Breaking the Mirror
The theory of the electroweak forces was formulated by Sheldon Glashow, Steven Weinberg and Abdus Salam, who won the 1979 Nobel Prize in Physics for their efforts. The weak force, which is involved in radioactive beta decay, does not act on all the quarks and leptons. Each of these particles comes in mirror-image varieties, termed left-handed and right-handed, and the beta-decay force acts only on the left-handed ones—a striking fact still unexplained 50 years after its discovery. The family symmetry among the left-handed particles helps to defi ne the electroweak theory.
In the initial stages of its construction, the theory had two essential shortcomings. First, it foresaw four long-range force particles—referred to as gauge bosons—whereas nature has but one: the photon. The other three have a short range, less than about 10–17 meter, less than 1 percent of the proton’s radius. According to Heisenberg’s uncertainty principle, this limited range implies that the force particles must have a mass approaching 100 billion electron volts (GeV). The second shortcoming is that the family symmetry does not permit masses for the quarks and leptons, yet these particles do have mass.
The way out of this unsatisfactory situation is to recognize that a symmetry of the laws of nature need not be reflected in the outcome of those laws. Physicists say that the symmetry is “broken.” The needed theoretical apparatus was worked out in the mid-1960s by physicists Peter Higgs, Robert Brout, François Englert and others. The inspiration came from a seemingly unrelated phenomenon: superconductivity, in which certain materials carry electric current with zero resistance at low temperatures. Although the laws of electromagnetism themselves are symmetrical, the behavior of electromagnetism within the superconducting material is not. A photon gains mass within a superconductor, thereby limiting the intrusion of magnetic fields into the material.
As it turns out, this phenomenon is a perfect prototype for the electroweak theory. If space is filled with a type of “superconductor” that affects the weak interaction rather than electromagnetism, it gives mass to the W and Z bosons and limits the range of the weak interactions. This super conductor consists of particles called Higgs bosons. The quarks and leptons also acquire their mass through their interactions with the Higgs boson. By obtaining mass in this way, instead of possessing it intrinsically, these particles remain consistent with the symmetry requirements of the weak force.
The modern electroweak theory (with the Higgs) accounts very precisely for a broad range of experimental results. Indeed, the paradigm of quark and lepton constituents interacting by means of gauge bosons completely revised our conception of matter and pointed to the possibility that the strong, weak and electromagnetic interactions meld into one when the particles are given very high energies. The electroweak theory is a stunning conceptual achievement, but it is still incomplete. It shows how the quarks and leptons might acquire masses but does not predict what those masses should be. The electroweak theory is similarly indefinite in regard to the mass of the Higgs boson itself: the existence of the particle is essential, but the theory does not predict its mass. Many of the outstanding problems of particle physics and cosmology are linked to the question of exactly how the electroweak symmetry is broken.
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Credit: Slim Films
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Where the Standard Model Tells Its Tale
Encouraged by a string of promising observations in the 1970s, theorists began to take the Standard Model seriously enough to begin to probe its limits. Toward the end of 1976 Benjamin W. Lee of Fermi National Accelerator Laboratory in Batavia, Ill., Harry B. Thacker, now at the University of Virginia, and I devised a thought experiment to investigate how the electroweak forces would behave at very high energies. We imagined collisions among pairs of W, Z and Higgs bosons. The exercise might seem slightly fanciful because, at the time of our work, not one of these particles had been observed. But physicists have an obligation to test any theory by considering its implications as if all its elements were real.
What we noticed was a subtle interplay among the forces generated by these particles. Extended to very high energies, our calculations made sense only if the mass of the Higgs boson were not too large—the equivalent of less than one trillion electron volts, or 1 TeV. If the Higgs is lighter than 1 TeV, weak interactions remain feeble and the theory works reliably at all energies. If the Higgs is heavier than 1 TeV, the weak interactions strengthen near that energy scale and all manner of exotic particle processes ensue. Finding a condition of this kind is interesting because the electroweak theory does not directly predict the Higgs mass. This mass threshold means, among other things, that something new—either a Higgs boson or other novel phenomena— is to be found when the LHC turns the thought experiment into a real one.
Experiments may already have observed the behind-the-scenes influence of the Higgs. This effect is another consequence of the uncertainty principle, which implies that particles such as the Higgs can exist for moments too fleeting to be observed directly but long enough to leave a subtle mark on particle processes. The Large Electron Positron collider at CERN, the previous inhabitant of the tunnel now used by the LHC, detected the work of such an unseen hand. Comparison of precise measurements with theory strongly hints that the Higgs exists and has a mass less than about 192 GeV.
For the Higgs to weigh less than 1 TeV, as required, poses an interesting riddle. In quantum theory, quantities such as mass are not set once and for all but are modified by quantum effects. Just as the Higgs can exert a behind-the-scenes influence on other particles, other particles can do the same to the Higgs. Those particles come in a range of energies, and their net effect depends on where precisely the Standard Model gives way to a deeper theory. If the model holds all the way to 1015 GeV, where the strong and electroweak interactions appear to unify, particles with truly titanic energies act on the Higgs and give it a comparably high mass. Why, then, does the Higgs appear to have a mass of no more than 1 TeV?
