Dark Matter:It’s the biggest puzzle in science: we see only five per cent of all matter. This is the epic tale of the unending hunt for dark matter.
Dark Matter:
“If the result confirms the hypothesis, then you've made a measurement. If the result is contrary to the hypothesis, then you've made a discovery.”
It’s the biggest puzzle in science: we see only five per cent of all matter. This is the epic tale of the unending hunt for dark matter.
Joe Walding, a physicist at Royal Holloway, University of London who often works at SNOLAB, enters the two-storey lift, he smiles broadly. The journey down takes about six minutes, and it’s not for the faint-hearted: the lift doesn’t have any lights, and the darkness is almost palpable; its top is half-open, and it shakes and bangs so much on the way down that people sometimes faint. “You certainly shouldn’t stick your arm out or it’ll get cut off,” says Walding, half-jokingly.
He’s in a good mood – like many other physicists here these days: in May, the scientists at SNOLAB got exciting news. The lab, operating since 2011, has just received funding approval from the US Department of Energy to build a brand-new dark matter experiment, scheduled to start operating in 2020. Mightier than any other (and there are many), the uber-sensitive SuperCDMS will cost some $34 million – and will be tasked with finally spotting what no other detector has spotted yet; no pressure.
Despite huge pots of money being poured since the 1970s into dark matter experiments on, under or above Earth, despite endless late nights spent doing calculations, and despite plenty of media coverage, researchers keep getting nowhere. Apart from SNOLAB, there is the LUX experiment in Lead, South Dakota, one mile underground in an abandoned gold mine. It has obtained zero results. In France, the EDELWEISS experiment in a lab under the French Alps, under 1.7 km of rock, has found nothing. The PandaX experiment in the Jin-Ping sub-terrain laboratory in China hasn't spotted any particles either. In India, Jaduguda Underground Science Laboratory opened last year, 550 meters below the surface at an operating uranium mine. So far, they have found nothing (well, they've only been looking for a year). And on, and on, and on.
The leading theory is that dark matter is made out of particles that interact with normal, atomic, matter or light only through gravity - by exerting a gravitational pull. SuperCDMS will be looking for a very specific type of such exotic particles, so-called WIMPs, or weakly interacting massive particles. That’s the main (some say most obvious) dark matter candidate several detectors are searching for. Scientists are even trying to create these particles in the largest and most powerful particle accelerator in the world, the Large Hadron Collider (LHC) near Geneva (which cost nearly $7 billion to build). But all in vain.
So just how much longer can researchers justify that they are looking for something unknown and finding nothing, but still get away with asking for more money to look for nothing... just a little bit longer? Well, turns out that for the researchers who have devoted their whole life to dark matter, null results are ultra-important – nearly as important as finding something.
“The only difference is that if you find dark matter, you get a Nobel Prize, but the importance in setting the limits [where there is no dark matter] is just the same,” says physicist Walter Fulgione from Istituto Nazionale di Fisica Nucleare (INFN) in Italy. That’s because setting tighter and tighter limits rules out hypotheses that seem viable, narrowing the window of search. After all, it took a century to detect gravitational waves, predicted by Albert Einstein in 1916, and nearly half a century to spot the Higgs boson in the LHC. Researchers knew what they were looking for and null results year after year helped them to better constrain their search limits. “The game is one about steady progress over years and decades, slowly chipping away at the possible range of models,” says Dan Hooper, an astrophysicist at the Fermi National Accelerator Laboratory (FNAL) just outside Chicago.
Avi Loeb, a Harvard astronomer, describes science as “an island of knowledge surrounded by an ocean of ignorance”. If you search for a lion in the desert, you can find it eventually by excluding successively bigger regions of the desert in which you do not find the lion until you narrow the region to the footprint of the lion itself, he says. But this is true only if you know that a lion lives in the desert. Well, the good news is that the majority of researchers do agree that, like a lion in the desert, dark matter should be out there... somewhere.
While the nickel mine has been around since the 1920s, the physics lab was only founded in 1992. The seemingly unusual location – deep underground – is not because of lack of space on the surface, but rather to shield sensitive detectors from energetic particles called cosmic rays. These are usually protons that come from faraway corners of the universe, and in the Earth’s atmosphere create a shower of other particles that rain down on us every second of every day. “We’re particularly worried about muons, which can interact with the matter surrounding our detectors and create neutrons, which can mimic a dark matter signal,” says Coderre. Thick slabs of rock can stop them in their tracks, though, helping scientists eliminate this interfering background noise.
While dark matter searches at SNOLAB (and elsewhere) have so far yielded nothing, the Creighton Mine did help one physicist, Art McDonald, get a Noble Prize in physics in 2015. That was for his work on neutrinos – ghostly, nearly massless particles that originate in the core of stars, in faraway cataclysmic events like supernovae, but can also be produced on Earth. Just like dark matter, they were theory once, first proposed in 1930; it took 26 years before the first neutrino was detected.
