From Sanford Underground Research Facility: “A new source for Majorana calibration”

From Sanford Underground Research Facility

February 4, 2019
Erin Broberg

Researchers recently got a special delivery: a hundred million atoms of Cobalt-56, an ideal calibration source.

A string of germanium detectors inside a cleanroom glovebox on the 4850 Level of Sanford Lab, before they were installed in the Majorana Demonstrator in 2016.
Photo by Matthew Kapust

U Washington Majorana Demonstrator Experiment at SURF

Researchers have not seen the copper glow of the Majorana Demonstrator’s internal detector since 2016. Sealed behind six layers, including 5,200 lead bricks and two heavy copper shields, the Majorana Demonstrator has recorded a steady stream of data that will inform the next-generation neutrinoless double-beta decay experiments. But how do researchers know if the information they’re receiving is accurate? How do they know something hasn’t gone amiss deep inside?

Simple. They use an advanced calibration system that allows them to monitor the performance of the germanium detectors that make up the heart of the demonstrator. Ralph Massarczyk, staff scientist at Los Alamos National Laboratory, designed and created the calibration system used by the Majorana Demonstrator collaboration.

“In a typical detector,” Massarczyk explains, “there is enough natural background that you can easily calibrate a detector. But with Majorana, you have a very minimal background, which is not enough to determine its performance.”

Without substantial background data, researchers don’t know if the background is stable or not. The detector could be reporting events at inaccurate energy levels or even missing them completely. So, to calibrate this extremely sensitive detector, a calibration source is used to produce a standard set of well-known physics events that researchers can use to understand detector performance.

Typically, the collaboration uses thorium, a naturally occurring, slightly radioactive material that creates signatures the Majorana Demonstrator can easily read. The only problem with this source is that the signatures it produces are at a slightly higher energy level than that at which neutrinoless double-beta decay is expected to occur.

For a more ideal calibration, Massarczyk and his team got a special delivery: a hundred million atoms of Cobalt-56, a slightly radioactive isotope created in particle accelerators and used mostly in the medical field. The source underwent several “swipe tests” to ensure no leaks had occurred.

“Cobalt-56 is an ideal source. It produces a lot of events, and those events are at the exact energy where we expect to see a neutrinoless double-beta decay event,” Massarczyk said.

If it is such a perfect indicator, why don’t researchers use it every time?

“Cobalt-56 has a really short half-life, only 77 days,” said Massarczyk. “This means that at the end of 77 days, only one-half of the source will be left. After waiting another 77 days, only one-fourth will be left. After a year, the source is gone.”

Thorium, on the other hand, lasts for years. Indeed, the collaboration has been using its thorium source for five years, Massarczyk said.

Delivery methods

To deliver a calibration source to the detector modules behind layers of shielding, Massarczyk designed a “line source.” In this system, a 5-meter long, half-inch thick plastic tube is inserted into a track from the outside of the shield. The tube, which carries the calibration source, is pushed along the “grooves” on the outside of each detector module, snaking its way around twice.

“It sort of resembles a helix,” Massarczyk said. “This way, the signals are distributed evenly, rather than coming from one point, allowing each detector within the modules to see activity from the same source.”

The normal rate for the Majorana Demonstrator is a few signature counts per hour. When a radioactive calibration source is included, researchers see a few thousand events per second. During its weekly calibration run, the Majorana Demonstrator sees more events in 3 hours than it would otherwise detect in the span of 120 years.

“If, while this source is inside, the demonstrator creates signals that correspond with known data, then we know the demonstrator is well-calibrated and on track,” Massarczyk said.

Looking to the future

The Majorana Demonstrator is expected to run for a few more years, so the short half-life of Cobalt-56 means it is not a sustainable calibration option for the team. That’s why this week’s calibration was so important. The data collected has been sent to analysts for interpretation.

“The main purpose for this data is to double-check the data analysis we do in the energy region 2MeV, where we expect the neutrinoless double-beta decay events to occur,” Massarczyk said.

