sit at the centre of the working experiment. Two remaining towers are scheduled to arrive in Sudbury later this fall.

Two detector towers that make up one half of SuperCDMS arrived at SNOLAB in May stand by for testing and installation in the experiment. The two remaining towers are expected this fall.

When operating, the towers will be cooled and shielded behind layers of lead, copper, polyethylene and water. The entire apparatus will also rest on a specially-designed platform that isolates it from vibrations in the lab. The point is to reduce any possible source of background noise that could mask a signal. If dark matter exists in the range that the experiment is built to search, it could record several detections per year.

The trick will be recognizing those detections so that team members can convince themselves, and others, that they have found something real.

Miriam Diamond, a team member and physicist at the University of Toronto who has been preparing for the analysis, said that the signals she and her colleague are interpreting from the experiment will be electronic in nature, but similar in concept to miniature earthquakes that move the needle on a seismograph.

“We have a template, which basically tells us what shape our signal event would be,” Dr. Diamond said. But the appearance of one such signal alone will not be sufficient to show that dark matter exists. Rather it will be a statistical accumulation of such events that will be the key to a positive result.

The full SuperCDMS test is still a year to 18 months from operation, but as the towers are cooled and tested during their installation, there is a chance they could spot something interesting as early as this fall. With the experiment nearing completion, the atmosphere around it has come to resemble the lead-up to a major Broadway production, Dr. Diamond said – a buildup of tension and excitement before the curtain rises.

Hunting ground

The ongoing search for dark matter has

already ruled out some possibilities, based

on the mass of the potential dark matter

particle and how close it would have to

approach a particle of ordinary matter for

an interaction to take place inside a detec-

tor. The SuperCDMS experiment is pushing

into new territory at the lighter end of

what has already been searched. As the

sensitivity of detectors improves, the

search may eventually hit the “neutrino

fog” — the limit at which interactions

between a detector and passing neutrinos

will mask the effects of dark matter.

Vertical axis: Size of area of interaction with ordinary matter (cm squared)

Horizontal axis: Mass of dark matter particle (relative to mass of a proton)

-42

10

Area to be

explored by

SuperCDMS

Already ruled

out by previous

experiments

-44

10

-46

10

Limit of

current

searches

-48

10

Neutrino fog

-50

10

1/10

10x

100x

1,000x

10,000x

1/100th

the mass

of a proton

Equal to the

mass of a

proton

100,000x

the mass of

a proton

MURAT YÜKSELIR / THE GLOBE AND MAIL, SOURCE: SUPERCDMS COLLABORATION

Hunting ground

The ongoing search for dark matter has already

ruled out some possibilities, based on the mass of

the potential dark matter particle and how close it

would have to approach a particle of ordinary

matter for an interaction to take place inside a

detector. The SuperCDMS experiment is pushing

into new territory at the lighter end of what has

already been searched. As the sensitivity of detec-

tors improves, the search may eventually hit the

“neutrino fog” — the limit at which interactions

between a detector and passing neutrinos will

mask the effects of dark matter.

Vertical axis: Size of area of interaction with ordinary matter (cm squared)

Horizontal axis: Mass of dark matter particle

(relative to mass of a proton)

-42

10

Area to be

explored by

SuperCDMS

Already ruled

out by previous

experiments

-44

10

-46

10

Limit of

current

searches

-48

10

Neutrino fog

-50

10

1/10

10x

100x

1,000x

10,000x

1/100th

the mass

of a proton

Equal to the

mass of a

proton

100,000x

the mass of

a proton

Hunting ground

The ongoing search for dark matter has already ruled out some possibilities, based on the mass of the

potential dark matter particle and how close it would have to approach a particle of ordinary matter for

an interaction to take place inside a detector. The SuperCDMS experiment is pushing into new territory at

the lighter end of what has already been searched. As the sensitivity of detectors improves, the search

may eventually hit the “neutrino fog” — the limit at which interactions between a detector and passing

neutrinos will mask the effects of dark matter.

Vertical axis: Size of area of interaction with ordinary matter (cm squared)

Horizontal axis: Mass of dark matter particle (relative to mass of a proton)

-42

10

Area to be explored

by SuperCDMS

Already ruled out by

previous experiments

-44

10

-46

10

Limit of current

searches

Neutrino fog

-48

10

-50

10

1/100th

the mass

of a proton

1/10

Equal to the

mass of a

proton

10x

100x

1,000x

10,000x

100,000x

the mass of

a proton

MURAT YÜKSELIR / THE GLOBE AND MAIL, SOURCE: SUPERCDMS COLLABORATION

Those involved with the experiment also know that it may find nothing at all. Dark matter particles simply be may be too light for SuperCDMS to detect or may not feel the weak force, counter to expectations. There is also the possibility, raised by some researchers, that dark matter does not exist and that the anomalous movements of galaxies could instead be cause to modify the current theory of gravity.

But every time a new detector is switched on, there is also a chance that a passing dark matter particle will reveal itself. And with SuperCDMS now partway through its installation, scientists at SNOLAB are hoping they may soon experience just such a moment.

For Dr. Cooley, who grew up in Wisconsin, moving to northern Ontario from her previous academic position in Texas has felt like something like a homecoming. But underground, it is all about pushing into unknown territory. After a career that has taken her as far as the South Pole to conduct particle physics experiments, the chance to be at SNOLAB during a time of a potential scientific breakthrough is exciting, regardless of the outcome.

“We’re trying to increase knowledge,” Dr. Cooley said. “But to me it’s not just about the answer at the end. It’s the entire journey.”