Neutrinos: Ghostly particles with unstable egos

September 6, 2011

So far it is unknown which rules neutrinos follow when they alter their identity. A study in which scientists of the Excellence Cluster Universe at the Technische Universitaet Muenchen, Germany, participated has now revealed that even the last of the three parameters, which describe the oscillation of neutrinos is most likely to be greater than zero. This may help to understand the development of the early Universe. The paper will be published in the journal Physical Review D.

They were always mysterious. 26 years had to pass before the prediction of was confirmed and the existence of neutrinos was finally proven experimentally in 1956. The reason for this ordeal: Neutrinos only interact by the weak interaction with other particles of matter. When a cosmic neutrino approaches the earth, it has the best chance of passing through the whole globe unhindered. It is correspondingly difficult to find direct evidence of neutrinos with the help of a detector. Further decades passed in the discussion about their masses: None or small but finite mass? In the meantime it is considered certain that the ghostly particles are carrying mass, if only a virtually infinitesimal amount: According to today's knowledge, no neutrino should exist that is heavier than 1 eV (an electron "weighs" about 500,000 eV). There are three types of neutrinos. This is also believed to be true today, so that neutrinos can each easily be classified in one of the three particle families in the framework of the .

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 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 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 were responsible for the minimal surplus of matter compared to anti-matter in the . 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.

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not rated yet Sep 06, 2011
"According to today's knowledge, no neutrino should exist that is heavier than 1 ev ", follows from astrophysical measmts. This is in gross disagreement w/Wikipedia, which lists their mass upper limits as 2 ev, 170 Kev, & 15 Mev (e,u,t resp.), which are presumably from indirect terrestrial measmts.
Something's got to give ! Which is correct ?
Also, how can the std.model accomodate Massive neutrinos ? The Higgs field does not couple to them.
Something's got to give ! Which is correct ?
5 / 5 (3) Sep 06, 2011
@JIMBO: There is nothing wrong with different experiments/calculations setting different upper bounds for mass. I weigh less than 2 thousand kilograms, but I also weigh less than 2 million kilograms.

The same wiki article you linked references many different experiments/calculations that establish different upper bounds. For instance, "In July 2010 the 3-D MegaZ experiment reported that they had measured the upper limit of the combined mass of the three neutrino varieties to be less than 0.28 eV."
not rated yet Sep 06, 2011
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.

Some backward numbers there. First the mixing angle (and its sine) may be negative. The experiments only look at the square of the sine, which will never be negative. The interesting question they are trying to answer is whether sin (Theta 13) = 0. Then the odds are stated backwards. Current data results in the conclusion that there is a one in 400 chance that Theta 13 DOES equal zero.
3 / 5 (2) Sep 06, 2011
This article mentions that there is an excess of electron neutrinos at the 400 meter detector compare to the 1050 meter one. From this they conclude some of the electron neutrinos are oscillating into muon and tau neutrinos.

This just may not be the case. What they need to do in order to prove that some of the electron neutrinos are oscillating into muon and tau neutrinos is to detect an excess of muon and tau neutrinos at the 1050 meter range over the 400 meter one equaling the missing electron neutrinos.
4 / 5 (2) Sep 06, 2011
If neutrinos change state and each state has a different mass, then where does the mass disappear to and where does the mass appear from?
4 / 5 (1) Sep 07, 2011
jsa, probably the mass gets converted back and forth into energy/angular momentum??? they should move in that case at slightly differend speeds, wich could be picked up by multiple detectors
not rated yet Sep 07, 2011
From the article:

Further decades passed in the discussion about their masses: None or small but finite mass?

I agree with the researchers that small but finite mass seems more likely than small but infinite.
5 / 5 (2) Sep 07, 2011
"About 1020 antineutrinos leave a typical reactor every second."
That would be 10^20, of course. I do wish PhysOrg's editors would proof-read their articles...
5 / 5 (1) Sep 07, 2011
then where does the mass disappear to and where does the mass appear from?
It doesn't is a possibility. The energies are so low that the Heisenberg Uncertainty Principle should cover it.


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