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The Stereo Experiment is a neutrino oscillation experiment designed to search for the existence of sterile neutrinos. It is located at very short baseline to the research reactor of the ILL in Grenoble, France. With its segmented detector, Stereo is probing our understanding of antineutrinos emitted by nuclear fissions. Its data allows to challenge the Standard Model of particle physics on currently unexplored territory.

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March 2019 — Stereo is moving up a gear

STEREO contour Figure 1: Exclusion contour drawn by the latest Stereo data in the plane of the amplitude of the oscillation toward an hypothetical 4th neutrino (horizontal axis) and the frequency of this oscillation (vertical axis). The blue area shows the expected exclusion coverage at the available statistical precision which would be obtained if all Stereo observables correspond exactly to the expectations without 4th neutrino. The red area is the actual exclusion contour based on the measured data resulting in statistical fluctuations around the blue limit. All points inside the red contour are excluded with at least 90% confidence level. This result rejects a large part of the domain of existence of the 4th neutrino predicted from the reactor neutrino anomaly (indicated by the black contours). © M. Vialat, ILL

STEREO rate Figure 2: Ratio of the neutrino rate measured by Stereo to the expected rate (blue point). This new result is in good agreement with the previous set of measurements at reactors operating with a highly enriched nuclear fuel (black points and purple average). The new world average including the Stereo result is shown in red. An independent extraction of the 235U neutrino rate from the Daya Bay and Reno measurements at commercial reactors operating with mixed fuel is shown for comparison (green point). © D. Lhuillier, CEA

STEREO shape Figure 3: Neutrino spectrum measured by Stereo (black points) compared to the normalized prediction (yellow line, the area of the predicted spectrum is set equal to the area of the measured spectrum). © L. Bernard, LPSC

The Stereo experiment releases new results based on the detection of about 65000 neutrinos at short distance from the research reactor of the ILL-Grenoble. The improved accuracy is rejecting the hypothesis of a 4th neutrino in a large fraction of the domain predicted from the reactor neutrino anomaly. Profiting from a good control of the detector response, Stereo now also releases its first absolute measurements of the neutrino rate and the spectrum shape.

Omnipresent particles, neutrinos are under scrutiny in all kinds of detectors to test the theory of the Standard Model, to witness the inside of reactors or stars, or to study the most violent and large-scale phenomena in the Universe. The detection of the faint signals left by the neutrinos has thus entered a high precision era, revealing new anomalies when comparing to expectations. The goal of the Stereo experiment is to perform a direct test of the existence of a hypothetical 4th neutrino, which could reconcile the so far unexplained deficit of neutrinos detected close to nuclear reactors (the reactor neutrino anomaly).

The Stereo detector is installed since end of 2016 at 10 m from the core of the reactor of the Institut Laue-Langevin (ILL) in Grenoble, France. It measures precisely the rates and energy spectra of the neutrinos emitted by the core in 6 identical detector cells. If a 4th neutrino exists, it will “oscillate” with the standard neutrinos, inducing a unique pattern of spectral distortions from one cell to another. However, the spectra measured in the 6 cells of the Stereo detector have compatible shapes and need a very careful analysis. The present result significantly shrinks the domain of existence of the 4th neutrino (Figure 1). As Stereo continues taking data it will improve its sensitivity and test the surviving zone, toward even smaller expected amplitudes of oscillations.

Beyond the cell-to-cell comparison, a more difficult task is the control of the absolute response of the detector. The Stereo result is of great interest because the nuclear fuel of the ILL core is highly enriched and detected neutrinos originate from the fission of a unique isotope, 235U, instead of a mix of 4 fissioning isotopes at commercial reactors. The absolute rate and spectrum shape have been kept hidden in the Stereo analysis. They are “un-blinded” for the first time after defining the evaluation of all systematics and the analysis procedure. Figure 2 shows that Stereo is actually among the most precise measurements of the 235U fission neutrino rate, adding valuable accuracy in the test of the reactor neutrino anomaly. The spectrum shape as measured by the sum of the 6 cells shows a remarkable agreement with the predicted shape for a pure 235U spectrum up to 6.3 MeV, but deviations beyond the estimated uncertainties are also seen at the highest energies (Figure 3). Stereo has not expressed its full potential yet. Complementary calibration observables are under study to reduce further the shape uncertainties and as many neutrinos as already acquired are expected by until mid-2020!

Stereo is a French-German experiment devised and operated by a team of scientists from Irfu-CEA in Saclay, the Institut Laue-Langevin in Grenoble, the Annecy’s Particle Physics Laboratory (LAPP), the Grenoble’s Subatomic Physics and Cosmology Laboratory (LPSC) and the Max-Planck-Institut für Kernphysik in Heidelberg, Germany (MPIK). 

March 2018 — Stereo constrains the existence of a 4th neutrino

STEREO detector Stereo is a neutrino detector made up of six scintillation liquid cells that takes data 10 m from the Grenoble high neutron flux reactor (ILL). © D. Lhuillier, CEA

RAA Figure 1: In the case of neutrinos emitted by nuclear reactors, a deficit has been identified by research works carried out at IRFU. Following a re-evaluation of the predicted neutrino rates, all the values measured between 10 and 100 m are clearly deficient compared to the prediction (red dotted line). The existence of a sterile neutrino could explain this deficit. © T. Lasserre, CEA

STEREO contour Figure 2: The possible values of the 4th neutrino parameters are delimited by the black curves, with the star marking the most likely case. The vertical axis is related to its mass and the horizontal axis to the amplitude of its mixing with the neutrinos emitted by the reactor. The red and green regions are rejected by the Stereo experiment measurements with different confidence levels (95% and 90%). The blue region represents the theoretical rejection sensitivity of the Stereo experiment for a statistical precision corresponding to 66 days of data. © T. Salagnac, LPSC

The Stereo experiment presented its first physics results at the 53rd Rencontres de Moriond. Stereo is a neutrino detector made up of six scintillation liquid cells that has been measuring, since November 2016, the electronic antineutrinos produced by the Grenoble high neutron flux reactor 10 metres from the reactor core. The existence of a fourth neutrino state, called sterile neutrino, could explain the deficit in neutrino flux detected at a short distance from nuclear reactors compared to the expected value. Indeed, this anomaly could result from a short-range oscillation that would result in less expected electronic antineutrinos being detected because they would disappear into sterile neutrinos. The first results obtained in 2018 after 66 days of data exclude a significant part of the parameter space. The experiment will continue to take data until the end of 2019. By multiplying the statistics by four and minimizing systematic analysis errors, Stereo will be able to shed light on the existence of this 4th neutrino family.

Sterile neutrinos
While they are among the most abundant particles in the universe, neutrinos are extremely difficult to detect. They originate in the heart of the stars or in the most violent phenomena of our universe, but can also be produced by particle accelerators or, as in the case of the Stereo experiment, in the heart of nuclear reactors.
Neutrinos have no electrical charge and interact very weakly with matter. Today we know 3 types or flavours: electronic neutrino, muonic neutrino and tauic neutrino. An amazing discovery made 20 years ago showed that neutrinos can change flavor, i.e. change from one flavor to another as they travel. This phenomenon, called "neutrino oscillation" was awarded the Nobel Prize in Physics in 2015.

Are there more than 3 types of neutrinos?
The interest in this issue gained new strength in 2011 when researchers noted that two sets of previously unexplained experimental results could be interpreted by transforming neutrinos into a 4th type of neutrino never observed before (Figure 1). The existence of a fourth neutrino state, called sterile neutrino, could explain the deficit in the neutrino flux detected at a short distance from nuclear reactors compared to the expected value. Indeed, this anomaly could result from a short-range oscillation that would result in less expected electronic antineutrinos being detected because they would disappear into sterile neutrinos.
This neutrino, called "sterile" because without direct interaction with matter, would have a mass around the eV, much larger than that of the other three neutrinos already known and its discovery would be a major advance in particle physics. Several experiments, including Stereo, aim to confirm or disprove this hypothesis.

Stereo: an experience as close as possible to reactors
This project consists of measuring the oscillation of electronic antineutrinos with a six-cell segmented detector, placed about 10 m from the core of the Grenoble high neutron flux reactor. The six Stereo cells are 40 cm wide, which makes it possible to follow the oscillation of the neutrinos over 2.4 m. The antineutrinos detection technology uses scintillation liquids doped with Gadolinium, as is the case with the Double Chooz and Nucifer detectors. The capture of a neutrino by a hydrogen atom in the liquid results in the emission of a positron and a neutron delayed by a few tens of microseconds. Since the end of 2016, data are being acquired with the detection of nearly 400 antineutrinos every day. The first results obtained in 2018 after 66 days of data exclude a significant part of the parameter space.
The IRFU through the Nuclear Physics Departments (DPhN), the Systems Engineering Department (DIS) and the Detector Electronics for Physics Department (DEDIP) is particularly involved in this project with responsibility for Stereo's core: the internal detector tank and its division into cells by reflective acrylic walls.

Results that weaken sterile neutrinos hypothesis
The first results of the Stereo experiment presented at the Moriond Meetings exclude a significant part of the parameter space expected for the existence of a hypothetical 4th neutrino (see Figure 2), but the worldwide quest for sterile neutrino continues.
Stereo will indeed gain in precision by accumulating 4 times more data by the end of 2019 and the competing projects currently underway will shed additional light on this hypothetical 4th neutrino.
Thanks to the characteristics of the ILL reactor core, highly enriched in 235U, the Stereo experiment will also be able to provide a new measurement of the neutrino spectrum emitted by the fission of this isotope, which is very important for all neutrino experiments with reactors.