The Higgs Boson: Searching for the God Particle
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Surprisingly (to the outsider) this was all considered old news. Repeatedly, the theorists joked that, with the exception of the actual CERN experimentalists present, all of us know that the Higgs has now been discovered with a mass of 125 GeV/c2. (It hasn’t, quite, but the hints are strong.) The message was clear: “We’ve known for decades that the Higgs is going to be found. So break open the champagne and get the celebrating over with, because what we really want to know is — which is the correct version of Beyond the Standard Model physics?” With a brief nod to large extra dimensions (a Dimopoulos, and associates A, D and also I, idea) and a fond farewell to Technicolor (another idea that Dimopoulos helped advance), the focus turned again and again to the likely suite of Supersymmetric (SUSY) particles (yet another stock in which Dimopoulos is heavily invested).
Supersymmetry – a theory that posits that for every known particle there is another (or more than one) yet-to-be-discovered partner particle – is the leading candidate for physics Beyond the Standard Model. It is central to string theory (a.k.a. super-string theory), required for gauge coupling unification (see below), useful for solving the Higgs Fine Tuning Problem (definitely see below) and also gives us the leading candidate for dark matter – the Lightest Supersymmetric Particle (LSP).
But I’m getting way ahead of myself, and probably you. Especially since I and my colleagues have come to believe that the principal indictment of the Standard Model, which has been used to argue so forcefully for Beyond the Standard Model (BSM) physics is, hmmm, dubious. Or as one of those colleagues would say – completely wrong. A main rationale for supersymmetry evaporates on closer inspection.
So what is Beyond The Standard Model (BSM) physics, why are people so convinced it is around the corner, and should they be?
At least since the discovery of the W and Z particles at CERN in 1983, physicists have been pretty much convinced that the Standard Model (SM) that emerged from the late 1960s and early 1970s is the correct model of fundamental physics. At least at energies below the so-called weak-scale – a few hundred GeV – or maybe a few times that. But particle theorists variously hoped/expected/knew that at higher energies the Standard Model was not the whole story, and a more fundamental theory would need to be found.
There are two types of reasons to doubt the completeness of the Standard Model – aesthetic (philosophical) and mathematical.
Aesthetic problem number one, physicists adore simplicity. Zero and one are our favorite numbers. Two can be suffered. After two comes “too many”, although identical copies (twins, triplets, …) may receive special dispensation. The Standard Model has too many too-many’s: three fundamental forces (a.k.a. gauge groups); way too many fundamental fermions (particles that make up matter)– three generations each with at least 5 representations (groups) of them — plus three sets of gauge bosons and the set of particles of which the Higgs boson is a member. It also has far too many (more than 20) independent parameters.
Aesthetic problem number two –– for no apparent reason the weak scale is much (as in about 1016 times) smaller than what we believe to be the fundamental energy scale of physics – the Planck scale (about 1019 GeV), a scale set by the strength of gravity (the one fundamental force not included in the Standard Model). This is known as the (Weak) Hierarchy Problem – and can also be understood in terms of the absolutely enormous strength of the three Standard Model forces compared to that of gravity between pairs of fundamental particles separated by appropriately microscopic scales.
It is however the technical problem that has carried the most weight in convincing people that there must be physics beyond the Standard Model. It is the story we tell our children — quantum mechanics makes the Standard Model unstable. Quantum mechanics teaches us that, as a particle such as a Higgs boson travels along, it can emit and reabsorb another particle. This process represents a “loop contribution” to the mass of the Higgs boson, so-called because a pictorial representation of the process – Feynman diagrams – depicts these processes as loops attached to the traveling Higgs boson.
Unfortunately, when you add up the loop contributions to the mass of the Higgs boson from all possible particles with all possible energies and momenta, they appear to be infinite or at least proportional to the maximum possible momentum that can be carried. For technical reasons these are called quadratic divergences and are widely derided. For the actual Higgs boson mass to be finite, there must apparently be subtle and precise cancellation of the loop contributions against the underlying “tree” (loop-free) mass. This Higgs Fine-Tuning Problem, so the lore tells us, must be remedied.
BSM physics is the proposed remedy. Supersymmetry cancels the loop of every known particle against the loop of an as-yet-to-be-discovered partner particle. Technicolor eliminates the Higgs boson – replacing it by a composite of new particles called techni-quarks. If there are large extra dimensions then the largest momentum that can circulate in a loop is actually only a little larger than the weak scale. Clearly BSM physics is not just desirable but essential.
Recently, however, my colleague Bryan Lynn suggested, and together with Katie Freese and Dmitry Podolsky, he and I explained, how the Standard Model actually comes up with a remedy all on its own.
The Higgs boson is one member of a set of quadruplets in the Standard Model. At energies below the weak scale, its three siblings get eaten – they get incorporated into the W and Z bosons. According to a famous theorem due to MIT’s Jeffrey Goldstone (hence “Goldstone’s Theorem”), the masses of the three siblings must be exactly zero. In particular, the quadratically divergent contribution to their masses are zero.
