What are Leptons?

CERN visualization showing two electrons (green), one to two muons (red lines) resulting from a collision between two Z bosons. Credit: CERN

During the 19th and 20th centuries, physicists began to probe deep into the nature of matter and energy. In so doing, they quickly realized that the rules which govern them become increasingly blurry the deeper one goes. Whereas the predominant theory used to be that all matter was made up of indivisible atoms, scientists began to realize that atoms are themselves composed of even smaller particles.

From these investigations, the Standard Model of Particle Physics was born. According to this model, all matter in the Universe is composed of two kinds of particles: hadrons – from which Large Hadron Collider (LHC) gets its name – and leptons. Where hadrons are composed of other elementary particles (quarks, anti-quarks, etc), leptons are elementary particles that exist on their own.


The word lepton comes from the Greek leptos, which means “small”, “fine”, or “thin”. The first recorded use of the word was by physicist Leon Rosenfeld in his book Nuclear Forces (1948). In the book, he attributed the use of the word to a suggestion made by Danish chemist and physicist Prof. Christian Moller.

The Standard Model of Elementary Particles. Image: By MissMJ - Own work by uploader, PBS NOVA [1], Fermilab, Office of Science, United States Department of Energy, Particle Data Group, CC BY 3.0
The Standard Model of Particle Physics, showing all known elementary particles. Credit: Wikipedia Commons/MissMJ/PBS NOVA/Fermilab/Particle Data Group
The term was chosen to refer to particles of small mass, since the only known leptons in Rosenfeld’s time were muons. These elementary particles are over 200 times more massive than electrons, but have only about one-ninth the the mass of a proton. Along with quarks, leptons are the basic building blocks of matter, and are therefore seen as “elementary particles”.

Types of Leptons:

According to the Standard Model, there are six different types of leptons. These include the Electron, the Muon, and Tau particles, as well as their associated neutrinos (i.e. electron neutrino, muon neutrino, and tau neutrino). Leptons have negative charge and a distinct mass, whereas their neutrinos have a neutral charge.

Electrons are the lightest, with a mass of 0.000511 gigaelectronvolts (GeV), while Muons have a mass of 0.1066 Gev and Tau particles (the heaviest) have a mass of 1.777 Gev. The different varieties of the elementary particles are commonly called “flavors”. While each of the three lepton flavors are different and distinct (in terms of their interactions with other particles), they are not immutable.

A neutrino can change its flavor, a process which is known as “neutrino flavor oscillation”. This can take a number of forms, which include solar neutrino, atmospheric neutrino, nuclear reactor, or beam oscillations. In all observed cases, the oscillations were confirmed by what appeared to be a deficit in the number of neutrinos being created.

Muons, a type of lepton, shown being produced by the Large Hadron Collider. Credit: CERN
Muons, a type of lepton, shown being produced by the Large Hadron Collider. Credit: CERN

One observed cause has to do with “muon decay” (see below), a process where muons change their flavor to become electron neutrinos  or  tau neutrinos – depending on the circumstances. In addition, all three leptons and their neutrinos have an associated antiparticle (antilepton).

For each, the antileptons have an identical mass, but all of the other properties are reversed. These pairings consist of the electron/positron, muon/antimuon, tau/antitau, electron neutrino/electron antineutrino, muon neutrino/muan antinuetrino, and tau neutrino/tau antineutrino.

The present Standard Model assumes that there are no more than three types (aka. “generations”) of leptons with their associated neutrinos in existence. This accords with experimental evidence that attempts to model the process of nucleosynthesis after the Big Bang, where the existence of more than three leptons would have affected the abundance of helium in the early Universe.


All leptons possess a negative charge. They also possess an intrinsic rotation in the form of their spin, which means that electrons with an electric charge – i.e. “charged leptons” – will generate magnetic fields. They are able to interact with other matter only though weak electromagnetic forces. Ultimately, their charge determines the strength of these interactions, as well as the strength of their electric field and how they react to external electrical or magnetic fields.

None are capable of interacting with matter via strong forces, however. In the Standard Model, each lepton starts out with no intrinsic mass. Charged leptons obtain an effective mass through interactions with the Higgs field, while neutrinos either remain massless or have only very small masses.

