麻豆传媒

PICK a word to describe dark matter. Mysterious? Elusive? Invisible? One you鈥檙e not likely to use is 鈥渇ound鈥�. But after 80 years of hunting for it, that may be about to change. Several experiments deep underground have recently seen signs of something that might 鈥� just might 鈥� be dark matter. In space, too, detectors are tracking radiation that could signal the very same dark particles colliding and annihilating in our galaxy. Is it just coincidence, or could these faint fingerprints really all be from the same dark hand?

Explore our interactive map: 鈥�Around the world with dark matter鈥�

, a theoretical astrophysicist at the University of Chicago, believes that we may already have glimpsed dark matter. 鈥淚 happen to be in the relatively rare minority of my colleagues who think we probably have,鈥� he says. 鈥淚鈥檓 not certain 鈥� I just think it鈥檚 likely.鈥� If Hooper鈥檚 hunch is right, the findings challenge our understanding of what dark matter is. The stuff that makes up 85 per cent of matter in the universe might be even stranger than we imagined.

Dark matter earned its name because it gives out no light for our telescopes to catch. Yet we see its gravitational footprints everywhere we look, from ripples in the cosmic microwave background to the largest galaxy clusters. If the big bang hadn鈥檛 created dark matter in abundance, there wouldn鈥檛 be enough gravity for gas and dust to coalesce into stars and galaxies. Remove dark matter and the spinning Milky Way would fly apart. 鈥淒ark matter is the scaffolding upon which galaxies are built,鈥� says , a cosmologist at the University of California, Irvine. 鈥淲ithout it, we really don鈥檛 think we would have much structure in the universe.鈥�

Astronomers discovered it, cosmologists map it, but it is particle physicists who most keenly want to understand it. They know it is something fundamentally different, beckoning from outside the standard model that describes the particles and forces of the visible universe. Many researchers believe that dark matter is the window to an uncharted world. But what is it?

The leading candidate for dark matter is a WIMP, or weakly interacting massive particle. WIMPs get their name because we suspect that they interact with visible matter only through gravity and the weak nuclear force. Slight as the weak interaction is, it should still make WIMPs detectable. In the decades-long search for them, dark matter hunters have taken three complementary approaches: direct detection, indirect detection and colliders.

The typifies direct detection 鈥� and it is one of the experiments that may have found dark matter. The idea is that every so often one of the billions of WIMPs whizzing through the galaxy interacts with the nucleus of an atom in the detector, giving it a tiny kick. The CDMS sandwiches targets made from silicon and germanium between sensors that register electrons and the faint vibrations produced in those collisions. To protect their detector from background radiation that might mimic a signal, the CDMS team has buried it 700 metres underground in a mine in Minnesota. They also chill it to one-hundredth of a degree above absolute zero to minimise another copycat signal from thermal noise.

In April, the CDMS team announced three collisions in silicon as likely WIMPs. These weighed in at 8 gigaelectronvolts (GeV), or around eight times the mass of a proton. But with just three the researchers can鈥檛 be sure.

Still, the CDMS findings added an extra frisson to the dark matter hunt. That鈥檚 because they are in line with an experiment called , also located in the Minnesota mine, that had seen particles with the same mass in 2011. Like CDMS, CoGeNT didn鈥檛 see them in sufficient numbers to claim an outright discovery, yet taken together the findings are tantalising. Especially when you consider that one of CoGeNT鈥檚 original goals was to settle an issue that is infamous in dark matter circles, known as the DAMA/LIBRA anomaly.

The lies 1400 metres beneath Italy鈥檚 Gran Sasso National Park and has been running for more than a decade. Its detectors measure a striking signal that ebbs and flows with the seasons, exactly as you would expect if our planet were sailing through a sea of dark matter particles on its way around the sun. DAMA/LIBRA implicates particles with a mass of around 10 GeV, similar to the mass of those found by CDMS and CoGeNT (see diagram). Unlike its counterparts, there鈥檚 no doubting the strength of the DAMA/LIBRA signal 鈥� in physics parlance, it has a standard deviation of 8.9 sigma, placing it well above the 5-sigma level considered as a discovery.