The information gained from these tests also is of interest to collaborators with LEGEND (Large Enriched Germanium Experiment for Neutrinoless ββ Decay), who are trying to perfect the next-generation neutrinoless double-beta decay experiment.

“As they plan a ton-scale experiment, researchers want to know if the materials are clean enough, if the shielding is working and how far underground they need to go,” said Massarczyk. “Understanding the backgrounds gives us important information to make those decisions.”

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About us.
The Sanford Underground Research Facility in Lead, South Dakota, advances our understanding of the universe by providing laboratory space deep underground, where sensitive physics experiments can be shielded from cosmic radiation. Researchers at the Sanford Lab explore some of the most challenging questions facing 21st century physics, such as the origin of matter, the nature of dark matter and the properties of neutrinos. The facility also hosts experiments in other disciplines—including geology, biology and engineering.

The Sanford Lab is located at the former Homestake gold mine, which was a physics landmark long before being converted into a dedicated science facility. Nuclear chemist Ray Davis earned a share of the Nobel Prize for Physics in 2002 for a solar neutrino experiment he installed 4,850 feet underground in the mine.

Homestake closed in 2003, but the company donated the property to South Dakota in 2006 for use as an underground laboratory. That same year, philanthropist T. Denny Sanford donated $70 million to the project. The South Dakota Legislature also created the South Dakota Science and Technology Authority to operate the lab. The state Legislature has committed more than $40 million in state funds to the project, and South Dakota also obtained a $10 million Community Development Block Grant to help rehabilitate the facility.

In 2007, after the National Science Foundation named Homestake as the preferred site for a proposed national Deep Underground Science and Engineering Laboratory (DUSEL), the South Dakota Science and Technology Authority (SDSTA) began reopening the former gold mine.

In December 2010, the National Science Board decided not to fund further design of DUSEL. However, in 2011 the Department of Energy, through the Lawrence Berkeley National Laboratory, agreed to support ongoing science operations at Sanford Lab, while investigating how to use the underground research facility for other longer-term experiments. The SDSTA, which owns Sanford Lab, continues to operate the facility under that agreement with Berkeley Lab.

The first two major physics experiments at the Sanford Lab are 4,850 feet underground in an area called the Davis Campus, named for the late Ray Davis. The Large Underground Xenon (LUX) experiment is housed in the same cavern excavated for Ray Davis’s experiment in the 1960s.
LUX/Dark matter experiment at SURF

In October 2013, after an initial run of 80 days, LUX was determined to be the most sensitive detector yet to search for dark matter—a mysterious, yet-to-be-detected substance thought to be the most prevalent matter in the universe. The Majorana Demonstrator experiment, also on the 4850 Level, is searching for a rare phenomenon called “neutrinoless double-beta decay” that could reveal whether subatomic particles called neutrinos can be their own antiparticle. Detection of neutrinoless double-beta decay could help determine why matter prevailed over antimatter. The Majorana Demonstrator experiment is adjacent to the original Davis cavern.

LUX’s mission was to scour the universe for WIMPs, vetoing all other signatures. It would continue to do just that for another three years before it was decommissioned in 2016.

In the midst of the excitement over first results, the LUX collaboration was already casting its gaze forward. Planning for a next-generation dark matter experiment at Sanford Lab was already under way. Named LUX-ZEPLIN (LZ), the next-generation experiment would increase the sensitivity of LUX 100 times.

SLAC physicist Tom Shutt, a previous co-spokesperson for LUX, said one goal of the experiment was to figure out how to build an even larger detector.
“LZ will be a thousand times more sensitive than the LUX detector,” Shutt said. “It will just begin to see an irreducible background of neutrinos that may ultimately set the limit to our ability to measure dark matter.”
We celebrate five years of LUX, and look into the steps being taken toward the much larger and far more sensitive experiment.

Another major experiment, the Long Baseline Neutrino Experiment (LBNE)—a collaboration with Fermi National Accelerator Laboratory (Fermilab) and Sanford Lab, is in the preliminary design stages. The project got a major boost last year when Congress approved and the president signed an Omnibus Appropriations bill that will fund LBNE operations through FY 2014. Called the “next frontier of particle physics,” LBNE will follow neutrinos as they travel 800 miles through the earth, from FermiLab in Batavia, Ill., to Sanford Lab.