History of Study:

The first lepton to be identified was the electron, which was discovered by British physicist J.J. Thomson and his colleagues in 1897 using a series of cathode ray tube experiments. The next discoveries came during the 1930s, which would lead to the creation of a new classification for weakly-interacting particles that were similar to electrons.

The first discovery was made by Austrian-Swiss physicist Wolfgang Pauli in 1930, who proposed the existence of the electron neutrino in order to resolve the ways in which beta decay contradicted the Conservation of Energy law, and Newton’s Laws of Motion (specifically the Conservation of Momentum and Conservation of Angular Momentum).

The positron and muon were discovered by Carl D. Anders in 1932 and 1936, respectively. Due to the mass of the muon, it was initially mistook for a meson. But due to its behavior (which resembled that of an electron) and the fact that it did not undergo strong interaction, the muon was reclassified. Along with the electron and the electron neutrino, it became part of a new group of particles known as “leptons”.

In 1962, a team of American physicists – consisting of Leon M. Lederman, Melvin Schwartz, and Jack Steinberger – were able to detect of interactions by the muon neutrino, thus showing that more than one type of neutrino existed. At the same time, theoretical physicists postulated the existence of many other flavors of neutrinos, which would eventually be confirmed experimentally.

The tau particle followed in the 1970s, thanks to experiments conducted by Nobel-Prize winning physicist Martin Lewis Perl and his colleagues at the SLAC National Accelerator Laboratory. Evidence of its associated neutrino followed thanks to the study of tau decay, which showed missing energy and momentum analogous to the missing energy and momentum caused by the beta decay of electrons.

In 2000, the tau neutrino was directly observed thanks to the Direct Observation of the NU Tau (DONUT) experiment at Fermilab. This would be the last particle of the Standard Model to be observed until 2012, when CERN announced that it had detected a particle that was likely the long-sought-after Higgs Boson.

Today, there are some particle physicists who believe that there are leptons still waiting to be found. These “fourth generation” particles, if they are indeed real, would exist beyond the Standard Model of particle physics, and would likely interact with matter in even more exotic ways.

We have written many interesting articles about Leptons and subatomic particles here at Universe Today. Here’s What are Subatomic Particles?, What are Baryons?First Collisions of the LHC, Two New Subatomic Particles Found, and Physicists Maybe, Just Maybe, Confirm the Possible Discovery of 5th Force of Nature.

For more information, SLAC’s Virtual Visitor Center has a good introduction to Leptons and be sure to check out the Particle Data Group (PDG) Review of Particle Physics.

Astronomy Cast also has episodes on the topic. Here’s Episode 106: The Search for the Theory of Everything, and Episode 393: The Standard Model – Leptons & Quarks.


What Are The Parts Of An Atom?

A depiction of the atomic structure of the helium atom. Credit: Creative Commons

Since the beginning of time, human beings have sought to understand what the universe and everything within it is made up of. And while ancient magi and philosophers conceived of a world composed of four or five elements – earth, air, water, fire (and metal, or consciousness) – by classical antiquity, philosophers began to theorize that all matter was actually made up of tiny, invisible, and indivisible atoms.

Since that time, scientists have engaged in a process of ongoing discovery with the atom, hoping to discover its true nature and makeup. By the 20th century, our understanding became refined to the point that we were able to construct an accurate model of it. And within the past decade, our understanding has advanced even further, to the point that we have come to confirm the existence of almost all of its theorized parts.

Continue reading “What Are The Parts Of An Atom?”

Shape-shifting neutrinos earn physicists the 2015 Nobel

Super-Kamiokande, a neutrino detector in Japan, holds 50,000 tons of ultrapure water surrounded by light tubes. Credit: Super-Kamiokande Observatory

What do Albert Einstein, Neils Bohr, Paul Dirac, and Marie Curie have in common? They each won the Nobel prize in physics. And today, Takaaki Kajita and Arthur McDonald have joined their ranks, thanks to a pioneering turn-of-the-century discovery: in defiance of long-held predictions, neutrinos shape-shift between multiple identities, and therefore must have mass.

