On January 17, the T2K collaboration announced the launch of the second phase of its experiment, as stated in a press release. This phase will exploit an upgrade of the beam, whose nominal power has been increased from 450 kW to 710 kW, with the aim of reaching 1.2 MW by 2027. An improved version of the experiment's near detector ND280 is also being implemented, incorporating new time-projection chambers using resistive-Micromegas technology designed and developed by the IRFU teams. The aim of this second phase is to collect more than twice the neutrino statistics recorded during the previous phase by 2027, and to reduce the uncertainty in the measured neutrino  interaction rate by a factor of two. The aim is to achieve a statistical significance of 3σ on the violation of Charge-Parity (CP) symmetry, in the event of maximum CP violation, as suggested by the results of the first phase of T2K. The discovery of CP symmetry violation in the lepton sector could explain one of the most fundamental mysteries of modern physics: the matter-antimatter asymmetry observed in the Universe.

A comprehensive revision of the summation method lays new and solid foundations for the calculation of antineutrino spectra emitted by a nuclear reactor. This major advance sheds new light on the origin of the reactor antineutrino anomalies, and will be

Supported by CEA's "digital simulation" cross-disciplinary program, Irfu, the Laboratoire National Henri Becquerel of DRT and the Service d'Étude des Réacteurs et de Mathématiques Appliquées of DES teamed up to carry out a thorough review of calculations of antineutrino spectra from nuclear reactors. A complete revision of the summation method lays a new and solid foundations for these calculations, and was featured as the Physical Review C  journal editor’s suggestion [1] on November 27, 2023. This revision incorporates numerous improvements in the beta decay modeling of the thousands of branches making up a reactor antineutrino spectrum, and in the use of nuclear evaluated data. It also quantifies all the systematic effects known to influence the calculations, providing for the first time a complete uncertainty model. This major advance now makes the summation model, long criticized for being approximate and incomplete, a robust tool for predicting reactor antineutrino spectra and for interpreting current and future experimental measurements. This work will likely stimulate targeted research to check and improve the experimental inputs, with potentially wide-ranging impact, from weak-interaction physics to many aspects of nuclear reactor science and technology. It also sheds interesting light on the origin of reactor antineutrino anomalies [2,3].

The collaboration has finalised a detailed background model offering the lowest index ever obtained, and has also adopted a new technology: NTL light detectors, which are much more effective at rejecting background noise.

Neutrino oscillations have confirmed that these mysterious particles have mass, contradicting the predictions of the Standard Model. The DPhP group at CEA/IRFU is seeking to solve this mystery by observing the very rare double-beta decay without neutrino emission of the Mo-100 nucleus using scintillating bolometers. Following the CUPID-Mo demonstration experiment at the Modane underground laboratory, the group has finalised a detailed background model that offers high precision for studying the 2v2β decay. The model thus achieves the lowest background index ever obtained by the scientific community for a 0ν2β bolometric experiment.

To achieve the target of 10-4 counts/keV/kg/year needed to detect this extremely rare decay, the CUPID experiment has also adopted a new detector technology: Nefanov-Trofimov-Luke (NTL) light detectors to improve background rejection. A measurement, using 10 identical light detectors coupled to Li2MoO4 and TeO2 crystals, was carried out at the Canfranc underground laboratory and demonstrated the applicability of this technology to CUPID detectors. Given the combination of low background, particle discrimination capability, high efficiency and high energy resolution, CUPID is recognised as one of the most promising next-generation 0ν2β search experiments. After a validation review, the experiment will begin its production and construction phase to obtain a complete detector from 2029.

Reactor antineutrino anomalies are a decade-long puzzle in neutrino physics. They are manifested by deviations of the order of a few percent between measurements and predictions. These deviations have been observed in the number of antineutrinos measured by more than a dozen experiments at nuclear reactors, and in the shape of the kinetic energy distributions by the seven most recent ones. They could have been the way to a new physics beyond the standard model, but the recent experiments, including the STEREO experiment carried by IRFU, have closed this door.

