Sterile neutrinos: The cosmic controllers 15 June 2006 New Scientist Search for the invisible WHEN all else fails, invent a new particle - after all isn't that what physicists do best? If there's mysterious stuff lurking somewhere in outer space, you can be sure someone will dream up a new particle to fit the bill. So it is no surprise to see yet another made-up particle on the block. This time, it is the "sterile neutrino", a ghostly particle so slippery that it might never be detected on Earth. Nonetheless, this elusive hypothetical particle is getting a surprisingly good reception. Most physicists seem rather keen on sterile neutrinos, which might just have played a cameo role in the universe's history billions of years ago. Equally, they could be centre-stage today, roaming space and healing all cosmology's woes. For instance, sterile neutrinos might account for all the invisible dark matter in space. They could explain why stars lit up the young cosmos so quickly, and why giant black holes emerged hot on their heels. "It's as if the pieces of the puzzle are suddenly coming together, and for the first time, things fit," says Peter Biermann from the Max Planck Institute for Radio Astronomy in Bonn, Germany. It's an impressive track record for a particle that seems to be little more than a figment of the imagination. Historically, many particles that physicists dreamed up to solve theoretical riddles were detected many years later. These include the ordinary neutrinos that flood out from nuclear reactions inside stars. Although neutrinos were first predicted in 1931, they were not detected for another 25 years. And now physicists say they have good reasons to think the ordinary neutrinos have invisible cousins called sterile neutrinos. These reasons stem from the surprise discovery in 1998 that the normal neutrinos have mass. Before then, scientists were not sure if the three types or "flavours" of neutrinos, dubbed electron, muon and tau, had any mass at all. Understanding neutrinos is especially tough because they are notoriously difficult to detect. They don't feel the strong or the electromagnetic force, so normally fly through anything, including people, stars and planets. Only very occasionally do neutrinos interact with an atom, via the weak force. Billions of neutrinos flood through the Earth all the time, but researchers have to build giant detectors weighing thousands of tonnes just to capture a tiny fraction of them. Since 1998 several experiments in the US and Japan have confirmed that the three neutrinos can "mix" or flip from one type into another in the course of their travels - something which can only happen if the neutrinos have mass. The experiments have allowed researchers to pin down the weight of neutrinos to no more than about 0.3 electronvolts, less than a millionth of the mass of an electron. Standard theories of particle physics sit comfortably with a massless neutrino. Having accepted that they do have mass, physicists now have a new puzzle to solve. Why are the masses of neutrinos so much tinier than those of other particles like electrons or quarks? One popular idea is that it is because they have undetected partners - enter the "sterile neutrinos". These particles would be sterile in the sense that they don't interact with normal matter at all, except through gravity. The idea is that neutrinos have a normal mass on average, except that it is shared between heavier sterile ones we can't detect and the familiar ordinary light neutrinos. "Most theorists believe that, given the experimental fact that neutrinos have masses, it is likely that sterile neutrinos do exist," says Boris Kayser, a neutrino theorist at Fermilab in Illinois. Another popular feature of sterile neutrinos is that they can help explain why matter dominates antimatter in our universe (New Scientist, 4 September 2004, p 37). Still, there's one thing that leaves scientists at odds. The role sterile neutrinos play in our universe depends crucially on their mass, and theory puts no limits on that at all. It could be anything. Nor does theory suggest how many types of sterile neutrinos can exist. Some scientists plump for three - just because particles seem to crop up in threes - but there could be any number. According to Janet Conrad, a neutrino experimenter at Fermilab, that has inspired a dismissive quip in neutrino circles: "Sterile neutrinos are like cockroaches - once you get one in your theory, there's no stopping them." For now, however, scientists are free to play with any number of sterile neutrinos with any mass they fancy, although it seems to have settled out into a three-horse race. One group, the particle theory buffs, tend to favour superheavy sterile neutrinos as massive as bacteria (see "Heavyweight bout"). Some other scientists extol the virtues of a much lighter sterile neutrino, which may have left its mark in an experiment in New Mexico (see "Lure of the lightweights"). Many astronomers and cosmologists have their eye on something in between - a middleweight sterile neutrino with a mass about a hundredth that of the electron. It is this middleweight sterile neutrino that can neatly resolve a host of cosmology's problems. The first of these problems is the nature of the curious dark matter in the universe. Around 90 per cent of