Tunneling for Answers The Search for Dark Matter
Teams of researchers working in underground laboratories across the world believe the decades-long hunt for dark matter might soon bear fruit. But their excitement about detecting these hypothetical ghost particles is only matched by growing doubt about whether they even exist at all.
A mysterious power has the universe in its grip and, like a giant fist, it bends entire groups of galaxies as if they had the soft consistency of pizza dough. Its power is greater than the gravitational force of all the tens of thousands of suns combined. The only problem is that there is no direct proof of its existence.
Physicists call this invisible substance dark matter, and it's estimated to make up more than 80 percent of all matter. But it only leaves behind traces, hints and signs of its existence.
Particle physicists hope that this could soon change. "The year 2012 could very well be the year of dark matter," says Rafael Lang, as he drives his rental car along a highway winding through the ravines of Italy's Abruzzo region. Lang, a physicist from Purdue University, in the US state of Indiana, is driving through the Italian mountains, where he hopes to achieve some clarity. "Soon we'll at least know whether we're on the right path," he says, "or whether we need to look in a completely different place."
Searching for Dark Matter in Deep Places
Thirty years ago, a tunnel was drilled into the 2,900-meter (9,500-foot) Gran Sasso Massif. It is 10 kilometers (6.2 miles) long and topped by 1,400 meters of rock at its midpoint. Here, scientists hope to uncover the secret of dark matter.
An exit sign appears about five kilometers into the tunnel, right in the middle of the mountain. Lang turns his blinker on and takes the exit. A loudspeaker squawks, but Lang knows the password. "I'm a particle physicist," he says in Italian. A gate opens with a groan, and security guards check his identification. "It's a little like being in a James Bond film," Lang says as he puts on a hardhat.
Lang's steps echo through the tunnel as he walks past experiment equipment as big as blast furnaces, in which ultra-sensitive detectors search for the traces of invisible particles. Almost 1,000 physicists from 32 nations are conducting experiments in the factory-like research facility at Italy's Gran Sasso National Laboratory. Technicians in blue overalls cycle through the hallways, crossing paths with trucks and forklifts.
There will be plenty of excitement as about 1,000 scientists come together in Stockholm this week for the triennial Marcel Grossmann Meeting on Recent Developments in Theoretical and Experimental General Relativity, Astrophysics and Relativistic Field Theories, which runs until Saturday. Dark matter will be one of the big topics, and there is a lot at stake: prestige, research funding and, to some extent, the creditability of particle physics.
Indeed, scientists haven't even been able to agree on what exactly dark matter is supposed to be, although most define it as a still unknown type of massive particle. The only thing that seems to be certain is that the galaxies couldn't exist without the ominous dark matter. Visible matter alone -- the myriads of stars and their planets -- would not be sufficient to hold the rotating islands of stars together with their reciprocal gravitational forces. It is only the mass of dark matter that prevents the stars in the Milky Way, for example, from being hurled away by centrifugal force like unsecured passengers on a carousel gone wild.
Scientists suspect that the answer to the question of what holds the galaxies together lies in so-called WIMPs, or "weakly interacting massive particles," which are massive hypothetical particles that could be heavier than the heaviest conventional atoms, though they hardly interact at all.
WIMPS have remained invisible to cameras and detectors, but they are presumably everywhere around us. Billions of them rush through people, buildings and entire planets every second as if they were mere phantoms.
For some time now, competing teams of scientists have been battling each another in the so-called "WIMP wars." The meeting in Stockholm also promises to be contentious, and the presence of Rita Bernabei, a professor from Rome, is a guarantee that it will be. The experiment she is conducting in a repurposed office container in the Gran Sasso tunnel is called DAMA. For years, Bernabei has claimed to be the first person to capture and measure dark matter particles, but many of her fellow scientists have their doubts.
The design of her experiment is impressive for its elegance. If dark matter fills the entire universe, Bernabei theorizes, the Earth, as it orbits the sun, would have to move sometimes with and sometimes against the current of massive, invisible particles. The highest speed of the Earth relative to this particle flow, she calculates, occurs around June 2.
