On July 4, 2012, the CERN auditorium was full. That’s not unusual; the room often hosts scientific presentations to packed houses. What was unusual was that this seminar was watched by millions of people worldwide, including reporters from high-impact media outlets like BBC, CNN, and The New York Times.
So what was the announcement that caused a hectic world to briefly pause and listen? A new subatomic particle had been discovered, and its properties were consistent with those predicted for the long-sought Higgs boson. The Higgs boson, if it exists, is the experimental evidence needed to confirm the existence of the Higgs field, which is thought to give fundamental subatomic particles their mass.
Physicists were careful to not claim that they had conclusively discovered the Higgs boson. The Higgs boson was predicted in 1964 to have a litany of very specific properties. Until scientists are able to demonstrate that the newly-discovered particle matches all of the predictions, there remains the possibility that the new particle is something wholly unexpected. Of the properties that had been tested prior to the seminar, all of them pointed to this being the Higgs, which is why scientists said “consistent with the Higgs boson.” Using a metaphor involving the senses, what was found looked and smelled like the Higgs boson, but nobody had been able to taste, feel and touch it. So some uncertainty remained. This uncertainty still remains today, and it will be some time before scientists can definitively state that the observed particle was the Higgs boson.
But let’s imagine that the discovered particle, which is a boson of mass about 125 times that of the proton, is the Higgs boson. What then?
You’d think scientists would celebrate (and we did…more than a few champagne corks were popped), but once the confetti settled, there were some furrowed brows. Nobody understood why the mass of the Higgs boson was so low. Here’s the source of the conundrum.
A Higgs boson doesn’t always exist as a Higgs boson. Like other quantum particles, it can change forms. For instance, it can briefly convert into a pair of top quarks before coalescing back into a Higgs boson. These evanescent top quarks are called “virtual particles” and are just an example of the several kinds of particles into which a Higgs boson can temporarily fluctuate. So, if you want to predict the mass of the Higgs, you have to take all of these possible forms into account.
Higgs bosons can spontaneously convert into pairs of other subatomic particles. These pairs exist only for a very short time, but their existence will alter the mass of the Higgs boson.
Mathematically, we split the mass of the Higgs into two parts: its “theoretical” mass—that is, the mass it would have if didn’t fluctuate into different particles—plus the effect of the fluctuations. (For the technically brave, I put the equation that describes this in a footnote1
To make things even more complicated, the effect of the fluctuations also comes in two pieces. These two terms are multiplied, not added, together. The first term involves the maximum energy for which the Higgs theory applies. This works out to be a huge number, about 1038 GeV2.
The second term is, roughly speaking, the sum of the effect of the bosons (W, Z & Higgs) minus the sum of the effect of the fermions (top quark). Let’s call this the fermion/boson sum.
So, let’s take a birds-eye view of the whole equation. The mass of the Higgs is equal to the theoretical mass plus a monstrously large number multiplied by the fermion/boson sum. Unless the fermion/boson sum is practically zero, the observed mass of the Higgs boson should be huge.
The only way to escape this conclusion is to somehow balance the fermion/boson sum to be exceedingly small. And to have the balance so perfect is utterly unnatural, as if we added up all the monthly paychecks of everyone in the United States and subtracted their monthly bills and those two huge numbers canceled out neatly.
That doesn’t happen in bookkeeping, and it shouldn’t happen in physics, either; unless, that is, there is some new and as-yet-undiscovered physical principle that enforces it. Thus, the small mass of the Higgs boson all but ensures that there is new physics to be discovered. Otherwise, we have to “tune” the masses of these particles to very precise values. Such precise balancing is utterly unnatural in physics theories, leading theoretical physicists to propose a series of ways in which this cancellation could occur naturally.
The most popular is a principle called supersymmetry. At the core of supersymmetry is the idea that, for every known fermion (quarks and leptons), there is a cousin boson (called squarks and sleptons) that we haven’t yet discovered. Similarly, for every known boson (e.g. photon, W, Z, gluon and Higgs boson), there is a cousin, also-undiscovered, fermion (called a photino, wino, zino, gluino and Higgsino). Because every fermion has a cousin boson (and vice versa), the fermion/boson sum is identically zero. Each particle cancels out exactly the effect of the cousin particle predicted by supersymmetry.
There are many technical issues that need to be addressed, not the least of which is that the predicted cousin particles have never been observed. But, so far, scientists can get around that problem. Thus supersymmetry remains an interesting idea.
If the particle found in July of 2012 is the Higgs boson, it definitely brings with it a very puzzling problem. As physicists begin to accept that the Higgs boson has likely been found, they are turning their attention to this most unnatural quandary. The main focus of the LHC is now becoming a search for a natural solution to this difficult question: Why is the Higgs so light?
The actual equation is the following: Mass(Higgs, observed)2 = Mass(Higgs, theoretical)2 + [k Λ]2 × [Mass(Z boson)2 + 2 × Mass(W boson)2 +Mass(Higgs, theoretical)2 – 4 × Mass(top quark)2 ]. k is a technical constant and Λ is the maximum energy that the theory applies.
