Physicists close in on a rare particle-decay process

June 4, 2012, California Institute of Technology
The Enriched Xenon Observatory 200 (EXO-200) is a neutrino experiment housed 2150 feet below ground in a salt basin at the Waste Isolation Pilot Plant (WIPP). The subterranean location isolates it from cosmic rays and other sources of natural radioactivity. Credit: EXO/WIPP/SLAC

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.

If discovered, the researchers say, this process could have profound implications for how scientists understand the fundamental laws of physics and help solve some of the universe's biggest mysteries—including why there is more matter than antimatter and, therefore, why regular matter like planets, stars, and humans exists at all.

The experiment, the Enriched Xenon Observatory 200 (EXO-200), is an international collaboration that includes the California Institute of Technology (Caltech) and is led by Stanford University and the SLAC National Accelerator Laboratory, a U.S. Department of Energy (DOE) National Laboratory.

The EXO-200 experiment has placed the most stringent constraints yet on the nature of a so-called neutrinoless double beta decay. In doing so, physicists have narrowed down the range of possible masses for the neutrino, a tiny uncharged particle that rarely interacts with anything, passing right through rock, people, and entire planets as it zips along at nearly the speed of light.

The collaboration, consisting of 80 researchers, has submitted a paper describing the results to the journal Physical Review Letters.

In a normal double beta decay, which was first observed in 1986, two neutrons in an unstable atomic nucleus turn into two protons; two electrons and two antineutrinos—the antimatter counterparts of neutrinos—are emitted in the process.

But physicists have suggested that two neutrons could also decay into two protons by emitting two electrons without producing any antineutrinos. "People have been looking for this process for a very long time," says Petr Vogel, senior research associate in physics, emeritus, at Caltech and a member of the EXO-200 team. "It would be a very fundamental discovery if someone actually observes it."

A neutrino is inevitably produced in a single beta decay. Therefore, the two neutrinos that are produced in a neutrinoless double beta decay must somehow cancel each other out. For that to happen, physicists say, a neutrino must be its own antiparticle, allowing one of the two neutrinos to act as an antineutrino and annihilate the other neutrino. That a neutrino can be its own antiparticle is not predicted by the Standard Model—the remarkably successful theory that describes how all elementary particles behave and interact.

If this neutrinoless process does indeed exist, physicists would be forced to revise the Standard Model.

This large copper cylindrical vessel is the Enriched Xenon Observatory 200's (EXO-200) time projection chamber, the part of the detector that contains the liquid xenon, isotopically enriched in xenon-136. The photo shows the chamber being inserted into the cryostat, which keeps the experiment at extremely low temperatures. Credit: EXO

The process also has implications for cosmology and the origin of matter, Vogel says. Right after the Big Bang, the universe had the same amount of matter as antimatter. Somehow, however, that balance was tipped, producing a slight surplus in matter that eventually led to the existence of all of the matter in the universe. The fact that the neutrino can be its own antiparticle might have played a key role in tipping that balance.

In the EXO-200 experiment, physicists monitor a copper cylinder filled with 200 kilograms of liquid xenon-136, an unstable isotope that, theoretically, can undergo neutrinoless double beta decay. Very sensitive detectors line the wall at both ends of the cylinder. To shield it from cosmic rays and other background radiation that may contaminate the signal of such a decay, the apparatus is buried deep underground in the DOE's Waste Isolation Pilot Plant in Carlsbad, New Mexico, where low-level radioactive waste is stored. The physicists then wait to see a signal.

The process, however, is very rare. In a normal double beta decay, half of a given sample would decay after 1021 years—a half-life roughly 100 billion times longer than the time that has elapsed since the Big Bang.

One of the goals of the experiment is to measure the half-life of the neutrinoless process (if it is discovered). In these first results, no signal for a neutrinoless double beta decay was detected in almost seven months' of data—and that non-detection allowed the researchers to rule out possible values for the half-life of the neutrinoless process. Indeed, seven months of finding nothing means that the half-life cannot be shorter than 1.6 × 1025 years, or a quadrillion times older than the age of the universe. With the value of the half-life pinned down, physicists can calculate the mass of a neutrino—another longstanding mystery. The new data suggest that a neutrino cannot be more massive than about 0.140 to 0.380 electron volts (eV, a unit of mass commonly used in particle physics); an electron, by contrast, is about 500,000 eV, or about 9 × 10-31 kilograms.