To accommodate SuperCDMS, the lab will be upgraded, to account for more electrical power, lighting and cooling. The apparatus will consist of a solid-state germanium and silicon detectors, cooled to extremely low temperatures of within a fraction of a degree above absolute zero. The capsule housing the detectors will be plunged into a water tank, with water also acting as shielding – to reduce the amount of background noise from the radioactivity of the mine. Many pieces of the experiment will be assembled and tested at surface facilities – famous physics laboratories like Fermilab, SLAC and PNNL – but the final assembly will be done underground at SNOLAB.
And what if SuperCDMS finds nothing? There are a couple of other experiments here at the mine such as DEAP-3600 that use a slightly different technology – namely, a nobel gas called argon to try and detect the WIMPs. That’s where Walding works, and those detectors will continue searching for WIMPs alongside the new experiment.
But for Coderre, null results provide important feedback about the theory – and help adjust it. “Finding nothing is obviously disappointing, but when searching for something completely new you sort of have to accept that as a likely outcome,” he says. “There is a correct answer to the dark matter problem that’s currently hidden to us and we’re doing all we can to find it.”
And that means attacking the problem on many fronts. Several experiments in former and current mines, under mountains and in space are searching for WIMPs – and it’s the steady progress over decades that counts, slowly chipping away at the possible range of dark matter models. “SuperCDMS will very likely make solid progress on this front,” says Hooper, adding that the detector will focus specifically on a new mass region for the WIMPs, which would assume that they are much lighter than what other detectors have been looking for. And just because other experiments have failed does not make the odds of SNOLAB’s future resident succeeding any smaller, he adds, narrowing the range of possible dark matter candidates.
With SuperCDMS still in the planning stage, spare a thought for the scientists who have been hunting for dark matter for many decades deep under the Italian Apennine Mountains, about an hour’s drive from Rome. Their dark matter detector is in a lab near the city of Aquila, which got ravaged by the 2009 earthquake (luckily, the lab wasn’t affected, but many physicists based in Aquila were). To get to the lab, the road takes you through a ten kilometre-long tunnel for about seven minutes, until you arrive deep below Gran Sasso, the highest peak. Turn off at a special exit, press a button, identify yourself in Italian over a speaker to the guard, and two huge metal double doors open to reveal a spacious cavern, the Gran Sasso National Laboratory – home to the most sensitive dark matter experiment in the world right now, XENON1T.
Founded in 1984, for the first two decades the researchers here were mainly studying neutrinos and cosmic rays. The dark matter hype caught on with the science crowd in mid-2000, and in 2002 the first detector – XENON10 – was switched on. It’s been getting upgrades ever since, morphing into XENON100, and now XENON1T, trying to catch the still hypothetical, but extremely rare interactions of WIMPs with ordinary matter.
The underground space is humongous. XENON1T is housed in one of three cavernous halls, each about 20 metres wide, 18 metres high and 100 metres long, all sporting their own experiments – 18 in total. They are designed for all kinds of science, from detecting solar neutrinos to study the inside of our Sun, to the biggest detector in the world that one day may catch the theoretical Majorana particle. About 950 researchers from 32 countries work in the lab.
In the dark matter hall, the most massive structure is a giant tank about 10m in diameter and 11m tall, resembling a large grain silo. It neighbours a three-storey glass building crammed with cryogenic pipes, pumps, coolers, subsystems and electronics, whose sole aim is to service the detector. The detector itself is inside the tank, sealed in a metre-thick stainless-steel cryostat. The detector and everything around is made of low-radioactivity materials, to reduce background ‘noise’ – just like at SNOLAB, radioactive materials emit electrons, gamma rays or neutrons that the detector is able to spot, messing up the picture of a potential arrival of a rare dark matter particle. To suppress any further unwanted background interactions, say from cosmic rays, the cryostat is immersed into about 700 tonnes of water in the outer tank. Some 1,400m of rock on top helps, too.
Inside the cryostat, the instrument is surrounded by two tonnes of ultra-cold liquified xenon gas (whereas the name of the experiment, slightly confusingly, is XENON1T – that’s because in the actual central core of the detector, the most sensitive part, there’s only one tonne of the liquified gas). Xenon is not just colourless and odourless, it’s also one of the rarest elements on Earth. Mainly produced in Russia, South Africa and Saudi Arabia, it’s a by-product of the steel-making process, where liquid oxygen is used to get rid of contaminants on the surface of molten iron.
When xenon atoms get excited by particles – and despite all the precautions, there are still electrons, gamma rays and an occasional atmospheric muon that make it into the detector – they emit tiny flashes of light, a property shared by several other rare gases. And liquified xenon has a great stopping power – it’s very sensitive to passing particles.