The neutrino, a slight whiff of a particle that is cast off in certain types of radioactive decay, nuclear reactions, and high-energy cosmic events, could be called… shy. Electrically neutral but enormously abundant, half the time a neutrino could pass through a lightyear of lead without interacting with a single other particle. According to the Standard Model of particle physics, it has a whopping mass of zero.

As you can imagine, neutrinos are notoriously difficult to detect.

But in 1956, scientists did exactly that. And just a few years later, a trio of physicists determined that neutrinos came in not just one, not two, but three different types, or flavors: the electron neutrino, the muon neutrino, and the tau neutrino.

The first annotated neutrino event. Image credit:
The neutrino was first detected in 1956 by Clyde Cowan and Frederick Reines. In 1970, scientists captured the first image of a neutrino track in a hydrogen bubble chamber. Image: Argonne National Laboratory

But there was a problem. Sure, scientists had figured out how to detect neutrinos—but they weren’t detecting enough of them. In fact, the number of electron neutrinos arriving on Earth due to nuclear reactions in the Sun’s core was only one-third to one-half the number their calculations had predicted. What, scientists wondered, was happening to the rest?

Kajita, working at the Super-Kamiokande detector in Japan in 1998, and McDonald, working at the Sudbury Neutrino Observatory in Canada in 1999, determined that the electron neutrinos were not disappearing at all; rather, these particles were changing identity, spontaneously oscillating between the three flavor-types as they traveled through space.

Moreover, the researchers proclaimed, in order for neutrinos to make such transformations, they must have mass.

This is due to some quantum funny business having to do with the oscillations themselves. Grossly simplified, a massless particle, which always travels at the speed of light, does not experience time—Einstein’s theory of special relativity says so. But change takes time. Any particle that oscillates between identities needs to experience time in order for its state to evolve from one flavor to the the next.

The interior structure of the Sun. Credit: Wikipedia Commons/kelvinsong
Neutrinos are produced in abundance during fusion reactions at the center of our Sun, and oscillate between three different types, or flavors, on their way to Earth. Image: Wikipedia Commons/kelvinsong

Kajita and McDonald’s work showed that neutrinos must have a mass, albeit a very small one. But neutrinos are abundant in the Universe, and even a small mass has a large effect on all sorts of cosmic phenomena, from solar nuclear physics, where neutrinos are produced en masse, to the large-scale evolution of the cosmos, where neutrinos are ubiquitous.

The neutrino, no longer massless, is now considered to play a much larger role in these processes than scientists had originally believed.

What is more, the very existence of a massive neutrino undermines the theoretical basis of the Standard Model. In fact, Kajita’s and McDonald’s discovery provided some of the first evidence that the Standard Model might not be as airtight as had been previously believed, nudging scientists ever more in the direction of so-called “new physics.”

This is not the first time physicists have been awarded a Nobel prize for research into the nature of neutrinos. In 1988, Leon Lederman, Melvin Schwartz, and Jack Steinberger were awarded the prize for their discovery that neutrinos come in three flavors; in 1995, Frederick Reines won a Nobel for his detection of the neutrino along with Clyde Cowan; and in 2002, a Nobel was awarded to Raymond David Jr., the oldest person ever to receive a the prize in physics, and Masatoshi Koshiba for their detection of cosmic neutrinos.

Kajita, of the University of Tokyo, and McDonald, of Queen’s University in Canada, were awarded the prestigious prize this morning at a news conference in Stockholm.

How are Energy and Matter the Same?

Picture of Albert Einstein, Physicist Credit: Public Domain

As Einstein showed us, light and matter and just aspects of the same thing. Matter is just frozen light. And light is matter on the move. How does one become the other?

Albert Einstein’s most famous equation says that energy and matter are two sides of the same coin.
But what does that really mean? And how are equations famous? I like to believe equations can be famous in the way a work of art, or a philosophy can be famous. People can have awareness of the thing, and yet never have interacted with it. They can understand that it is important, and yet not understand why it’s so significant. Which is a little too bad, as this is really a lovely mind bending idea.

The origin of E=mc2 lies in special relativity. Light has the same speed no matter what frame of reference you are in. No matter where you are, or how fast you’re going. If you were standing still at the side of the road, and observed a car traveling at ¾ light speed, you would see the light from their headlights traveling away from them at ¼ the speed of light.

