Shape-shifting neutrinos earn physicists the 2015 Nobel

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

What’s the Big Deal About the Pentaquark?

“Three quarks for Muster Mark!,” wrote James Joyce in his labyrinthine fable, Finnegan’s Wake. By now, you may have heard this quote – the short, nonsensical sentence that eventually gave the name “quark” to the Universe’s (as-yet-unsurpassed) most fundamental building blocks. Today’s physicists believe that they understand the basics of how quarks combine; three join up to form baryons (everyday particles like the proton and neutron), while two – a quark and an antiquark – stick together to form more exotic, less stable varieties called mesons. Rare four-quark partnerships are called tetraquarks. And five quarks bound in a delicate dance? Naturally, that would be a pentaquark. And the pentaquark, until recently a mere figment of physics lore, has now been detected at the LHC!

So what’s the big deal? Far from just being a fun word to say five-times-fast, the pentaquark may unlock vital new information about the strong nuclear force. These revelations could ultimately change the way we think about our superbly dense friend, the neutron star – and, indeed, the nature of familiar matter itself.

Physicists know of six types of quarks, which are ordered by weight. The lightest of the six are the up and down quarks, which make up the most familiar everyday baryons (two ups and a down in the proton, and two downs and an up in the neutron). The next heaviest are the charm and strange quarks, followed by the top and bottom quarks. And why stop there? In addition, each of the six quarks has a corresponding anti-particle, or antiquark.

Six types of quark, arranged from left to right by way of their mass, depicted along with the other elementary particles of the Standard Model. The Higgs boson was added to the right side of the menagerie in 2012. (Image Credit: Fermilab)

An important attribute of both quarks and their anti-particle counterparts is something called “color.” Of course, quarks do not have color in the same way that you might call an apple “red” or the ocean “blue”; rather, this property is a metaphorical way of communicating one of the essential laws of subatomic physics – that quark-containing particles (called hadrons) always carry a neutral color charge.

For instance, the three components of a proton must include one red quark, one green quark, and one blue quark. These three “colors” add up to a neutral particle in the same way that red, green, and blue light combine to create a white glow. Similar laws are in place for the quark and antiquark that make up a meson: their respective colors must be exactly opposite. A red quark will only combine with an anti-red (or cyan) antiquark, and so on.

The pentaquark, too, must have a neutral color charge. Imagine a proton and a meson (specifically, a type called a J/psi meson) bound together – a red, a blue, and a green quark in one corner, and a color-neutral quark-antiquark pair in the other – for a grand total of four quarks and one antiquark, all colors of which neatly cancel each other out.

Physicists are not sure whether the pentaquark is created by this type of segregated arrangement or whether all five quarks are bound together directly; either way, like all hadrons, the pentaquark is kept in check by that titan of fundamental dynamics, the strong nuclear force.

The strong nuclear force, as its name implies, is the unspeakably robust force that glues together the components of every atomic nucleus: protons and neutrons and, more crucially, their own constituent quarks. The strong force is so tenacious that “free quarks” have never been observed; they are all confined far too tightly within their parent baryons.

But there is one place in the Universe where quarks may exist in and of themselves, in a kind of meta-nuclear state: in an extraordinarily dense type of neutron star. In a typical neutron star, the gravitational pressure is so tremendous that protons and electrons cease to be. Their energies and charges melt together, leaving nothing but a snug mass of neutrons.

Physicists have conjectured that, at extreme densities, in the most compact of stars, adjacent neutrons within the core may even themselves disintegrate into a jumble of constituent parts.

The neutron star… would become a quark star.

The difference between a neutron star and a quark star (Chandra)
The difference between a neutron star and a quark star. (Image Credit: Chandra)

Scientists believe that understanding the physics of the pentaquark may shed light on the way the strong nuclear force operates under such extreme conditions – not only in such overly dense neutron stars, but perhaps even in the first fractions of a second following the Big Bang. Further analysis should also help physicists refine their understanding of the ways that quarks can and cannot combine.

The data that gave rise to this discovery – a whopping 9-sigma result! – came out of the LHC’s first run (2010-2013). With the supercollider now operating at double its original energy capacity, physicists should have no problem unraveling the mysteries of the pentaquark even further.

A preprint of the pentaquark discovery, which has been submitted to the journal Physical Review Letters, can be found here.

Why Can’t We See the Big Bang?

Since telescopes let us look back in time, shouldn’t we be able to see all the way back to the very beginning of time itself? To the moment of the Big Bang?