The light flashes are registered by photo devices and later analysed. So far, all the signals have been discarded. “We are searching for a signal nobody has seen before and this only works if our background is both well-understood and as low as possible,” says Coderre, who’s the former analysis coordinator of XENON1T.
XENON1T has been running since 2016, and its latest batch of data, published in May, reveals the familiar null results. However, say scientists, the limits they managed to put on the effective size of dark matter particles are the most stringent ever – one-trillionth of one-trillionth of a centimetre squared, or 4.1x10-47 square cm. Effective size, or in science talk ‘cross-section,’ is how strongly WIMPs are thought to be interacting with normal matter – the atoms of xenon. To determine it, researchers sifted through 279 days of data, during which time they were expecting to register up to ten dark matter events. None happened – which means that WIMPs must be even smaller than previously believed.
There are always two people on shift – ‘shifters’ – either in the underground lab or in the research facility near the mountains, which sports a multitude of offices, a canteen and even a bar. The shifters operate the detector remotely using tablets, constantly keeping an eye on any interesting data readings, to make sure a dark matter particle isn’t missed. In total, about 160 scientists work on the experiment from 26 scientific institutions in the US, EU and Asia.
To keep the detector extremely clean and minimise contamination by radioactive impurities that can be carried by dust on or in the detector materials, researchers need full clean room garb during assembly, with coats, masks, boots, and so on. Even a fingerprint could ruin years of work.
Very soon, they will do it all over again – over December and January, XENON1T will be switched off. Researchers will take the detector out, only to replace it over the following months with a much larger one, called XENONnT. The upgrade will cost in the tens of millions of euros, split between the collaboration members. What helps is that the outer tank will stay the same. The inner chamber, though, will be filled with six tonnes of liquid xenon instead of two. The instrument should start processing data by the end of 2019.
If that detector still finds nothing, researchers here at Gran Sasso are likely to try just one more time with an even larger instrument before moving to different technologies. “The hunt for traditional heavy WIMPs will reach a natural conclusion with the next stage of the LHC and the big XENON dark matter experiments,” says Daniel Bauer from FNAL. “It is still possible these will be found in the next few years but, if they are not, then the standard WIMP hypothesis looks like the wrong answer.”
While the WIMP detectors keep on searching, more and more researchers are developing other theories about the nature of dark matter and working on experiments to catch it. It’s multi-faceted interrogation of the dark matter puzzle. In the frame are much lighter hypothetical dark matter particles, axions, and neutral particles called sterile neutrinos.
It’s not that axions are a particularly new idea. First thought of in 1977, these particles are supposed to be much lighter than WIMPs but have been in and out of fashion for decades. But now, in light of continuous nothingness from all the various WIMPs catchers, many scientists are considering axions their second-best bet.
The theory goes that the axion would interact with photons – very weakly, but interact nonetheless. This is how the Axion Dark Matter Experiment (ADMX), housed at the Centre for Experimental Nuclear Physics and Astrophysics at the University of Washington, is trying to catch them. Unlike SNOLAB and XENON1T, ADMX is in a typical physics lab, and consists of a detector in a tank about four metres high. It consists of a very large superconducting magnet and a microwave cavity, and works similarly to a radio receiver, says Gray Rybka, a physicist at the University of Washington. It’s like if the researchers had a radio that’s searching for a radio station, but they have no idea about its frequency – so they are turning the knob, slowly, trying to hear a signal when the frequency is just right.
The magnet generates a strong magnetic field, and receivers should register the specific electromagnetic radiation produced if an axion were to pass through the field, converting its energy into a weak microwave signal that researchers can detect with quantum electronics. Because the signal the experiment is looking for is expected to be much lower in energy than those produced by cosmic rays or radioactivity, ADMX doesn't have to be shielded by rock or water. It does have to be shielded from mobile phones, Wi-Fi, and television signals. “It literally is a radio receiver. The only difference is that we convert axions into radio waves in the first step of the experiment,” says Rybka.
ADMX isn’t new – it was built more than two decades ago, in 1995. In 2010, it was moved to the University of Washington from the Lawrence Livermore National Laboratory. It’s been constantly upgraded though – and finally now, the scientists say, it’s sensitive enough to detect the weak interactions caused by axions.
The temperature of the ADMX is -273C, only 0.15C above absolute zero – colder than deep space. “There's a great deal of work in the refrigeration operation and moving liquid helium around,” says Rybka. It's liquid helium that keeps the detector so cold, and it's important because the temperature has to be cold enough for the superconducting magnet and quantum electronics to work. Also, the colder the experiment is, the lower the noise, and the clearer a detected signal will be.
