Ghost traps: the hunt for dark matter
We have no idea what most of the matter in the Universe is made of. Are we finally closing in on dark matter? Robin McKie reports.
If you were designing a villain’s lair for a James Bond movie, you would be hard pushed to create one as spectacular as Italy’s Gran Sasso Laboratory.
To reach it, you follow the A24 motorway west towards Rome as it plunges through a 10-kilometre tunnel drilled below the Gran Sasso National Park, a mountain range that is home to bears, wildcats, wolves, chamois and thousands of summer tourists.
Half way along, an unsigned tunnel branches to the right. Travel 100 metres along this passageway and you reach a four-metre high, solid stainless steel door topped with barbed wire. As it swings open, a labyrinth of tunnels, uniformed guards and glittering racks of equipment appear before you. All the scene lacks is the appearance of a mysterious figure, clutching a white Persian cat, to let you know: “We’ve been expecting you, Mr Bond.”
You won’t find a Bond villain down here. But you will find physicists, burrowed under the mountains in their hunt for dark matter. This elusive material makes up 85% of the matter in the Universe. We know it must be out there – without it galaxies would fly apart and we would not exist. But so far what it’s made of remains a mystery; we’ve been unable to detect the stuff.
That is about to change. Researchers working in several scientific arenas – including Gran Sasso – are now confident that within the next two or three years they will make the breakthrough that will reveal the truth about dark matter.
Something else was glueing the galaxy together
“There are two ways we’re likely to succeed,” says Chamkaur Ghag, who leads the “direct” dark matter search at University College London. Direct searches involve placing detectors deep underground at places such as Gran Sasso. Here 1,400 metres of rock shelters the detectors from the cacophony of particles pelting the Earth’s surface. If the physicists pick up any hint of a signal way down there, “we will know we have hit pay dirt”, Ghag says.
The other method of revealing dark matter is more dramatic. Scientists believe they should soon be able to make the invisible material inside the world’s most powerful atom-smasher, the Large Hadron Collider at the European Organisation for Nuclear Research (CERN), near Geneva. The collider recently reopened with double the power it employed during its successful hunt for the Higgs boson. Since matter and energy are interchangeable, at these massive energies particles can be created.
More power means a greater chance of creating more massive particles – possibly dark ones. It also creates more interactions so if the creation of dark matter is extremely rare, the chances of picking it up are greater.
“I would have thought that if dark matter exists we will be well placed to make it at CERN in the very near future,” says John Ellis, one of the organisation’s key theoretical physicists. The dark matter will be detected at CERN by its absence – the energy and mass that is missing after physicists account for all the particles in the debris of smashed protons.
Who is likely to get there first? Will it be the subterranean teams manning underground detectors in their lairs, or will it be the collider physicists smashing protons into each other?
“It’s going to be a very close call,” admits Ghag, who previously worked on the Xenon100 and DarkSide-50 projects at Gran Sasso and now works at a similar facility called SURF (the Sanford Underground Research Facility) in the US. “But even if CERN gets there before us, we would still need to confirm with direct detection that the particle they make is entirely responsible for all dark matter,” he says. The CERN result – which will narrow down the energy and mass of the dark particle – would “indicate we are on the right track and are closing in on our target”, he says.
The gravity of dark matter is warping the fabric of space.
The story of dark matter goes back to the first half of the 20th century when astronomers realised galaxies were spinning so fast that they should have flung themselves apart.
Think of a stone tied to a piece of string. If the string is too weak, when you whizz it round your head it will snap and your stone will hurtle into the distance. And so it is with galaxies. A galaxy needs a great deal of mass to generate a gravitational field powerful enough to hold on to its rapidly rotating stars. In 1932, Dutch astronomer Jan Oort pointed his optical telescope at the Milky Way and from its luminosity and redshift (a way of measuring how fast stars are receding), estimated the mass and rotation speed of the stars. He concluded there were too few stars to glue the spinning galaxy together.
A year later, Fritz Zwicky at the California Institute of Technology reported a similar conundrum while observing a large group of galaxies known as the Coma Cluster. At their speed of rotation, the outer galaxies ought to have been flung out. Oort and Zwicky referred to the missing galactic material as “dark matter”.
The idea that some exotic form of invisible matter existed was hotly contested. Surely it could be explained by the inability of light telescopes to detect faint galactic objects. There was no shortage of mundane candidates: tiny stars, large dark neutron stars, brown dwarfs (small failed stars) or clouds of diffuse gas.
But the idea of dark matter refused to go away. In the 1970s Vera Rubin and colleagues at the Carnegie Institution of Washington made more rigorous measurements of the rotation speeds and matter content of a number of galaxies. In every galaxy measured, there were far too few stars to account for the speed of the galaxy’s rotation. Something else was generating a powerful gravitational field that held each galaxy together.