More than ten years ago, the collaboration behind the Heidelberg-Moscow Double Beta Decay Experiment controversially claimed to have discovered neutrinoless double beta decay using germanium-76 isotopes. But now, the EXO-200 researchers say, their new data makes it highly unlikely that those earlier results were valid.

The EXO-200 experiment, which started taking data last year, will continue its quest for the next several years.

Explore further: Investigating the Neutrino Mass Scale with the ultra-low background KamLAND-Zen detector

More information: The paper is available on arXiv at

Related Stories

Calcium isotope holds the secret to the mass of neutrinos

April 12, 2016

Scientists around the world are being kept in suspense by the negligible mass of neutrinos, subatomic particles that could be matter and antimatter at the same time. Now, researchers from the University of Tokyo, in collaboration ...

NEMO closes in on neutrino mass

June 20, 2014

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 ...

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. ...

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 ...

Mysterious neutrinos take the stage at SLAC

September 24, 2015

Of all known fundamental particles, neutrinos may be the most mysterious: Although they are highly abundant in the universe and were discovered experimentally in 1956, researchers still have a lot left to learn about them. ...

Recommended for you

Sculpting stable structures in pure liquids

February 21, 2019

Oscillating flow and light pulses can be used to create reconfigurable architecture in liquid crystals. Materials scientists can carefully engineer concerted microfluidic flows and localized optothermal fields to achieve ...

How to freeze heat conduction

February 21, 2019

Physicists have discovered a new effect, which makes it possible to create excellent thermal insulators which conduct electricity. Such materials can be used to convert waste heat into electrical energy.

Water is more homogeneous than expected

February 21, 2019

In order to explain the known anomalies in water, some researchers assume that water consists of a mixture of two phases, even under ambient conditions. However, new X-ray spectroscopic analyses at BESSY II, ESRF and Swiss ...

Correlated nucleons may solve 35-year-old mystery

February 20, 2019

A careful re-analysis of data taken at the Department of Energy's Thomas Jefferson National Accelerator Facility has revealed a possible link between correlated protons and neutrons in the nucleus and a 35-year-old mystery. ...


Adjust slider to filter visible comments by rank

Display comments: newest first

3.8 / 5 (4) Jun 04, 2012
wow. To sum it up, they don't even attempt to answer any questions a layman science enthusiast like you or I would have.

Now, before you comment on the dozens of apparent inconsistencies in this article, ask yourself honestly, do you really know anything deeper than the surface of what they're talking about? Unless you hold an actual PHD in the relevant field, then I'm going to guess no.
1 / 5 (4) Jun 04, 2012
But have annihilations of neutrinos and anti-neutrinos been observed? Neutrinos being true neutral particles, they might have no annihilation mechanism... In my opinion, neutrinos are their own anti-particles and they are probably the missing dark matter in a cold form (slowed down mainly by black holes). Supernovae produce a huge amount of them and there was, possibly, remnants from a pre-bigbang universe...

After all, the only difference between a neutrino and its counter part is its helicity, left-handed vs right-handed...
4.2 / 5 (5) Jun 04, 2012
I just want to be sure I read this right-- in order to protect the sensors from unwanted radiation, they are buried deep underground in a nuclear waste repository?
5 / 5 (5) Jun 04, 2012
The decay from thoses wastes is relatively easy to contain compared to cosmic rays (impossible with known materials). So yes, it can make sense in that context.
2.3 / 5 (3) Jun 05, 2012
Neutrinos being true neutral particles, they might have no annihilation mechanism
Neutrinos aren't quite neutral, as they exhibit weak nuclear charge. It's a tiny charge which is manifesting at short distance scale only (up to 10-18 meters). On the contrary, the existence of fully neutral neutrino (so-called the sterile or Majorana neutrinos) is a subject of intensive research by now.

The XENON collaboration was originally dedicated to search of WIMPS predicted with string theory - and they found nothing, so they apparently changed subject for not to lost the interest of grant agencies completely. IMO this version or research is substantially more realistic and it has some hope for success, albeit it's not so groundbreaking. Maybe they find something unexpected during it.
1 / 5 (3) Jun 05, 2012
By the way, nowadays we still could not clearly explain the elusive neutrino in the conventional way. May be this unconventional view could give some hint.