You’ve probably heard that looking out into space is like looking back in time. As it takes light 1 second to get from the Moon to us. Whenever we view it, we’re seeing it 1 second in the past. The Sun is 8 light minutes away, and the light we see from it is from 8 minutes into the past.

A better example might be Andromeda, it’s 2.5 million light years away… and you guessed it, we’re seeing it 2.5 million years in the past. Since the Big Bang happened 13.7 billion years ago, using this idea, shouldn’t we be able look all the way back to the beginning of time, even if we’ve misplaced the key to our Tardis?

At the very beginning of the Universe, seconds after the Big Bang, everything was mushed together. Energy and matter were the same thing. Dogs and cats lived together. There was no difference between light and radiation, it was all just one united force.

You couldn’t see it, because light didn’t actually exist. There were no such thing as photons.

However, if you’re still insisting there’s no such thing as photons, you might want to check yourself. After these things started to separate. Photons and particles became actual things. Electromagnetism and the weak nuclear force split off and formed new bands, but could never quite get the momentum of the original lineup.

By the end of the first second, neutrons and protons were around, and they were getting mashed by the intense heat and pressure into the first elements. But you still couldn’t see that because the whole Universe was like the inside of a star. Everything was opaque. It was Scarlett Johansson hot, and too crazy to form stable atoms with electrons as we see today.

Artist's conception of Planck, a space observatory operated by the European Space Agency, and the cosmic microwave background. Credit: ESA and the Planck Collaboration - D. Ducros
Artist’s conception of Planck, a space observatory operated by the European Space Agency, and the cosmic microwave background. Credit: ESA and the Planck Collaboration – D. Ducros

After the Universe was about 380,000 years old, it had cooled down to the point that proper atoms could form. This is the moment when light could finally move, and travel distances across the Universe to you and get caught up in your light buckets. In fact, this light is known as the cosmic microwave background radiation.

So, how come we don’t see all this freed light in all directions with our eyes? It’s because the region of space where it exists is so far away, and travelling away from us so quickly. The light’s wavelengths have been stretched out to the point that light has been turned into microwaves. It’s only with sensitive radio telescopes and space missions that astronomers can even detect it.

Unfortunately, we’ll never be able to see the Big Bang. Even though we’re looking back in time, right to the edge of the observable Universe, it’s just beyond our reach. If you could look at the Universe at any point in time, what would it be? Tell us in the comments below.

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Researchers May Have Finally Detected a Dark Matter Particle

Dark matter: it’s invisible, it’s elusive, it’s controversial… and it’s everywhere — in the Universe, yes, but especially in the world of astrophysics, where researchers have been exhaustively trying to reveal its true identity for decades.

Now, scientists with the international Super Cryogenic Dark Matter Search (SuperCDMS) experiment are reporting the detection of a particle that’s thought to make up dark matter: a weakly-interacting massive particle, or WIMP. According to a press release from Texas A&M University (whose high-energy physicist Rupak Mahapatra is a principal investigator in the experiment) SuperCDMS has identified a WIMP-like signal at the 3-sigma level, which indicates a 99.8 percent chance of an actual discovery — a “concrete hint,” as it’s being called.

“In high-energy physics, a discovery is only claimed at 5-sigma or better,” Mahapatra said. “So this is certainly very exciting, but not fully convincing by the standards. We just need more data to be sure. For now, we have to live with this tantalizing hint of one of the biggest puzzles of our time.”

If this is indeed a WIMP it will be the first time such a particle has been directly observed, lending more insight into what dark matter is… or isn’t.

Notoriously elusive, WIMPs rarely interact with normal matter and therefore are difficult to detect. Scientists believe they occasionally bounce off, or scatter like billiard balls from, atomic nuclei, leaving behind a small amount of energy capable of being tracked by detectors deep underground, particle colliders such as the Large Hadron Collider at CERN and even instruments in space like the Alpha Magnetic Spectrometer (AMS) mounted on the International Space Station.

A stack of crystal germanium CDMS detectors (Fermilab)
A stack of crystal germanium CDMS detectors (Fermilab)

The CDMS experiment, located a half-mile underground at the Soudan mine in northern Minnesota and managed by the United States Department of Energy’s Fermi National Accelerator Laboratory, has been searching for dark matter since 2003. The experiment uses very sophisticated detector technology and advanced analysis techniques to enable cryogenically cooled (almost absolute zero temperature at -460 degrees F) germanium and silicon targets to search for the rare recoil of dark matter particles.

