Read more: 鈥�Instant Expert: Neutron science鈥�
Our view of what makes up the universe at its most fundamental level is described by the standard model of particle physics. But this theory is incomplete and does not, for example, include an explanation for gravity. One way we might find answers is by studying the subtle changes in how neutrons behave at very low energies.
At a handful of neutron facilities around the world, researchers are slowing down neutrons until they have lost most of their energy. Known as ultra-cold neutrons, they move no faster than you can run. More importantly, they can be stored in special traps, enabling researchers to perform high-precision measurements and look for signs of new physics.
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Tying in gravity
In his general theory of relativity, Albert Einstein explained gravity as the curvature of space and time. But this description starts to fall down on the tiniest scales, and the search is on for a new theory that describes gravity at the quantum level and unites it with the other forces. Various theories exist and they predict some curious effects on extremely small length scales, which may throw new light on the nature of the universe.
Neutrons are ideal for investigating gravity at the microscopic scale because they are electrically neutral and do not experience stray electromagnetic fields that might spoil experiments.
In 2002, researchers at the ILL demonstrated that neutrons falling in a gravitational field do not drop like a stone but instead descend in distinct quantum leaps. Quantum mechanics predicts such behaviour, and Isaac Newton鈥檚 laws of gravity predict the height of the leaps. Then last year, a research team from the Vienna University of Technology in Austria and the ILL repeated the experiment using neutrons boosted to higher quantum energy states 鈥� a technique they called gravity spectroscopy.
Using this method, future research will be able to measure the energies of the various quantum states of a neutron in Earth鈥檚 gravitational field with great precision. This will in turn help to accurately model gravitational interactions over very short distances.
Some physicists believe that more accurate measurement of these energies will reveal a slight divergence from Newtonian laws. Detecting and quantifying this disparity could provide evidence of theoretical dark matter particles known as axions, or the extra dimensions suggested by string theory.
It could also test the equivalence principle, a 16th-century law stating that gravity accelerates all objects equally, regardless of their mass. Apollo astronaut Dave Scott famously demonstrated the principle on the moon in 1972, when . Millions watching at home saw the two land simultaneously. Researchers hope to use gravity spectroscopy to test the principle for quantum particles.
Universal theory
The unification of gravity with the other fundamental forces 鈥� electromagnetism and the strong and weak nuclear forces 鈥� would take us far beyond current particle physics as described by the standard model. Several theories are being actively explored, many based on a concept favoured by physicists: symmetry.
A number of symmetries predicted by the standard model are broken by experimental or theoretical evidence. One is CP-symmetry, which states that the laws of physics should apply equally to matter and antimatter particles and their mirror images. Many scientists agree that CP-symmetry must be violated in order to explain the survival of matter at the expense of antimatter after the big bang. Although a small amount of CP violation has been observed in certain particles, it isn鈥檛 enough to explain why the matter we are made of survived. Where is the rest?
One of the most promising avenues is being investigated with neutrons. Although the neutron is neutral overall, there are small opposing charges deep within it. If the average position of these charges does not coincide, they do not cancel each other out. The neutron then has what is known as an electric dipole moment and would be affected by an electric field.
If the neutron has an electric dipole moment, it violates CP-symmetry (see diagram). The amount of CP violation in the standard model predicts an electric dipole moment so small that it is beyond the scope of modern measuring equipment. That hasn鈥檛 stopped us looking for a larger than expected value.
Experiments look for telltale signs of an electric dipole moment by storing neutrons in a special cell and applying a combination of a weak magnetic and strong electric field. So far, they cannot rule out the possibility that the neutron does have an electric dipole moment. If it does, it must be very small. If we blew a neutron up to the size of Earth, the current limit would correspond to the neutron鈥檚 charges being separated by less than a hair鈥檚 width. After decades working to develop instruments sufficiently sensitive to measure at this level, the neutron electric dipole moment remains elusive, but the search continues at ever higher precision.
One further hint of new physics is the neutrality of the neutron itself. According to the standard model, there is no reason why the neutron鈥檚 charge should be zero. However, some theorists have suggested that unifying the forces does require the neutron to be exactly neutral. A high-precision experiment carried out at the ILL has so far proved this to be the case up to 21 decimal places.
Life and decay
Although stable for billions of years within their atoms, free neutrons quickly decay with a mean half-life of about 15 minutes. They release an electron and an electron-antineutrino to become protons. However, after 60 years of experiments, we still don鈥檛 know exactly how long it takes.
鈥淎fter 60 years of experiments, the half-life of the neutron remains uncertain鈥�
Interest in this precise value has grown substantially because it could help answer fundamental questions about the origins of the first elements, the balance between ordinary matter and invisible dark matter, and the force that powers stars.
Two experiments are attempting to improve the accuracy of this measurement using different observational techniques. At the US National Institute of Standards and Technology in Gaithersburg, Maryland, researchers count the neutrons and then determine the number of protons left behind after their decay. At the ILL, the team is bottling ultra-cold neutrons and counting those that survive after different storage times. Between them, these approaches have reduced the discrepancy to a couple of seconds.