This tension is known as the hierarchy problem. One resolution would be a precarious balance of additions and subtractions of large numbers, standing for the contending contributions of different particles. Physicists have learned to be suspicious of immensely precise cancellations that are not mandated by deeper principles. Accordingly, in common with many of my colleagues, I think it highly likely that both the Higgs boson and other new phenomena will be found with the LHC.
Supertechnifragilisticexpialidocious
Theorists have explored many ways in which new phenomena could resolve the hierarchy problem. A leading contender known as supersymmetry supposes that every particle has an as yet unseen superpartner that differs in spin. If nature were exactly supersymmetric, the masses of particles and superpartners would be identical, and their influences on the Higgs would cancel each other out exactly. In that case, though, physicists would have seen the superpartners by now. We have not, so if supersymmetry exists, it must be a broken symmetry. The net influence on the Higgs could still be acceptably small if superpartner masses were less than about 1 TeV, which would put them within the LHC’s reach.
Another option, called technicolor, supposes that the Higgs boson is not truly a fundamental particle but is built out of as yet unobserved constituents. (The term “technicolor” alludes to a generalization of the color charge that defines the strong force.) If so, the Higgs is not fundamental. Collisions at energies around 1 TeV (the energy associated with the force that binds together the Higgs) would allow us to look within it and thus reveal its composite nature. Like supersymmetry, technicolor implies that the LHC will set free a veritable menagerie of exotic particles.
A third, highly provocative idea is that the hierarchy problem will go away on closer examination, because space has additional dimensions beyond the three that we move around in. Extra dimensions might modify how the forces vary in strength with energy and eventually meld together. Then the melding—and the onset of new physics—might not happen at 1012 TeV but at a much lo
wer energy related to the size of the extra dimensions, perhaps only a few TeV. If so, the LHC could offer a peek into those extra dimensions.
One more piece of evidence points to new phenomena on the TeV scale. The dark matter that makes up the bulk of the material content of the universe appears to be a novel type of particle. If this particle interacts with the strength of the weak force, then the big bang would have produced it in the requisite numbers as long as its mass lies between approximately 100 GeV and 1 TeV. Whatever resolves the hierarchy problem will probably suggest a candidate for the dark matter particle.
Revolutions on the Horizon
Opening the TeV scale to exploration means entering a new world of experimental physics. Making a thorough exploration of this world— where we will come to terms with electroweak symmetry breaking, the hierarchy problem and dark matter—is the top priority for accelerator experiments. The goals are well motivated and matched by our experimental tools, with the LHC succeeding the current workhorse, Fermilab’s Tevatron collider. The answers will not only be satisfying for particle physics, they will deepen our understanding of the everyday world.
But these expectations, high as they are, are still not the end of the story. The LHC could well find clues to the full unification of forces or indications that the particle masses follow a rational pattern. Any proposed interpretation of new particles will have consequences for rare decays of the particles we already know. It is very likely that lifting the electroweak veil will bring these problems into clearer relief, change the way we think about them and inspire future experimental thrusts.
Cecil Powell won the 1950 Nobel Prize in Physics for discovering particles called pions— proposed in 1935 by physicist Hideki Yukawa to account for nuclear forces—by exposing highly sensitive photographic emulsions to cosmic rays on a high mountain. He later reminisced: “When [the emulsions] were recovered and developed in Bristol, it was immediately apparent that a whole new world had been revealed. . . . It was as if, suddenly, we had broken into a walled orchard, where protected trees had flourished and all kinds of exotic fruits had ripened in great profusion.” That is just how I imagine our first look at the TeV scale.
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Sidebar: Hidden Symmetry That Shapes Our World
If there were no Higgs mechanism, what a different world it would be! Elementary particles of matter such as quarks and electrons would have no mass. Yet that does not mean the universe would contain no mass. An underappreciated insight from the Standard Model is that particles such as the proton and neutron represent matter of a novel kind. The mass of a proton, in contrast to macroscopic matter, is only a few percent of its constituent masses. (In fact, quarks account for not more than 2 percent of the proton’s mass.) Most of the mass arises through the original form of Albert Einstein’s famous equation, m = E/c2, from the energy stored up in confining the quarks in a tiny volume. In identifying the energy of quark confinement as the origin of proton and neutron mass, we explain nearly all the visible mass of the universe, because luminous matter is made mostly of protons and neutrons in stars.