In subsequent decades, astrophysicists have eliminated virtually all the mundane candidates for dark matter. Spinning neutron stars were detected by their radio waves. Infrared detectors picked up dim stars and brown dwarfs. And space-based telescopes such as NASA’s Chandra X-ray Observatory measured the vast mass of gas clouds, such as the one that engulfs our Milky Way and weighs as much as all the stars inside. Yet when all the dim and ethereal matter is added up, it is still not enough to glue galaxies together.
“We’ve spent more than 30 years trying to pin down objects that might account for dark matter, and have had no success,” says astrophysicist Gerry Gilmore of Cambridge University.
Other evidence for dark matter comes from gravitational “lensing”. Sometimes, as a galaxy spins through an apparently empty stretch of space, multiple images appear – as if it were passing behind a warped lens. The lens in this case is inferred to be dark matter: its gravity is warping the fabric of space.
But the strongest evidence for dark matter “is that we’re here at all”, says Alan Duffy, a theoretical physicist at Melbourne’s Swinburne University of Technology. His supercomputer-based models of the formation of the Universe show that the plasma created by the Big Bang was too hot and too smoothly distributed to have collapsed into galaxies 13 billion years later.
But dark matter is not subject to the same frenetic interactions. It would have settled down early on, forming “wells” that ordinary detectable matter – also known as baryonic matter – could fall into.
Having ruled out baryonic matter as the source of the missing mass, the only remaining option was that dark matter is composed of an exotic subatomic particle. It is also abundant: five times more plentiful than the baryonic matter scattered throughout the Universe.
Physicists took to calling these numerous but mysterious bits of matter weakly interacting massive particles – or WIMPS.
So how are we to detect a WIMP? Via one of the fundamental forces. There are four: gravity; the electromagnetic force; the strong nuclear force that glues the nuclei of atoms together; and the weak nuclear force that transforms particles from one type to another and drives radioactive decay.
Scientists have ruled out the idea that dark matter interacts in any way with either electromagnetism or the strong nuclear force. “If it did we would have seen the results – bursts of light or radiation,” says Ghag. These would be produced whenever dark matter and regular matter particles collide.
We know dark matter does interact with gravity. But gravity exerts a virtually undetectable force at subatomic scales. For an electron and proton, the gravitational force is 39 orders of magnitude weaker than the electromagnetic force – so gravity is not a helpful way to detect a dark matter particle.
That means all our hopes are pinned on the last remaining force, the weak force. “And that is what we really mean when we call them weakly interacting massive particles – it’s because they may interact with the weak force,” says Ghag.
Space must be saturated with WIMPS. Katherine Freese, a theoretical physicist at Michigan University and author of The Cosmic Cocktail: Three Parts Dark Matter, believes billions of these particles must pass through the human body every second – rather like neutrinos. Indeed neutrinos seem to fit the bill for dark matter since they have mass but only interact via the weak force. But they cannot account for dark matter. Although they are the most abundant particle in the cosmos – with one billion cosmic neutrinos for every atom – their mass is less than a billionth the mass of a proton. They are far too light to account for the missing mass in the Universe.
Hunting for WIMPS has been “like searching for a particular kind of fish in the ocean”, says the slim, amiable Ghag, with his typical rapid-fire verbal delivery. “At first, you put on goggles and dive down just below the surface to see if you can see it. If that does not work, you try scuba gear and go deeper. Then you try a submarine until, eventually, you find it.” Or so he hopes.
Hence the detectors installed under Gran Sasso and at several other underground laboratories have been built thousands of feet below the surface, usually in old mines. These include the Stawell gold mine near the Grampians National Park in Victoria, Australia; the old Homestake gold mine in South Dakota; and the Boulby salt mine in north England.
The subterranean locations are important. The Earth’s surface is bombarded by sub-atomic particles called muons. These energetic, charged particles are the byproducts of high-energy cosmic rays which slam into our atmosphere so hard they smash oxygen, nitrogen and other atmospheric gases into showers of subatomic particles. “Muons light up our detectors like Christmas trees,” says Ghag. “They are so numerous they would blind us to anything else.”
Down in Gran Sasso, shielded by 1,400 metres of rock, muon levels are one million times lower than at the surface. More of them are filtered out by placing the particle detectors in huge vats containing thousands of cubic metres of pure water. Inside the tank, a huge sphere containing a device called a scintillator is used to cut out any stray particles that make it through the water jacket. “We are putting one device inside another like a set of Russian dolls to get rid of every possible spurious signal,” says Frank Calaprice of Princeton University, who co-leads the DarkSide-50 experiment at Gran Sasso.