But the driver of the car would still see that the light moving ahead of them at the speed of light. This is only possible if their time appears to slow down relative to you, and you and the people in the car can no longer agree on how long a second would take to pass.

Einstein's famous equation. Image via Pixabay.
Einstein’s famous equation. Image via Pixabay.

So the light appears to be moving away from them more slowly, but as they experience things more slowly it all evens out. This also affects their apparent mass. If they step on the gas, they will speed up more slowly than you would expect. It’s as if the car has more mass than you expect. So relativity requires that the faster an object moves, the more mass it appears to have. This means that somehow part of the energy of the car’s motion appears to transform into mass. Hence the origin of Einstein’s equation. How does that happen? We don’t really know. We only know that it does.

The same effect occurs with quantum particles, and not just with light. A neutron, for example, can decay into a proton, electron and anti-neutrino. The mass of these three particles is less than the mass of a neutron, so they each get some energy as well. So energy and matter are really the same thing. Completely interchangeable. And finally, Although energy and mass are related through special relativity, mass and space are related through general relativity. You can define any mass by a distance known as its Schwarzschild radius, which is the radius of a black hole of that mass. So in a way, energy, matter, space and time are all aspects of the same thing.

What do you think? Like E=mc2, what’s the most famous idea you can think of in physics?

And if you like what you see, come check out our Patreon page and find out how you can get these videos early while helping us bring you more great content!

NOvA Experiment Nabs Its First Neutrinos

The NUmI (Neutrinos from the Main Injector) horn at Fermilab, which fires protons that degrade into neutrinos. (Image: Caltech)

Neutrinos are some of the most abundant, curious, and elusive critters in particle physics. Incredibly lightweight — nigh massless, according to the Standard Model — as well as chargeless, they zip around the Universe at the speed of light and they don’t interact with any other particles. Some of them have been around since the Big Bang and, just as you’ve read this, trillions of them have passed through your body (and more are on the way.) But despite their ubiquitousness neutrinos are notoriously difficult to study precisely because they ignore pretty much everything made out of anything else. So it’s not surprising that weighing a neutrino isn’t as simple as politely asking one to step on a scale.

Thankfully particle physicists are a tenacious lot, including the ones at the U.S. Department of Energy’s Fermilab, and they aren’t giving up on their latest neutrino safari: the NuMI Off-Axis Electron Neutrino Appearance experiment, or NOvA. (Scientists represent neutrinos with the Greek letter nu, or v.) It’s a very small-game hunt to catch neutrinos on the fly, and it uses some very big equipment to do the job. And it’s already captured its first neutrinos — even before their setup is fully complete.

Created by smashing protons against graphite targets in Fermilab’s facility just outside Chicago, Illinois, resulting neutrinos are collected and shot out in a beam 500 miles northwest to the NOvA far detector in Ash River, Minnesota, located along the Canadian border. The very first beams were fired in Sept. 2013, while the Ash River facility was still under construction.

One of the first detections by NOvA of Fermilab-made neutrinos (Image courtesy of NOvA collaboration)
One of the first detections by NOvA of Fermilab-made neutrinos (Image courtesy of NOvA collaboration)

“That the first neutrinos have been detected even before the NOvA far detector installation is complete is a real tribute to everyone involved,” said University of Minnesota physicist Marvin Marshak, Ash River Laboratory director. “This early result suggests that the NOvA collaboration will make important contributions to our knowledge of these particles in the not so distant future.”

The 500-mile (800 km) path of the NOvA neutrino beam (Fermilab)
The 500-mile (800 km) subterranean path of the NOvA neutrino beam (Fermilab)

The beams from Fermilab are fired in two-second intervals, each sending billions of neutrinos directly toward the detectors. The near detector at Fermilab confirms the initial “flavor” of neutrinos in the beam, and the much larger far detector then determines if the neutrinos have changed during their three-millisecond underground interstate journey.

Again, because neutrinos don’t readily interact with ordinary particles, the beams can easily travel straight through the ground between the facilities — despite the curvature of the Earth. In fact the beam, which starts out 150 feet (45 meters) below ground near Chicago, eventually passes over 6 miles (10 km) deep during its trip.

