NEMO closes in on neutrino mass

June 20, 2014, CNRS
NEMO closes in on neutrino mass
View of the inside of the NEMO detector - Central tower Credit: Laboratoire souterrain de Modane (LSM)

The NEMO (Neutrino Ettore Majorana Observatory) experiment, whose goal was to elucidate the nature of neutrinos and measure their mass, yielded very positive results. The product of an extensive international collaboration including seven CNRS joint laboratories1, the detector, installed in the Modane Underground Laboratory (CNRS/CEA) in the Fréjus road tunnel, ran from 2003 to 2011. The observation, in seven different isotopes, of an extremely rare radioactive decay event, the so-called 'allowed' double-beta decay, helped improve our understanding of the atomic nucleus. In addition, the data collected during the search for the so-called 'forbidden' double-beta decay enabled the researchers to establish a range (0.3-0.9 eV) for the upper limit on the mass of the neutrino.

These findings, just published in the journal Physical Review Letters shed new light on neutrino physics and cosmological models. The technology chosen for NEMO opens the way for the SuperNEMO detector, which will be 100 times as sensitive and may even be able to detect so-called 'forbidden' double-beta decay, which would usher in a new era in physics.

The NEMO detector was aimed at observing an extremely rare radioactive phenomenon, double-beta decay, which only occurs in a few isotopes with half-lives as long as 100 billion times the age of the Universe. In 'allowed' double-beta decay, two neutrons are simultaneously converted into two protons, and two electrons and two neutrinos are emitted. During its eight years of operation, NEMO detected a million such events in seven different isotopes, thus helping to elucidate the structure of the .

Some theories predict the existence of a double decay with no emission of . This decay is said to be 'forbidden', since it violates the Standard Model on which the whole of particle physics is based. If such decay actually exists it would mean that the neutrino is a so-called Majorana particle, in other words, a particle that is its own antiparticle. According to cosmologists, this could explain why matter was created in the early Universe and why it prevailed over antimatter. NEMO was unable to detect . However, the data collected enabled the researchers to establish that the upper limit on the mass of the neutrino must be in the range 0.3-0.9 eV, depending on the nuclear model considered. It also allowed them to set the most accurate limits to date for certain neutrinoless double-beta decay modes, in particular that involving supersymmetric particles.

The main goal of the NEMO experiment was to detect an extremely rare signal, double-beta decay, which is normally hidden by stray radiation and natural radioactivity. To protect it from this background radiation, the NEMO-3 detector was set up under around 2,000 m of rock, in the Fréjus road tunnel, and built using materials with very low radioactivity. As a result, total radioactivity levels inside the dectector are 10 million times weaker than natural radioactivity.

NEMO closes in on neutrino mass
View of the inside of the NEMO detector - Central tower. Credit: Laboratoire souterrain de Modane (LSM)

Another characteristic that makes the NEMO instrument unique is its ability to identify the particles emitted in double-beta decay while at the same time using calorimeters to measure their energy. The quality of the data obtained thanks to these technologies opens the way for SuperNEMO, a detector that will be 100 times as sensitive and may be able to detect neutrinoless double-beta decay. With this future instrument, expected to be up and running in 2018, the scientists hope to usher in a new physics that goes beyond the Standard Model.

Explore further: No evidence of the double nature of neutrinos

More information: "Search for Neutrinoless Double-Beta Decay of 100Mo with the NEMO-3 Detector" ; R. Arnold, C. Augier, et al. Physical Review Letters ; 12 June 2014.

Related Stories

No evidence of the double nature of neutrinos

June 4, 2014

After two years of searching for a special radioactive decay that would provide an indication of new physics beyond the standard model, an experiment deep under ground near Carlsbad has so far found no evidence of its existence. ...

News about ghost particles

July 17, 2013

( —Neutrinos are the most elusive particles having extremely weak interactions with all other particles. They have rather unusual properties and are even expected to be identical with their own antiparticles. So ...

Physicists close in on a rare particle-decay process

June 4, 2012

In the biggest result of its kind in more than ten years, physicists have made the most sensitive measurements yet in a decades-long hunt for a hypothetical and rare process involving the radioactive decay of atomic nuclei.

In neutrino-less double-beta decay search, physicists excel

July 19, 2012

Physicists Andrea Pocar and Krishna Kumar of the University of Massachusetts Amherst, part of an international research team, recently reported results of an experiment conducted at the Enriched Xenon Observatory (EXO), located ...

Muon makes tracks in EXO-200 detector

February 1, 2011

( -- The Enriched Xenon Observatory-200, a prototype observatory that will search for exotic decays of fundamental particles of matter, passed a significant if unofficial milestone last month: its detector registered ...

Stalking the Neutrinoless Double Beta Decay

February 12, 2010

( -- The hunt for the elusive neutrino mass has officially begun. This difficult-to-detect elementary particle travels close to the speed of light, is electrically neutral, and can pass through ordinary matter ...

Recommended for you

Zirconium isotope a master at neutron capture

January 17, 2019

The probability that a nucleus will absorb a neutron is important to many areas of nuclear science, including the production of elements in the cosmos, reactor performance, nuclear medicine and defense applications.

Mechanism helps explain the ear's exquisite sensitivity

January 16, 2019

The human ear, like those of other mammals, is so extraordinarily sensitive that it can detect sound-wave-induced vibrations of the eardrum that move by less than the width of an atom. Now, researchers at MIT have discovered ...


Adjust slider to filter visible comments by rank

Display comments: newest first

Jun 20, 2014
This comment has been removed by a moderator.
not rated yet Jun 20, 2014
The bound 0.3-0.9 eV refers to the Majorana mass, not to the Dirac mass. Hence a larger mass remains possible, of Dirac type.
Jun 21, 2014
This comment has been removed by a moderator.
Jul 23, 2014
This comment has been removed by a moderator.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.