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Posts Tagged ‘neutrinos’

Costumes to make zombie Einstein proud

Wednesday, October 29th, 2014

This article appeared in symmetry on Oct. 21, 2014.

These physics-themed Halloween costume ideas are sure to entertain—and maybe even educate. Terrifying, we know. Image: Sandbox Studio, Chicago with Corinne Mucha

These physics-themed Halloween costume ideas are sure to entertain—and maybe even educate. Terrifying, we know. Image: Sandbox Studio, Chicago with Corinne Mucha


So you haven’t picked a Halloween costume, and the big night is fast approaching. If you’re looking for something a little funny, a little nerdy and sure to impress fellow physics fans, look no further. We’ve got you covered.

1. Dark energy

This is an active costume, perfect for the party-goer who plans to consume a large quantity of sugar. Suit up in all black or camouflage, then spend your evening squeezing between people and pushing them apart.

Congratulations! You’re dark energy: a mysterious force causing the accelerating expansion of the universe, intriguing in the lab and perplexing on the dance floor.

2. Cosmic inflation

Theory says that a fraction of a second after the big bang, the universe grew exponentially, expanding so that tiny fluctuations were stretched into the seeds of entire galaxies.

But good luck getting that costume through the door.

Instead, take a simple yellow life vest and draw the cosmos on it: stars, planets, asteroids, whatever you fancy. When friends pull on the emergency tab, the universe will grow.

3. Heisenberg Uncertainty Principle

Here’s a great excuse to repurpose your topical Breaking Bad costume from last year.

Walter White—aka “Heisenberg”—may have been a chemistry teacher, but the Heisenberg Uncertainty Principle is straight out of physics. Named after Werner Heisenberg, a German physicist credited with the creation of quantum mechanics, the Heisenberg Uncertainty Principle states that the more accurately you know the position of a particle, the less information you know about its momentum.

Put on Walter White’s signature hat and shades (or his yellow suit and respirator), but then add some uncertainty by pasting Riddler-esque question marks to your outfit.

4. Bad neutrino

A warning upfront: Only the ambitious and downright extroverted should attempt this costume.

Neutrinos are ghostly particles that pass through most matter undetected. In fact, trillions of neutrinos pass through your body every second without your knowledge.

But you aren’t going to go as any old neutrino. Oh no. You’re a bad neutrino—possibly the worst one in the universe—so you run into everything: lampposts, trees, haunted houses and yes, people. Don a simple white sheet and spend the evening interacting with everyone and everything.

5. Your favorite physics experiment

You physics junkies know that there are a lot of experiments with odd acronyms and names that are ripe for Halloween costumes. You can go as ATLAS (experiment at the Large Hadron Collider / character from Greek mythology), DarkSide (dark matter experiment at Gran Sasso National Laboratory / good reason to repurpose your Darth Vader costume), PICASSO (dark matter experiment at SNOLAB / creator of Cubism), MINERvA (Fermilab neutrino experiment / Roman goddess of wisdom), or the Dark Energy Survey (dark energy camera located at the Blanco Telescope in Chile / good opportunity for a pun).

Physics-loving parents can go as explorer Daniel Boone, while the kids go as neutrino experiments MicroBooNE and MiniBooNE. The kids can wear mini fur hats of their own or dress as detector tanks to be filled with candy.

6. Feynman diagram

You might know that a Feynman diagram is a drawing that uses lines and squiggles to represent a particle interaction. But have you ever noticed that they sometimes look like people? Try out this new take on the black outfit/white paint skeleton costume. Bonus points for going as a penguin diagram.

7. Antimatter

Break out the bell-bottoms and poster board. In bold letters, scrawl the words of your choosing: “I hate things!,” “Stuff is awful!,” and “Down with quarks!” will all do nicely. Protest from house to house and declare with pride that you are antimatter. It’s a fair critique: Physicists still aren’t sure why matter dominates the universe when equal amounts of matter and antimatter should have been created in the big bang.

Fortunately, you don’t have to solve this particular puzzle on your quest for candy. Just don’t high five anyone; you might annihilate.

8. Entangled particles

Einstein described quantum entanglement as “spooky action at a distance”—the perfect costume for Halloween. Entangled particles are extremely strange. Measuring one automatically determines the state of the other, instantaneously.

Find someone you are extremely in tune with and dress in opposite colors, like black and white. When no one is observing you, you can relax. But when interacting with people, be sure to coordinate movements. They spin to the left, you spin to the right. They wave with the right hand? You wave with the left. You get the drill.

You can also just wrap yourselves together in a net. No one said quantum entanglement has to be hard.

9. Holographic you(niverse)

The universe may be like a hologram, according to a theory currently being tested at Fermilab’s Holometer experiment. If so, information about spacetime is chunked into 2-D bits that only appear three-dimensional from our perspective.

Help others imagine this bizarre concept by printing out a photo of yourself and taping it to your front. You’ll still technically be 3-D, but that two-dimensional picture of your face will still start some interesting discussions. Perhaps best not to wear this if you have a busy schedule or no desire to discuss the nature of time and space while eating a Snickers.