Quark masses do account for an important detail of the real world: that the neutron is slightly more massive than the proton. One might expect the proton to be the more massive one, because its electric charge contributes to its intrinsic energy—a source of self-energy the neutron lacks. But quark masses tip the balance the other way. In the no-Higgs zone, the proton would outweigh the neutron. Radioactive beta decay would be turned on its head. In our world, a neutron sprung from a nucleus decays into a proton, electron and antineutrino in about 15 minutes, on average. If quark masses were to vanish, a free proton would decay into a neutron, positron and neutrino. Consequently, hydrogen atoms could not exist. The lightest “nucleus” would be one neutron rather than one proton.
In the Standard Model, the Higgs mechanism differentiates electromagnetism from the weak force. In the absence of the Higgs, the strong force among quarks and gluons would induce the distinction. As the strong interaction confined the colored quarks into colorless objects like the proton, it would also act to distinguish the weak and electromagnetic interactions, giving small masses to the W and Z bosons while leaving the photon massless. This manifestation of the strong force would not give any appreciable mass to the electron or the quarks. If it, rather than the Higgs, operated, beta decay would operate millions of times faster than in our world. Some light nuclei would be produced in the early no-Higgs universe and survive, but they would not form atoms we would recognize. An atom’s radius is inversely proportional to the electron’s mass, so if the electron has zero mass, atoms— less than a nanometer across in our world—would be infinitely big. Even if other effects gave electrons a tiny mass, atoms would be macroscopic. A world without compact atoms would be a world without chemistry and without stable composite structures like our solids and liquids.
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Credit: Ian Worpole
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-Originally published: Scientific American 298(2) 46-53 (February 2008)
Fermilab Provides More Constraints on the Elusive Higgs Boson
by John Matson
The Higgs particle, the last piece of the Standard Model of particle physics menagerie that has yet to be observed, is running out of places to hide—if, that is, it exists at all. Fermi National Accelerator Laboratory in Batavia, Ill., today narrowed the range of mass where the Higgs might be found.
The Higgs boson, named for British physicist Peter Higgs, is believed to give other elementary particles, such as the heavy W and Z bosons, their mass, so finding it or proving it does not exist would have major implications in ground-up interpretations of how the world works.
"This is a very interesting time in particle physics, because we have this Standard Model, which explains everything we've observed and everything we know about for the last 30 years with no significant deviations. And, yet, we know that the Standard Model can't be the whole story of nature," says John S. Conway, a physicist at the University of California, Davis, and a member of the Collider Detector at Fermilab (CDF) collaboration, one of two teams involved in the new mass-range results. Many of the lingering questions in physics could be answered or at least clarified when the model's missing piece is located. "Whatever we discover," Conway adds, "it's going to be astounding."
Previous collider experiments had placed a lower bound of 114 giga-electron volts (GeV), a measure that can be used for particle mass, on the Higgs, and theoretical calculations require it to be less than 185 GeV. The new Fermilab results, from its Tevatron collider, rule out a Higgs mass between 160 and 170 GeV. (All of these constraints are at the 95 percent confidence level, according to Fermilab.)
Collider experiments such as those at the Tevatron smash particles together at extremely high energies and observe what is produced, including some exotic but short-lived particles. "We look for the signature of things we know are there and things we think might be there, like the Higgs," says physicist Craig Blocker of Brandeis University in Waltham, Mass., also a member of the CDF team. "If the Higgs had a mass in this fairly narrow range" of 160 to 170 GeV, he says, "we should have seen it, we had a good chance to see it."
Conway says the extension of the excluded Higgs masses at Fermilab is "a really exciting development." All the same, he thinks the Higgs, if it is to be found, will be first seen at the more powerful Large Hadron Collider (LHC) near Geneva, Switzerland, which is slated to come back online later this year after an aborted start-up last September. (Both Conway and Blocker are also working on physics projects at the LHC.) "It is a bit of a race" to find the Higgs, Conway says, "but if I had to bet money, I would bet on the LHC."
There are some scenarios, however, in which Fermilab—enjoying its continuing status as particle physics top dog while the LHC is sidelined—might win that race. If the Higgs happened to have a mass around 150 GeV, which Conway believes is unlikely—evidence points to a lighter particle in the neighborhood of 120 GeV, he says—the Tevatr
on could find it relatively soon. Alternately, with more time to gather data, the Tevatron could close in even tighter on the Higgs by inching its lower mass bound upward.
But what if the entire mass window were exhausted—if experiments showed that the Higgs, or something like it, didn't exist at all? "That would basically mean we have a very deep and fundamental lack of understanding of what is going on in the Standard Model," Blocker says. "If there's not something like the Higgs or something similar giving masses to the W and Z, we have no clue as to how that's happening."
-Originally published: Scientific American online, March 13, 2009.
A Higgs Setback: Did Steven Hawking Just Win the Most Outrageous Bet in Physics History?
by Amir Aczel
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CERN
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A few years ago, celebrated British physicist Stephen Hawking was widely reported in the press to have placed a provocative public bet that the LHC (along with all particle accelerators that preceded it) would never find the Higgs boson, the so-called “God particle” believed responsible for having imbued massive particles with their mass when the universe was very young.
The Higgs Boson: Searching for the God Particle Page 25