Editor's picks for further reading
The Nature of Reality: Bittersweet Victory: Physics After the Higgs
A look at the implications of the Higgs on the future directions of physics research.
The Nature of Reality: Thanks, Mom! Finding the Quantum of Ubiquitous Resistance
In this blog post, physicist Frank Wilczek celebrates the July 4 Higgs announcement.
Quantum Diaries: Why The Higgs Boson Should Not Exist and Why This Is a Good Thing
Physicist Richard Ruiz asks why the Higgs boson is so light.
When I see those victorious Olympic athletes all bedecked on the podium, beaming their gold-medal smiles and crying their gold-medal tears, I can’t help thinking: Now what?
And now that the coming-out party for the Higgs (or the Higgs-like boson, if you must) is over—the bubbly popped, the headlines receded—are physicists asking themselves the same question?
Certainly, physicists are not crying into their champagne. The discovery of a new boson right where the Higgs should be is a scientific tour-de-force. “It confirms, as it completes, the Standard Model of fundamental physics,” Frank Wilczek wrote here on the morning of the announcement.
And yet, science thrives on observations that don’t match up with predictions. Dark energy and dark matter, two of the greatest discoveries in a century of astrophysics, were hit upon because of the yawning gap between prediction and observation. If the universe is a puzzle, dark energy and dark matter are odd-shaped pieces that puckishly refuse to be wedged into place and, in their refusal, open up the possibility that the puzzle is actually richer and more complex than we ever anticipated. The Higgs, on the other hand, snaps right into place with a satisfying “Eureka!”
But if the puzzle of the Standard Model is now complete, where does that leave physics?
“There’s this huge looming question: The Standard Model works impeccably, but it leaves a lot of things unexplained,” says David Kaiser, a physicist and science historian at MIT. The Standard Model does not account for gravity, for instance, and it provides no explanation for why the physical constants take the particular values that they do. Like the periodic table of the elements, the Standard Model is an utterly faithful census of the ingredients that make up our universe. But while we know the elegant atomic underpinnings of the motley periodic table, we are still seeking the deeper laws that are expressed in the Standard Model.
“I always felt the best possible thing for the LHC would be to not see the Higgs,” says Peter Woit, a theorist at Columbia University. That would have cracked the Standard Model wide open, perhaps giving scientists a glimpse of the deeper physics underlying it. In this sense, says Woit, “The Standard Model is a victim of its own success.” Though it fails to answer some fundamental questions about out universe, it is so impervious to experimental contradiction—so perfect in its predictions—that physicists may soon find themselves at an impasse.
“If this is really the Higgs, then we have completed the Standard Model,” says physicist Peter Fisher of MIT. “We have created this model that describes exquisitely the world around us. We could legitimately say that, as a field of endeavor, we’ve done all there is to be done, and ask: Is this a place to stop and reassess?”
Physicists do have some guesses at what may lie beyond the Standard Model. There’s supersymmetry, for one, which suggests that elementary particles have mirror-image “superpartners” that differ in spin. Yet, to the surprise of some physicists, even the LHC has been unable to turn up any evidence of these superpartners. That suggests that, if superpartners are out there, they don’t possess the neat mirror-image symmetry we expected. Instead, the mirror that divides “us” from “them” may be warped.
“With the Higgs, you knew exactly what to look for,” says Woit. But the mirror of supersymmetry, if it exists, “could be warped in any arbitrary way,” leaving physicists to pursue an almost limitless game of hide-and-seek. And what if the superpartners—or other hints of new physics—are hiding where the LHC can’t find them?
But the story of the Higgs isn’t over yet. Over the coming months, physicists on the CMS and ATLAS teams will look to see whether this thing they have found decays in the ways they expect. Perhaps the new boson will turn out to be not so “vanilla” after all. Historically, it is often the “one last measurement to nail it down” that ends up taking physics in a new direction, Kaiser points out.
To Nobel prize-winning physicist Frank Wilczek, finding the new boson is just the beginning. “Having won this glorious battle, I'm psyched up for complete victory. We need to see some of the new particles that low-energy supersymmetry predicts. I think that will eventually happen at the LHC.”
“There is also room for gratuitous, but not perverse, speculation about the Higgs being a ‘portal’ into hidden sectors—hypothetical worlds of particles that have neither strong nor weak nor electromagnetic interactions,” adds Wilczek.
Yet Steve Ahlen, a Boston University physicist who helped build the ATLAS detector, thinks that the story of the quest for the Higgs has a somewhat different moral: “The most impressive thing about the success of the LHC, CMS and ATLAS is that thousands of people from all over the world, supported by tax dollars from many hundreds of millions of people, achieved success without the promise of fortune, power or fame, but for the simple joy of observing the beautiful world we live in. I think there is an important lesson to be learned from that.”