10. Your favorite particle

There are many ways to dress up as a fundamental particle. Bring a lamp along to trick-or-treat to go as the photon, carrier of light. Hand out cookies to go as the Higgs boson, giver of mass. Spend the evening attaching things to people to go as a gluon.

To branch out beyond the Standard Model of particle physics, go as a supersymmetric particle, or sparticle: Wear a gladiator costume and shout, “I am Sparticle!” whenever someone asks about your costume.

Or grab a partner to become a meson, a particle made of a quark and antiquark. Mesons are typically unstable, so whenever you unlink arms, be sure to decay in a shower of electrons and neutrinos—or candy corn.

Lauren Biron


This Fermilab press release came out on Oct. 6, 2014.

With construction completed, the NOvA experiment has begun its probe into the mysteries of ghostly particles that may hold the key to understanding the universe. Image: Fermilab/Sandbox Studio

With construction completed, the NOvA experiment has begun its probe into the mysteries of ghostly particles that may hold the key to understanding the universe. Image: Fermilab/Sandbox Studio

It’s the most powerful accelerator-based neutrino experiment ever built in the United States, and the longest-distance one in the world. It’s called NOvA, and after nearly five years of construction, scientists are now using the two massive detectors – placed 500 miles apart – to study one of nature’s most elusive subatomic particles.

Scientists believe that a better understanding of neutrinos, one of the most abundant and difficult-to-study particles, may lead to a clearer picture of the origins of matter and the inner workings of the universe. Using the world’s most powerful beam of neutrinos, generated at the U.S. Department of Energy’s Fermi National Accelerator Laboratory near Chicago, the NOvA experiment can precisely record the telltale traces of those rare instances when one of these ghostly particles interacts with matter.

Construction on NOvA’s two massive neutrino detectors began in 2009. In September, the Department of Energy officially proclaimed construction of the experiment completed, on schedule and under budget.

“Congratulations to the NOvA collaboration for successfully completing the construction phase of this important and exciting experiment,” said James Siegrist, DOE associate director of science for high energy physics. “With every neutrino interaction recorded, we learn more about these particles and their role in shaping our universe.”

NOvA’s particle detectors were both constructed in the path of the neutrino beam sent from Fermilab in Batavia, Illinois, to northern Minnesota. The 300-ton near detector, installed underground at the laboratory, observes the neutrinos as they embark on their near-light-speed journey through the Earth, with no tunnel needed. The 14,000-ton far detector — constructed in Ash River, Minnesota, near the Canadian border – spots those neutrinos after their 500-mile trip and allows scientists to analyze how they change over that long distance.

For the next six years, Fermilab will send tens of thousands of billions of neutrinos every second in a beam aimed at both detectors, and scientists expect to catch only a few each day in the far detector, so rarely do neutrinos interact with matter.

From this data, scientists hope to learn more about how and why neutrinos change between one type and another. The three types, called flavors, are the muon, electron and tau neutrino. Over longer distances, neutrinos can flip between these flavors. NOvA is specifically designed to study muon neutrinos changing into electron neutrinos. Unraveling this mystery may help scientists understand why the universe is composed of matter and why that matter was not annihilated by antimatter after the big bang.

Scientists will also probe the still-unknown masses of the three types of neutrinos in an attempt to determine which is the heaviest.

“Neutrino research is one of the cornerstones of Fermilab’s future and an important part of the worldwide particle physics program,” said Fermilab Director Nigel Lockyer. “We’re proud of the NOvA team for completing the construction of this world-class experiment, and we’re looking forward to seeing the first results in 2015.”

The far detector in Minnesota is believed to be the largest free-standing plastic structure in the world, at 200 feet long, 50 feet high and 50 feet wide. Both detectors are constructed from PVC and filled with a scintillating liquid that gives off light when a neutrino interacts with it. Fiber optic cables transmit that light to a data acquisition system, which creates 3-D pictures of those interactions for scientists to analyze.

The NOvA far detector in Ash River saw its first long-distance neutrinos in November 2013. The far detector is operated by the University of Minnesota under an agreement with Fermilab, and students at the university were employed to manufacture the component parts of both detectors.

“Building the NOvA detectors was a wide-ranging effort that involved hundreds of people in several countries,” said Gary Feldman, co-spokesperson of the NOvA experiment. “To see the construction completed and the operations phase beginning is a victory for all of us and a testament to the hard work of the entire collaboration.”

The NOvA collaboration comprises 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.

For more information, visit the experiment’s website: http://www-nova.fnal.gov.

Note: NOvA stands for NuMI Off-Axis Electron Neutrino Appearance. NuMI is itself an acronym, standing for Neutrinos from the Main Injector, Fermilab’s flagship accelerator.

