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Posts Tagged ‘neutrinos’

The dawn of DUNE

Wednesday, March 25th, 2015

This article appeared in symmetry on March 25, 2015.

A powerful planned neutrino experiment gains new members, new leaders and a new name. Image: Fermilab

A powerful planned neutrino experiment gains new members, new leaders and a new name. Image: Fermilab

The neutrino experiment formerly known as LBNE has transformed. Since January, its collaboration has gained about 50 new member institutions, elected two new spokespersons and chosen a new name: Deep Underground Neutrino Experiment, or DUNE.

The proposed experiment will be the most powerful tool in the world for studying hard-to-catch particles called neutrinos. It will span 800 miles. It will start with a near detector and an intense beam of neutrinos produced at Fermi National Accelerator Laboratory in Illinois. It will end with a 10-kiloton far detector located underground in a laboratory at the Sanford Underground Research Facility in South Dakota. The distance between the two detectors will allow scientists to study how neutrinos change as they zip at close to the speed of light straight through the Earth.

“This will be the flagship experiment for particle physics hosted in the US,” says Jim Siegrist, associate director of high-energy physics for the US Department of Energy’s Office of Science. “It’s an exciting time for neutrino science and particle physics generally.”

In 2014, the Particle Physics Project Prioritization Panel identified the experiment as a top priority for US particle physics. At the same time, it recommended the collaboration take a few steps back and invite more international participation in the planning process.

Physicist Sergio Bertolucci, director of research and scientific computing at CERN, took the helm of an executive board put together to expand the collaboration and organize the election of new spokespersons.

DUNE now includes scientists from 148 institutions in 23 countries. It will be the first large international project hosted by the US to be jointly overseen by outside agencies.

This month, the collaboration elected two new spokespersons: André Rubbia, a professor of physics at ETH Zurich, and Mark Thomson, a professor of physics at the University of Cambridge. One will serve as spokesperson for two years and the other for three to provide continuity in leadership.

Rubbia got started with neutrino research as a member of the NOMAD experiment at CERN in the ’90s. More recently he was a part of LAGUNA-LBNO, a collaboration that was working toward a long-baseline experiment in Europe. Thomson has a long-term involvement in US-based underground and neutrino physics. He is the DUNE principle investigator for the UK.

Scientists are coming together to study neutrinos, rarely interacting particles that constantly stream through the Earth but are not well understood. They come in three types and oscillate, or change from type to type, as they travel long distances. They have tiny, unexplained masses. Neutrinos could hold clues about how the universe began and why matter greatly outnumbers antimatter, allowing us to exist.

“The science is what drives us,” Rubbia says. “We’re at the point where the next generation of experiments is going to address the mystery of neutrino oscillations. It’s a unique moment.”

Scientists hope to begin installation of the DUNE far detector by 2021. “Everybody involved is pushing hard to see this project happen as soon as possible,” Thomson says.

Jennifer Huber and Kathryn Jepsen

Image: Fermilab

Image: Fermilab

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Expanding the cosmic search

Friday, March 20th, 2015

This article appeared in Fermilab Today on March 20, 2015.

The South Pole Telescope scans the skies during a South Pole winter. Photo: Jason Gallicchio, University of Chicago

The South Pole Telescope scans the skies during a South Pole winter. Photo: Jason Gallicchio, University of Chicago

Down at the South Pole, where temperatures drop below negative 100 degrees Fahrenheit and darkness blankets the land for six months at a time, the South Pole Telescope (SPT) searches the skies for answers to the mysteries of our universe.

This mighty scavenger is about to get a major upgrade — a new camera that will help scientists further understand neutrinos, the ghost-like particles without electric charge that rarely interact with matter.

The 10-meter SPT is the largest telescope ever to make its way to the South Pole. It stands atop a two-mile thick plateau of ice, mapping the cosmic microwave background (CMB), the light left over from the big bang. Astrophysicists use these observations to understand the composition and evolution of the universe, all the way back to the first fraction of a second after the big bang, when scientists believe the universe quickly expanded during a period called inflation.

One of the goals of the SPT is to determine the masses of the neutrinos, which were produced in great abundance soon after the big bang. Though nearly massless, because neutrinos exist in huge numbers, they contribute to the total mass of the universe and affect its expansion. By mapping out the mass density of the universe through measurements of CMB lensing, the bending of light caused by immense objects such as large galaxies, astrophysicists are trying to determine the masses of these elusive particles.

A wafer of detectors for the SPT-3G camera undergoes inspection at Fermilab. Photo: Bradford Benson, University of Chicago

A wafer of detectors for the SPT-3G camera undergoes inspection at Fermilab. Photo: Bradford Benson, University of Chicago

To conduct these extremely precise measurements, scientists are installing a bigger, more sensitive camera on the telescope. This new camera, SPT-3G, will be four times heavier and have a factor of about 10 more detectors than the current camera. Its higher level of sensitivity will allow researchers to make extremely precise measurements of the CMB that will hopefully make it possible to cosmologically detect neutrino mass.