LBNE

The MAJORANA DEMONSTRATOR will contain 40 kg of germanium; up to 30 kg will be enriched to 86% in 76Ge. The DEMONSTRATOR will be deployed deep underground in an ultra-low-background shielded environment in the Sanford Underground Research Facility (SURF) in Lead, SD. The goal of the DEMONSTRATOR is to determine whether a future 1-tonne experiment can achieve a background goal of one count per tonne-year in a 4-keV region of interest around the 76Ge 0νββ Q-value at 2039 keV. MAJORANA plans to collaborate with GERDA for a future tonne-scale 76Ge 0νββ search.

CASPAR is a low-energy particle accelerator that allows researchers to study processes that take place inside collapsing stars.

The scientists are using space in the Sanford Underground Research Facility (SURF) in Lead, South Dakota, to work on a project called the Compact Accelerator System for Performing Astrophysical Research (CASPAR). CASPAR uses a low-energy particle accelerator that will allow researchers to mimic nuclear fusion reactions in stars. If successful, their findings could help complete our picture of how the elements in our universe are built. “Nuclear astrophysics is about what goes on inside the star, not outside of it,” said Dan Robertson, a Notre Dame assistant research professor of astrophysics working on CASPAR. “It is not observational, but experimental. The idea is to reproduce the stellar environment, to reproduce the reactions within a star.”

From SURF: “The MAJORANA DEMONSTRATOR”

Sanford Underground Research facility

4.4.18
No writer credit found

In 1937, Italian physicist Ettore Majorana hypothesized the Majorana fermion—a particle that could be its own antiparticle. If the theory proves true, it could unlock one of the greatest mysteries of the universe: why there is more matter than anti-matter—and why we exist at all.

The MAJORANA DEMONSTRATOR Project, located deep underground at Sanford Lab, uses 44 kilograms of natural and enriched germanium crystals placed inside two cryostats in the hopes of finding this particle, a rare form of decay called neutrinoless double-beta decay. The experiment is called a demonstrator because the collaboration needed to prove it could create a quiet enough environment to find what it is looking for. A unique shield and 4,850 feet of rock help block cosmic and terrestrial radiation from this highly sensitive experiment.

Now, after years of planning, designing and building the experiment, the collaboration has something to celebrate. In a study published in March 2018, the Majorana Collaboration showed it can shield a sensitive, scalable, 44-kilogram germanium detector from background radioactivity, which is critical to developing a proposed ton-scale experiment.

“We know that we created an environment that is incredibly clean and quiet,” said Vincent Guiseppe, a co-spokesperson for the Majorana Demonstrator and an assistant professor of physics and astronomy at the University of South Carolina. “These results give us a much better understanding of the always-elusive neutrino and how it shaped the universe.”

Guiseppe credits the results to the design of the experiment and the stringent cleanliness protocols put in place.

Growing copper

In its finished form, Majorana is made up of more than 6,600 pounds of copper and more than 5,000 parts and pieces—some as tiny as the head of a pen; others measuring 2 feet square—nearly all of which were made from ultra-pure copper grown on the 4850 Level of Sanford Lab. The first step to building this highly sensitive experiment? Electroforming the purest copper in the world. It’s a simple, but slow process.

Copper nuggets were dissolved in acid baths to remove trace impurities. Then, an electric current was added causing the copper atoms to adhere to a stainless steel cylinder called a mandrel, growing to a thickness of about 5/8 of an inch over a 14 month-period—approximately 33 millionth of a meter per day. Once electroformed the copper was taken to the world’s deepest clean machine shop a kilometer away in the Davis Campus.

“Majorana went to great lengths to ensure the materials used in the experiment would not contribute to backgrounds,” said Cabot-Ann Christofferson, chemist for the Majorana and the South Dakota School of Mines & Technology. “The copper is such an integral part of low-background experiments, that it will be one of the technologies used going forward.”