The recent buzz over the discovery of a new boson that might be the long-sought quantum of the Higgs field has led some to forget that the Large Hadron Collider at CERN isn’t a single-purpose facility. Two large experiments each engage approximately 3,000 physicists in a concentrated effort to better understand the rules that govern the universe in which we live. These collaborations can study many different phenomena. One of the most tantalizing of these phenomena is called supersymmetry—and it could pick up where the Higgs left off.
There are many mysteries remaining in physics. While the Standard Model states that the Higgs boson gives subatomic particles their mass, it is quite silent on the specific masses held by each particle. Further, it doesn’t explain why so many different types of particles exist, nor does it explain why there are three forces and not two or 20.
Subatomic particles have a property called spin which can be usefully (and misleadingly) imagined as each particle being a tiny spinning ball, though the reality is that spin is an inherent property of these particles in the same way that electric charge is. Particles are divided into two classes based on their spin: Particles with half-number spin (1/2, 3/2, 5/2 and so on) are called fermions, and particles with whole-number spin are called bosons.
Supersymmetry proposes a new rule to govern the relationship between fermions and bosons. According to supersymmetry, the equations that describe the universe should work in exactly the same way if all fermion and boson terms are swapped. This implies that, for every particle known in the Standard Model, there should be an as-yet-undiscovered cousin particle. These cousin particles are identical to the known particles in every way except that they have different spin.
If supersymmetry is right, then the existing fermion quarks have cousin bosons called “squarks”; the lepton has a supersymmetric cousin called a slepton. For bosons, the naming convention is a little different: The bosons of the standard model (the gluon, photon and W and Z boson) have supersymmetric fermion cousins called the gluino, photino, wino and zino.
Though none of these particles has yet been observed, their very obscurity does offer us one important insight: If supersymmetry exists, it is not, in fact, symmetric. Recall that I said that the supersymmetric cousins of the familiar particles of the Standard Model were the same in every way except for their spin. This means that the selectron would have the same mass as the familiar electron and the up squark would have the same mass of the up quark. However, were this true, we would have discovered them already. Given that we haven’t, we can categorically say that supersymmetry in its ideal form has already been falsified.
However it could be that supersymmetry is mostly true, but “broken.” In the same way that an imperfect top might spin reasonably well, only to wobble a bit and end with a preferred side always touching the ground when it stops, perhaps the universe might have a supersymmetry that is mostly true. Just what mechanism breaks the symmetry between the Standard Model particles and the supersymmetric cousins is not known, although many ideas have been proposed.
So with this additional consideration, you are to be forgiven if you are suspicious of the whole idea. What is the reason for the interest in the idea of supersymmetry? Why have over ten thousand scientific papers (both experimental and theoretical) been written on the subject?
While there are several reasons to find the idea intriguing, one topical example is the way in which supersymmetry is thought to be linked to the Higgs boson. While we remain unsure if the boson we found in July is the Higgs boson, the new boson has a mass of about 125 GeV, or about 133 times heavier than a proton. This is an utterly unnatural value for the mass of the Higgs boson.
Why unnatural? The Higgs boson gains its own mass (in part) through its interaction with the other subatomic particles: the quarks and leptons and force carrying bosons. These particles should have a huge influence on the mass of the Higgs—on the scale of 1015 GeV. That’s over ten trillion times the observed mass of the new boson. So why isn’t the Higgs weighing in at that enormous mass?
First, we are helped because the contribution from the fermions and bosons are of opposite sign, so they can cancel each other out. But without invoking supersymmetry, it seems pretty suspicious that they would be so close in value. It’s uncanny, like a big bank simultaneously taking in a deposit of about a trillion dollars and making a loan of almost exactly the same amount down to a few bucks.
Supersymmetry can explain this quite easily, though. After all, for each particular fermion (say an electron), there is a corresponding boson (a selectron). Given the symmetry and the fact that fermions and bosons contribute with opposite signs, it is easier to see how these two corresponding particles could cancel each other out exactly. If supersymmetry were in fact perfectly symmetric, they would cancel each other perfectly and mass of the Higgs boson would be caused solely by its interaction with other Higgs bosons.
This example is but one in the myriad of phenomena which can be explained by supersymmetry. You should remember that we don’t know that supersymmetry is actually present in the universe; just because it works on paper doesn’t make it real. It makes it a cool idea. However scientists at the Large Hadron Collider are hot on supersymmetry’s trail. If supersymmetry is the answer to why the mass of the Higgs boson is small but not zero, we will find it at the LHC.
Editor's picks for further reading
Nature's Blueprint: Supersymmetry and the Search for a Unified Theory of Matter and Force
Theoretical astrophysicist Dan Hooper's book on supersymmetry and the LHC's role in the search for evidence of supersymmetry.
Scientific American: Is Supersymmetry Dead?
Davide Castelvecchi asks what it will mean for physics if the LHC does not turn up evidence of supersymmetry.
Supersymmetry: Unveiling the Ultimate Laws of Nature
Physicist Gordon Kane's accessible 2001 book on supersymmetry.