In a work just published in Physical Review Letter [1], a team of physicists from IRFU and the Laboratoire National Henri Becquerel of DRT have shown that these anomalies could come from biases in the measurements of fission electrons used as a reference for the prediction. They have developed a beta strength function  model to reduce the biases in the calculation of the energy spectra of electrons from fission of fissile reactor nuclei. The two "anomalies" on the antineutrino flux and the "bump" at 5 MeV in the antineutrino energy spectrum are now reproduced by their model. This allows to propose an explanation to solve an enigma of more than 10 years.

The final results of the Stereo experiment have just been published in the journal Nature. A record of precision is established for the spectrum of neutrinos emitted by the fission of 235U, measured between 9 and 11m distance from the ILL reactor core in Grenoble. The hypothesis of a sterile neutrino to explain the reactor neutrino anomaly is rejected. The quality of these direct neutrino measurements now surpasses that of the underlying nuclear data describing the beta decays of fission products. Stereo provides the community with a fission neutrino spectrum corrected for all detection effects, which will serve as a reference for future reactor experiments and points out residual biases in the nuclear databases.

The Stereo experiment has just completed a beautiful scientific adventure that began in 2011 with the revelation by the IRFU group of the "reactor antineutrino anomaly". Physicists were left with a significant deficit of 6% between the flux of neutrinos measured at the reactors and the predicted flux. The history of science has taught us enough about the potential richness of new phenomena that can hide behind an anomaly. What was going on here was the possible existence of a new type of neutrino that would open up a new area of physics beyond the standard model. Without any direct interaction with matter, this neutrino, described as "sterile", could however mix with "standard" neutrinos and thus leave an imprint of its existence through ... a deficit of count rate in our detectors.

The analysis on data recorded in 2019, led by an IRFU physicist, on relic neutrinos improves on previous limits by two orders of magnitude.

The cosmic background neutrino is one of the predictions of the standard cosmological model, but it has never been directly observed. These so-called "relic neutrinos" could be captured on a radioactive nucleus like tritium. The resulting capture rate depends on the local density of relic neutrinos. Since massive neutrinos get caught by the gravitational potential of our galaxy and cluster locally, a modest local overdensity of relic neutrinos should exist on Earth. More exotic considerations could lead to more substantial overdensities. The KATRIN experiment published in June 2022 in Physical Review Letters its first search for relic neutrinos based on analysis of data recorded in 2019.  The analysis, led by an IRFU physicist, improves on previous limits by two orders of magnitude.

The KATRIN collaboration has just recently reported a new upper limit of 0.8 eV/c2 on the mass of neutrinos. The KATRIN spectrometer also has a strong potential to search for new, so-called "sterile" neutrinos, based on a fine analysis of the tritium beta decay spectrum. The collaboration has just published its new results in Physical Review D based on the first two data campaigns acquired in 2019. This work reveals no evidence of a fourth neutrino, and KATRIN may well be a key player in clarifying the anomalies observed by some neutrino oscillation experiments over the past 20 years or so.

KATRIN has just crossed a symbolic threshold and reveals a new upper limit!

The KATRIN (KArlsruhe TRItium Neutrino Experiment) located at the Karlsruhe Institute of Technology (KIT) has just crossed a symbolic threshold. In a paper published in the prestigious journal Nature Physics, the collaboration reveals a new upper limit of 0.8 eV/c^2 for the mass of neutrinos. This result is of fundamental interest for both particle physics and cosmology.

At Irfu, neutrino physics is studied using different sources such as reactors, accelerators and radioactive sources.

Irfu teams have been engaged for several decades in a long quest to study the neutrino in all its aspects, to understand its place in the Standard Model of particle physics and even more, but also its contribution to the evolution of the Universe since its first moments. The traditional summer conferences organized last year were an opportunity to measure the progress made by the armada of international experiments with which our institute is working to achieve this ultimate goal. A look back at year 2021, full of lessons and promises for the future... 

