EAGER to track down one of the most exotic imaginings of the cosmological mind, six teams of particle physicists have set out to detect the existence of a hypothetical form of matter called weakly interacting massive particles, or WIMPs. Next month, in California, researchers will start up a cryogenic detector containing a crystal of germanium cooled to almost absolute zero, in the hope of recording the tiny rise in temperature that would occur if a WIMP struck. Early next year, a team in Germany will be doing the same, using a single crystal of pure sapphire. Two more teams in Europe plan to use photodetectors to pick up the tiny flashes of light that they expect WIMPs to generate in large crystals of sodium iodide. A fifth group, based in California, hopes to develop a gas tube that will register the scintillations of a series of strikes from individual WIMPs and thereby reveal the direction of travel. And another team in California has already started studying samples of ancient mica, on the lookout for evidence of WIMP impacts throughout the past 500 million years.
These unprecedented efforts are driven by the pressing desire to find the missing dark matter that, cosmologists say, makes up 90 per cent of the Universe (鈥淪ome of our Universe is missing鈥�, New 杏吧原创, 8 July). They speculate that this invisible material might be clumped together as MACHOs (massive astrophysical compact halo objects), or consist of even more exotic particles such as neutrinos, WIMPs and axions.
Neutrinos and MACHOs have been convincingly detected in our Galaxy. But this has not resolved the issue. Nobody has yet confirmed whether neutrinos have any mass at all. Even if they do, they cannot account for more than about one-fifth of the missing matter. MACHOs, meanwhile, can account for no more than 5 to 20 per cent of the other four-fifths of dark matter that some cosmologists claim must exist, which leaves WIMPs and perhaps axions to make up a huge shortfall(see 鈥淲hy we need WIMPS鈥� and 鈥淭he axion alternative鈥�). The trouble is that no form of WIMP, which cosmologists hypothesise to be a relatively slow moving particle of large mass that rarely interacts with ordinary matter, has yet been detected. Nevertheless, many cosmologists are convinced that WIMPs exist in abundance, and particle physicists are keen to discover whether the cosmologists are right because of the impact such a discovery could have on the fundamental issues of particle physics. 鈥淭he gravity of this problem is such that some of us feel compelled, despite the difficulties, to do experiments to look for this missing mass,鈥� says Susan Cooper, professor of experimental particle physics at the University of Oxford. Cooper is also project leader of the European team at the Max Planck Institute for Physics in Munich that has developed one of the two cryogenic detectors intended to detect the existence of WIMPs.
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No one underestimates the difficulty of detecting a WIMP. The particles are expected to interact with the experimental targets no more than once a day -possibly no more than once a year 鈥� and their effects could easily be swamped by background radiation. At the heart of the two cryogenic experiments are crystals of germanium or aluminium oxide 鈥� in the form of sapphire 鈥� that have been cooled to within 15 to 20 thousandths of a degree of absolute zero. A WIMP striking the crystal lattice will strip electrons from its atoms, and also spread energy into nearby regions as vibrations, called phonons. The phonons will raise the temperature by a tiny amount, either within the crystal itself or at the crystal鈥檚 edges. The low temperatures increase the ratio by which the temperature rises, making the detector more sensitive. Keeping the crystal cold also reduces vibrations in the lattice that could generate signals that would mimic those of WIMPs.
The first cryogenic experiment, the Cold Dark Matter Search (CDMS), is due to begin next month at Stanford University, California. It brings together physicists from Stanford, the University of California at Berkeley and at Santa Barbara, the Lawrence Berkeley Laboratory and the Baksan Laboratory in the Caucasus Mountains in Russia. The second experiment, planned to start early next year, is dubbed the Cryogenic Rare Event Search Using Superconducting Thermometers (CRESST). It involves researchers from the University of Oxford, the Technical University of Munich, the Max Planck Institute for Physics in Munich, and the Gran Sasso National Laboratories in Italy.
Ingenious strip
Both teams of experimenters have had to dream up some ingenious techniques for measuring the scanty record of a WIMP strike. In the case of CRESST鈥檚 sapphire crystal, the tiny rise in temperature will be just sufficient to alter the state of a strip of tungsten on the crystal鈥檚 surface from superconducting to normally conducting, leading to a sizable jump in its electrical resistance.
The CDMS researchers, meanwhile, have chosen to record all the changes that occur in their germanium crystal when a WIMP strikes. They are designing instruments that will register the tiny bursts of heat energy generated by the phonons that will follow an impact, as well as the charge generated when the impacts strip electrons away form their atoms. By measuring both the rise in temperature and the degree of ionisation, the researchers expect to be able to distinguish WIMP impacts from other events. Particles produced by the radioactive decay of nearby nuclei, for example, will yield a different ratio of the number of charges released by ionisation to impact energy.