Fermilab is America’s premier national laboratory for particle physics and accelerator research. A U.S. Department of Energy Office of Science laboratory, Fermilab is located near Chicago, Illinois, and operated under contract by the Fermi Research Alliance, LLC. Visit Fermilab’s website at www.fnal.gov and follow us on Twitter at @FermilabToday.

The DOE Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.


This article appeared in Fermilab Today on Sept. 5, 2014.

This aerial view shows the Neutrino Area under construction in May 1971. The 15-foot bubble chamber, pictured on the left, would later be moved to the present-day location of Lab D.  Photo: Fermilab

This aerial view shows the Neutrino Area under construction in May 1971. The 15-foot bubble chamber, pictured on the left, would later be moved to the present-day location of Lab B. Photo: Fermilab

It was called Target Station C. One of three stations north of Wilson Hall at the end of beamlines extending from the Main Ring (later replaced by the Tevatron), Target Station C was assigned to experiments that would require high beam intensities for investigating neutrino interactions, according to a 1968 design report.

Within a few years, Target Station C was officially renamed the Neutrino Area. It was the first named fixed-target area and the first to be fully operational. Neutrinos and the Intensity Frontier had an early relationship with Fermilab. But why is it resurfacing now?

“The experimental program is driven by the current state of knowledge, and that’s always changing,” said Jeffrey Appel, a retired Fermilab physicist and assistant laboratory director who started research at the lab in 1972.

When Appel first arrived, there was intense interest in neutrinos because the weak force was poorly understood, and neutral currents were still a controversial idea. Fermilab joined forces with many institutions both in and outside the United States, and throughout the 1970s and early 1980s, neutrinos generated from protons in the Main Ring crashed through a 15-foot bubble chamber filled with super-heated liquid hydrogen. Other experiments running in parallel recorded neutrino interactions in iron and scintillator.

“The goal was to look for the W and Z produced in neutrino interactions,” said Appel. “So the priority for getting the beam up first and the priority for getting the detectors built and installed was on that program in those days.”

It turns out that the W and Z bosons are too massive to have been produced this way and had to wait to be discovered at colliding-beam experiments. As soon as the Tevatron was ready for colliding beams in 1985, the transition began at Fermilab from fixed-target areas to high-energy particle colliding.

More recent revelations have shown that neutrinos have mass. These findings have raised new questions that need answers. In 1988, plans were laid to add the Main Injector to the Fermilab campus, partly to boost the capabilities of the Tevatron, but also, according to one report, because “intense beams of neutral kaons and neutrinos would provide a unique facility for CP violation and neutrino oscillation experiments.”

Although neutrino research was a smaller fraction of the lab’s program during Tevatron operations, it was far from dormant. Two great accomplishments in neutrino research occurred in this time period: One was the most precise neutrino measurement of the strength of the weak interaction by the NuTeV experiment. The other was when the DONUT experiment achieved its goal of making the first direct observation of the tau neutrino in 2000.

“In the ’90s most evidence of neutrinos changing flavors was coming from natural sources. But this inspired a whole new generation of accelerator-based neutrino experiments,” said Deborah Harris, co-spokesperson for the MINERvA neutrino experiment. “That’s when Fermilab changed gears to make lower-energy but very intense neutrino beams that were uniquely suited for oscillation physics.”

In partnership with institutions around the globe, Fermilab began planning and building a suite of neutrino experiments. MiniBooNE and MINOS started running in the early 2000s and MINERvA started in 2010. MicroBooNE and NOvA are starting their runs this year.

Now the lab is working with other institutions to establish a Long-Baseline Neutrino Facility at the laboratory and advance its short-baseline neutrino research program. As Fermilab strengthens its international partnerships in all its neutrino experiments, it is also working to position itself as the home of the world’s forefront neutrino research.

“The combination of the completion of the Tevatron program and the new questions about neutrinos means that it’s an opportune time to redefine the focus of Fermilab,” Appel explained.

“Everybody says: ‘It’s not like the old days,’ and it’s always true,” Appel said. “Experiments are bigger and more expensive, but people are just as excited about what they’re doing.”

He added, “It’s different now but just as exciting, if not more so.”

Troy Rummler

Special thanks go to Fermilab archivists Valerie Higgins and Adrienne Kolb for helping navigate Fermilab’s many resources on early neutrino research at the laboratory.


What are Sterile Neutrinos?

Sunday, July 27th, 2014

Sterile Neutrinos in Under 500 Words

Hi Folks,

In the Standard Model, we have three groups of particles: (i) force carriers, like photons and gluons; (ii) matter particles, like electrons, neutrinos and quarks; and (iii) the Higgs. Each force carrier is associated with a force. For example: photons are associated with electromagnetism, the W and Z bosons are associated with the weak nuclear force, and gluons are associated with the strong nuclear force. In principle, all particles (matter, force carries, the Higgs) can carry a charge associated with some force. If this is ever the case, then the charged particle can absorb or radiate a force carrier.