The last Russian doll is a stainless steel sphere containing argon or xenon liquid, with some gas on top. If a WIMP passing through hits an argon or xenon atom directly, the weak nuclear interaction between atom and particle might bump out an electron or spit out a photon. The characteristics of that signal will tell scientists if a muon has slipped through the screens or whether they have actually detected a WIMP.
To date, despite some tantalising signals at detectors such as DAMA [see box, page 69], physicists are yet to be convinced that any WIMP has announced its existence. While researchers are continually refining the sensitivities of their machines, it is possible that we will never catch a WIMP. Sadly (for all the billions spent on the detectors) it may be that dark matter does not interact via any of the known forces other than gravity. “One way or other, we are going to find out very soon if that is the case,” says Ghag.
Gran Sasso’s detectors might never trap a WIMP. But scientists at CERN are confident they can create dark matter.
For a start, their base of operations utterly dwarfs those at Gran Sasso or South Dakota’s Homestake mine. CERN is a sprawling suburb of Geneva stacked with laboratories, dormitories, restaurants and control rooms all built over a giant circular tunnel, 27 kilometres in circumference, that makes up the Large Hadron Collider.
The collider is roughly the size of the London Underground’s Circle Line, and has been constructed with nanometre precision. Its magnets – which guide beams of protons round its tunnel – are chilled to within two degrees above absolute zero (a temperature at which electricity flows without resistance), making the collider the coldest place on Earth. At the same time, the tube that carries those beams of protons has been sucked of virtually every atom or molecule, creating a vacuum that is purer than that found in space.
Three years ago, CERN scientists used their astonishingly powerful and precise instrument to discover the Higgs boson. Then they shut down for two years to upgrade the machine. The LHC “is almost like a new machine now”, says CERN’s Frederick Bordry, director of accelerators and technology.
When it discovered the Higgs, the LHC could generate energy bursts of up to eight trillion electron volts (8 TeV). Now, it can produce collisions with energies of up to 13 TeV.
The more energy, the more likely this matter-energy soup will create new massive particles. Just as two protons collided with enough energy to form the Higgs boson – with far more than twice the mass-energy of two protons – the hope is the higher energy will create a massive dark matter particle.
“When we batter the beams of protons into each other, we will make particles of a mass we think could be similar to those that we think account for dark matter,” says John Ellis, one of CERN’s key theorists.
Ellis has been hunting for dark matter all his working life, albeit indirectly at first. He is an amiable, slightly shambolic figure with a massive white beard, who has been compared to Santa Claus, Dumbledore and Gandalf.
A Cambridge University graduate, Ellis began his research on supersymmetry in the early 1970s. Supersymmetry predicts that versions of the particles that make up normal matter possess mirror, or “supersymmetrical”, versions. Thus there could be supersymmetrical quarks – or squarks – out there. Or supersymmetrical electrons – selectrons. Ellis’s field became embroiled in the hunt for dark matter when physicists realised some types of supersymmetrical particles and WIMPS could be one and the same.
If the Large Hadron Collider wins the battle to find these elusive entities, it will be a fitting tribute to Ellis who has directed much of CERN’s research effort over the past couple of decades. These efforts have revealed the nature of the Standard Model of Matter, which could finally be completed by incorporating the ideas of supersymmetry.
Most CERN physicists believe supersymmetrical particles – and therefore, WIMPS – are likely to lie within the range of the upgraded collider. As Dave Charlton, head of the collider’s ATLAS detector, puts it: “The supersymmetry particles that we think we will soon be producing provide a perfectly natural explanation for dark matter – in the form of WIMPS.” He predicts that “the next couple of years promise to be extremely interesting”.
Because dark matter rarely interacts with regular matter, it won’t appear directly in the collider’s detectors, Ellis explains. “If WIMPS are created … they will escape through the collider’s detectors unnoticed.” But when they go, they will carry away energy and momentum with them. “We will be to able to infer their existence from the amount of energy and momentum missing after a collision.”
But there’s no guarantee says Duffy. “The collider might not be capable of producing a dark matter particle, regardless of the energies it reaches. And if it does, it has to make enough to notice that some matter is missing.”
So who will be the first to snare these ghostly particles? Duffy has no qualms about backing the physicists at Gran Sasso. “It’s a race. But just like the tortoise and the hare, I’d rather go with the slower, surer runner.”
Striking dark matter in an Aussie gold mine
Elizabeth Finkel Reports
The residents of the Victorian country town of Stawell are a resourceful lot. The town was founded in the 1853 gold rush and has long been sustained by gold. But as profits dwindle, the mine is being revived in a remarkable way: by hunting for dark matter.
The 10,000 residents of Stawell may not be particle physicists but they have been quick to realise their mine’s potential.