Most times the lab is quiet and empty, because the data acquisition is largely automated. Researchers control and monitor it via the internet day and night, though. About once a year, operation stops, scientists warm the system up to room temperature and pull the internal detector out of the magnet. “This is the most spectacular time to see the lab: everyone is wearing their helmets and cryo-safety gloves as we carefully move the system into a clean room, where we install the new upgrades over the course of a few months,” says Rybka.
He is convinced that so far, the searches for dark matter “have been looking for the wrong thing”. With ADMX, he says, the equipment is sensitive enough to find just the right signals, and all that is left is to slowly look over the plausible masses. “We've finally got the volume knob turned up enough, and now we just need to turn the frequency knob until we hear the signal,” he says. “Our chances are good. Certainly better than ever before.”
At the moment, ADMX is looking at frequencies corresponding to the 4G-LTE mobile phone band, but researchers are also developing technologies to explore the Wi-Fi band at higher frequencies and lower frequencies like AM radio. In April, ADMX delivered its latest results – null results, that is. For Rybka, they are important though, because it shows that the detector actually has the correct sensitivity to find the axion. “If we end up with a null result after having swept over all the plausible masses, it would become important in a different way: axions are important to explain some phenomena in nuclear physics, so we expect axions to exist,” he says. “If we don't find axion dark matter, then we're faced with the problem that either we don't understand our nuclear physics, or we don't understand how the early universe works well enough. So a null result would cause our questions to multiply.”
Plenty of scientists are busy developing alternative approaches to netting dark matter, and many are even looking at alternative theories of gravity that would be able to explain the way our universe works without any need for the exotic dark stuff. However, the recent direct observation of gravitational waves coming from a collision of two neutron stars helped bury a number of such theories, so the idea that dark matter is out there somewhere is still very much alive.
If we don’t find axions or WIMPs, there’s another back-up: sterile neutrinos. These are theoretical particles supposed to interact only via gravity as opposed to ‘normal’ neutrinos that interact – very weakly – with normal matter.
The idea is that sterile neutrinos of the right mass might decay producing an X-ray spectral feature – so would be detectable with an X-ray spectrometer. Just like with other experiments, until very recently such instruments did not have sufficient sensitivity and spectral resolution to accurately measure the predicted feature. Then the Japanese space agency, JAXA, together with Nasa, sent its Hitomi satellite into orbit on February 17, 2016, carrying what they hoped would be just the right type of spectrometer. Except it didn’t stay there long – having made some initial observations of the Perseus cluster, Hitomi suddenly died three weeks later, breaking up into five pieces.
That was a big blow – it cost $273 million to build the doomed satellite. But scientists don’t get fazed easily, and both the Americans and the Japanese have already cobbled together a replacement, a satellite called XRISM (the X-Ray Imaging and Spectroscopy Mission). On July 1, it was approved in Japan, and will sport two instruments, Resolve – a high-resolution X-ray spectrometer, and Xtend – an imaging instrument. The cost to rebuild the spectrometer has been quoted at $70 million to $90 million.
The team decided to call the new spectrometer Resolve “to give it a more meaningful name – it resolves X-ray light into its component colours, and to give it a name that reflects the determination of our team to get back to where we were with Hitomi as quickly as possible,” says Richard Kelley, the principal investigator for the US part of the instrument on the Hitomi recovery mission at Nasa. The new spectrometer will detect photons by how much heat they deposit in the detector and will operate at just 0.06 degrees above absolute zero. It will observe clusters of galaxies and individual galaxies, aiming to detect that very specific spectral feature that has the right energy and strength to be the predicted signature of the decay of a sterile neutrino.
It’s based on the assumption that X-rays are absorbed in small pixels, and convert their energies to heat, which can be precisely measured with microscopic thermometers. Resolve will detect individual X-rays, one at a time, and make a histogram of the energies, which is the spectrum. “We would expect the sterile neutrinos to have a certain amount of energy, and if this energy is in the X-ray band, we can use Resolve to detect individual neutrinos and make a histogram, or spectrum, of their energies,” explains Kelley. If the neutrinos have a very well-defined energy, it will be possible to see this as a narrow feature in the spectrum.
XRISM should be launched between April 2020 and March 2021, on a JAXA H-IIA rocket from the Tanagashima Space Centre, following the path of Hitomi. And what if XRISM doesn’t find the spectral feature? As usual, detecting nothing is still something. “Rejecting a sterile neutrino as a possible dark matter candidate would reduce the candidate ideas considerably,” says Richard Mushotzky, an astronomer at University of Maryland.
It seems that with the hunt for dark matter, for the researchers involved, there are two possible outcomes. To quote the famous physicist Enrico Fermi: “If the result confirms the hypothesis, then you've made a measurement. If the result is contrary to the hypothesis, then you've made a discovery.” In the seemingly unending hunt for dark matter, the discovery of nothing means a lot.'
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Article/Extract: Katia Moskvitch for
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