SM Credit: Wiki

Credit: Wikipedia

As a concrete example, consider electrons and top quarks. Electrons carry an electric charge of “-1″ and a top quark carries an electric charge of “+2/3″. Both the electron and top quark can absorb/radiate photons, but since the top quark’s electric charge is smaller than the electron’s electric charge, it will not absorb/emit a photon as often as an electron. In a similar vein, the electron carries no “color charge”, the charge associated with the strong nuclear force, whereas the top quark does carry color and interacts via the strong nuclear force. Thus, electrons have no idea gluons even exist but top quarks can readily emit/absorb them.

Neutrinos  possess a weak nuclear charge and hypercharge, but no electric or color charge. This means that neutrinos can absorb/emit W and Z bosons and nothing else.  Neutrinos are invisible to photons (particle of light) as well as gluons (particles of the color force).  This is why it is so difficult to observe neutrinos: the only way to detect a neutrino is through the weak nuclear interactions. These are much feebler than electromagnetism or the strong nuclear force.

Sterile neutrinos are like regular neutrinos: they are massive (spin-1/2) matter particles that do not possess electric or color charge. The difference, however, is that sterile neutrinos do not carry weak nuclear or hypercharge either. In fact, they do not carry any charge, for any force. This is why they are called “sterile”; they are free from the influences of  Standard Model forces.

Credit: somerandompearsonsblog.blogspot.com

Credit: somerandompearsonsblog.blogspot.com

The properties of sterile neutrinos are simply astonishing. For example: Since they have no charge of any kind, they can in principle be their own antiparticles (the infamous “sterile Majorana neutrino“). As they are not associated with either the strong nuclear scale or electroweak symmetry breaking scale, sterile neutrinos can, in principle, have an arbitrarily large/small mass. In fact, very heavy sterile neutrinos might even be dark matter, though this is probably not the case. However, since sterile neutrinos do have mass, and at low energies they act just like regular Standard Model neutrinos, then they can participate in neutrino flavor oscillations. It is through this subtle effect that we hope to find sterile neutrinos if they do exist.

Credit: Kamioka Observatory/ICRR/University of Tokyo

Credit: Kamioka Observatory/ICRR/University of Tokyo

Until next time!

Happy Colliding,

Richard (@bravelittlemuon)



This article appeared in symmetry on June 4, 2014.

Data collected at the long-running MINOS experiment stacks evidence against the existence of these theoretical particles. Photo: Reidar Hahn

Data collected at the long-running MINOS experiment stacks evidence against the existence of these theoretical particles. Photo: Reidar Hahn

If you’re searching for something that may not exist, and can pass right through matter if it does, then knowing where to look is essential.

That’s why the search for so-called sterile neutrinos is a process of elimination. Experiments like Fermilab’s MiniBooNE and the Liquid Scintillator Neutrino Detector (LSND) at Los Alamos National Laboratory have published results consistent with the existence of these theoretical particles. But a new result from the long-running MINOS experiment announced this week severely limits the area in which they could be found and casts more doubt on whether they exist at all.

Scientists have observed three types or “flavors” of neutrinos—muon, electron and tau neutrinos—through their interactions with matter. If there are other types, as some scientists have theorized, they do not interact with matter, and the search for them has become one of the hottest and most contentious topics in neutrino physics. MINOS, located at Fermilab with a far detector in northern Minnesota, has been studying neutrinos since 2005, with an eye toward collecting data on neutrino oscillation over long distances.

MINOS uses a beam of muon neutrinos generated at Fermilab. As that beam travels 500 miles through the earth to Minnesota, those muon neutrinos can change into other flavors.

MINOS looks at two types of neutrino interactions: neutral current and charged current. Since MINOS can see the neutral current interactions of all three known flavors of neutrino, scientists can tell if fewer of those interactions occur than they should, which would be evidence that the muon neutrinos have changed into a particle that does not interact. In addition, through charged current interactions, MINOS looks specifically at muon neutrino disappearance, which allows for a much more precise measurement of neutrino energies, according to João Coelho of Tufts University.

“Disappearance with an energy profile not described by the standard three-neutrino model would be evidence for the existence of an additional sterile neutrino,” Coelho says.

The new MINOS result, announced today at the Neutrino 2014 conference in Boston, excludes a large and previously unexplored region for sterile neutrinos. To directly compare the new results with previous results from LSND and MiniBooNE, MINOS combined its data with previous measurements of electron antineutrinos from the Bugey nuclear reactor in France. The combined result, says Justin Evans of the University of Manchester, “provides a strong constraint on the existence of sterile neutrinos.”

“The case for sterile neutrinos is still not closed,” Evans says, “but there is now a lot less space left for them to hide.”