Precision machining

Every part of the Majorana experiment was machined underground to minimize exposure to cosmic radiation. And every part had to fit perfectly to ensure the experiment runs correctly.

“If it’s just one or two thousandths of an inch off, it’s not close enough,” said project engineer Matthew Busch of Duke University/Triangle University Nuclear Laboratories.

Inside the clean machine shop, machinist Randy Hughes used a lathe to machine the outer layer of the copper, a slitting saw to cut the copper cylinders in half, a 70-ton press to flatten the copper pieces and a wire EDM—electrical discharge machine— to vaporize copper as it cut hundreds of tiny identical parts. If things didn’t fit right, they had to get creative, Busch said.

“We couldn’t buy more tools because there was no more room. So, we modified the tool or the design if things didn’t fit the way we needed them to,” Busch said.

The science

The Majorana Demonstrator collaboration believes germanium is the best material to detect neutrinoless double-beta decay. During the decay process, two electrons are ejected in the germanium. The electrons ionize the germanium, creating a very specific amount of electric charge that can be measured with special equipment. If they discover it, it could tell us why matter—planets, stars, humans and everything else in the universe—exists.

The process is so rare, the slightest interference could render the experiment useless. That’s why it was built deep underground, using electroformed copper that never saw daylight. Still, that wasn’t enough. To achieve the quietest background possible, they built the experiment inside a glovebox in a class-1,000 cleanroom, then surrounded it with a six-layered shield designed to protect it from any stray cosmic or terrestrial radiation.

Assembling Majorana

Having the world’s cleanest copper isn’t enough if you can’t keep your experiment clean. That’s why the experiment was assembled deep underground in a nitrogen-filled glovebox housed in a class-1,000 cleanroom.

Before entering the cleanroom, scientists donned cleanroom garb—Tyvek suits, masks, hoods, special shoe coverings and two pairs of gloves. Once inside the cleanroom, they replaced the outer glove with a new one then headed to the glovebox where they placed their already gloved hands inside huge black rubber gloves covered with another pair of latex gloves. This was done to protect the experiment, not the researchers. Once fully garbed, they began assembling the strings of detectors that reside inside two cryostats. Each cryostat contains about seven strings of 4-5 germanium crystals.

It was challenging and delicate work, involving hundreds of custom-made parts for each string. And everything had to be assembled in a particular order. Each detector is encapsulated in copper then stacked in strings and tied together with cables—most of which are no thicker than a strand of hair—and attached to the cryostat. Many of the parts connect everything to a data collection system inside the cleanroom.

“It’s a detailed, highly specialized procedure that came out of many revisions of the experiment,” said Tom Gillis, a graduate student at the University of South Carolina.

Shields are like onions

In the movie “Shrek,” the title character tells Donkey, “Ogres are like onions! … They have layers.” The same can be said of Majorana’s six-layered shield, said Guiseppe who oversaw the construction of the shield.

Designed to keep out as much radiation as possible, each layer is cleaner as it gets closer to the heart of the experiment. The outer layer is polyethylene, which slows neutrons. The second layer is scintillating plastic, which detects muons. The third layer is an aluminum radon enclosure that keeps out room air, while the fourth layer is made of lead bricks to block gamma rays. Finally, a rectangular box of ultrapure commercial copper surrounds the electroformed copper shield.

But the most critical layer—the one closest to the experiment and the last to be installed—is made of electroformed copper: two five-sided boxes made of 40, 1/2-inch thick plates that, together, weigh about a ton. Majorana began collecting data long before the shield was completed and released positive results as early as 2015.

“Just two months after installing the electroformed shield, we saw a huge difference,” said Guiseppe. “It was like night and day.”

5,500 parts
Ultra-pure copper
5,500 electroformed copper parts were used in the experiment, all were machined underground.

144,500 pounds
Total weight of the shield

The breakdown:
Lead: 108,000 pounds
Poly shield: 31,000 pounds
Copper shielding: 5,500 pounds

The Majorana Demonstrator was designed to lay the groundwork for a ton-scale experiment by demonstrating that backgrounds can be low enough to justify building a larger detector.