In fact, since early June, Bernabei's detector has registered an increase in particle collisions, just as it has for a decade. The DAMA team can even specify the weight of the WIMPs with some accuracy, calculating that they are about as heavy as helium atoms. But the fact that this value is significantly lower than what most theorists expect can mean one of two things for Bernabei: a measuring error or a Nobel Prize.
Her colleagues are suspicious because DAMA captures far more potential dark matter particles than theorists have predicted. They suspect that DAMA is essentially fishing too much extraneous data out of a sea of particles. Critics want to see reference samples taken from the Southern Hemisphere. They say that this is the only way to rule out the possibility that Bernabei's detector is merely measuring the side-effects of summer warming over Italy instead of traces of the massive, ghost-like particles.
The 'Xenon' Experiment
"We are basically searching for tiny needles in a giant haystack, and we don't even know what they look like," says Lang, one of the most ardent rivals of DAMA.
The project Lang is involved in is called Xenon. As a relatively young experiment, it is housed in a remote tunnel far from the large chambers of the Gran Sasso National Laboratory. Moisture drips from the mossy tunnel walls, and a small drainage ditch gurgles behind the laboratory container. The lights in the container are on day and night since even something as minor as flicking a light switch could produce a small electric spark that could disturb the sensitive measurements.
The experiment uses one of the world's most sensitive cameras, though it only has 178 pixels. These light sensors are focused on a small barrel holding 100 kilograms (220 pounds) of the noble gas xenon in liquefied form -- hence the name of the experiment.
A cooling pump and an air-conditioner hum inside the laboratory container. To keep it from returning to gaseous form, the xenon has to be cooled to minus 100 degrees Celsius (minus 148 degrees Fahrenheit). The assumption is that a small flash of light will be produced if a WIMP collides with a xenon atom, thereby making the presence of dark matter indirectly perceptible. It's as if a referee were to observe the reactions of fans in the stands to determine whether a goal had been scored.
Filtering Out Radiation
The problem is that a constant electrical storm is taking place within the xenon container. "We have an event about every second," Lang says. To see what's happening in the black box, he has connected an oscilloscope to the data stream. With each flash of light, the green line on his screen makes a blip, like a heart monitor in a hospital.
"The overwhelming majority of events cannot be caused by collisions with dark matter, but by interfering radiation sources that we have to filter out," Lang says. Of the millions of flashes they have registered, the Xenon researchers have only accepted three -- not as evidence of dark matter, but merely as possible candidates that merit careful future analysis.
The reason is simple: Everything produces radiation, including rock, the container and the air. Thus, finding traces of dark matter requires being a master of radiation-blocking. As with a Matryoshka doll, the Russian set of dolls that fit within each other, there are multiple protective layers surrounding the dark camera. One layer of plastic, for example, protects against radiation from space. But, since the plastic also emits radiation, there is also a copper shield in front of it. The setup is supported by a steel frame, but since even the steel emits radiation, it also has to be shielded with a layer of lead.
Unfortunately, the lead also emits radiation, especially if it has been recently mined. But, in another experiment (called "Cuore"), Italian scientists have found a solution to even this problem: They use old lead from a sunken Roman trading ship. After lying on the seafloor for almost 2,000 years, the lead is almost radiation-free.
But the most important protection against particle radiation from space is the 1,400 meters of dark dolomite rock separating the laboratory from the sky above. The rock shields the experiment as effectively as a 100-meter-thick slab of lead or 3,700 meters of water. Down in the tunnels, cosmic radiation is a million times weaker than it is on the Earth's surface -- though even that still causes disturbances.
In addition, the radioactive noble gas radon is evaporating underground. To protect against radon, filtered air is pumped into the containers at high pressure, as is done in the so-called "clean rooms" of microchip factories. Under ideal circumstance, only dark matter -- which penetrates everything -- should reach the reaction container.
- Part 1: The Search for Dark Matter
- Part 2: Pursuing an Invisible Enigma
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