The search for double beta decay without neutrino emission (0νββ) is one of the major challenges of contemporary physics, because its observation would make a clear statement about the nature of the neutrino itself and potentially on the origin of the matter/antimatter asymmetry of our universe. The CUPID collaboration, in which several researchers from IRFU and IN2P3 are involved, is actively researching this process using scintillating bolometers as detectors. In June 2020, the CUPID-Mo demonstrator experiment, which is located at the Modane underground laboratory, demonstrated the excellent potential of this detection method with only 2.264 kg of 100Mo and one year of data collection. In the coming years, the objective of the CUPID collaboration is to design one of the most sensitive experiments ever built by increasing the total mass of 100Mo to 250 kg. Three articles have just been published on the technological and methodological choices needed for this change of scale, while maintaining the required performances of the final experiment.

The main objective of the KATRIN experiment is the measurement of the mass of the three neutrinos of the Standard Model of Particle Physics. But the analysis of the beta decay spectrum of tritium also allows to search for the trace of a hypothetical fourth neutrino, called sterile neutrino. The collaboration has just published its first analysis in Physical Review Letters (see article) based on four weeks of data acquired in 2019. There is no trace of this fourth neutrino, but this is only the beginning as sensitivity will rapidly improve. The KATRIN spectrometer shows a strong potential to study this possible new facet of the neutrino.

In its standard form, double beta decay is a process in which a nucleus decays into a different nucleus and emits two electrons and two antineutrinos (2νββ). This nuclear transition is very rare, but it was detected in several nuclei with sophisticated experiments. If neutrinos are their own antiparticles, it’s possible that the antineutrinos emitted during double beta decay annihilate one another and disappear. This is called neutrinoless double beta decay (0νββ), a phenomenon never observed so far. If 0νββ is detected, we will ascertain that neutrinos are their own antiparticles, and this would be a clue as to why they get their tiny masses—and whether they played a part in the existence of our matter-dominated universe. 

The CUPID-Mo experiment, installed at the Modane Underground Laboratory, after one year of data between March 2019 and April 2020 has just set a new global limit for the detection of the signature 0νββ.

The international CUPID-Mo experiment conducted by French laboratories of IN2P3, CEA/IRFU and CEA/IRAMIS has been testing the use of Molybdenum-based crystals since last April to detect double beta decay without neutrino emission. The experiment is gradually gaining strength and already shows a near-zero background in the region of interest, which is very promising. The scientists of the collaboration made an update in the occasion of the official inauguration on 11 and 12 December 2019.

A team from IRFU's Department of Particle Physics (DPhP) has just conducted the most accurate study to date of the mass of cosmic neutrinos, including both standard model neutrinos and sterile neutrinos contributing to dark matter.

The researchers used the spectra of nearly 200,000 distant quasars measured by the Sloan Digital Sky Survey (SDSS) eBOSS project to map the distribution of hydrogen at very remote times in the history of our universe, ten to twelve billion years ago.

Neutrinos, propagating at relativistic speeds for billions of years, prevent gravity from acting on small scales and smooth the structures (clusters of galaxies, filaments, ...) revealed by the spectra of quasars. Thanks to the accuracy of the measurements, researchers have been able to narrow the possible range for the mass of cosmic neutrinos, to the point of being able to have their word on how the different masses of the three neutrinos of the Standard Model are ordered.

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 separate topics developed at CEA, the first at DEN, the second at DRF/Irfu, 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 The European Physical Journal A [1].

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.

Neutrinos from the Big Bang have been traveling the Universe for more than 13 billion years. They are almost undetectable but their footprint on the formation of large structures in the Universe, such as galaxies, can be detected. For the first time, this trace of the "diffuse neutrino background" from the Big Bang on the "baryonic acoustic oscillations" (BAO) has been deduced from the survey of 1.2 million galaxies of the "Sloan Digital Sky Survey" (SDSS). These data correspond to 5 years of observations from the Baryon Oscillation Spectroscopic Survey (BOSS) experiment, a ground-based telescope installed in New Mexico. The result, published in the journal Nature Physics, shows how the BAO phase can constrain the number of neutrino species in the Standard Model of Particle Physics.

The DPhP group has been involved in this project for more than 10 years and is currently working on its extension, the eBOSS project. In the very near future, the DESI project will be able to study even more precisely this cosmic neutrino background produced by the Big Bang.


