Web edition: July 20, 2012
You know that a scientific idea has penetrated popular culture when people start making jokes about it. Like the one about the priest who changed his name to Higgs so he could be better at giving mass. Or some that say a gadget for negating the Higgs field would be a weapon of mass destruction. Most are variants on a Higgs boson walks into a bar … finally.
Such jokes celebrated the Fourth of July announcement of a new subatomic particle, detected amid the debris of proton-proton collisions at a powerful atom smasher in Europe known as the Large Hadron Collider. Headlines worldwide proclaimed success in creating the Higgs boson, the only missing member on the roster of the universe’s basic building blocks.
In terms usually reserved for athletic achievements, news reports described the finding as a monumental milestone in the history of science. The Higgs confers mass on other particles, makes atoms and molecules possible, and deepens humankind’s understanding of the cosmos, media consumers learned. Most of that hype was roughly true (sometimes very roughly). But in the wake of Higgs hysteria, some nuances were washed away with the deluge of atom-smashing euphoria.
One subtlety, for instance, concerns whether the particle discovered actually is “the” Higgs boson, rather than perhaps just “a” Higgs boson. And it’s a shortcut to say the Higgs particle gives mass to other particles — it’s really the “Higgs field,” the underlying stuff the boson is created from, that makes other particles massive. In any case, the Higgs is certainly not responsible for all mass. Most of the mass of protons and neutrons, for instance, derives from the energetic interaction of quarks and the gluons that hold them together.
Yet while some of the hype was exaggerated, many aspects of the Higgs real value to science and society went un- or understated. It’s quite fair to say that the Higgs field makes the universe the way it is, with material structure and all the complexities of biology. Add its significance for understanding the secrets of the cosmos and its illustration of the value of science, and the discovery of the Higgs particle, along with the search itself, has contributed value to society that’s difficult to overstate.
Whether the Higgs discovery itself was overstated hinges on a distinction between reasonable inference and scientific caution. “As a layman I would say we have it,” says Rolf-Dieter Heuer, director general of the CERN physics laboratory that operates the LHC. “But as a scientist I can only say what do we have.… What we have announced is the discovery of a new particle which is consistent with the Higgs boson.”
In fact, most experts seem to believe it most likely is the boson that Peter Higgs predicted in 1964. “It’s almost certainly the case that that boson was made at CERN,” University of Oxford physicist Frank Close said in a talk July 16 in Dublin at the Euroscience Open Forum 2012 conference.
After all, particle accelerator searches over the last two decades had narrowed the range of possible masses for the Higgs; if it existed at all, it had to weigh in at between 114 billion and 143 billion electron volts or GeV (1 GeV is slightly more than the mass of a hydrogen atom). So when two LHC experiments both spotted a boson weighing 125-126 GeV, inferring it to be the Higgs seemed reasonable. It was like Thoreau’s comment that finding a trout in the milk is strong evidence that the milk has been diluted by river water.
In the Higgs case, its discoverers can say for sure that they’ve found a trout, but DNA tests will be needed to confirm that it’s the right species.
That it’s a trout — well, boson — is certain. A boson is defined by specific quantum properties; in the interplay of nature’s particles, bosons are typically transmitters of forces. In LHC collisions, a surefire signature of a boson is the appearance of two photons after a collision, the end products of one of the several ways that the unstable Higgs can decay. Evidence of such two-photon decay established the particle’s boson status indisputably.
“We know there’s a new particle, we know it is a boson,” Heuer told reporters in Dublin during ESOF. “But we don’t know if we can say Higgs boson. It looks very much like the Higgs, but scientists are sometimes very careful and very cautious.”
In principle there could be an impostor. Theorists have constructed a standard set of equations that describe all of nature’s particles and forces (except gravity) with extraordinary precision; a Higgs boson with very specific properties is necessary for this standard model to hold together mathematically. But as all physicists know, the standard model doesn’t explain everything — it accounts for less than 20 percent of the matter in the universe, for instance — the rest is invisible or “dark” and cannot be made of the ordinary matter particles found on Earth. Theoretical efforts to describe new types of “dark matter” particles imply that the standard model’s Higgs could have several cousins.
