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In search of the ghost particle

A team of drillers at the IceCube observatory in 2010

The summer of 2012 marked a triumphant capstone for physics. Two separate experiments at the gigantic Large Hadron Collider (LHC) at the CERN laboratory revealed compelling evidence of the Higgs boson, one of the most elusive subatomic particles that theorists had ever concocted. With this discovery, the crucial final piece of the grand edifice known as the standard model of particle physics fell into place.

“Higgs is the culmination of a long, arduous and enthralling journey, but it is not the next great scientific adventure”

Finding the Higgs boson, or ruling out its existence, was a top priority for the LHC, built over a decade at a cost of nearly $10 billion with the help of thousands of scientists and engineers. Not surprisingly, when CERN scheduled a press conference on 4th July 2012, many people expected it to announce the discovery of this long-sought particle. Hundreds of people lined up overnight to get into the auditorium. Journalists reported that the atmosphere at the laboratory was reminiscent of a rock concert. At the event, scientists presented results from two LHC experiments, showing significant peaks above the noise in each data set. The peaks appeared at an energy of about 125 GeV (giga–electron volts), corresponding to a particle about 130 times more massive than the proton. The researchers had little doubt the bumps signalled the discovery of the Higgs boson.

Peter Higgs, who was in his eighties by this point, was a guest of honour at the announcement along with two other theorists who had predicted the particle’s existence, and attendees witnessed him wiping away a tear of joy. “It’s certainly been a long wait,” he said at a press conference in Edinburgh a couple of days later. “At the beginning I had no idea whether a discovery would be made in my lifetime because we knew so little at the beginning about where this particle might be in mass, and therefore how high an energy machine would have to go before it could be discovered.” Stephen Hawking – who had once wagered a hundred dollars that the particle would not be found – agreed that tracking down the Higgs boson was a major milestone in the history of physics. However, in an interview with the BBC, he also noted the flip side of what the discovery meant: “But it is a pity in a way because the great advances in physics have come from experiments that gave results we didn’t expect.”

New data from the LHC, since the initial announcement, have firmed up the Higgs detection beyond a reasonable doubt. Scientists are scrutinising the data for the slightest anomalies that could hint at unexpected phenomena, like the possibility that more than one Higgs particle exists. However, as Hawking lamented, while finding Higgs in some sense marks the culmination of a long, arduous and enthralling journey, it does not point the way to the next great scientific adventure, because so far it has behaved as expected. As Steven Weinberg of the University of Texas at Austin, one of the architects of the standard model and a Nobel laureate, put it to me, “Higgs is the last missing piece of the standard model, but it doesn’t take us beyond.” For that, more and more physicists are looking elsewhere.

The IceCube observatory at the South Pole

“If you’re trying to listen to a whisper, you don’t want a lot of noise around

The most ambitious, and unconventional, of the scientific instruments near the South Pole is buried permanently deep under the ice, and it looks down, not up. Its construction — or burial, to be more accurate — was completed in 2010. All that can be seen above ground is a rectangular office trailer on stilts, filled with cables and computers. There is little sign of what lies beneath but for the small flags that scientists have planted helpfully on the ice to mark its mammoth footprint.

IceCube is an observatory like no other. The glacial ice itself, transparent and cleared of air bubbles by extreme pressure at depths greater than a mile, serves the same purpose as the smooth primary mirror of a conventional astronomical telescope. Buried in it are 86 long steel cables standing vertically, with 60 basketball-size globes hanging on each at regular intervals. Every one of the 5,160 globes contains optical sensors and electronics. The sensors, called phototubes, act like lightbulbs in reverse: they collect light and generate electric signals. In the case of IceCube, these sensors scrutinize the subterranean ice for faint blue flashes that occasionally shimmer in the dark stillness. Whenever a sensor detects a flash, it sends a signal to computers on the surface.

IceCube’s digital optical modules are lowered into an array beneath the ice

The blue flickers mark the passage of elementary particles known as muons, which belong to the same family as electrons but are about two hundred times more massive. By combining signals from the different nodes of this deeply buried sensor network, physicists can trace a muon’s path in 3D. But the researchers are not after the muons themselves. They are hunting for neutrinos, by far the most elusive and the weirdest of all known denizens of the subatomic world. These ghostly particles interact every once in a while with protons within ice molecules to release muons, thus betraying their presence as the muons in turn light up the ice. Since a newly created muon travels through ice along the same path as the incoming neutrino did, researchers can tell which direction the neutrino came from by examining the muon’s trail.

Neutrinos are elementary particles, just like electrons that buzz about atomic nuclei or quarks that combine to make protons and neutrons. They are fundamental building blocks of matter, but they don’t remain trapped inside atoms. Also unlike their subatomic cousins, neutrinos carry no electric charge, have a tiny mass, and hardly ever interact with other particles. A typical neutrino can travel through a light-year’s worth of lead without interacting with any atoms. Therein lies the snag: neutrinos are pathologically shy. Their severe reluctance to mingle makes these particles hard to pin down, so neutrino hunting is a tricky business. But every so often, a neutrino does collide with something, such as a proton inside a water molecule, essentially by accident. It is to raise the odds of accidental collisions, and thus to increase our chances of observing neutrinos, that scientists build extremely large detectors like IceCube.