According to a press release from Fermilab, neutrinos “come in three types, called flavors (electron, muon, or tau), and change between them as they travel. The two detectors of the NOvA experiment are placed so far apart to give the neutrinos the time to oscillate from one flavor to another while traveling at nearly the speed of light. Even though only a fraction of the experiment’s larger detector, called the far detector, is fully built, filled with scintillator and wired with electronics at this point, the experiment has already used it to record signals from its first neutrinos.”

The 50-foot (15 m) tall detector blocks are filled with a liquid scintillator that’s made of 95% mineral oil and 5% liquid hydrocarbon called pseudocumene, which is toxic but “imperative to the neutrino-detecting process.”  The mixture magnifies any light that hits it, allowing the neutrino strikes to be more easily detected and measured. (Source)

“NOvA represents a new generation of neutrino experiments,” said Fermilab Director Nigel Lockyer. “We are proud to reach this important milestone on our way to learning more about these fundamental particles.”

One of NOvA's 28 detectors  (Fermilab)
One of NOvA’s 28 far detector blocks (Fermilab)

After completion this summer NOvA’s near and far detectors will weigh 300 and 14,000 tons, respectively.

The goal of the NOvA experiment is to successfully capture and measure the masses of the different neutrino flavors and also determine if neutrinos are their own antiparticles (they could be the same, since they lack  specific charge.) By comparing the oscillations (i.e., flavor changes) of muon neutrino beams vs. muon antineutrino beams fired from Fermilab, scientists hope to determine their mass hierarchy — and ultimately discover why the Universe currently contains much more matter than antimatter.

Read more: Neutrino Detection Could Help Paint an Entirely New Picture of the Universe

Once the experiment is fully operational scientists expect to catch a precious few neutrinos every day — about 5,000 total over the course of its six-year run. Until then, they at least now have their first few on the books.

“Seeing neutrinos in the first modules of the detector in Minnesota is a major milestone. Now we can start doing physics.”
– Rick Tesarek, Fermilab physicist

Learn more about the development and construction of the NoVA experiment below:

(Video credit: Fermilab)

Find out more about the NOvA research goals here.

Source: Fermilab press release

The NOvA collaboration is made up of 208 scientists from 38 institutions in the United States, Brazil, the Czech Republic, Greece, India, Russia and the United Kingdom. The experiment receives funding from the U.S. Department of Energy, the National Science Foundation and other funding agencies.

Polar Telescope Casts New Light On Dark Energy And Neutrino Mass

The 10-meter South Pole Telescope in Antarctica at the Amundsen-Scott Station. (Daniel Luong-Van, National Science Foundation)


Located at the southermost point on Earth, the 280-ton, 10-meter-wide South Pole Telescope has helped astronomers unravel the nature of dark energy and zero in on the actual mass of neutrinos — elusive subatomic particles that pervade the Universe and, until very recently, were thought to be entirely without measureable mass.

The NSF-funded South Pole Telescope (SPT) is specifically designed to study the secrets of dark energy, the force that purportedly drives the incessant (and apparently still accelerating) expansion of the Universe. Its millimeter-wave observation abilities allow scientists to study the Cosmic Microwave Background (CMB) which pervades the night sky with the 14-billion-year-old echo of the Big Bang.

Overlaid upon the imprint of the CMB are the silhouettes of distant galaxy clusters — some of the most massive structures to form within the Universe. By locating these clusters and mapping their movements with the SPT, researchers can see how dark energy — and neutrinos — interact with them.

“Neutrinos are amongst the most abundant particles in the universe,” said Bradford Benson, an experimental cosmologist at the University of Chicago’s Kavli Institute for Cosmological Physics. “About one trillion neutrinos pass through us each second, though you would hardly notice them because they rarely interact with ‘normal’ matter.”

If neutrinos were particularly massive, they would have an effect on the large-scale galaxy clusters observed with the SPT. If they had no mass, there would be no effect.

The SPT collaboration team’s results, however, fall somewhere in between.

Even though only 100 of the 500 clusters identified so far have been surveyed, the team has been able to place a reasonably reliable preliminary upper limit on the mass of neutrinos — again, particles that had once been assumed to have no mass.