“If it’s not quite the standard model Higgs boson—that means if its properties are slightly different to the one which it needs to have for the standard model Higgs boson—that could indicate physics beyond the standard model,” says Heuer. “Only once we understand how it behaves, we know if it’s the Higgs boson or not.… If it’s not, then it is maybe one of a family of Higgs bosons.”
Theorists have considered other possible particles that could mimic the Higgs in collider experiments. But most of those can already be ruled out by the LHC results, physicists Ian Low, Joseph Lykken and Gabe Shaughnessy point out in a paper online at arXiv.org (arxiv.org/abs/1207.1093). One impostor remains possible, but the researchers write that the standard Higgs boson “gives a slightly better overall fit” to the LHC data. Another analysis reached a similar conclusion: “We find that the current Higgs data are consistent with the Standard Model Higgs boson,” Dean Carmi of Tel Aviv University and collaborators reported online July 9 at arXiv.org (arxiv.org/abs/1207.1718).
“Consistent with,” isn’t the same as identification with certainty, but in this case it’s like looking for your best friend, in a place where you would expect him to be, and seeing someone there that looks just like him. It’s a reasonable inference that you’ve found your friend. But as Heuer notes, a cautious scientist will point out that your friend may have a twin, and you’ll need to look closer for a scar or some other identifying feature to be sure.
In the Higgs case, one such feature would be the particle’s spin. Bosons are permitted various amounts of spin, a quantum quantity roughly analogous to the angular momentum of an ordinary-sized rotating object. But the standard model’s Higgs particle would be spinless — spin 0. That means the field it originates from is scalar — without direction. Other common fields in nature, such as the electromagnetic field, are vectors, with both a strength and a direction in which the force acts. Heuer offered an analogy comparing the vector flow of a river with the still water of a swimming pool.
“If you swim in a river, the force of the water is always different on you depending on which direction you swim,” he said. “The force of the water in a large swimming pool when you swim is always the same in any direction, and that’s a scalar. In all directions the same.”
While scalar fields are widely used by theorists in contemplating cosmic phenomena, no experiment has been able to detect one. “If it’s a scalar,” says Heuer, “that would be the first fundamental scalar ever discovered.”
If the new particle truly is the Higgs, its verification of real scalar fields would be nearly as important as its role in generating mass.
In fact, the Higgs’ mass-generating job is very important. Without the Higgs, nobody would be around to search for it anyway. “Because then, fundamental particles would not have mass,” notes Heuer, “and then it’s difficult to get them together because they would all fly around at the velocity of light.”
But the Higgs’ role in making mass is often imprecisely described. It’s not really the Higgs boson that confers mass on other particles, but the field of force from which the boson is made. Particles that interact with the Higgs field acquire mass because the field resists change in their state of motion; such resistance is inertia, the hallmark of mass.
As originally described by Higgs in 1964, that field could form in the early universe in a symmetry-breaking process similar to the sudden appearance of superconductivity in metals cooled below a certain critical temperature. In this process, some particles — those responsible for the weak nuclear force — would acquire mass, while photons (particles of light) would remain massless. Others published similar work in the same year (and in a different context, the same idea had been described by Philip Anderson in 1958). But Higgs went one step further, noting that his theory implied the existence of a new scalar boson. In 1967, Steven Weinberg applied the Higgs idea to the unification of the weak force with electromagnetism; he showed how the Higgs mechanism could cause the two forces — indistinguishable at the universe’s birth — to split into the versions observed today. Weinberg also showed that the Higgs field could confer mass on matter particles, such as quarks and electrons (but exactly how that mass-giving process would work remains mysterious).
Even if quarks and electrons would be massless without the Higgs field, that’s not the same thing as saying all the mass observed in everyday life is due to the Higgs. Most ordinary-world mass is contained in protons and neutrons, the constituents of the atomic nucleus, which are in turn made of quarks. But quark mass from the Higgs field would contribute very little to a proton’s or neutron’s mass. More than 95 percent is generated from quark interactions with gluons, the particles that hold quarks (and the nucleus itself) together, as Nobel laureate Frank Wilczek points out in a recent paper (arxiv.org/abs/1206.7114).