Andre Salles

The vertical axis shows the possible mass regions for the sterile neutrinos. The horizontal axis shows how likely it is that a muon neutrino will turn into a sterile neutrino as it travels. The new MINOS result excludes everything to the right of the black line. The colored areas show limits by previous experiments. Image courtesy of MINOS collaboration

The vertical axis shows the possible mass regions for the sterile neutrinos. The horizontal axis shows how likely it is that a muon neutrino will turn into a sterile neutrino as it travels. The new MINOS result excludes everything to the right of the black line. The colored areas show limits by previous experiments.
Image courtesy of MINOS collaboration

This graph shows the combined MINOS/Bugey result (the red line) in comparison with the results from LSND and MiniBooNE (the green areas). The vertical axis shows the possible mass regions for sterile neutrinos. The new MINOS/Bugey result excludes everything to the right of the red line. Image courtesy of MINOS collaboration

This graph shows the combined MINOS/Bugey result (the red line) in comparison with the results from LSND and MiniBooNE (the green areas). The vertical axis shows the possible mass regions for sterile neutrinos. The new MINOS/Bugey result excludes everything to the right of the red line.
Image courtesy of MINOS collaboration


Today at the Neutrino2014 conference in Boston, the IceCube collaboration showed an analysis looking for standard atmospheric neutrino oscillations in the 20-30 GeV region. Although IceCube has seen oscillations before, and reported them in a poster at the last Neutrino conference, in 2012, this plenary talk showed the first analysis where the IceCube error bands are becoming competitive with other oscillation experiments.

IC86Multi_NuMuOsc_results_Pscan_V1Neutrino oscillation is a phenomenon where neutrinos change from one flavor to another as they travel; it’s a purely quantum phenomenon. It has been observed in several contexts, including particle accelerators, nuclear reactors, cosmic rays hitting the atmosphere, and neutrinos traveling from our Sun. This is the first widely accepted phenomenon in particle physics that requires an extension to the Standard Model, the capstone of which was the observation of the Higgs boson at CERN. Neutrinos and neutrino oscillations represent the next stage of particle physics, beyond the Higgs.

IC86Multi_NuMuOsc_results_LEOf the parameters used to describe neutrino oscillations, most have been previously measured. The mixing angles that describe oscillations are the most recent focus of measurement. Just two years ago, the last of the neutrino mixing angles was measured by the Daya Bay experiment. Of the remaining mixing angles, the atmospheric angle accessible to IceCube remains the least constrained by experimental measurements.  

IceCube, because of its size, is in a unique position to measure the atmospheric mixing angle. Considering neutrinos that traverse the diameter of the Earth, the oscillation effect is the strongest in the energy region from 20 to 30 GeV, and an experiment that can contain a 20 GeV neutrino interaction must be very large. The Super Kamiokande experiment in Japan, for example, also measures atmospheric oscillations, but because of its small size relative to IceCube, Super Kamiokande can’t resolve energies above a few GeV. At any higher energies, the detector is simply saturated. Other experiments can measure the same mixing angle using accelerator beamlines, like the MINOS experiment that sends neutrinos from Fermilab to Minnesota. Corroborating these observations from several experimental methods and separate experiments proves the strength of the oscillation framework.

The sheer size of IceCube means that neutrinos have many chances to interact and be observed within the detector, giving IceCube a statistical advantage over other oscillation experiments. Even after selecting only the best reconstructed events, the experimental sample remaining still has over five thousand events from three years of data. Previous atmospheric oscillation experiments base analysis on hundreds or fewer events, counting instead on precise understanding of systematic effects. 

The IceCube collaboration is composed of more than 250 scientists from about 40 institutions around the world, mostly from the United States and Europe. The current results are possible because of decades of planning and construction, dedicated detector operations, and precise calibrations from all over the IceCube collaboration.

IceCube has several major talks at the Neutrino conference this year, the first time that the collaboration has had such a prominent presence. In addition to the new oscillations result, Gary Hill spoke in the opening session about the high energy astrophysical neutrinos observed over the last few years. Darren Grant spoke about the proposed PINGU infill array, which was officially encouraged in the recent P5 report. IceCube contributed nine posters on far-ranging topics from calibration and reconstruction methods to a neutrino-GRB correlation search. The conference-inspired display at the MIT museum is about half IceCube material, including an 8-foot tall LED model of the detector. One of three public museum talks on Saturday will be from (yours truly) Laura Gladstone about the basics of IceCube science and life at the South Pole.

One new aspect of the new oscillation analysis is that it uses an energy reconstruction designed for the low end of the energy range available to IceCube, in the tens-of-GeV range. In this range, only a handful of hits are visible for each event, and reconstructing directional information can be tricky. “We took a simple but very clever idea from the ANTARES Collaboration, and rehashed it to tackle one of our biggest uncertainties: the optical properties of the ice. It turned out to work surprisingly well,” says IceCuber Juan Pablo Yanez Garza, who brought the new reconstruction to IceCube, and presented the result in Boston.  By considering only the detector hits that arrive without scattering, the reconstruction algorithm is more robust against systematic errors in the understanding of the glacial ice in which IceCube is built. 


Since IceCube was proposed, people have been claiming that you can get a new view of astrophysics by using particles instead of light, and we were pretty sure what the journey would look like. It hasn’t gone quite in the order we expected, but we’re getting that new view of astrophysics, and also, a few years later, filling in the steps we expected to fill first. When we find bits of scientific evidence in a different order than we expected, does that change how excited we get about them?

Sunrise over the IceCube laboritory

The sunrise at the South Pole over the IceCube laboratory, the central building on top of the IceCube Neutrino Observatory.

We been expanding astronomy since it started. First, astronomers used telescopes to resolve visible light better. Later, they expanded to different regions of the light spectrum like x-rays and gamma rays.  Then, it was a small step to expand from gamma rays, which are easier to think of as particles than as waves, to particles like the atomic nuclei that make up cosmic rays. Neutrinos are another kind of particle we can use for astronomy, and they have unique advantages and challenges.

The hard part about using neutrinos as a messenger between the stars and us is that neutrinos very rarely interact with matter. This means that if thousands pass through our detector, we might only see a few. There are some ways around this, and the biggest trick IceCube uses is to look in a very large volume. If we look for more neutrinos at a time, we have more of a chance of seeing the few that interact. The other trick is that we concentrate on high energies, where the neutrinos have a higher chance of interacting in our detector.

The great thing about using neutrinos as a messenger is that they hardly ever interact, so almost nothing can stop them from arriving at our door. If we see a neutrino in IceCube, it came to us directly from something interesting. We know that its direction wasn’t deflected in any magnetic fields, and it wasn’t dimmed by dust clouds or even asteroid clouds. Every (rare) time we see a high-energy neutrino, it tells us something about the stars, explosions, or black holes that created it.

That’s the story that people like Francis Halzen used to get funding for IceCube originally, and around Madison we still get to hear him tell this story, with his inimitable accent, when he speaks at museums or banquets.

Comparing neutrino astronomy to other new 20th century advances in astronomy, we expected the development of the field to follow a certain story.

We expected that first we would see a “diffuse” signal. This would be part of a large sample including a lot of background events, but some component would only be explained by including astrophysical sources. In IceCube, one of the best ways of reducing background noise is to look for events traveling up through the Earth, since only neutrinos can pass through the Earth. We could also look at high energies, since backgrounds like atmospheric neutrinos fall off exponentially with energy. So we thought the first diffuse astrophysics signal would come from the high-energy tail of an upgoing sample.

After that, we expected to resolve the diffuse sample into some clusters, and after a few of the clusters remained consistent, to declare them astrophysical sources.

What we did instead was to skip to the end of this story. We found astrophysical neutrinos first, and then a diffuse upgoing signal only two years after that (just this past spring). The exciting part about finding this recent diffuse signal isn’t that it’s the first detection of astrophysics, or even the strongest. It’s exciting because it follows the story we thought neutrino astronomy was going to follow.

The first detection was exciting too. That used a different kind of analysis: we identified only a few events (28 in two years) that were extremely likely to be from astrophysical sources. These were so special that each one got a name, using the theme of the Muppets, from Sesame Street and the Muppet Show. One is named Bert, one Ernie, one Mr. Snuffleupagus, one Oscar the Grouch. If we keep analyzing our data this way and eventually get enough events, we can expand to the Muppet Babies cartoons and various muppet movies, even including things like Labyrinth that used Jim Henson’s talents but not the muppets specifically. I’m personally a big fan of the muppet naming scheme, partly because it draws from a cannon recent enough that it includes several women and many kinds of diversity. When naming events is our biggest problem, it will be a great day for neutrino astrophysics. For formal publications, we usually say “HESE” for “High Energy Stating Event,” instead of “muppets.”

The two bedrock assumptions of the muppet analysis were that (1) we’re the most interested in the highest energy events, and (2) the events must have started within the detector; they must be “contained.” That containment requirement means that they must have been neutrinos and not cosmic rays, since comic ray showers contain lots of stuff besides neutrinos that arrives at the same time. We can assume at the highest energies that no cosmic ray could make it through the outer layers of our detector without leaving a trace (unpacked: cosmic rays must leave a trace) but at lower energies some cosmic ray muons can steak through. For the first muppet analysis, we get around this by just looking at the highest energies.

This is backwards from what we expected in two ways: first, the sample we get is mostly from neutrinos coming from above the detector, and second, there are almost no background events in our sample, so we don’t have to include directional clustering to know that we’ve seen astrophysics.

The sample is mostly downgoing because the highest energy neutrinos are blocked by the Earth. Higher energy neutrinos are more likely to interact than low-energy neutrinos; it’s the opposite of our momentum-based intuition from faster cars slamming through walls without stopping. It’s a popular trivium that neutrinos can pass through lightyears of lead without interacting, but that’s only true at low energy scales like the neutrinos from nuclear reactors. At IceCube astrophysics scales, it takes only our tiny planet to stop a neutrino. So the muppet events we do see are mostly ones that don’t pass through the Earth.

Since the muppets sample has almost no background events (at the very most, 10 of the 28, but we don’t know which 10), we don’t need to do a clustering analysis. Traditionally, we thought this was the most promising way to find neutrino point sources, and the background would be neutrinos from interactions in the Earth’s atmosphere. But at PeV energies, there aren’t enough atmospheric neutrinos to explain what we saw, so each event in the new analysis is potentially as interesting as a cluster would be in the old analysis.