Yet for many years, no one believed the DAMA/LIBRA result was down to dark matter, because the experiment was one of a kind. Then came CoGeNT. at the University of Chicago, who leads the CoGeNT team, was as surprised as anyone when it didn鈥檛 rule out the DAMA/LIBRA findings. 鈥淩ather than excluding the DAMA halo, we found an excess at low energies that we can鈥檛 explain away, and hints of an annual modulation,鈥� he says. The CoGeNT team will publish four years of results soon. 鈥淚 can鈥檛 tell you what鈥檚 in there,鈥� Collar says, 鈥渂ut I can tell you that the data are very interesting.鈥�

The discovery of dark matter would be the find of the century. So why aren鈥檛 physicists celebrating? One catch is that DAMA/LIBRA records far more strikes than other experiments expect. That鈥檚 a problem that needs to be solved before other researchers will accept the team鈥檚 claims. For James Boyce, an experimental physicist at (JLab) in Newport News, Virginia, the need for caution matches the significance of the goal. 鈥淐laiming dark matter has been found is such a serious and monumental step in our exploration of the universe, we dare not trivialise the discovery,鈥� he says.

鈥淭he discovery of dark matter would be the find of the century. So why aren鈥檛 physicists celebrating?鈥�

Another issue is that for decades theorists have favoured heavier WIMPs, in the 100 GeV range. Such heavy WIMPs fall out naturally from supersymmetry and other theories that attempt to go beyond the standard model. A lightweight WIMP ran counter to many of their expectations.

However, the biggest problem is that the world鈥檚 largest and most sensitive direct detection experiment, XENON, has seen nothing, casting doubt on all these findings. Like an overzealous cleaning crew, the team has swept away one potential observation after another, including most

10 GeV WIMPs. As they say in the polite world of physics, there鈥檚 鈥渢ension鈥� between XENON鈥檚 negative results and those other experiments.

Exclusion zone

, who leads the XENON collaboration, feels that tension acutely. 鈥淚t鈥檚 a nightmare to exclude and exclude and never find something,鈥� she says. She hopes her team鈥檚 results will challenge other experimenters to build bigger and better detectors to find or rule out particles with lower mass or even weaker interactions. Her own team is building a version that will be 100 times more sensitive than the existing XENON detector. It is scheduled to start in 2015.

Still, researchers who think a WIMP lurks in the 7 to 10 GeV range haven鈥檛 given up. They know that underlying every experiment鈥檚 conclusions are theoretical assumptions about how WIMPs interact with different nuclei, about dark matter鈥檚 distribution and velocity in our galaxy, and just how each detector functions. In a recent paper, for example, Hooper argues that if XENON鈥檚 detector responded slightly differently to what was expected, its results would He hopes that a huge project called , which will take the title of world鈥檚 most sensitive dark matter detector when it starts running later this year, will resolve the controversy.

The lightweight WIMP scenario has also been boosted by events happening out in space. Several instruments are hunting for signs of dark matter particles smacking into each other and annihilating. These indirect searches look for a distinctive pattern of gamma rays or antimatter forged from the annihilation energy. One of the most promising sightings comes from the . In 2010, it found two huge gamma-ray producing bubbles that tower 25,000 light years above and below the Milky Way鈥檚 disc of stars. Although other sources are possible, Hooper and , a theoretical physicist at the Massachusetts Institute of Technology, think the bubbles show dark particles in the 7 to 10 GeV range annihilating. 鈥淭his 10 GeV-ish window is starting to be pretty suggestive,鈥� Hooper says.

But indirect searches have their fair share of disappointments, too. Another Fermi signal once hailed as a 鈥渟moking gun鈥� faded when more measurements were made. Budding signs from the PAMELA satellite and the Alpha Magnetic Spectrometer (AMS) on the International Space Station suggest that dark matter particles are heavyweight brutes, while results from another gamma ray satellite are suggestive of WIMPs a thousand times lighter.

Ultimately, if WIMPs do exist, it should be possible to make them by smashing together particles at high energies. That means they should turn up at colliders like the Large Hadron Collider near Geneva in Switzerland. So far, however, they have been a no-show.

With the picture so bewildering, everyone agrees that what they need is more time to collect data and to build bigger and better detectors. 鈥淲e want to be 100 per cent sure about what we鈥檙e saying,鈥� says Roberto Battiston, deputy spokesperson for the AMS.

While experimentalists like Aprile describe failing to find WIMPs as a 鈥渘ightmare scenario鈥�, many theorists find the field鈥檚 confusion liberating. 鈥淭o solve the dark matter mystery, we have to think as broadly as possible about what dark matter could be, so we don鈥檛 miss what it is,鈥� says , a high-energy theorist at the University of Michigan in Ann Arbor.