“When we started this project, there were many risks and no guarantee that we could achieve our goals, as we were pushing into unexplored territory,” said John Wilkerson, principal investigator of the experiment and the John R. and Louise S. Parker Distinguished Professor in the Department of Physics and Astronomy at the University of North Carolina.

“It’s very exciting to see these world-leading results. We’ve achieved the best energy resolution of any double-beta decay experiment and are among the lowest backgrounds ever seen.”

With 30 times more germanium than the current experiment, the ton-scale, called LEGEND (Large Enriched Germanium Experiment for Neutrinoless Double-Beta Decay), could more easily see the rare decay it seeks. Abstract on Legend.

The plan is to partner with GERDA (GERmanium Detector Array), a sister experiment located at Gran Sasso in Italy, and other researchers in the field.

MPG GERmanium Detector Array (GERDA) at Gran Sasso, Italy

Gran Sasso LABORATORI NAZIONALI del GRAN SASSO, located in the Abruzzo region of central Italy

“This merger leverages public investments by combining the best technologies of each,” said LEGEND Collaboration co-spokesperson Steve Elliott of Los Alamos National Laboratory.

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LBNE

From CNN: “Why the universe shouldn’t exist at all”

CNN

April 1, 2018

Don Lincoln, a senior physicist at Fermilab, does research using the Large Hadron Collider. He is the author of The Large Hadron Collider: The Extraordinary Story of the Higgs Boson and Other Stuff That Will Blow Your Mind, and produces a series of science education videos. Follow him on Facebook. The opinions expressed in this commentary are his.

Why is there something, rather than nothing?” could be the oldest and deepest question in all of metaphysics. Long exclusively the province of philosophy, in recent years this question has become one that can be addressed by scientific methods. What’s more, a new scientific advance has made it more likely that we will finally be able to answer this cosmic conundrum. This is a big deal, because the simplest scientific answer to that question is “We shouldn’t exist at all.”

Obviously, we know that there must be something, because we’re here. If there were nothing, we couldn’t ask the question. But why? Why is there something? Why is the universe not a featureless void? Why does our universe have matter and not only energy? It might seem surprising, but given our current theories and measurements, science cannot answer those questions.

However, give some scientists 65 pounds of a rare isotope of germanium, cool it to temperatures cold enough to liquify air, and place their equipment nearly a mile underground in an abandoned gold mine, and you’ll have the beginnings of an answer. Their project is called the Majorana Demonstrator and it is located at the Sanford Underground Research Facility, near Lead, South Dakota.

Science paper om Majorana Demonstrator project
Initial Results from the Majorana Demonstrator
Journal of Physics: Conference Series

SURF-Sanford Underground Research Facility

SURF circular wooden frame was built to form a concrete ring to hold the 72,000-gallon (272,549 liters) water tank that would house the LUX dark matter detector

SURF LUX water tank was transported in pieces and welded together in the Davis Cavern

U Washington LUX Xenon experiment at SURF

To grasp why science has trouble explaining why matter exists — and to understand the scientific achievement of Majorana — we must first know a few simple things. First, our universe is made exclusively of matter; you, me, the Earth, even distant galaxies. All of it is matter.

However, our best theory for explaining the behavior of the matter and energy of the universe contradicts the realities that we observe in the universe all around us. This theory, called the Standard Model, says that the matter of the universe should be accompanied by an identical amount of antimatter, which, as its name suggests, is a substance antagonistic to matter. Combine equal amounts of matter and antimatter and it will convert into energy.

And the street goes both ways: Enough energy can convert into matter and antimatter. (Fun fact: Combining a paper clip’s worth of matter and antimatter will result in the same energy released in the atomic explosion at Hiroshima. Don’t worry though; since antimatter’s discovery in 1931, we have only been able to isolate enough of it to make about 10 pots of coffee.)

An enigma about the relative amounts of matter and antimatter in the universe arises when we think about how the universe came to be. Modern cosmology says the universe began in an unimaginable Big Bang — an explosion of energy. In this theory, equal amounts of matter and antimatter should have resulted.

So how is our universe made exclusively of matter? Where did the antimatter go?