“Neutrinos are among the ‘most wanted’ of all cosmic messengers for the secrets they hold about the nature
of matter”

You still can’t see neutrinos directly, but you can get a whiff of their presence from the clues they leave behind. On the rare occasions that neutrinos do interact with matter, they produce charged particles such as muons that physicists can detect with their instruments. But distinguishing neutrino signals from unrelated ‘noise’ poses a challenge: cosmic rays, fast-moving particles that arrive from deep space, also produce muons, which might be confused with muons produced by neutrino interactions. Neutrino hunters place their equipment deep underground, or under a thick layer of ice, so that cosmic ray muons cannot get through. As Janet Conrad of the Massachusetts Institute of Technology explains, “If you’re trying to listen to a whisper, you don’t want a lot of noise around.”

Neutrinos are hard to catch, but they are also among the ‘most wanted’ of all cosmic messengers for the secrets they hold about the nature of matter, the pyrotechnics of exploding stars, and the structure of the universe itself. Besides, in the words of theorist Boris Kayser of the Fermi National Accelerator Laboratory (Fermilab) near Chicago, which is home to several neutrino experiments, “If neutrinos didn’t exist, we wouldn’t be here.” He explains that “the sun produces energy through nuclear reactions on which life on Earth depends, and those reactions could not occur without neutrinos.” Moreover, the nuclear burning in previous generations of stars, which produced the heavy elements necessary for life, would not have been possible without neutrinos, either. Therefore, he argues that “to make sense of the universe we need to understand neutrinos well.”

Thankfully, neutrinos are as ubiquitous as they are cagey. In fact, neutrinos are the most abundant matter particles in the universe. According to Hitoshi Murayama of the University of Tokyo and the University of California, Berkeley, there are a billion neutrinos for every atom in the universe. He contends that “their sheer number means they have an important role. The contribution of neutrinos to the cosmic energy budget is comparable to that of all the stars.” In fact, about a hundred trillion neutrinos produced in the nuclear furnace at the Sun’s core pass through your body every second of the day and night, yet they do no harm and leave no trace. During your entire lifetime, perhaps one single neutrino would interact with an atom in your body. Neutrinos travel right through the Earth unhindered, like bullets cutting through fog. Besides, the Earth’s bowels generate neutrinos, as radioactive elements decay, and so do collisions of energetic particles from space in the upper levels of the atmosphere. Cataclysmic deaths of massive stars set off tremendous bursts of neutrinos, which escape these sites of mayhem unscathed and bring us news of awesome celestial events millions of light-years away. Moreover, our planet is immersed in a sea of cosmic neutrinos, which sprang forth when the infant universe was barely two seconds old.

The Daya Bay Reactor Neutrino Experiment in China

The Daya Bay Reactor Neutrino Experiment in China

Super Kamiokande near Hida, Japan

The end of the standard model

Over the years, neutrinos have drawn the attention of some of the most brilliant minds and colourful personalities in the history of physics. The cast of historical characters associated with neutrinos included the sharp-witted Wolfgang Pauli, who invoked these particles in the first place to dodge a crisis in physics; the troubled genius Ettore Majorana, who theorised about neutrinos’ mirror twins before disappearing without a trace at the age of 32; and the committed socialist Bruno Pontecorvo, who realised that neutrinos might morph between different types and caused a Cold War ruckus by defecting to the Soviet Union. Some neutrino hunters built experiments deep underground to peer into the heart of the Sun, while others set up traps next to powerful nuclear reactors to catch neutrinos changing form. During the past two decades, many more scientists have caught the neutrino bug and joined the quest.

That’s because, for neutrino hunters, the best is yet to come. These shadowy particles promise to unlock some of the greatest secrets of the universe. Most exciting, if not unsettling, is the prospect of physics beyond the so-called standard model. Formulated in the early 1970s, the standard model incorporates two dozen elementary particles of matter and their antimatter twins, three types of interactions among them, and the symmetries that govern those interactions. It is the best description of the subatomic world that we have, and countless experiments over three decades have verified its predictions with exquisite precision. When the LHC confirmed the existence of the Higgs boson, a particle hypothesised to be responsible for endowing other elementary particles with mass, it appeared to nail down the final missing piece of the theory.

The standard model, however, presumed that neutrinos have no mass, come in three flavours, and cannot change form. So the discovery that neutrinos do have a very small but non-zero mass, and a chameleonlike tendency to morph among the three types, has exposed a crack in the model’s elegant edifice. If it turns out that there are more than three neutrino flavours, as some data hint, such a revelation could shatter the very foundations of physics. As physicist Kate Scholberg of Duke University puts it, “We’re right on the verge of exploring a new regime in physics. Several unknowns out there are teasing us.” She points out that “neutrinos provide us with a whole new sector of phenomena that we can measure to investigate the nature of the universe.”

The starring role of neutrinos in a great many sagas unfolding across physics, cosmology, and astronomy explains why scientists make considerable efforts to trap these minuscule particles. IceCube is just he most exotic of a new generation of neutrino facilities with unprecedented sensitivity. Some, like IceCube  itself and an even bigger network to be deployed on the Mediterranean seafloor, are, as we’ve seen, designed to catch neutrinos coming from outer space or produced when cosmic rays hit the Earth’s atmosphere.  Others, such as the cathedral scale detector under Mount Kamioka in Japan and another, weighing nearly as much as 5,000 automobiles,  tucked away in a Minnesota mine, measure neutrino beams generated by giant particle accelerators  hundreds of miles away. Experiments of yet another type, located at the village of Chooz in France and at Daya Bay in China, harness neutrinos produced in commercial nuclear power plants.

Together, these facilities make up the formidable arsenal of today’s neutrino hunters. Their advent signals that neutrino chasing, once an esoteric sideline, is ready for prime time.

Ray Jayawardhana is the author of The Neutrino Hunters: The Chase for the Ghost Particle and the Secrets of the Universe published by Oneworld at £8.39

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