Previous tests have also assigned a lower limit to the mass of neutrinos, thus narrowing the anticipated mass of the subatomic particles to between 0.05 – 0.28 eV (electron volts). Once the SPT survey is completed, the team expects to have an even more confident result of the particles’ masses.

“With the full SPT data set we will be able to place extremely tight constraints on dark energy and possibly determine the mass of the neutrinos,” said Benson.

“We should be very close to the level of accuracy needed to detect the neutrino masses,” he noted later in an email to Universe Today.

The South Pole Telescope's unique position allows it to watch the night sky for months on end. (NSF)

Such precise measurements would not have been possible without the South Pole Telescope, which has the ability due to its unique location to observe a dark sky for very long periods of time. Antarctica also offers SPT a stable atmosphere, as well as very low levels of water vapor that might otherwise absorb faint millimeter-wavelength signals.

“The South Pole Telescope has proven to be a crown jewel of astrophysical research carried out by NSF in the Antarctic,” said Vladimir Papitashvili, Antarctic Astrophysics and Geospace Sciences program director at NSF’s Office of Polar Programs. “It has produced about two dozen peer-reviewed science publications since the telescope received its ‘first light’ on Feb. 17, 2007. SPT is a very focused, well-managed and amazing project.”

The team’s findings were presented by Bradford Benson at the American Physical Society meeting in Atlanta on April 1.

Read more on the NSF press release here.

Underwater Neutrino Detector Will Be Second-Largest Structure Ever Built

Artist's rendering of the KM3NeT array. (Marco Kraan/Property KM3NeT Consortium)


The hunt for elusive neutrinos will soon get its largest and most powerful tool yet: the enormous KM3NeT telescope, currently under development by a consortium of 40 institutions from ten European countries. Once completed KM3NeT will be the second-largest structure ever made by humans, after the Great Wall of China, and taller than the Burj Khalifa in Dubai… but submerged beneath 3,200 feet of ocean!

KM3NeT – so named because it will encompass an area of several cubic kilometers – will be composed of lengths of cable holding optical modules on the ends of long arms. These modules will stare at the sea floor beneath the Mediterranean in an attempt to detect the impacts of neutrinos traveling down from deep space.

Successfully spotting neutrinos – subatomic particles that don’t interact with “normal” matter very much at all, nor have magnetic charges – will help researchers to determine which direction they originated from. That in turn will help them pinpoint distant sources of powerful radiation, like quasars and gamma-ray bursts. Only neutrinos could make it this far and this long after such events since they can pass basically unimpeded across vast cosmic distances.

“The only high energy particles that can come from very distant sources are neutrinos,” said Giorgio Riccobene, a physicist and staff researcher at the National Institute for Nuclear Physics. “So by looking at them, we can probe the far and violent universe.”

Each Digital Optical Module (DOM) is a standalone sensor module with 31 3-inch PMTs in a 17-inch glass sphere.

In effect, by looking down beneath the sea KM3NeT will allow scientists to peer outward into the Universe, deep into space as well as far back in time.

The optical modules dispersed along the KM3NeT array will be able to identify the light given off by muons when neutrinos pass into the sea floor. The entire structure would have thousands of the modules (which resemble large versions of the hovering training spheres used by Luke Skywalker in Star Wars.)

In addition to searching for neutrinos passing through Earth, KM3NeT will also look toward the galactic center and search for the presence of neutrinos there, which would help confirm the purported existence of dark matter.

Read more about the KM3NeT project here, and check out a detailed article on the telescope and neutrinos on Popsci.com.

Height of the KM3NeT telescope structure compared to well-known buildings

Images property of KM3NeT Consortium 

Breaking the Speed of Light

Particle Collider
Today, CERN announced that the LHCb experiment had revealed the existence of two new baryon subatomic particles. Credit: CERN/LHC/GridPP


It’s been a tenet of the standard model of physics for over a century. The speed of light is a unwavering and unbreakable barrier, at least by any form of matter and energy we know of. Nothing in our Universe can travel faster than 299,792 km/s (186,282 miles per second), not even – as the term implies – light itself. It’s the universal constant, the “c” in Einstein’s E = mc2, a cosmic speed limit that can’t be broken.