Zurek and others are exploring a much richer dark sector, with multiple dark particles and interactions and even dark atoms, dark chemistry, and ultimately a mirror universe matching ours particle for particle and force for force.

鈥淭he dark sector could be much richer, with dark atoms, dark forces and even a mirror universe matching ours鈥�

The simplest models incorporate a new dark force that lets dark particles interact, meaning that the dark sector would include dark electromagnetism and dark light, an idea first explored by , at Durham University, UK. In parallel with quantum theory, which calls for photons mediating the electromagnetic force, Boehm鈥檚 idea implies dark photons. 鈥淭he dark photon would be a force of nature beyond the ones we know about right now,鈥� says , a particle physicist at the State University of New York in Stony Brook.

A dark force has major implications, from how visible and dark matter emerged from the big bang to how galaxies and larger structures formed. In a , Slatyer, along with theorists Nima Arkani-Hamed, Douglas Finkbeiner and Neal Weiner argued that a long-range dark force could boost the rate of dark matter collisions and neatly account for DAMA/LIBRA, PAMELA and related findings. 鈥淚t鈥檚 totally reasonable to assume that the dark sector has its own set of forces,鈥� says Weiner. 鈥淵ou put that in and you find it does many things simultaneously, which is a bonus.鈥�

That opens the door to a dazzling array of possibilities. David Kaplan, a theorist at Johns Hopkins University in Baltimore, Maryland, notes that a dark force implies dark charges, which could produce dark atoms, hydrogen-like chemistry, and perhaps even dark stars. Earlier this year, Andrey Katz and his colleagues at Harvard University, in Massachusetts, even suggested that some dark atoms would cool and settle into galactic discs. That 鈥渄ouble-disc dark matter鈥� could explain several experimental anomalies and should be easily testable.

Dark light

Dark antimatter is a possibility too. According to a theory put forward by Kaplan, Zurek and Markus Luty, at the University of California, Davis, a link between normal antimatter and dark antimatter in the early universe could neatly explain the amount of dark matter we observe now.

Perhaps most exciting of all, it might be possible to make dark light in the lab. Theorist Bob Holdom at the University of Toronto in Canada showed in 1986 that dark photons can interact with normal photons and electrons, leading some researchers to think it might be easier to find dark photons than heavier dark particles. 鈥淭he dark photon can be quite light and can be seen by existing accelerators,鈥� says experimentalist Harald Merkel at Johannes Gutenberg University, in Mainz, Germany.

That prospect has inspired a new raft of experiments across Europe and the US. The , which Merkel works on, has started searching for dark photons, but has not yet seen one. 鈥淚f we had found something, you would know,鈥� says Merkel. 鈥淚t would be as important as the discovery of the Higgs boson. It would be the beginning of the new physics.鈥�

On the other side of the Atlantic, three experiments 鈥� , and the 鈥� have passed preliminary tests at JLab. Each one is sensitive to a different mass range, so the three will complement each other. The Heavy Photon Search will be the first to start taking data in 2015, when JLab鈥檚 accelerator fires up after an upgrade, followed by APEX and DarkLight. John Jaros, a high-energy particle physicist at Jlab cautions that even if these experiments discover a dark photon, finding how it links to dark matter will still be challenging.

Such frenzied activities beg a question: what will it take to declare dark matter found? Researchers agree that a convincing discovery of a dark matter particle or force will require at least two different experiments whose findings match perfectly in terms of mass and willingness to interact with normal matter, and which have high statistical reliability. Collar adds two caveats. 鈥淚deally, compatible 鈥榤an-made鈥� dark matter should be found at accelerators,鈥� he says. 鈥淎nd the cherry on top would be a theory that generates the particle in question, and yields other testable predictions.鈥�

Whatever the outcome of these experiments, dark matter is spurring what physicist at Purdue University in West Lafayette, Indiana, calls a new Copernican revolution. If a dark matter particle is found, it will be the first piton anchoring a long, hard climb into the dark sector. If it鈥檚 not found 鈥� and soon 鈥� everything is up for grabs, from how gravity works to the whole of physics beyond the standard model.

This article appeared in print under the headline 鈥淧ortraits of darkness鈥