The simplest answer is that we don’t know. In fact, it remains one of the biggest unanswered problems of modern physics.

Just because the question of missing antimatter is unanswered doesn’t mean that scientists are completely clueless. Beginning in 1964 and continuing through to the present day, physicists have studied the problem and we have found out that early in the universe there was a slight asymmetry in the laws of nature that treated matter and antimatter differently.

Very approximately, for every billion antimatter subatomic particles that were made in the Big Bang, there were a billion-and-one matter particles. The billion matter and antimatter particles were annihilated, leaving the small amount of leftover matter (the “one”) that went on to make up the universe we see around us. This is accepted science.

However, we don’t know the process whereby the asymmetry in the laws of the universe arose. One possible explanation revolves around a class of subatomic particles called leptons.

The most well-known of the leptons is the familiar electron, found around atoms. However, a less known lepton is called the neutrino. Neutrinos are emitted in a particular kind of nuclear radiation, called beta decay. Beta decay occurs when a neutron in an atom decays into a proton, an electron, and a neutrino.

Neutrinos are fascinating particles. They interact extremely weakly; a steady barrage of neutrinos from the nuclear reactions in the sun pass through the entire Earth essentially without interacting. Because they interact so little, they are very difficult to detect and study. And that means that there are properties of neutrinos that we still don’t understand.

Still a mystery to scientists is whether there is a difference between neutrino matter and neutrino antimatter. While we know that both exist, we don’t know if they are different subatomic particles or if they are the same thing. That’s a heavy thought, so perhaps an analogy will help.

Imagine you have a set of twins, with each twin standing in for the matter and antimatter neutrinos. If the twins are fraternal, you can tell them apart, but if they are identical, you can’t. Essentially, we don’t know which kind of twins the neutrino matter/antimatter pair are.

If neutrinos are their own antimatter particle, it would be an enormous clue in the mystery of the missing antimatter. So, naturally, scientists are working to figure this out.

The way they do that is to look first for a very rare form of beta decay, called double beta decay. That’s when two neutrons in the nucleus of an atom simultaneously decay. In this process, two neutrinos are emitted. Scientists have observed this kind of decay.

However, if neutrinos are their own antiparticle, an even rarer thing can occur called “neutrinoless double beta decay.” In this process, the neutrinos are absorbed before they get outside of the nucleus. In this case, no neutrinos are emitted. This process has not been observed and this is what scientists are looking for. The observation of a single, unambiguous neutrinoless double beta decay would show that matter and antimatter neutrinos were the same.

If indeed neutrinoless double beta decay exists, it’s very hard to detect and it’s important that scientists can discriminate between the many types of radioactive decay that mimic that of a neutrino. This requires the design and construction of very precise detectors.

So that’s what the Majorana Demonstrator scientists achieved. They developed the technology necessary to make this very difficult differentiation. This demonstration paints a way forward for a follow-up experiment that can, once and for all, answer the question of whether matter and antimatter neutrinos are the same or different. And, with that information in hand, it might be possible to understand why our universe is made only of matter.

For millennia, introspective thinkers have pondered the great questions of existence. Why are we here? Why is the universe the way it is? Do things have to be this way? With this advance, scientists have taken a step forward in answering these timeless questions.

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From Ethan Siegel: “How Neutrinos Could Solve The Three Greatest Open Questions In Physics”

Ethan Siegel
Dec 12, 2017

Dark matter, dark energy, and why there’s more matter than antimatter? There’s an experiment to explore if neutrinos could solve all three.

A detailed look at the Universe reveals that it’s made of matter and not antimatter, that dark matter and dark energy are required, and that we don’t know the origin of any of these mysteries. Image credit: Chris Blake and Sam Moorfield.

When you take a look at the Universe in great detail, a few facts jump out at you that might be surprising. All the stars, galaxies, gas, and plasma out there are made of matter and not antimatter, even though the laws of nature appear symmetric between the two. In order to form the structures we see on the largest scales, we require a huge amount of dark matter: about five times as much as all the normal matter we possess. And to explain how the expansion rate has changed over time, we need a mysterious form of energy inherent to space itself that’s twice as important (as far as energy is concerned) as all the other forms combined: dark energy. These three puzzles may be the greatest cosmological problems for the 21st century, and yet the one particle that goes beyond the standard model — the neutrino — just might explain them all.