That is, until now.

An international team of scientists at the Gran Sasso research facility outside of Rome announced today that they have clocked neutrinos traveling faster than the speed of light. The neutrinos, subatomic particles with very little mass, were contained within beams emitted from CERN 730 km (500 miles) away in Switzerland. Over a period of three years, 15,000 neutrino beams were fired from CERN at special detectors located deep underground at Gran Sasso. Where light would have made the trip in 2.4 thousandths of a second, the neutrinos made it there 60 nanoseconds faster – that’s 60 billionths of a second – a tiny difference to us but a huge difference to particle physicists!

The implications of such a discovery are staggering, as it would effectively undermine Einstein’s theory of relativity and force a rewrite of the Standard Model of physics.

The OPERA Neutrino Detector. Credit: LGNS.

“We are shocked,” said project spokesman and University of Bern physicist Antonio Ereditato.

“We have high confidence in our results. We have checked and rechecked for anything that could have distorted our measurements but we found nothing. We now want colleagues to check them independently.”

Neutrinos are created naturally from the decay of radioactive materials and from reactions that occur inside stars. Neutrinos are constantly zipping through space and can pass through solid material easily with little discernible effect… as you’ve been reading this billions of neutrinos have already passed through you!

The experiment, called OPERA (Oscillation Project with Emulsion-tRacking Apparatus) is located in Italy’s Gran Sasso facility 1,400 meters (4,593 feet) underground and uses a complex array of electronics and photographic plates to detect the particle beams. Its subterranean location helps prevent experiment contamination from other sources of radiation, such as cosmic rays. Over 750 scientists from 22 countries around the world work there.

Ereditato is confident in the results as they have been consistently measured in over 16,000 events over the past two years. Still, other experiments are being planned elsewhere in an attempt to confirm these remarkable findings. If they are confirmed, we may be looking at a literal breakdown of the modern rules of physics as we know them!

“We have high confidence in our results,” said Ereditato. “We have checked and rechecked for anything that could have distorted our measurements but we found nothing. We now want colleagues to check them independently.”

A preprint of the OPERA results will be posted on the physics website ArXiv.org.

Read more on the Nature article here and on Reuters.com.

UPDATE: The OPERA team paper can be found here.



Cosmologists Provide Closest Measure of Elusive Neutrino

Slices through the SDSS 3-dimensional map of the distribution of galaxies. Earth is at the center, and each point represents a galaxy, typically containing about 100 billion stars. Galaxies are colored according to the ages of their stars, with the redder, more strongly clustered points showing galaxies that are made of older stars. The outer circle is at a distance of two billion light years. The region between the wedges was not mapped by the SDSS because dust in our own Galaxy obscures the view of the distant universe in these directions. Both slices contain all galaxies within -1.25 and 1.25 degrees declination. Credit: M. Blanton and the Sloan Digital Sky Survey.


Cosmologists – and not particle physicists — could be the ones who finally measure the mass of the elusive neutrino particle. A group of cosmologists have made their most accurate measurement yet of the mass of these mysterious so-called “ghost particles.” They didn’t use a giant particle detector but used data from the largest survey ever of galaxies, the Sloan Digital Sky Survey. While previous experiments had shown that neutrinos have a mass, it is thought to be so small that it was very hard to measure. But looking at the Sloan data on galaxies, PhD student Shawn Thomas and his advisers at University College London put the mass of a neutrino at no greater than 0.28 electron volts, which is less than a billionth of the mass of a single hydrogen atom. This is one of the most accurate measurements of the mass of a neutrino to date.

Their work is based on the principle that the huge abundance of neutrinos (there are trillions passing through you right now) has a large cumulative effect on the matter of the cosmos, which naturally forms into “clumps” of groups and clusters of galaxies. As neutrinos are extremely light they move across the universe at great speeds which has the effect of smoothing this natural “clumpiness” of matter. By analysing the distribution of galaxies across the universe (i.e. the extent of this “smoothing-out” of galaxies) scientists are able to work out the upper limits of neutrino mass.

A neutrino is capable of passing through a light year –about six trillion miles — of lead without hitting a single atom.