1 / 5 (4) Jun 05, 2012
Dense aether theory explains the neutrinos as a supersymmetric counterparts of photons. In the water surface analogy of space-time the photons are Russel's solitons of surface ripples, whereas the neutrinos are solitons of underwater waves, so called the Falaco solitons. They're solitons of gravitational waves instead of light waves.

Both photons, both neutrinos deform the space-time a bit, so they have a rest mass, but they're of opposite gravitational charge. Illustratively speaking, the Russel's soliton deform the water surface upward, the Falaco solitons downward. Both they're slowing the surface wave spreading, but they behave differently during acceleration or inside of gravitational field.
1 / 5 (3) Jun 05, 2012
The question, whether the neutrino can form its own antiparticle is not transparent even in dense aether model, because whole the neutrino itself is antiparticle by its very character. Illustratively speaking, it behaves like tiny bubble inside of space-time stuffed with gravitational waves - but this bubble has a sufficiently thick and dense walls formed with "photons" for to behave like the particle of positive rest mass as a whole. The right-hand form has more antiparticle character pronounced, than the less common left-hand form. If Majorana neutrino would exist, than it must be a product of temporal transition between left-hand and right hand form, in this sense the left hand neutrino would be real, albeit probably not very stable. We can detect such a transition with deviation of common colour oscillations of neutrinos from theory. In recent experiment it seems, such a deviation may really exist.
5 / 5 (2) Jun 05, 2012
To sum it up, they don't even attempt to answer any questions a layman science enthusiast like you or I would have.

Scientific papers aren't written for the layman. A certain amount of background knowledge is expected.
But if they kept to the usual style then it goes something like this: Within timeX you have not detected any decay. Chances for a decay are Y (which is a function of the energy released among other things). So you can say that with 95% probability (the usual confidence interval) the half life is longer than Z and that the energy interval cannot be more than U.

in order to protect the sensors from unwanted radiation, they are buried deep underground in a nuclear waste repository?

Which is not a problem in this case. They are detecting betas - which can be shielded against easily. If it were gammas things would look differently.

dozens of apparent inconsistencies in this article

Could you name one? I fail to see any.
5 / 5 (3) Jun 05, 2012
wow. To sum it up, they don't even attempt to answer any questions a layman science enthusiast like you or I would have.

This is particle physics at its best - just raw data without any "analysis" or "theories" by armchair physicists.

It is the way it should be done - observe the results of carefully controlled experiments and then try to draw up theory to explain them. Unfortunately too much theory is not worth the paper it is written on when no experiment can back it up with facts.
2.5 / 5 (2) Jun 05, 2012
Which is not a problem in this case. They are detecting betas - which can be shielded against easily. If it were gammas things would look differently.

There wouldn't be any difference.

Halving thickness in water for gammas from mixed fission products is about 20 cm. Salt is denser and should be even better shielding.

To a first approximation the tunnels are about 80 meters apart. That's ~320 halving thicknesses, 2^-320 is 5*10^-97. There's only ~10^80 atoms in the universe. If every atom in the universe was stored in the adjacent tunnel and they all emitted a gamma it is one in a quadrillion that a gamma makes it to the detector in the other tunnel.
not rated yet Jun 05, 2012
In 1 atom, 2 neutrons simultaneously transmutate becoming 2 protons (releasing 1 beta particle and 1 neutrino each). 2 electrons escape the nucleus of the 1 atom but whenever a beta particle emerges so does a complimentary neutrino. 2 neutrinos and 2 electrons emerge from the 1 atom.

Now, 2 electrons don't annihilate (electrons aren't their own antiparticles). IF the nucleus releases 2 electrons that are detected AND there are no 2 neutrinos to be found in the decay product THEN the 2 electron antineutrinos annihilated in the transmutation process.
not rated yet Jun 10, 2012
IF the nucleus releases 2 electrons that are detected AND there are no 2 neutrinos to be found in the decay product THEN the 2 electron antineutrinos annihilated in the transmutation process.

I wonder if the anti-neutrinos annihilate one another by possessing opposite vectors and cancelling one another that way. I know a photon is it's own anti-particle. When two of them are created together as their own anti-particles, they race off in opposite directions, reflecting the symmetry that makes each the opposite of the other.

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.