Neutrinos: Ghostly particles with unstable egosSeptember 06, 2011
The knowledge of the neutrino mass is based on numerous experiments, in which so-called neutrino oscillations were observed. Neutrinos freely flying through the space of a particular family (i.e. the electron neutrino) can transform themselves spontaneously into a neutrino of another family affiliation (the muon neutrino or tau neutrino). One refers to an oscillation because the neutrino may change its family affiliation periodically during an extended journey. Such oscillations are only possible if the particles are carrying mass. The experimental evidence of neutrino oscillations (and thus a neutrino mass other than zero) is among the greatest breakthroughs of modern particle physics in the past 20 years.
The conversion process among different neutrino flavors depends on three so-called mixing angles Theta 12, Theta 23 and Theta 13. In interplay with the neutrino mass-squared differences they regulate the transition probabilities among different flavors. Of the three mixing angles only two are well known and have large values, while the third one Theta 13 is at the focus of current searches. So far, it was known that its value had to be small compared to the other two neutrino mixing angles. That is, Theta 13 = 0 could not be excluded. In the past, several independent projects have tried to measure this elusive parameter without success. The most important piece of information came in 1998 from the Chooz experiment in France, which established that the oscillation evoked by Theta 13 cannot be larger than approximately one tenth of those induced by the each of the other two neutrino mixing angles.
Three years ago a group of theoretical physicists of whom one, Antonio Palazzo, is now at the Excellence Cluster Universe, the others at the University and INFN of Bari, evidenced for the first time a weak hint of non-zero Theta 13 thanks to an accurate work of global analysis of all the existing neutrino oscillation data. In the meantime, two accelerator experiments (MINOS and T2K) were at work to nailing down Theta 13 and they have recently released their results. Notably, both experiments point towards a non-zero Theta 13, in agreement with the hint evidenced by the group of theorists. By combining their previous findings with the new accelerator data, in June 2011 the same group came for the first time to a statistically clear conclusion according to which sin² Theta 13 ≈ 0.02 with a confidence level of at least 3 Sigma. This means that the odds against Theta 13 > zero are 1:400.
However, physicists are very prudent and, before claiming a discovery, need to have a higher confidence level of 5 Sigma, diminishing the odds against Theta 13 > zero to 1:1 million. In order to provide secure evidence, the researchers are performing other experiments. Among these, the reactor experiment Double-Chooz, in which physicists of the Universe Clusters are strongly involved, will have a crucial role. For this purpose, it has been developed a particularly effective, terrestrial neutrino source: The particles (more precisely: anti-neutrinos) are generated and emitted during the fission processes in a nuclear power plant in particularly high flux. About 1020 antineutrinos leave a typical reactor every second. For this reason, a new experiment, the inheritor of the forerunning Chooz experiment, has started in the vicinity of the nuclear power plant in the French municipality Chooz. Thanks to this setup the value of Theta 13 will be measured with a precision that hitherto has not been achieved.
The principle behind the Double-Chooz experiment is very simple: Immediately after their generation in the reactor, several anti-neutrinos collide with a detector located 400 meters away. The spatial proximity ensures that no oscillations (or only extremely few) occur between emission and initial detection. The first detector thus measures the electron anti-neutrinos, which haven't transformed to muon and or tau neutrinos yet. A second detector of identical construction is located approximately 1,050 meters away from the reactor. If the value of the neutrino mixing angle Theta 13 is large enough, a part of the electron anti-neutrinos will become muon or tau anti-neutrinos as a result of the oscillations. The electron-anti-neutrino rate observed at the second detector therefore is much smaller than expected without oscillations.
Both detectors are filled with about 10 tons of scintillation fluid. If an electron-anti-neutrino interacts with a proton within the fluid, this will lead to inverse-beta decay: The proton captures the electron-anti-neutrino thereby transforming into one neutron by emitting one positron. Both particles generate one quick flash each in the liquid in a set time sequence. 390 photo sensors mounted on the walls of the vessel record the events. The Double Chooz experiment started physics data taking in April 2011 and will search for corresponding signals for five years. The detector performance and the status of data taking will be reported at the TAUP conference in Munich from 5 to 9 September 2011. First results are expected by the end of this year.
Establishing that Theta 13 is effectively different from zero would entail that all the three mixing angles are non-vanishing. This would provide the three neutrino flavors with maximal freedom of flipping one to each other. In turn, such a high degree of freedom is the necessary condition to generate CP-violation in the leptonic sector, i.e. to give rise to a different behavior of neutrinos and anti-neutrinos. The observation of CP-violation is now the next target of neutrino physicists as it would have significant consequences for several unanswered questions of modern physics. It could soon be clarified, in particular, whether neutrinos were responsible for the minimal surplus of matter compared to anti-matter in the early Universe. Without this asymmetry, all matter would have been transformed to radiation shortly after the birth of the Universe. There would be no galaxies, no stars or planets and no one who could measure Theta 13.
Technische Universitaet Muenchen
Related Neutrino Current Events and Neutrino News Articles
New data from Antarctic detector firms up cosmic neutrino sighting
Researchers using the IceCube Neutrino Observatory have sorted through the billions of subatomic particles that zip through its frozen cubic-kilometer-sized detector each year to gather powerful new evidence in support of 2013 observations confirming the existence of cosmic neutrinos.
Solving a long-standing atomic mass difference puzzle paves way to the neutrino mass
To measure the mass of neutrinos, scientists study radioactive decays in which they are emitted. An essential ingredient is the decay energy which corresponds to the mass difference between the mother and daughter nuclei.
Fermilab experiment sees neutrinos change over 500 miles
Scientists on the NOvA experiment saw their first evidence of oscillating neutrinos, confirming that the extraordinary detector built for the project not only functions as planned but is also making great progress toward its goal of a major leap in our understanding of these ghostly particles.
NuSTAR provides explosive evidence for supernova asymmetry
New results from the NASA NuSTAR telescope show that a supernova close to our galaxy experienced a single-sided explosion.
The microwave oven has been around for almost 80 years. When it heats food or liquid, the frequency of electrons increases but their energy slows down due to their own microwave emissions. Until now, scientists have only been able to observe this phenomenon in a group of electrons.
Electron chirp: Cyclotron radiation from single electrons measured directly for first time
A year before Albert Einstein came up with the special theory of relativity, or E=mc2, physicists predicted the existence of something else: cyclotron radiation.
New tabletop detector 'sees' single electrons
MIT physicists have developed a new tabletop particle detector that is able to identify single electrons in a radioactive gas.
Detector at the South Pole explores the mysterious neutrinos
Neutrinos are a type of particle that pass through just about everything in their path from even the most distant regions of the universe.
Cosmic debris: Study looks inside the universe's most powerful explosions
A new study provides an inside look at the most powerful explosions in the universe: gamma-ray bursts.
Decoding the gravitational evolution of dark matter halos
Researchers at Kavli IPMU and their collaborators have revealed that considering environmental effects such as a gravitational tidal force spread over a scale much larger than a galaxy cluster is indispensable to explain the distribution and evolution of dark matter halos around galaxies.
More Neutrino Current Events and Neutrino News Articles