Central to this new calculation is the existence of the largest ever 3D map of galaxies, called Mega Z, which covers over 700,000 galaxies recorded by the Sloan Digital Sky Survey and allows measurements over vast stretches of the known universe.

“Of all the hypothetical candidates for the mysterious Dark Matter, so far neutrinos provide the only example of dark matter that actually exists in nature,” said Ofer Lahav, Head of UCL’s Astrophysics Group. “It is remarkable that the distribution of galaxies on huge scales can tell us about the mass of the tiny neutrinos.”

The Cosmologists at UCL were able to estimate distances to galaxies using a new method that measures the colour of each of the galaxies. By combining this enormous galaxy map with information from the temperature fluctuations in the after-glow of the Big Bang, called the Cosmic Microwave Background radiation, they were able to put one of the smallest upper limits on the size of the neutrino particle to date.

“Although neutrinos make up less than 1% of all matter they form an important part of the cosmological model,” said Dr. Shaun Thomas. “It’s fascinating that the most elusive and tiny particles can have such an effect on the Universe.”

“This is one of the most effective techniques available for measuring the neutrino masses,” said Dr. Filipe Abadlla. “This puts great hopes to finally obtain a measurement of the mass of the neutrino in years to come.”

The authors are confident that a larger survey of the Universe, such as the one they are working on called the international Dark Energy Survey, will yield an even more accurate weight for the neutrino, potentially at an upper limit of just 0.1 electron volts.
The results are published in the journal Physical Review Letters.

Source: University College London


IceCube neutrino detector.

The antineutrino (or anti-neutrino) is a lepton, an antimatter particle, the counterpart to the neutrino.

Actually, there are three distinct antineutrinos, called types, or flavors: electron antineutrino (symbol ̅νe), muon antineutrino (symbol ̅νμ), and tau antineutrino (symbol ̅ντ).

Beta Decay which produces electrons also produces (electron) antineutrinos. Wolfgang Pauli proposed the existence of these particles, in 1930, to ensure that beta decay conserved energy (the electrons in beta decay have a continuum of energies) and momentum (the momentum of the electron and recoil nucleus – in beta decay – do not add up to zero); Enrico Fermi – who developed the first theory of beta decay – coined the word ‘neutrino’, in 1934 (it’s actually a pun, in Italian!). It would be a quarter of a century before the (electron) antineutrino was confirmed, via direct detection (Cowan and Reines did the experiment, in 1956, and later got a Nobel Prize for it).

Another Nobel Prize – for Leon Lederman, Melvin Schwartz, and Jack Steinberger, in 1988 – came from experimental work in the 1960s which showed that muon antineutrinos are not the same as electron antineutrinos.

And in 2002, Davis and Koshiba shared the Nobel Prize (with Giacconi, for work in x-ray astronomy) for their detection of cosmic antineutrinos (a 40-year task!), which lead to the discovery of flavor oscillations (in which an antineutrino of one kind changes into another – electron antineutrino to muon antineutrino, for example).

Are neutrinos their own antiparticles? No … but perhaps there is an as yet undiscovered kind of neutrino that is (called a Majorana neutrino)? So β (electron) decay produces antineutrinos (lepton number is conserved: 1 + (-1) = 0), and β+ (positron) decay produces neutrinos.

No Guide to Space article would be complete without some ‘Further Reading’, would it? KamLAND (the Kamioka Liquid-scintillator Anti-Neutrino Detector) is a wonderful place to start! For one of the greatest physics detective stories of the 20th century, check out my idol John Bahcall’s webpage. Applied Antineutrino Physics (Lawrence Livermore National Laboratory) – great stuff there too.

You won’t find ‘antineutrino’ in many Universe Today articles … but you’ll find plenty on neutrinos! That’s OK … remember that it’s very common to use the word ‘neutrino’ in a generic sense, one that includes the meaning ‘antineutrino’. Some examples: Neutrino Evidence Confirms Big Bang Predictions , Seeing Inside the Earth with Neutrinos, and Do Advanced Civilizations Communicate with Neutrinos?

Two Astronomy Cast episodes give you more insight into the antineutrino, Antimatter, and The Search for Neutrinos.

Stanford University KamLAND