the matter in the universe is in some strange, invisible form. It clumps into big dark balls centred on galaxies and we know it is there because its gravity tugs on stars and galaxies. In 1993 Scott Dodelson from Fermilab and his colleague Lawrence Widrow pointed out that the dark matter could be made up of middleweight sterile neutrinos. If copious numbers of sterile neutrinos were churned out in the hot big bang 14 billion years ago, theory predicts that they would still be hanging around today. Unlike normal neutrinos from the sun that zip around close to the speed of light, these middleweight sterile neutrinos would be relatively sluggish. Hence, they could easily clump together under their mutual gravity to form the dark balls of matter that glue galaxies together. This fits with observations of dwarf galaxies in which the dark matter seems to be made of "warm" particles milling around at speeds of several kilometres per second (New Scientist, 11 February 2006, p 7). Dodelson simply chose the mass of the sterile neutrinos he believed would best solve the dark matter problem. Now Alexander Kusenko of the University of California, Los Angeles has shown that these middleweight tailor-made particles have an uncanny knack of solving other mysteries as well. "We're surprised to find that the same neutrinos explain some other astrophysical puzzles," he says. "There are several coincidences that make me very excited about this idea." Earlier this year, for instance, Kusenko and Biermann reported that the sterile middleweights can explain why the first stars in the universe formed so quickly. The latest observations by NASA's Wilkinson Microwave Anisotropy Probe (WMAP) suggest stars must have been burning just 400 million years after the big bang. However, star formation theories have struggled to explain how gas could have clumped together and shrunk into stars so quickly. Middleweight sterile neutrinos come to the rescue. While most of the ones churned out just after the big bang would still be around today, their lifetime, like that of a radioactive nucleus, isn't fixed. A tiny fraction would have decayed into normal neutrinos during the first few million years after the big bang. As they did so, they would have released X-rays that then ionised hydrogen atoms. That would have encouraged them to bind to other hydrogen atoms to form molecular hydrogen. All that molecular hydrogen is just the ticket for star formation, because the molecules efficiently radiate away heat, allowing hot clouds of gas and dust to cool and lose pressure. Only then can they stop fighting and collapse under gravity into dense stars. Biermann and Kusenko calculate that the decays of middleweight sterile neutrinos should indeed have allowed stars to form by 400 million years after the big bang, exactly as WMAP measures. Seeding black holes Biermann and his colleague Faustin Munyaneza have shown that middleweight sterile neutrinos can also help build black holes quickly by clumping happily together into little nuggets of dark matter at the centres of galaxies, ready to "seed" the supermassive black holes that inhabit the cores of galaxies. Observations reveal that black holes millions of times the mass of the sun formed just 800 million years after the big bang. Sterile neutrinos help this happen naturally because they clump together easily into superdense matter. Because they don't emit any light, they wouldn't generate bright radiation that would resist the collapse. "Suddenly, it becomes obvious that the sterile neutrino is a good candidate to solve all kind of problems," says Biermann. Kusenko has noticed that middleweight sterile neutrinos have another talent. In 1996, he and his colleague Gino Segrč showed that they could explain why neutron stars move so fast. A neutron star forms when a massive star explodes at the end of its life, leaving behind a superdense ball of neutrons about the size of a city. These weird stars have solid crusts of iron nuclei, and some have been spotted barrelling through our galaxy at speeds of more than 1000 kilometres per second. Why so fast? As the neutron star forms, some kind of "kick" must accelerate it up to that enormous speed. Kusenko and Segrč argue that middleweight sterile neutrinos could do this very efficiently. A newborn neutron star is incredibly hot, with a temperature of about 1012 kelvin. Energetic neutrons and protons undergo several chain reactions that produce neutrinos. For instance, a single neutron can decay to make a proton, an electron and an electron neutrino. Neutron stars have superstrong magnetic fields too. And the neutrons, protons and electrons involved in neutrino generation all have their spins aligned with the magnetic field. The upshot is that the newly created neutrinos preferentially speed out towards the star's south pole. These normal neutrinos don't get far because they constantly bump into neutrons, scatter and fly off in random directions. However, if a small fraction suddenly changes into sterile neutrinos, which don't interact with matter at all, they maintain their initial direction and whoosh out from the neutron star's south pole, giving it an almighty kick. "They transfer momentum to the neutron star just like a rocket," says Kusenko. His calculations suggest sterile neutrinos are quite