Double Chooz experiment

Logo of the experiment

In the Double Chooz experiment , the special peculiarity of neutrinos to transform from one kind to another ( neutrino oscillation ) is examined . The experiment is being carried out as part of an international collaboration in France at the Chooz nuclear power plant , in which - as in every nuclear reactor - beta decay produces large numbers of antineutrinos. To determine the conversion probability, two identical detectors were set up 400 and 1050 m away from the reactor. Since neutrinos have a very low reaction probability, measurements must be carried out for several years in order to detect enough neutrinos and to determine the small conversion probability.

This experiment is the successor to the Chooz experiment , which also detected neutrinos at the Chooz nuclear power plant. The original Chooz experiment was able to determine the most accurate upper limit until 2012 for the conversion probability of the electron neutrinos, which is referred to as. It was hoped that Double Chooz would provide a further greatly improved limit or even an exact value. A first result presented in November 2011 indicated a non-zero conversion probability. This indication, which was not yet statistically significant , later proved to be consistent with the significant results of the Daya Bay experiments and, shortly thereafter, RENO . Double Chooz has also been delivering statistically significant results since 2012. ${\ displaystyle \ theta _ {13}}$

Concept of the double chooz experiment

Oscillation probability of electron antineutrinos

The radioactive decay of fission products in the nuclear reactor provides a by-product of anti-electron neutrinos that fly in all directions. One of two detectors is placed relatively close to the reactor. The antineutrinos do not yet have the opportunity to transform themselves into another species until the nearby detector. The second detector, on the other hand, is placed at a greater distance where conversions are more likely. The detectors can only measure the anti-electron neutrinos generated in the reactor. If one measures fewer neutrinos in the distant detector than expected due to the dilution of the distance, one can assume that the anti-electron neutrinos have partially converted into a different species. From the number of neutrino events in the distant detector compared to the nearby detector, one can conclude how great the conversion probability is.

Neutrinos have no electrical charge and are therefore difficult to detect. In the case of the Double Chooz experiment, the neutrino detection takes place via the inverse beta decay , in which an anti-electron neutrino converts a proton into a neutron and a positron :

${\ displaystyle {\ bar {\ nu _ {e}}} + p ^ {+} \ rightarrow n ^ {0} + e ^ {+}}$

The resulting positron generates scintillation light in the detector. In order to detect the neutron, gadolinium is added to the liquid scintillator , which with a high degree of probability captures the neutron and changes into an excited state. The excited gadolinium nucleus can then pass into the ground state with emission of gamma radiation, which leads to the production of scintillation light again. The light is then registered by the photomultiplier units .

Detector principle

The detector consists of different parts that perform special tasks. Inside the detector, the neutrino reactions are to be detected in a liquid scintillator . A neutrino meets a proton and a neutron and a positron (the antiparticle of the electron ) are created. Both reaction products are detected in order to obtain a clearly defined neutrino signal: the positron annihilates together with an electron from the environment and thus generates two high-energy photons . To capture the neutron, the scintillator contains gadolinium . This also creates high-energy photons, which appear somewhat delayed in relation to the positron signal. In the liquid scintillator of the inner two volumes of the detector, the energy of the photons is gradually converted until visible light is obtained at the end. This light is detected with the help of photo multipliers. Photomultiplier devices are devices that can convert single photons in the visible range into an electrical signal. The outer part of the detector is used to shield against natural ionizing radiation from the environment. The fourth volume is used for active background suppression. In particular, cosmic muons , which can interfere with the measurement, should be recognized in this part of the detector.

Results so far

The first results of the Double Chooz experiment were presented at the LowNu conference in Seoul in November 2011. The most likely value of θ ₁₃ was therefore:

${\ displaystyle \ sin ^ {2} (2 \ theta _ {13}) = 0 {,} 085 \ pm 0 {,} 051}$

First results suggested oscillations, but could not yet rule out the possibility of no oscillation (θ ₁₃ = 0) ( statistical significance of 1.7 standard deviations ). Double Chooz was the first reactor neutrino experiment that was able to measure the oscillation of electron antineutrinos over short distances. On March 8, 2012, the Daya Bay collaboration published the first measurement of the mixing angle θ ₁₃ above the detection threshold with a significance of more than five standard deviations . Daya Bay gave a value of 0.092 for the mixing angle, a measurement consistent with the result of Double Chooz.

In 2012 and 2014, the Double Chooz collaboration announced improved results based on larger data sets with significantly reduced uncertainties. All results published up to 2014 relate to analyzes of the data with the remote detector and, within the measurement accuracy, agree well with the results of the other θ ₁₃ experiments. The second nearby detector was completed in a specially built underground laboratory at the end of 2014 and has been collecting data since then. By measuring with two detectors, the dominant uncertainty due to the predicted reactor neutrino flow should be significantly reduced.

Institutes from Germany

In addition to numerous other institutes from France, Spain, Russia, the USA and Brazil, four German institutes are also involved in Double Chooz:

Individual evidence

↑ Indication of Reactor ν̅e Disappearance in the Double Chooz Experiment , Phys. Rev. Lett. 108, 131801 (2012)
↑ Reactor ν̅ e disappearance in the Double Chooz experiment , Phys. Rev. D 86, 052008 (2012)
↑ Improved measurements of the neutrino mixing angle θ ₁₃ with the Double Chooz detector , JHEP10 (2014) 086
^ Double Chooz lecture, Neutrino Telescopes 2015, Venice

literature

M. Apollonio et al. Search for neutrino oscillations on a long base-line at the CHOOZ nuclear power station , Eur.Phys.J.C27: 331-374,2003
Proposal Double Chooz: A Search for the Neutrino Mixing Angle ${\ displaystyle \ theta _ {13}}$

Web links

Homepage of the project

[1] Indication of Reactor ν̅e Disappearance in the Double Chooz Experiment , Phys. Rev. Lett. 108, 131801 (2012)

[2] Reactor ν̅ e disappearance in the Double Chooz experiment , Phys. Rev. D 86, 052008 (2012)

[3] Improved measurements of the neutrino mixing angle θ ₁₃ with the Double Chooz detector , JHEP10 (2014) 086

[4] Double Chooz lecture, Neutrino Telescopes 2015, Venice