By Pallab Ghosh
Image source, Fermilab/Reidar HahnImage caption, Microboone's 12m-long detector is contained inside a large cryogenic tank, filled with 150 tonnes of liquid argon at -186C
A new chapter in physics has opened, according to scientists who have been searching for a vital building block of the Universe.
A major experiment has been used to search for the elusive sub-atomic particle: a key component of the matter that makes up our everyday lives.
The search failed to find the particle, known as the sterile neutrino.
This will now direct physicists towards even more interesting theories to help explain how the Universe came to be.
Prof Mark Thomson, the executive chair of the Science and Technology Facilities Council (STFC), which funds the UK's contribution to the Microboone experiment, described the result as ''pretty exciting''.
That is because a sizeable proportion of physicists have been developing their theories on the basis that the existence of the sterile neutrino was a possibility.
''This has been out there for a long time now and generated a lot of interest,'' he told BBC News.
''The result is really interesting because it has an influence on emerging theories in particle physics and cosmology.''
The Microboone experiment is based at the US Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois - just outside Chicago. But physicists from many countries are involved with the project.
Image source, Fermilab/Reidar HahnImage caption, Microboone's electronics racks are located just above the detector, on a platform that blocks significant amounts of cosmic radiation that could affect the accuracy of the results
Neutrinos are ghostly sub-atomic particles that permeate the Universe, but barely interact with the everyday world around us. Each second, billions of them pass right through the Earth - and everyone living on it.
Neutrinos come in three known types, or flavours - the electron, muon and tau. In 1998, Japanese researchers discovered that neutrinos changed from one flavour to another as they travelled.
This flavour-flipping cannot fully be explained by the current "big theory" of sub-atomic physics - called the Standard Model. Some physicists believe that finding out why the neutrino has such a tiny mass - which is what allows them to change flavour - will give them a deeper understanding of how the Universe works and specifically how it came into being.
Current theories suggest that, shortly after the Big Bang, there were equal amounts of matter and its shadowy mirror-image anti-matter. However, when matter collides with anti-matter, they violently annihilate each other, releasing energy. If there were equal amounts in the early Universe, they should have cancelled each other out.
Instead, most of the Universe today is made of matter, with much smaller amounts of anti-matter.
Some scientists believe that, contained within the neutrino's flavour-changing, is the cosmic sleight-of-hand that enabled some matter to survive after the Big Bang and create the planets, stars and galaxies that make up the Universe.
In the 1990s, an experiment called the Liquid Scintillator Neutrino Detector experiment at the US Department for Energy's Los Alamos National Laboratory in New Mexico saw the production of more electron neutrinos than could be explained by the three-neutrino flavour-flipping theory. That result was confirmed by a separate experiment tin 2002.
Physicists proposed the existence of a fourth flavour called the sterile neutrino. They believed this form of the particle could explain the over-production of electron neutrinos and, crucially, give an insight into why the particles change flavour.
They were named sterile neutrinos because they are predicted not to interact with matter at all, whereas other neutrinos can - though very rarely. Detecting a sterile neutrino would have been a bigger discovery in sub-atomic physics than the Higgs boson because, unlike other forms of neutrino and the Higgs particle, it is not part of the current Standard Model of physics.
A team involving nearly 200 scientists from five countries developed and built the Micro Booster Neutrino Experiment, or Microboone, in order to find it. Microboone consists of 150 tonnes of hardware in a space that's the size of a lorry.
Its detectors are highly sensitive: its observations of the sub-atomic world have been likened to looking in ultra-high definition.
The team has now announced that four separate analyses of data gathered by the experiment show "no hint" of the sterile neutrino.
A new chapter
But this result is not so much the end of the story, but the beginning of a new chapter.
Dr Sam Zeller from Fermilab says that the non-detection does not have to contradict previous findings.
"The earlier data doesn't lie," she said.
"There's something really interesting happening that we still need to explain. Data is steering us away from the likely explanations and pointing toward something more complex and interesting, which is really exciting."
Prof Justin Evans, from the University of Manchester, believes that the puzzle posed by the latest findings marks a turning point in neutrino research.
"Every time we look at neutrinos, we seem to find something new or unexpected," he said.
"Microboone's results are taking us in a new direction, and our neutrino programme is going to get to the bottom of some of these mysteries."
Related Internet Links
The BBC is not responsible for the content of external sites.