This newly-announced detection actually comes from data acquired during an earlier phase of the experiment.

“This result is from data taken a few years ago using silicon detectors manufactured at Stanford that are now defunct,” Mahapatra said. “Increased interest in the low mass WIMP region motivated us to complete the analysis of the silicon-detector exposure, which is less sensitive than germanium for WIMP masses above 15 giga-electronvolts [one GeVa is equal to a billion electron volts] but more sensitive for lower masses. The analysis resulted in three events, and the estimated background is 0.7 events.”

Although Mahapatra says the result is certainly encouraging and worthy of further investigation, he cautions it should not be considered a discovery just yet.

“We are only 99.8 percent sure, and we want to be 99.9999 percent sure,” Mahapatra said. “At 3-sigma, you have a hint of something. At 4-sigma, you have evidence. At 5-sigma, you have a discovery.”

“In medicine, you can say you are curing 99.8 percent of the cases, and that’s OK. When you say you’ve made a fundamental discovery in high-energy physics, you can’t be wrong.”

– Dr. Rupak Mahapatra, SuperCDMS principal investigator, Texas A&M University

Advanced 6-inch silicon detectors developed by Mahapatra's lab at Texas A&M
Advanced 6-inch silicon detectors developed by Mahapatra’s lab at Texas A&M

The collaboration will continue to probe this WIMP sector using the SuperCDMS Soudan experiment’s operating germanium detectors and is considering using larger, more advanced 6-inch silicon detectors developed at the Texas A&M’s Department of Electrical Engineering in future experiments.

The team has detailed its results in a paper published in arXiv that eventually will appear in Physical Review Letters. Mahapatra will also announce the results today at 12 p.m. CDT in a talk at the Mitchell Institute for Fundamental Physics and Astronomy.

Source: Texas A&M University

(Read more about dark matter here and here.)

Tevatron Targets Higgs Mass

Today, researchers from Fermilab announced they have zeroed in further on the mass of the Higgs boson, the controversially-called “God particle”* that is thought to be the key to all mass in the Universe. This news comes just two days before a highly-anticipated announcement by CERN during the ICHEP physics conference in Melbourne, Australia (which is expected by many to confirm actual proof of the Higgs.)

Even after analyzing the data from 500 trillion collisions produced over the past decade at Fermilab’s Tevatron particle collider the Higgs particle has not been identified directly. But a narrower range for its mass has been established with some certainty: according to the research the Higgs, if it exists, has a mass between 115 and 135 GeV/c2.

“Our data strongly point toward the existence of the Higgs boson, but it will take results from the experiments at the Large Hadron Collider in Europe to establish a discovery,” said Fermilab’s Rob Roser, cospokesperson for the CDF experiment at DOE’s Fermi National Accelerator Laboratory.

Researchers hunt for the Higgs by looking for particles that it breaks down into. With the Large Hadron Collider at CERN, scientists look for energetic photons, while at Fermilab CDF and DZero collaborators have been searching for bottom quarks. Both are viable results expected from the decay of a Higgs particle, “just as a vending machine might return the same amount of change using different combinations of coins.”

Fermilab’s results have a statistical significance of 2.9 sigma, meaning that there’s a 1-in-550 chance that the data was the result of something else entirely. While a 5-sigma significance is required for an official “discovery”, these findings show that the Higgs is running out of places to hide.

“We have developed sophisticated simulation and analysis programs to identify Higgs-like patterns,” said Luciano Ristori, co-spokesperson of the CDF experiment. “Still, it is easier to look for a friend’s face in a sports stadium filled with 100,000 people than to search for a Higgs-like event among trillions of collisions.”

“We achieved a critical step in the search for the Higgs boson. Nobody expected the Tevatron to get this far when it was built in the 1980s.”

– Dmitri Denisov, DZero cospokesperson and physicist at Fermilab

Nearly 50 years since it was proposed, physicists may now be on the edge of exposing this elusive and essential ingredient of… well, everything.

See the Fermilab press release here.

Read Fermilab’s FAQs on the Higgs boson

Top image: The Tevatron typically produced about 10 million proton-antiproton collisions per second. Each collision produced hundreds of particles. The CDF and DZero experiments recorded about 200 collisions per second for further analysis. Sub-image: The three-story, 6,000-ton CDF detector recorded snapshots of the particles that emerge when protons and antiprotons collide.(Fermilab)

*And why is it often called the God particle? Because of this book.