This approach is important for the CDMS team because the experiment occupies a site underground on the Stanford campus, where the effects of background radiation will be significant. The CRESST researchers plan to minimise this interference by setting up the experiment in a cavern 2 kilometres below the Apennine mountains near the laboratory at Gran Sasso.
Background radiation is a big problem for all the WIMP searches. As cosmic rays hit the atmosphere, or any object on Earth, they can start a chain of collisions that produces high-energy neutrons and other particles. When these particles strike the WIMP detector they will produce signals that could distort records of WIMP impacts, which is why the CRESST researchers have gone underground to escape them. But being below ground does not help when it comes to the gamma rays, electrons and fast-moving neutrons generated by radioactive matter on Earth, so detectors must be insulated.
Scintillating search
As the two cryogenic detectors prepare to begin work, two other groups of researchers are stepping up their search for WIMPs, using large crystals of sodium iodide as their detectors. When a WIMP strikes one of the sodium nuclei in the crystal, its impact will produce a scintillation 鈥� a microscopic flash of light that occurs when an electron is stripped off one or more of the sodium atoms. Special light guides at each end of the crystal direct these flashes into photomultiplier tubes that amplify them.
One of the scintillation experiments unites scientists at the Rutherford Appleton Laboratory in Oxfordshire, Imperial College in London, and the University of Sheffield. They have put their experiment one kilometre below ground in the Boulby potash mine on the moors of north Yorkshire. The second group comprises physicists from Beijing, Paris and Rome, whose equipment nestles close to the cryogenic experiment in the cavern below Gran Sasso in Italy.
In an effort to reduce local background effects as far as possible, the team operating the Boulby experiment house their detector in a 200-tonne tank of ultrapure water 6 metres across and 6 metres deep, and encase it in cylinders of ultrapure copper that have been kept in the mine for many years. In this way, any radioactivity generated in the copper by cosmic-ray bombardment near the surface has had time to decay. The sodium iodide crystals must likewise avoid impurities that would produce radiation. 鈥淲e preselect the raw powder to make our crystals,鈥� says Peter Smith, project leader of the experimental group at the Rutherford Appleton Laboratory. Every other component in the detector is also pretested, he says, including the pure silica used for the guides that channel light to the photodetectors.
Like all the WIMP-detecting experiments to date, the Boulby and Gran Sasso scintillation detectors have yet to register a single WIMP. This simply encourages the experimenters to work harder to increase the sensitivity of their instruments. The immediate aim at Boulby is to boost the sensitivity by a factor of 10. 鈥淚 estimate that it will take two years,鈥� Smith says, 鈥渁nd we鈥檙e getting into the interesting region now.鈥� With enough funding, Smith expects to be detecting WIMPs by 2000 or earlier.
The fifth WIMP detector, still in the prototype phase, aims to determine the direction from which the WIMPs arrive. This effort, led by George Masek, of the University of California, San Diego, uses a low-pressure gas 鈥� either methane or a methane-argon mixture 鈥� inside a metre-long cylinder. With collaborators from the Rutherford Appleton Laboratory and Temple University in Philadelphia, Masek hopes to observe the tracks left by WIMPs as they pass through the gas, interacting with atoms one after another. For the moment, the group is short of funds to develop its experiment and, in particular, to go underground.
Diminishing funds are a worldwide problem in WIMP detection. Earlier this year, for example, government funds for the Boulby experiment were cut by 60 per cent. Nevertheless, Smith remains optimistic. 鈥淲e are continuing to make significant improvements,鈥� he says. 鈥淲e still hope to have some big gains in sensitivity from these this year and next.鈥�
With the five larger groups of WIMP hunters likely to remain underfunded while budgets are tight, they might take heart from the work being done to look for the tracks of long-gone WIMPs. At Berkeley, physicists Daniel Snowden-Ifft, Eric Freeman and Buford Price realised that WIMPs should leave tracks in materials such as mica, which would be revealed by etching with an acid. Because these tracks would be only a few micrometres long and not even that wide, they are using an atomic force microscope (AFM) that has a resolution of one micrometre or better.
In their initial efforts over the past year, the Berkeley team used an AFM to scan several dozen areas, each about 40 micrometres square. Although this amounts to using a target with a mass of less than one-millionth of a gram, this particular target has been exposed to possible WIMP impacts for 500 million years, not just the few days or years of conventional experiments. Nevertheless, the Berkeley physicists have still to find anything that looks like a WIMP track.