The particles and antiparticles of the Standard Model of particle physics are exactly in line with what experiments require, with only massive neutrinos providing a difficulty. Image credit: E. Siegel / Beyond the Galaxy.

Standard Model of Particle Physics from Symmetry Magazine

Here in the physical Universe, we have two types of Standard Model:

The Standard Model of particle physics (above), with six flavors of quarks and leptons, their antiparticles, the gauge bosons, and the Higgs.
The Standard Model of cosmology (below), with the inflationary Big Bang, matter and not antimatter, and a history of structure formation that leads to stars, galaxies, clusters, filaments, and the present-day Universe.

The matter and energy content in the Universe at the present time (left) and at earlier times (right). Note the presence of dark energy, dark matter, and the prevalence of normal matter over antimatter, which is so minute it does not contribute at any of the times shown. Image credit: NASA, modified by Wikimedia Commons user 老陳, modified further by E. Siegel.

Both Standard Models are perfect in the sense that they explain everything we can observe, but both contain mysteries we cannot explain. From the particle physics side, there’s the mystery of why the particle masses have the values that they do, while on the cosmology side, there are the mysteries of what dark matter and dark energy are, and why (and how) they came to dominate the Universe.

The Universe according to the Standard Model

The big problem in all of this is that the Standard Model of particle physics explains everything we’ve ever observed — every particle, interaction, decay, etc. — perfectly. We’ve never observed a single interaction in a collider, a cosmic ray, or any other experiment that runs counter to the Standard Model’s predictions. The only experimental hint we have that the Standard Model doesn’t give us everything we observe is the fact of neutrino oscillations: where one type of neutrino transforms into another as it passes through space, and through matter in particular. This can only happen if neutrinos have a small, tiny, non-zero mass, as opposed to the massless properties predicted by the Standard Model.

If you begin with an electron neutrino (black) and allow it to travel through either empty space or matter, it will have a certain probability of oscillating into one of the other two types, something that can only happen if neutrinos have very small but non-zero masses. Image credit: Wikimedia Commons user Strait.

So, then, why and how do neutrinos get their masses, and why are those masses so tiny compared to everything else?

The mass difference between an electron, the lightest normal Standard Model particle, and the heaviest possible neutrino is more than a factor of 4,000,000, a gap even larger than the difference between the electron and the top quark. Image credit: Hitoshi Murayama.

There’s even more bizarreness afoot when you take a closer look at these particles. You see, every neutrino we’ve ever observed is left-handed, meaning if you point your left-hand’s thumb in a certain direction, your fingers curl in the direction of the neutrino’s spin. Every anti-neutrino, on the other hand (literally), is right-handed: your right thumb points in its direction of motion and your fingers curl in the direction of the anti-neutrino’s spin. Every other fermion that exists has a symmetry between particles and antiparticles, including equal numbers of left-and-right-handed types. This bizarre property suggests that neutrinos are Majorana (rather than the normal Dirac) fermions, where they behave as their own antiparticles.

Why could this be? The simplest answer is through an idea known as the see-saw mechanism.

If you had “normal” neutrinos with typical masses — comparable to the other Standard Model particles (or the electroweak scale) — that would be expected. Left-handed neutrino and right-handed neutrinos would be balanced, and would have a mass of around 100 GeV. But if there were very heavy particles, like the yellow one (above) that existed at some ultra-high scale (around 10¹⁵ GeV, typical for the grand unification scale), they could land on one side of the see-saw. This mass would get mixed together with the “normal” neutrinos, and you’d get two types of particles out:

. a stable, neutral, weakly interacting ultra-heavy right-handed neutrino (around 10¹⁵ GeV), made heavy by the heavy mass that landed on one side of the see-saw, and
. a light, neutral, weakly interacting left-handed neutrino of the “normal” mass squared over the heavy mass: about (100 GeV)²/(10¹⁵ GeV), or around 0.01 eV.