This photo shows an up-close look at a single SPT-3G detector. Photo: Volodymyr Yefremenko, Argonne National Laboratory

This photo shows an up-close look at a single SPT-3G detector. Photo: Volodymyr Yefremenko, Argonne National Laboratory


“In the next several years, we should be able to get to the sensitivity level where we can measure the number of neutrinos and derive their mass, which will tell us how they contribute to the overall density of the universe,” explained Bradford Benson, the head of the CMB Group at Fermilab. “This measurement will also enable even more sensitive constraints on inflation and has the potential to measure the energy scale of the associated physics that caused it.”

SPT-3G is being completed by a collaboration of scientists spanning the DOE national laboratories, including Fermilab and Argonne, and universities including the University of Chicago and University of California, Berkeley. The national laboratories provide the resources needed for the bigger camera and larger detector array while the universities bring years of expertise in CMB research.

“The national labs are getting involved because we need to scale up our infrastructure to support the big experiments the field needs for the next generation of science goals,” Benson said. Fermilab’s main role is the initial construction and assembly of the camera, as well as its integration with the detectors. This upgrade is being supported mainly by the Department of Energy and the National Science Foundation, which also supports the operations of the experiment at the South Pole.

Once the camera is complete, scientists will bring it to the South Pole, where conditions are optimal for these experiments. The extreme cold prevents the air from holding much water vapor, which can absorb microwave signals, and the sun, another source of microwaves, does not rise between March and September.

The South Pole is accessible only for about three months during the year, starting in November. This fall, about 20 to 30 scientists will head down to the South Pole to assemble the camera on the telescope and make sure everything works before leaving in mid-February. Once installed, scientists will use it to observe the sky over four years.

“For every project I’ve worked on, it’s that beginning — when everyone is so excited not knowing what we’re going to find, then seeing things you’ve been dreaming about start to show up on the computer screen in front of you — that I find really exciting,” said University of Chicago’s John Carlstrom, the principal investigator for the SPT-3G project.

Diana Kwon

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This article appeared in Fermilab Today on Nov. 26, 2014

The plastic scintillator extrusion line, shown here, produces detector material for export to experiments around the world. Photo: Reidar Hahn

The plastic scintillator extrusion line, shown here, produces detector material for export to experiments around the world. Photo: Reidar Hahn

Small, clear pellets of polystyrene can do a lot. They can help measure cosmic muons at the Pierre Auger Observatory, search for CP violation at KEK in Japan or observe neutrino oscillation at Fermilab. But in order to do any of these they have to go through Lab 5, located in the Fermilab Village, where the Scintillation Detector Development Group, in collaboration with the Northern Illinois Center for Accelerator and Detector Design (NICADD), manufactures the exclusive source of extruded plastic scintillator.

Like vinyl siding on a house, long thin blocks of plastic scintillator cover the surfaces of certain particle detectors. The plastic absorbs energy from collisions and releases it as measurable flashes of light. Fermilab’s Alan Bross and Anna Pla-Dalmau first partnered with local vendors to develop the concept and produce cost-effective scintillator material for the MINOS neutrino oscillation experiment. Later, with NIU’s Gerald Blazey, they built the in-house facility that has now exported high-quality extruded scintillator to experiments worldwide.

“It was clear that extruded scintillator would have a big impact on large neutrino detectors,” Bross said, “but its widespread application was not foreseen.”

Industrially manufactured polystyrene scintillators can be costly — requiring a labor-intensive process of casting purified materials individually in molds that have to be cleaned constantly. Producing the number of pieces needed for large-scale projects such as MINOS through casting would have been prohibitively expensive.

Extrusion, in contrast, presses melted plastic pellets through a die to create a continuous noodle of scintillator (typically about four centimeters wide by two centimeters tall) at a much lower cost. The first step in the production line mixes into the melted plastic two additives that enhance polystyrene’s natural scintillating property. As the material reaches the die, it receives a white, highly reflective coating that holds in scintillation light. Two cold water tanks respectively bathe and shower the scintillator strip before it is cool enough to handle. A puller controls its speed, and a robotic saw finally cuts it to length. The final product contains either a groove or a hole meant for a wavelength-shifting fiber that captures the scintillation light and sends the signal to electronics in the most useful form possible.

Bross had been working on various aspects of the scintillator cost problem since 1989, and he and Pla-Dalmau successfully extruded experiment-quality plastic scintillator with their vendors just in time to make MINOS a reality. In 2003, NICADD purchased and located at Lab 5 many of the machines needed to form an in-house production line.

“The investment made by Blazey and NICADD opened extruded scintillators to numerous experiments,” Pla-Dalmau said. “Without this contribution from NIU, who knows if this equipment would have ever been available to Fermilab and the rest of the physics community?”

Blazey agreed that collaboration was an important part of the plastic scintillator development.

“Together the two institutions had the capacity to build the resources necessary to develop state-of-the-art scintillator detector elements for numerous experiments inside and outside high-energy physics,” Blazey said. “The two institutions remain strong collaborators.”

Between their other responsibilities at Fermilab, the SDD group continues to study ways to make their scintillator more efficient. One task ahead, according to Bross, is to work modern, glass wavelength-shifting fibers into their final product.

“Incorporation of the fibers into the extrusions has always been a tedious part of the process,” he said. “We would like to change that.”

Troy Rummler

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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

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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.

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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.

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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)

 

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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

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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. 

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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.

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