“The next chapter in Stawell’s story is an exciting one, home to hundreds more local jobs and, possibly, the world’s next great scientific discovery,” enthused Victorian Premier Daniel Andrews last February as he announced his State Government would invest $1.75 million to kickstart the gold mine’s conversion. The Federal Government matched the funding, and the Australian Research Council contributed $1.18 million.
The funding is remarkable given how difficult it has been of late to extract any government money for new science projects. Clearly resuscitating regional towns is good politics. And Melbourne University’s particle physicists have also made a persuasive argument.
“For a few million, this is a cheap Nobel prize,” says physicist Elisabetta Barberio, in charge of building the detector at the bottom of the kilometre-deep gold mine. After decades of hunting, dark matter might finally be captured here.
So why is Stawell so significant? Barberio and her colleagues jumped at the opportunity because hunting for dark matter in the Southern Hemisphere might resolve a scientific impasse.
Theory tells us the Universe contains five times more dark matter than the ordinary atoms surrounding us. “Here, right in the space between you and I, could be this alternative world of dark matter,” says the otherwise sane-seeming Geoff Taylor, director of the Centre of Excellence for Particle Physics at the University of Melbourne.
Particle physicists joined the hunt for the ghostly stuff in the 1990s, building detectors that rely on the hope that when a dark matter particle – or weakly interacting massive particle (WIMP) – bumps into an atom’s nucleus, it gives it a glancing blow, rather like a gentle cue ball. The WIMP slips off in its ghostly way while the recoiling nucleus emits a photon.
Different types of traps are a good strategy, says Barberio, since
“we don’t really know what we’re looking for”.
Difficulties arise because many things can interact with an atom and cause a photon to be emitted. The trick is to eliminate as many of these pretenders as possible. So detectors are located deep underground to shield them from space muons and surrounded by multiple screens to protect them from rock-borne radioactive particles.
Many detector designs have been tried. Different types of traps are a good strategy, says Barberio, since “we don’t really know what we’re looking for”. The main requirement is that detector materials be extremely pure to eliminate radioactive noise. The first models used pure crystals of sodium iodide (NaI).
Later generations used liquefied xenon, at first containing modest amounts because it was expensive to produce. XENON100 at Gran Sasso now contains 100 kilograms, while the Large Underground Detector Dark Matter Experiment (LUX) at a mine in South Dakota contains 350 kilograms – and they’re heading to seven tonnes in South Dakota with the Lux-Zeplin (LZ) detector. The more xenon, the more shielded the core of the detector, and the greater the sensitivity. Argon detectors such as DarkSide-50 are also being tried, but so far have yet to match the quiet background and sensitivity delivered by xenon.
Despite dozens of experiments most detectors have failed. A notable exception is DAMA (DArk MAtter) at Gran Sasso. It was one of the first detectors, and in 1998 it detected something.
It has continued to detect that signal with ever greater statistical significance (now with nine standard deviations) even though the XENON100 detector, in the same location and with more than 1,000 times greater sensitivity, has not.
Many physicists have concluded DAMA is detecting some sort of artefact. On the other hand, perhaps DAMA happens to have the right sort of trap? Even Chamkaur Ghag, a dark matter hunter and DAMA sceptic at University College London, acknowledges, “they do have spectacularly good crystals”.
To find out if the ultrapure NaI crystals are the right trap for dark matter, other traps need to incorporate crystals of this quality and replicate the experiment. That’s what will happen at Stawell and Gran Sasso, when identical twin detectors are placed in these locations as part of an international project dubbed SABRE and headed by Frank Calaprice from Princeton University.
So why is Stawell the linchpin?
As our Sun zooms though the Milky Way at 220 kilometres per second, it is thought to encounter a stream of dark matter particles known as the WIMP wind. For half the year, Earth’s orbital motion around the Sun means the two sail into the wind together.
But for the other half of the year, Earth tacks back around the Sun and sails with the wind. The prediction then, is that the Earth will be battered by WIMPs while sailing into the wind, and experience relative calm while sailing with it. This could be what DAMA measures: its signal peaks around June and is weakest around December.
But there’s a problem: these fluctuations also correspond to the Northern Hemisphere summer and winter: Earth’s atmosphere is thicker in the summer and generates more muons than in winter. So is DAMA merely detecting summer muons, or the WIMP wind? The debate has raged for years.
Scientists hope an identical detector in the Southern Hemisphere will resolve it. If the Gran Sasso detector is truly identifying changes in the WIMP wind then the Southern Hemisphere detector will feel the same effect at the same time.
But if the seasons are the explanation, then the Stawell detector should see the opposite fluctuation to its Northern Hemisphere twin.
Meanwhile, the good people of Stawell need to be patient about their Nobel. The detector should take five years to build and the results should be in hand three to five years after that.