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.

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.

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).

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. 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!

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).

Figure 3: Neutrino spectrum measured by STEREO (black points) compared to the normalized prediction (blue line, the area of the predicted spectrum is set equal to the area of the measured spectrum).

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).

The Laboratory of Subatomic Physics and Cosmology (LPSC, University of Grenoble Alpes/ CNRS/ Grenoble INP/ LPSC-IN2P3) is deeply involved in this experiment since its conception. The LPSC was responsible for protecting the detector from background (muon detector, mu-metal and polyethylene shielding), electronics and data acquisition and the LED calibration system. Physicists take most of the responsibility of the data taking and have a major contribution in the data analysis. They will continue their efforts to further improve the accuracy of the measurement.

Link to the presentation given by Laura Bernard, phd at LPSC, at the 54rd Rencontres de Moriond Electroweak session

Contact: Anne Stutz

Conférence mm Universe

La première édition de la conférence internationale "Observing the mm Universe with the NIKA2 camera" se tiendra au LPSC du 3 au 7 juin 2019.

Cette conférence est consacrée à l'exploitation scientifique de la caméra NIKA2 au télescope de 30 m de l'IRAM.

Les sujets couverts par la conférence incluent tous les sujets liés à NIKA2 :

- Cosmologie avec des amas de galaxies

- Physique des amas de galaxies (SZ, X, visible)

- Synergie avec Planck, Euclid, LSST, ...

- Instrumentation

- ...

La conférence mm Universe est gratuite. Toutefois, la préinscription est obligatoire.

Date limite d'inscription et de soumission des résumés :  4 mai.

Les actes seront publiés dans : European Physical Journal Web of Conferences.

La conférence mm Universe est financée par l'ANR NIKA2Sky, le labex Focus et le LPSC

Plus d'informations : https://lpsc-indico.in2p3.fr/Indico/event/1765/

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Une nouvelle machine de métrologie pour la construction du détecteur ATLAS-ITk

Une machine à mesurer tridimensionnelle (MMT) vient d'être installée dans la nouvelle salle blanche du groupe ATLAS au LPSC.
Cette machine à commande numérique de grande vitesse et de haute précision permettra une mesure précise des assemblages de modules à pixels du futur détecteur ITk de l'expérience ATLAS au CERN.

Lors de la phase à haute luminosité du LHC qui doit commencer en 2026, la plus grande intensite des collisions entrainera une plus grande densité de traces à mesurer, et un plus grand taux de radiations auquel résister. Il faudra donc procéder au remplacement complet du trajectographe d'ATLAS.

Formation à l'utilisation de la MMT sur un prototype d'échelle d'ITk

Dans le cadre d'une collaboration avec les laboratoires de l'IN2P3 CPPM et LAPP, le LPSC va procéder à l'assemblage d'un millier de modules de capteurs à pixels sur leur support mécanique, de 2021 à 2023. Ces modules devront être placés avec une précision de 0.05 mm, soit l'épaisseur d'un cheveu.

Contact : Elisabeth Petit, Ludovic Eraud

Prêt pour une exploration de l’Univers ? Cosmic Tour VR

Découvrez la toile cosmique au travers de COSMIC TOUR VR, une nouvelle animation de réalité virtuelle réalisée à l’initiative du LAPP, du LPSC et du CCIN2P3, et financée par le labex ENIGMASS en collaboration avec LSST France et Euclid France.

La bande annonce est disponible à l’URL : http://lpsc.in2p3.fr/upload/doc/7d1cbf/CosmicTourVR-Trailer.mp4

Réalisée à partir d’images simulées du projet Horizon cette animation sera au programme du Dark Matter Day 2018 au Muséum de Grenoble le mercredi 31 octobre 2018.

Deux casques de réalité virtuelle permettront au personnel du LPSC de découvrir en avant-première le COSMIC TOUR.
=> RDV à l’accueil entre 10h et 12h du 15 au 26 octobre 2018.