Of course, that realization doesn’t diminish the Higgs importance; it only clarifies it. And the Higgs is entirely responsible for the mass of some particles, notably the W and Z bosons that transmit the weak nuclear force. That force plays a key role in the nuclear reactions that power the sun. Fortunately for life on Earth, fusion reactions providing the sun’s energy proceed fairly slowly. Consequently the sun is still glowing strong after nearly 5 billion years, providing life plenty of time to evolve. As Close of Oxford points out, the sun’s leisurely pace depends on the weak force being weak, and it is only weak because the W bosons that transmit it are massive.
“So the slow-burning sun is critical for our existence,” says Close, “and the slow-burning sun is because the weak force is so feeble, and it’s feeble because the W boson is very massive, and the W boson is very massive because of all this Higgs business we’ve been talking about.”
Besides illuminating the origin of mass, studying the Higgs has other implications for physics. For one thing, its reported mass is compatible with a popular theory for explaining the dark matter in space. Inspired by the mathematical symmetry of the standard model, theorists have proposed an analogous mathematical framework called supersymmetry. For each particle in the standard model, supersymmetry requires a partner particle, much more massive. Most such particles would have disintegrated in the universe’s earliest moments, but the lightest remaining superpartner should still be around in large quantities, accounting for the dark matter.
Understanding the Higgs field more deeply might also aid physicists in their quest to understand the universe’s beginning, when a brief burst of inflationary expansion planted the seeds for the galaxies decorating the cosmos today. That burst of expansion was supposedly caused by a scalar field in space, much like the Higgs field. In fact, some physicists have even proposed that the Higgs field itself might have been the inflation instigator. In any event, finally having a scalar particle to study should shed light on that universe-initiating event. It may even be that “dark energy,” the mysterious force that even now drives the universe to expand at an accelerating rate, is itself a scalar field.
Beyond purely scientific rewards, the Higgs — and the search for it — benefits society in many ways already evident and others unforeseeable. CERN, of course, was the site of the invention of the World Wide Web, whose economic impact has already paid for all the atom smashers ever built. And deepening knowledge of the physical world has always in the past produced unforeseen future societal benefits. When James Clerk Maxwell predicted, and then Heinrich Hertz discovered, electromagnetic waves, nobody foresaw the economic consequences of television, microwave ovens and Wi-Fi. Particles of antimatter, discovered in cosmic rays, have found uses in medical imaging. Nobody knew at first that Einstein’s relativity theory would allow your car to tell you when to turn. Or that the mysterious math of quantum mechanics would empower lasers and computer chips, revolutionizing everything from grocery stores and music players to mass communication.
Of course, you won’t ever video chat with a Higgs-powered smartphone, and there certainly will be no “bosonic bomb” based on the Higgs envisioned in Herman Wouk’s 2004 novel A Hole in Texas, about how the Chinese discovered the Higgs in cosmic rays. There’s no good business plan for a particle that shows up only about once for every trillion proton collisions.
But studying it will serve to satisfy the human quest to understand nature more deeply. Such deeper understanding always leads to more material advances, eventually.
And a deeper understanding of the laws of nature and the mysteries of the universe is precisely what the Higgs boson offers. It made the universe fit for human habitation, and now it may enable humans to comprehend their cosmic home more fully.
“The properties of this boson might open up a window into the DNA of the universe,” comments Heuer. No joke.
S. Weinberg. A model of leptons. Physical Review Letters. Vol. 19, November 20, 1967, p. 1264-1266. [Go to]
P. Higgs. Broken symmetries and the masses of gauge bosons. Physical Review Letters. Vol. 13, October 19, 1964, p. 508. [Go to]
C. Quigg. Particle Physics in a Season of Change. arXiv:1202.4391v1. Posted February 20, 2012. [Go to]
F. Wilczek. Origins of Mass. arXiv:1206.7114v1. Posted June 20, 2012. [Go to]
D. Carmi et al. Higgs after the discovery: a status report. arXiv:1207.1718v2. Posted July 9, 2012. [Go to]
I. Low et al. Have we observed the Higgs (imposter)? arXiv:1207.1093v1. Posted July 4, 2012. [Go to]