That first type of particle could easily be the mass of the dark matter particle we need: a member of a class of cold dark matter candidates known as WIMPzillas. This could successfully reproduce the large-scale structure and gravitational effects we need to recover the observed Universe. Meanwhile, the second number lines up extremely well with the actual, allowable mass ranges of the neutrinos we have in our Universe today. Given the uncertainties of one or two orders of magnitude, this could describe exactly how neutrinos work. It gives a dark matter candidate, an explanation for why neutrinos would be so light, and three other interesting things.

The expected fates of the Universe (top three illustrations) all correspond to a Universe where the matter and energy fights against the initial expansion rate. In our observed Universe, a cosmic acceleration is caused by some type of dark energy, which is hitherto unexplained. Image credit: E. Siegel / Beyond the Galaxy.

Dark energy. If you try and calculate what the zero-point energy, or vacuum energy, of the Universe is, you get a ridiculous number: somewhere around Λ ~ (10¹⁹ GeV)⁴. If you’ve ever heard of people saying that the prediction for dark energy is too large by about 120 orders of magnitude, this is where they get that number from. But if you replace that number of 10¹⁹ GeV with the mass of the neutrino, at 0.01 eV, you get a number that’s right around Λ ~ (0.01 eV)⁴, which comes out to match the value we measure almost exactly. This isn’t a proof of anything, but it’s extremely suggestive.

When the electroweak symmetry breaks, the combination of CP-violation and baryon number violation can create a matter/antimatter asymmetry where there was none before, owing to the effect of sphaleron interactions working on a neutrino excess. Image credit: University of Heidelberg.

A baryon asymmetry. We need a way to generate more matter than antimatter in the early Universe, and if we have this see-saw scenario, it gives us a viable way to do it. These mixed-state neutrinos can create more leptons than anti-leptons through the neutrino sector, giving rise to a Universe-wide asymmetry. When the electroweak symmetry breaks, a series of interactions known as sphaleron interactions can then give rise to a Universe with more baryons than leptons, since baryon number (B) and lepton number (L) aren’t individually conserved: just the combination B — L. Whatever lepton asymmetry you start with, they’ll get converted into equal parts baryon and lepton asymmetry. For example, if you start with a lepton asymmetry of X, these sphalerons will naturally give you a Universe with an “extra” amount of protons and neutrons that equals X/2, while giving you that same X/2 amount of electrons and neutrinos combined.

When a nucleus experiences a double neutron decay, two electrons and two neutrinos get emitted conventionally. If neutrinos obey this see-saw mechanism and are Majorana particles, neutrinoless double beta decay should be possible. Experiments are actively looking for this. Image credit: Ludwig Niedermeier, Universitat Tubingen / GERDA.

A new type of decay: neutrinoless double beta decay. The theoretical idea of a source for dark matter, dark energy, and the baryon asymmetry is fascinating, but you need an experiment to detect it. Until we can directly measure neutrinos (and anti-neutrinos) left over from the Big Bang, a feat that’s practically impossible due to the low cross-section of these low-energy neutrinos, we won’t know how to test whether neutrinos have these properties (Majorana) or not (Dirac). But if a double beta decay that emits no neutrinos occurs, we’ll know that neutrinos do have these (Majorana) properties after all, and all of this suddenly could be real.

Perhaps ironically, the greatest advance in particle physics — a great leap forward beyond the Standard Model — might not come from our greatest experiments and detectors at high-energies, but from a humble, patient look for an ultra-rare decay. We’ve constrained neutrinoless double beta decay to have a lifetime of more than 2 × 10²⁵ years, but the next decade or two of experiments should measure this decay if it exists. So far, neutrinos are the only hint of particle physics beyond the Standard Model. If neutrinoless double beta decay turns out to be real, it might be the future of fundamental physics. It could solve the biggest cosmic questions plaguing humanity today. Our only choice is to look. If nature is kind to us, the future won’t be supersymmetry, extra dimensions, or string theory. We just might have a neutrino revolution on our hands.

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“Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan