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Januar 2023 — Final results from the Stereo experiment reject sterile neutrino hypothesis

After several years of operation, the Stereo collaboration published final results of the antineutrino studies. With this data, hints for the existence of sterile neutrinos, an additional neutrino state expected in many theories, are excluded. The result which appeared in the January 11 issue of Nature has important implications for many areas of fundamental physics.

To unambiguously test the hypothesis of sterile neutrinos and determine their properties, the Stereo experiment was taking data over 4 years at the high-flux nuclear research reactor at the ILL Grenoble. The project profited from the experience collected over several generations of reactor neutrino experiments. Shielded from the external environment, the cells of the detector were ideally positioned to search with unprecedented precision for the signature of sterile neutrinos: position dependent distortions in their energy distribution should appear at a short distance from the reactor.

Now, the Stereo collaboration published their most recent results combining the full data set: An anomaly in the antineutrino flux emitted by nuclear reactors was confirmed, but sterile neutrinos are, however, not the cause of this. Stereo was able to observe a total of more than 100 000 neutrinos in the years 2017 to 2020, but could not find any trace of potential sterile neutrinos within these measurements. The previously observed anomalies result most likely from underestimated uncertainties in the nuclear data from the radioactive decays used for the flux prediction rather than the neutrino experiments itself. While this result rejects the sterile neutrino hypothesis quite strongly, it serves as a further support of the Standard Model and its neutrino content.

Besides the search for sterile neutrinos, the Stereo experiment also provides the most accurate measurement to date of the antineutrino spectrum from the fission of Uranium-235. It is intended to be used as a reference spectrum for future high precision reactor experiments, such as the determination of the mass hierarchy of neutrinos or the low-energy tests of the Standard Model. In addition, precision measurements of this kind might help to better understand the phenomena happening during a reactor shutdown, for instance.

References:
Stereo Collaboration, “Stereo neutrino spectrum of 235U fission rejects sterile neutrino hypothesis”, Nature, 613 (2023) 257–261
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October 2019 — FIFRELIN and Stereo at the Crossroads of Cultures

The FIFRELIN code simulates nuclear fission and de-excitation of the nuclei produced therein. Stereo is a compact neutrino detector that looks for a hypothetical sterile neutrino. Two a-priori disjoint topics, which have however recently met to achieve unprecedented precision on a crucial ingredient in the detection of neutrinos: the de-excitation of a gadolinium nucleus after the capture of a neutron. The results of this meeting have just been published in the journal EPJA.

Figure 1: Neutrino detection process in Stereo. The incident neutrino interacts with a hydrogen nucleus of the liquid scintillator to form a positron (e⁺) and a neutron (n) which, in most cases, will be captured by a gadolinium nucleus. © D. Lhuillier, CEA

Figure 2: Illustration of the distribution of excited levels of a nucleus. The density increases very rapidly with the excitation energy. In this example, the initial state, whose energy S_n (neutron separation energy) is of the order of 8 MeV, is de-excited by emitting 3 gamma rays to the ground state (G.S.). © O. Litaize, CEA

Figure 3: Comparison between experimental energy (points) and simulated (histograms) distributions of gamma-rays detected in Stereo after a neutron capture. Neutrons are generated by a radioactive source placed halfway up a cell. © H. Almazán Molina, MPIK

The Stereo experiment aims to test the existence of a sterile neutrino by looking for oscillations of the standard electron anti-neutrinos produced by the research reactor of ILL Grenoble. Although 4·10¹⁵ neutrinos, resulting from the beta decays of the fission products, pass each second through the 2 m³ of scintillator liquid of the detector, only one neutrino is intercepted every 4 minutes. If a sterile neutrino exists, it will induce an oscillation in the number of active neutrinos detected, visible by comparing the spectra measured in the 6 cells of Stereo (read about the detector and its detection principle here).

With such a low rate of neutrinos, one must be able to reject all other signals (natural radiation, reactor activity) that could mimic the signal of a neutrino. Fortunately, physicists have a rather unique signature of the interaction of a neutrino with a proton of the detector: the detection of a positron followed by the capture of a neutron (Figure 1). To make this process more efficient, gadolinium (Gd) is blended with the scintillator liquid. This element holds the record of appetite for neutrons. In just a few microseconds, it will capture the neutron produced by a neutrino and emit a cascade of gamma rays to signal its capture with a total energy of 8 MeV, well above most disturbing signals.

The oscillations sought by Stereo develop at the scale of meters. The detector is therefore compact and a non-negligible part of the gammas of the cascade will escape the liquid scintillator. The nice signal expected at 8 MeV will thus be adorned with a broad low energy tail, filled with all these partial energy deposits of the cascade. A detection cutoff is applied to stay above the background (typically, energies above 4.5 MeV are selected). But Stereo wants to control its detection efficiency to the %-level, so it is necessary to describe very precisely the gamma cascades of Gd.

At this point FIFRELIN, being well acquainted with the complex terrains of gamma cascades, enters the stage. It is a Monte Carlo code developed at CEA / DEN Cadarache that simulates the production and de-excitation of fission fragments to meet nuclear data needs for reactor physics [1]. In particluar, FIFRELIN is capable of modelling the emission cascade of gammas and electrons resulting from the de-excitation of a nucleus created by neutron capture. To do this, it uses all available nuclear structure data that describes the first excited levels. But after absorption of a neutron, the excitation energy of the Gd nucleus reaches a continuum of levels (Figure 2). FIFRELIN is then based on level-density models to complete the level schemes. After calculating all the probabilities of partial inter-level transitions, the code samples millions of electromagnetic cascades, while simultaneous controlling the number and energy of the gamma rays. These cascades are then used in the simulation of the Stereo detector response.

Figure 3 illustrates the improvement obtained, thanks to FIFRELIN, in the description of the energy measured after the neutron captures. Capture peaks on Gd (8 MeV) and to a lesser extent on hydrogen (2.2 MeV) are clearly visible. The initial agreement obtained with the GEANT4 simulations, illustrated by the graph in blue, seems satisfactory, but the residual distortions were nevertheless sufficient to generate 4.5% difference between the simulated and measured detection efficiencies for a neutron source in the center of a cell. Thanks to the FIFRELIN simulations (red graph), the agreement becomes almost perfect. This is the case for the alignment of the peaks as well as the distribution of intermediate energies, very sensitive to the description of the cascades. The agreement between simulation and data reaches down to only 0.5%, with an associated sub-% level uncertainty [2].

A fruitful meeting that comes at the right time for Stereo's pursuit of high precision! The technology of Gd-doped scintillators is widely used for neutrino detection. This advancement towards high precision will be beneficial to several other ongoing projects. In parallel with the publication of these results, 10 million Gd cascades have been made available to the scientific community [3].

References:
[1] O. Litaize et al., “Fission modelling with FIFRELIN”, EPJA, 51 (2015) 177
[2] H. Almazán et al., “Improved STEREO simulation with a new gamma ray spectrum of excited gadolinium isotopes using FIFRELIN”, EPJA, 55 (2019) 183
[3] H. Almazán et al., “Data from: Improved STEREO simulation with a new gamma ray spectrum of excited gadolinium isotopes using FIFRELIN”, Zenodo, 2653786 (2019)

March 2019 — Stereo is moving up a gear

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 4^th 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.

Figure 1: Exclusion contour drawn by the latest Stereo data in the plane of the amplitude of the oscillation toward an hypothetical 4^th 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 4^th 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 4^th neutrino predicted from the reactor neutrino anomaly (indicated by the black contours). © M. Vialat, ILL

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

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

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 4^th 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 4^th 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 4^th 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). 

References:
[1] L. Bernard (for the STEREO Collaboration), arXiv:1905.11896 [hep-ex]

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

The Stereo experiment presented its first physics results at the 53^rd 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 4^th neutrino family.

Figure 1: 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

Figure 2: 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

Figure 3: The possible values of the 4^th 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

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 4^th type of neutrino never observed before (Figure 2). 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 4^th neutrino (see Figure 3), 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 4^th 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.

References:
[1] H. Almazán et al. (Stereo Collaboration), Phys. Rev. Lett. 121, 161801