In 2018, IRFU is participating  in a publication CUPID-0: the first array of enriched scintillating bolometers for 0νββ decay investigations which reviews a first matrix of bolometers installed in the Gran Sasso laboratory in Italy, with the objective of tracking the double beta decay without neutrino emission (0νββ) that will reveal the nature of neutrinos. This publication describes the integration of the detectors, their testing and commissioning for a first data collection that began in 2017 with the participation of IRFU and IN2P3 researchers at various stages. The test results show a very good response of the electronics and cryogenic systems. The mean value in energy resolution proves the unmatched efficiency of this technique for measuring radioactivity, which makes it possible to dissociate beta decay from alpha decay, a source of background  in this research.

What is the mass of neutrinos? To answer this fundamental question, the KATRIN experiment was designed and built by an international collaboration at the Karlsruhe Institute of Technology. On June 11, 2018, an international symposium marked the beginning of data acquisition. The first electron spectra from tritium decay have been analyzed with an analysis chain developed at IRFU. Everything conforms to the required specifications and the first long data taking campaign for physics can start.  First results expected in 2020.

A hundred years old mystery might get resolved with the detection of neutrinos by the IceCube collaboration coming from a known active black hole. Irfu, which coordinate those observations with the H.E.S.S. telescope, did not detect anything for now but the multi-messenger astronomy has just begun…

The STEREO experiment presented its first physics results at the 53rd Rencontres de Moriond1. 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.

153rd Rencontres de Moriond Electroweak session


The T2K collaboration, whose goal is to study and measure neutrino oscillations, is publishing new results on the interaction of neutrinos with nuclei. This study, in which the T2K group of the IRFU plays a major role, is crucial in that it allows the dominant uncertainty on the oscillation parameters to be restrained. For the first time, protons emerging from the neutrino-nucleus interaction have been characterized using new variables capable of exposing and characterizing nuclear effects.

The data collected between 2010 and 2017 by the T2K collaboration (Tokai To Kamiokande) and the reactor neutrino experiments strengthens the trend announced a year ago—neutrinos and antineutrinos have seemingly different behavior.

The new-generation liquid argon detector used in the WA105 experiment at CERN has collected its first signals. This prototype is used in preparation of the Deep Underground Neutrino Experiment (DUNE) for neutrino observations on a mass scale, which is due to start in 2026 in the USA. This research involving IRFU aims, in particular, to shed light on the origin of matter and antimatter.

After four years of study, the Luminescent Underground Molybdenum Investigation for Neutrino mass and nature (LUMINEU) collaboration has selected lithium molybdate for the manufacture of scintillating bolometers. These ultrasensitive particle detectors will be used for neutrinoless double-beta-decay searches. Should evidence of the latter be highlighted, neutrinos would merge with their antiparticle and the absolute mass of the neutrino would become accessible.

IRFU's Double Chooz group has just published some surprising results regarding the flux of antineutrinos generated by uranium and plutonium fission products in nuclear power reactors. A more precise estimate of this flux has revealed a +3% shift with respect to the predictions considered as the benchmark for the past 25 years. The re-analysis of the most important past reactor neutrino experimental results, in the light of this new flux prediction, lead to the so called 'reactor antineutrino anomaly'. Including other effects such as the evolution of the neutron lifetime and the presence of long-lived fission isotopes, the averaged shortfall in the number of antineutrinos detected at short baseline reactor experiments is almost 6%. The hypothesis according to which this anomaly can be explained by the existence of a new particle, a 4th "sterile” neutrino, ties in surprisingly well with other independent results. 
If its existence is proven by forthcoming experiments this 4th neutrino, interacting only with gravity, should be added to the bestiary of the Standard Model of particle physics. It could have some impact on cosmology as well. Nonetheless, neutrino flux measurements will have to be performed less than ten meters from reactor cores to obtain irrefutable proof that the new particle exists. This is within the grasp of present-day techniques used in neutrino detection experiments, particularly in the case of Nucifer, a detector that is about to be used in a data-gathering mission on Osiris, the research reactor in Saclay. Other new proposals are being discussed by the particle physics community to test this anomaly, with new neutrino beam experiment (at CERN) or by deploying a Mci neutrino generator in large liquid scintillator detectors (like Borexino).