Cooper, for one, is not downhearted. Nor does she feel too many resources are chasing too elusive a prize. 鈥淎 positive signal, if seen, is hard to prove, and would need to be confirmed by other detectors,鈥� she says. 鈥淪o it is important to have different approaches to looking for WIMPs 鈥� and for any other particle hypothesised to explain the dark matter.鈥� And she is convinced that an answer will be found, though not straight away. 鈥淚 expect another 10 years will go by before we have dark matter.鈥�
Why we need WIMPs
SUPERSYMMETRY, an extension of the standard model of particle physics, predicts the WIMPs should exist. Supersymmetry views the two distinct classes of subatomic particles 鈥� fermions and bosons 鈥� as two faces of the same coin. Every boson has a fermionic partner: for instance, photons and gluons are twinned with photinos and gluinos. And every fermion has a bosonic partner: electrons and quarks are twinned with selectrons and squarks.
The theory鈥檚 most striking feature, however, is that it predicts a whole new family of weakly interacting massive particles or WIMPs. If supersymmetry is right, then these supersymmetric particles would have been created in the big bang along with ordinary particles.
In the simplest versions of the theory, the lightest supersymmetric particle 鈥� the neutralino 鈥� is stable, which means that it could still be around. This makes the neutralinos 鈥� particles with the multiple personalities of a photino. Higgsino and Zino 鈥� candidates for being the WIMPs that may make up about four-fifths of the mass of the dark halo that is thought to surround our galaxy.
The axion alternative
APART from WIMPs, another likely candidate for the Universe鈥檚 missing dark matter is the axion. However, it would take 1012 axions to equal the mass of just one proton, so the number of axions would have to be tremendous to explain dark matter. And like WIMPs, axions have never actually been detected.
Axions interact so weakly with ordinary matter that picking them up is tremendously difficult, says Karl van Bibber of the Lawrence Livermore National Laboratory in California. With physicists from MIT led by Leslie Rosenberg, his search for axions began last month. A similar search is due to begin in December at Kyoto University.
Theoretical models predict that when axions interact with a magnetic field they produce a photon 鈥� that is, electromagnetic radiation of a particular frequency. The rate of this interaction, and hence the intensity of the radiation, should increase in proportion to the square of the magnetic field strength times the field volume.
The Lawrence Livermore experiment creates an 8-tesla magnetic field 鈥� about one hundred thousand times as strong as the Earth鈥檚 鈥� within a cylindrical cavity 1 metre long and half a metre in diameter. At one end of this cavity lies a cable leading to a sensitive detector called a high-electron-mobility transistor (HEMT). If an axion produces a photon in the magnetic field, the photon will slightly change the stream of electrons from the HEMT detector, much as photons from a radio broadcast station affect the motion of electrons in an ordinary antenna.
To reduce extraneous noise, and thus to increase the chances of detecting a weak signal, the experimenters cool the cavity to 1.6 kelvin. 鈥淚ncreasing the signal-to-noise ratio is the name of the game in any detector system,鈥� van Bibber notes. 鈥淲e are already planning for the next generation of detectors, the superconducting quantum interferometer devices (SQUIDS) that will do an even better job.鈥�
The size of the cavity in the Lawrence Livermore experiment determines the mass of the axions that the apparatus can detect. An axion of a particular mass will produce a photon of a particular energy, because when the axion interacts with the magnetic field, it converts all its energy 鈥� which is almost entirely its rest-mass energy, given by Einstein鈥檚 formula E = mc2 鈥� into the energy of the photon it produces. An axion of mass m will yield a photon whose energy is proportional to m.
This photon is also a vibration of an electromagnetic field, and a cavity of a particular size is most effective at detecting photons that have its resonant frequency. Experimenters 鈥渢une鈥� their cavity to different frequencies by inserting conducting rods made of aluminium, or dielectric rods made of sapphire crystal, which change the cavity鈥檚 effective size and shape. 鈥淚t鈥檚 like tuning your radio dial to different stations,鈥� van Bibber says, 鈥渆xcept that the dial is enormous, and you don鈥檛 know if any radio stations exist at all.鈥�
Three years of experimentation will cover a range of masses from the lower limit of the possible axion mass up to ten times that value. This would still leave masses up to a thousand times the lower mass limit for exploration in the future.
Difficult though the search for axions may be, the experimenters will have a simple way of confirming that a promising signal comes from axions rather than some other type of particle, because these other types do not interact with magnetic fields in the same way. 鈥淲e鈥檒l just turn off the magnet,鈥� van Bibber says. 鈥淚f it鈥檚 axions, the signal should disappear.鈥