We haven’t yet seen clusters using the old techniques, and when we do, it will probably be celebrated by a small party, an email around our collaboration, some nights out for the people involved, and a PhD for someone (or a few someones). But it won’t be the same cover-of-Science-Magazine celebration (that was Mr. Snuffalupagus on the cover) and press coverage that we had for the first discovery. It will be a quiet victory, as it was for the recent diffuse result.

While it doesn’t have to follow the script we expect it to, science can still sometimes choose to follow a familiar plotline. And we are comforted by the familiarity.


Massive thoughts

Thursday, April 24th, 2014

This article appeared in symmetry on April 24, 2014.

The Higgs boson and the neutrino fascinate the general public and particle physicists alike. Why is that?

The Higgs boson and the neutrino fascinate the general public and particle physicists alike. Why is that?

If there are two particles that everyone has read about in the news lately, it’s the Higgs boson and the neutrino. Why do we continue to be fascinated by these two particles?

As just about everyone now knows, the Higgs boson is integrally connected to the field that gives particles their mass. But the excitement of this discovery isn’t over; now we need to figure out how this actually works and whether it explains everything about how particles get their mass. With time, this knowledge is likely to affect daily life.

One way it could possibly bridge the gap between fundamental research and the commercial market, I believe, is in batteries. The ultimate battery in nature is mass. The expression E=mc2 is a statement of that fact. During the early moments of the universe, all particles were massless and traveling at the speed of light. Once the Higgs mechanism turned on, particles suddenly began interacting with the field and, in this process, converted their energy into what we now refer to as mass. In a recent address to the Canadian Nuclear Society, I suggested that if engineers of the future could learn how to manipulate the Higgs field (to “turn it on and off”), then we would have developed the ultimate energy source and the best battery nature has created. This idea definitely belongs in the science-fiction category, but remember that progress in science is driven by thinking “outside the box!”

This sort of thinking comes from looking at the Higgs from another angle. According to the Standard Model, many particles come in left-handed and right-handed versions (in the former, the particle’s direction of spin matches its direction of motion, while in the latter, they are opposite).

Keeping this fact in mind, let’s look at the mass of the familiar electron as an example. When we say that the mass of the electron is created by interactions with the Higgs field, we can think of this as the Higgs field rapidly changing a left-handed electron into a right-handed electron, and vice versa. This switching back and forth is energy and, through E=mc2, energy is mass. A heavier particle like the top quark would experience this flipping at a much higher frequency than a lighter particle like the electron. As we learn more about how this process works, I encourage physicists to also seek applications of that knowledge.

And what about neutrinos? Do they get their mass from the Higgs field or in a completely different way? Once thought to be massless, neutrinos are now known to have a tiny mass. If the Higgs mechanism is responsible for that mass, there must exist both a left-handed and a right-handed neutrino. A good number of physicists think that both are out there, but we do not yet know. That knowledge may help us understand why the neutrino mass is tiny, as well as why there is more matter than antimatter in the universe—one of the most important questions facing our field of particle physics.

But since the neutrino is a neutral particle, the story gets more interesting. It may instead be possible that there is another type of mass. Referred to as a Majorana mass, it is not a mass described by the flipping of left- and right-handed neutrinos back and forth, but it is “intrinsic,” not derived from any kind of “motional energy.” I expect that the efforts by our field of particle physics, in the collective sense, will pursue the questions associated with both the Higgs boson and the neutrino with enthusiasm, and that the results will lead to advancements we can’t even imagine today.

Nigel Lockyer, Fermilab director


Fermilab released this press release on Feb. 11, 2014.

Workers at the NOvA hall in northern Minnesota assemble the final block of the far detector in early February 2014, with the nearly completed detector in the background. Each block of the detector measures about 50 feet by 50 feet by 6 feet and is made up of 384 plastic PVC modules, assembled flat on a massive pivoting machine. Photo courtesy of NOvA collaboration

Workers at the NOvA hall in northern Minnesota assemble the final block of the far detector in early February 2014, with the nearly completed detector in the background. Each block of the detector measures about 50 feet by 50 feet by 6 feet and is made up of 384 plastic PVC modules, assembled flat on a massive pivoting machine. Photo courtesy of NOvA collaboration

Scientists on the world’s longest-distance neutrino experiment announced today that they have seen their first neutrinos.

The NOvA experiment consists of two huge particle detectors placed 500 miles apart, and its job is to explore the properties of an intense beam of ghostly particles called neutrinos. Neutrinos are abundant in nature, but they very rarely interact with other matter. Studying them could yield crucial information about the early moments of the universe.

“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.”