The Double Chooz collaboration recently completed its neutrino detector which will see anti-neutrinos coming from the Chooz nuclear power plant in the French Ardennes. The experiment is now ready to take data in order to measure fundamental neutrino properties with important consequences for particle and astro-particle physics.












Christian VEYSSIERE 


In August 2010 at CERN in Geneva, a team of physicists from SEDI and SPP working in collaboration with a group from ETH-Zurich obtained the first successful results from a MicroMegas detector operating in a time projection chamber filled with pure cryogenic argon at a temperature of 87.2 kelvin. 




Thierry Lasserre, a physicist at IRFU, has received the Bronze medal in the CNRS awards for 2009

On April 14, Thierry Lasserre received the CNRS bronze medal from the new director of the In2p3, Jacques Martino. Since 1954, CNRS has awarded three medals each year to renowned researchers or promising young scientists. This Bronze Medal rewards a researcher's first work, which marks that person as a promising specialist in his or her field. The work of Thierry Lasserre concerned the most abundant massive particle in the universe: the neutrino.

In Japan at the end of January 2010, the detectors of the Tokai to Superkamiokande (T2K, [ti:tu:kei]), developed at Saclay, observed their first neutrinos. These detectors consist of two large chambers where the tracks of charged particles are able to be reconstructed and the neutrino beam can be characterized. In this experiment, neutrinos are created by a proton beam coming from the Tokai accelerator. These same neutrinos are then measured 300 km away, at Kamioka, in a large water vessel 40 m in diameter and 40 m high, which was previously used to study neutrinos coming from cosmic ray interactions in the atmosphere and to definitively prove the phenomena of neutrino oscillation (leading to a Nobel Prize for Masatoshi Koshiba in 2002). The first interaction with a neutrino coming from Tokai was observed at the end of February in the detector at Kamioka, marking the beginning of a very exciting new phase in neutrino physics.

A company from the Vosges Department in France, NEOTEC, received the 2009 "Outstanding Implementations" award, at the International MIDEST Exhibition attended by the Industry Minister, Christian Estrosi, for their production of very special chambers. This equipment forms part of an important component of the Double-Chooz experiment which, before the end of the year, will measure neutrinos emitted by the reactor at the Chooz nuclear power station in the Ardennes.




Engineers and physicists from IRFU have successfully assembled and commissioned three large chambers designed to reconstruct charged particle tracks. The chambers will characterize the neutrino beam used in the T2K (Tokai to Kamiokande) experiment. They are the first large Time Projection Chambers (TPCs) to be equipped with micromesh gas detectors (Micromegas). The chambers have a very large sensitive area (nearly 9m²) and a correspondingly high number of electronic channels (124,000). IRFU built the entire detection system of the three TPCs, comprising 72 Micromegas detectors and all the front-end electronics. Engineers from SEDI, a department specialised in detector, electronics and information technology, specially designed a new chip called AFTER and two printed circuit boards for sending digitised signals to the acquisition system via an array of 72 gigabit optical links. The three chambers were tested with a particle beam at TRIUMF in Canada and have been installed in JPARC (Tokai, Japan) at the end of 2009.

The first beam and cosmic ray tests demonstrated that tracks could be reconstructed with the required degree of precision. This achievement represents a major step forward in the construction of a vital detector for the T2K experiment.


The TPC are now detecting tracks produced by neutrino interaction in the near detector of T2K and provide superior information to measure the momentum and to identify these particles. 





Marco ZITO

The second phase of the Double Chooz international experiment officially began on Wednesday 20 May. The Declaration of Intent signed by the four partners (CEA, CNRS, EDF, Champagne-Ardenne Region) is the first step in the plan to build a second detector devoted to neutrino research next to the Chooz nuclear power plant.

Prior to signing the DOI, the participants visited the site of the first detector, currently under construction. By the end of the year, the detector should pick up the first neutrinos emitted by the plant and attempt to measure the disappearance of primary flux neutrinos. The second detector, which will be operational two years from now, will provide precise measurements of the flux and spectrum of the neutrinos emitted and greatly enhance measurement control and precision.









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