capable of providing that giant kick. As astronomical trouble-shooters, middleweight sterile neutrinos have a lot going for them, according to Conrad. "I think it's wonderful science, it's really a very nice marriage of the things that go on in particle physics, nuclear physics and astrophysics," she says. "I think there's room for this model to make sense." ?The pieces of the puzzle are coming together and, for the first time, things fit? What's lacking is experimental proof, which will be fiendishly difficult to find given that sterile neutrinos are totally indifferent to the real world. No one can imagine any way to detect them directly, though there are possibilities for seeing them indirectly (see Graphic). Many theorists find that a turn-off. "It is bad," says Joe Lykken, a Fermilab theorist, "because you will never truly believe a particle-physics explanation of astrophysical puzzles until you observe the relevant particle in the laboratory." Conrad argues that someone might think up a way to detect sterile neutrinos directly in future. "People are very smart and if they think they're onto something, they sniff around until they finally figure out how to get there," she says. Kusenko adds that the night sky might hold the answer. If middleweight sterile neutrinos do make up dark matter, their decays today will produce X-rays at a telltale wavelength in a pattern that mirrors the distribution of dark matter in the universe. That signal could be detected by NASA's Constellation-X project, a fleet of X-ray telescopes that could launch a decade from now if NASA budgets allow. "Constellation-X would be fantastic, it would really do a very good job searching for these sterile neutrinos," says Kusenko. He is also discussing with other astronomers the prospects of piggybacking a tailor-made, cheap X-ray detector onto an earlier space mission. One way or the other, we may be entering new territory for physics. With some ingenious thinking, it might be possible to prove that the imperceptible cousins of ordinary neutrinos shaped the cosmos profoundly. Then again, sterile neutrinos are not the first made-up particles dreamed up to plaster over cosmological cracks, and they probably won't be the last. From issue 2556 of New Scientist magazine, 15 June 2006, page 46 Heavyweight bout In the world of sterile neutrinos, size matters. Boris Kayser, a neutrino theorist at Fermilab in Illinois, says that most of the theoretical clan favours the idea that the masses of sterile neutrinos are enormous - about 1020 electronvolts, or about 1011 times the proton mass. That's as heavy as an E. coli bacterium. Theorists favour such heavy sterile neutrinos because they think they neatly explain why the three normal neutrinos are so light. Neutrinos, like other matter particles, may acquire their mass by interacting with as-yet-undetected particles called Higgs bosons. That process would give the neutrinos a mass that's fairly normal compared with other subatomic particles. However, that mass would be shared out between normal neutrinos and their hefty sterile cousins that only live for a fleeting instant. Making the sterile partners superheavy most easily explains why the normal neutrinos are so very light. Lure of the lightweights Wearing a gold wedding ring? Pricey, was it? Some scientists suspect it would have cost a good deal more if it weren't for sterile neutrinos. Very light sterile neutrinos could help to account for large amounts of heavy elements, like gold and uranium, in the universe. These elements mainly come from supernova explosions at the end of huge stars' lives. The crushing pressures in supernovae are thought to force neutrons to gang together into heavy nuclei. However, the explosions also create zillions of electron neutrinos. Electron neutrinos tend to kill off neutrons, by interacting with them to make protons and electrons. So much so that there shouldn't be enough neutrons left to build all the heavy elements we see around us. George Fuller from the University of California in San Diego has suggested that a light sterile neutrino with a mass of only 1 electronvolt - just a little heavier than its normal cousins - could come to the rescue. If a small fraction of electron neutrinos inside supernovae flip into these light sterile neutrinos and escape for good, that could explain why more neutrons survive to form copious heavy nuclei like gold. What's more, there's a whiff of experimental evidence for this light sterile neutrino. The results come from an experiment called the Liquid Scintillator Neutrino Detector (LSND) at Los Alamos National Laboratory in New Mexico. During the 1990s, this experiment measured muon neutrinos flipping into electron neutrinos, but it measured more conversions than expected, if results from other experiments are also correct. One possible reason was that at least one light sterile neutrino with a mass of about 1 electronvolt was joining in with the three familiar neutrinos. The LSND result is controversial and has never been confirmed. Things might change soon, however. The MiniBoone experiment at Fermilab is gathering data that will either confirm or reject the LSND results once and for all. Janet Conrad of Fermilab, a spokeswoman for MiniBoone, says her team will announce the results during the next few months.