Scientists generate a beam of the particles for the NOvA experiment using one of the world’s largest accelerators, located at the Department of Energy’s Fermi National Accelerator Laboratory near Chicago. They aim this beam in the direction of the two particle detectors, one near the source at Fermilab and the other in Ash River, Minn., near the Canadian border. The detector in Ash River is operated by the University of Minnesota under a cooperative agreement with the Department of Energy’s Office of Science.

Billions of those particles are sent through the earth every two seconds, aimed at the massive detectors. Once the experiment is fully operational, scientists will catch a precious few each day.

Neutrinos are curious particles. They come in three types, called flavors, 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.

“That the first neutrinos have been detected even before the NOvA far detector installation is complete is a real tribute to everyone involved. That includes the staff at Fermilab, Ash River Lab and the University of Minnesota module facility, the NOvA scientists, and all of the professionals and students building this detector,” 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.”

Once completed, NOvA’s near and far detectors will weigh 300 and 14,000 tons, respectively. Crews will put into place the last module of the far detector early this spring and will finish outfitting both detectors with electronics in the summer.

“The first neutrinos mean we’re on our way,” said Harvard physicist Gary Feldman, who has been a co-leader of the experiment from the beginning. “We started meeting more than 10 years ago to discuss how to design this experiment, so we are eager to get under way.”

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.

The NOvA experiment is scheduled to run for six years. Because neutrinos interact with matter so rarely, scientists expect to catch just about 5,000 neutrinos or antineutrinos during that time. Scientists can study the timing, direction and energy of the particles that interact in their detectors to determine whether they came from Fermilab or elsewhere.

Fermilab creates a beam of neutrinos by smashing protons into a graphite target, which releases a variety of particles. Scientists use magnets to steer the charged particles that emerge from the energy of the collision into a beam. Some of those particles decay into neutrinos, and the scientists filter the non-neutrinos from the beam.

Fermilab started sending a beam of neutrinos through the detectors in September, after 16 months of work by about 300 people to upgrade the lab’s accelerator complex.

“It is great to see the first neutrinos from the upgraded complex,” said Fermilab physicist Paul Derwent, who led the accelerator upgrade project. “It is the culmination of a lot of hard work to get the program up and running again.”

Different types of neutrinos have different masses, but scientists do not know how these masses compare to one another. A goal of the NOvA experiment is to determine the order of the neutrino masses, known as the mass hierarchy, which will help scientists narrow their list of possible theories about how neutrinos work.

“Seeing neutrinos in the first modules of the detector in Minnesota is a major milestone,” said Fermilab physicist Rick Tesarek, deputy project leader for NOvA. “Now we can start doing physics.”

Note: NOvA stands for NuMI Off-Axis Electron Neutrino Appearance. NuMI is itself an acronym, standing for Neutrinos from the Main Injector, Fermilab’s flagship accelerator.


This article appeared in symmetry on Nov. 6, 2013.

Scientists planning the next decade in US particle physics consider what we can learn from fundamental particles called neutrinos.

Scientists planning the next decade in US particle physics consider what we can learn from fundamental particles called neutrinos.

We live in a galaxy permeated with tiny particles called neutrinos. Trillions of them stream through each of us each second. They are everywhere, but much remains a mystery about these particles, which could be key to understanding our universe.

During the first weekend of November, a couple of hundred scientists gathered at Fermilab to discuss ways to unravel the mystery of neutrinos.

The meeting was part of the process of planning the next decade of particle physics research for the United States. A group of 25 scientists on the Particle Physics Project Prioritization Panel, or P5, is studying an abundance of research opportunities in particle physics. In spring they will make recommendations about which of these opportunities should take priority in the United States.

In their first town-hall meeting, the group dedicated a full day to discussing neutrino research.

“Neutrinos have already revealed many properties of the universe, some of them unexpected,” says Antonio Masiero, the vice president of Italy’s National Institute of Nuclear Physics, who provided an international perspective at the meeting. “They still keep secrets which could reveal aspects which are new and answer questions which are still open.”

Neutrinos might help scientists understand what caused the imbalance between matter and antimatter that allowed our universe to form. They could give insight into why particles seem naturally to be organized into three generations. They could help reveal undiscovered principles of nature.

“The neutrino is still a mysterious particle,” says Fermilab physicist Vaia Papadimitriou, pictured above giving a presentation at the meeting. “When I was a graduate student, we didn’t even know neutrinos had masses.”

The next generations of neutrino experiments could reveal other surprises. For example, says Northwestern physicist Andre de Gouvea, neutrinos could turn out to be identical to antineutrinos. They could give scientists clues to the existence of undiscovered types of neutrinos, such as massive ones theorists think might have had a great influence early in the formation of the universe. Neutrinos could turn out to be the only fundamental particles that gain their mass from a source other than the just-discovered Higgs field.

Scientists have proposed a number of experiments to learn more about the properties and behaviors of neutrinos. Those answers could lead to even deeper insights.

P5 will hold at least two more town-hall meetings to discuss additional opportunities in particle physics—including dark matter and dark energy, the Higgs boson, new hidden dimensions of space and time, and the imbalance between matter and antimatter.

Kathryn Jepsen