Although this doesn’t force the mass of the Higgs boson to be zero (a good thing, since it seems likely to be about 125 GeV/c2), it does mean that the quadratic divergences in the Higgs mass that have worried us for decades are not a problem of the Standard Model after all.
Now, not everybody buys our argument. Some of them prefer to focus on the aesthetic challenge of the Weak Hierarchy Problem, while others argue that we have no choice but to add quantum gravity to the Standard Model, inevitably resurrecting the Higgs Fine Tuning Problem.
We would counter that the absence of a Higgs Fine Tuning Problem in the Standard Model is such a virtue that, absent any hard evidence for BSM physics, preserving the Standard Model’s Goldstone miracle should be taken as a requirement of any proposed BSM theories.
The implication is clear. If there is no problem, there may be no need for a solution. Beyond the Standard Model Physics isn’t ruled out by the absence of a Higgs Fine Tuning Problem in the Standard Model, but it does mean that the Standard Model may well be the whole story, or at least the whole story at the energies that the LHC can command. In short, don’t be surprised if the Higgs is the last new particle discovered by the LHC. Theorists may hunger for physics beyond the Standard Model, but nature may be quite content without it, thank you very much.
-Originally published: Scientific American online, June 20, 2012
If You Want More Higgs Hype, Don’t Read This Column
By John Hogan
So it’s finally, probably, maybe, happened. Although they are still hedging a bit, physicists at CERN, the European Organization for Nuclear Research, announced in July that they had found the long-sought Higgs boson. First postulated almost a half century ago by physicist Peter Higgs (who attended the press conference at CERN) and others, the Higgs particle is believed to confer mass to quarks, electrons and other building blocks of our world.
After presentations by two groups gathering data from CERN’s Large Hadron Collider, CERN director general Rolf Dieter Heuer said, according to The Independent, “As a layman, I would say that I think we have it. Do you agree?” After the audience erupted into applause, Heuer added, “We have a discovery. We have observed a new particle consistent with a Higgs Boson…but which one, it remains open.” “If scientists are lucky,” Dennis Overbye wrote in The New York Times, “the discovery could lead to a new understan
ding of how the universe began.”
But a few reports were tinged with gloom. Physicist-journalist Adrian Cho noted in Science that “even as physicists celebrate, the discovery raises worries among some that there may remain no new physics that can be discovered with the atom-smasher.” Cho quoted Nobel laureate Steven Weinberg: “My nightmare, and it’s not just me, but a lot of us [in particle physics], is that the LHC discovers the Higgs boson and nothing else… That would be like closing a door.”
I offered my glum take on the Higgs and the future of physics last December, after reports that the LHC had turned up “tantalizing hints” of the Higgs unleashed the hounds of hype. I was especially annoyed by a Wall Street Journal essay, “The ‘God Particle’ and the Origins of the Universe,” in which physicist Michio Kaku exulted, “Physicists around the world have something to celebrate this Christmas.” Responding to my column, Kaku called me “an agent provocateur, throwing flames in all directions, and hoping that some of them may start a fire.” But he conceded that I raised “real and thoughtful scientific questions.” Because my views haven’t changed—and because I think they are still relevant—I’m reprinting an edited version of that December column. Here goes:
The Higgs has long been a mixed blessing for particle physics. In the early 1990s, when physicists were pleading—ultimately in vain–with Congress not to cancel the Superconducting Supercollider, which was sucking up tax dollars faster than a black hole, the Nobel laureate Leon Lederman christened the Higgs “the God particle.” This is scientific hype at its most outrageous. If the Higgs is the “God Particle,” what should we call an even more fundamental particle, like a string? The Godhead Particle? The Mother of God Particle?
Lederman himself confessed that “the Goddamn Particle” might have been a better name for the Higgs, given how hard it had been to detect “and the expense it is causing.” A more fundamental problem is that discovering the Higgs would be a modest, even anti-climactic achievement, relative to the grand ambitions of theoretical physics. The Higgs would serve merely as the capstone of the Standard Model of particle physics, which describes the workings of electromagnetism and the strong and weak nuclear forces. The Standard Model, because it excludes gravity, is an incomplete account of reality; it is like a theory of human nature that excludes sex. Even Kaku has called the Standard Model “rather ugly” and “a theory that only a mother could love.”
Our best theory of gravity is still general relativity, which does not mesh mathematically with the quantum field theories that comprise the Standard Model. Over the past few decades, theorists have become increasingly obsessed with finding a unified theory, a “theory of everything” that wraps all of nature’s forces into one tidy package. Hearing all the hoopla about the Higgs, the public might understandably assume that it represents a crucial step toward a unified theory–and perhaps at least tentative confirmation of the existence of strings, branes, hyperspaces, multiverses and all the other fantastical eidolons that Kaku, Stephen Hawking, Brian Greene and other unification enthusiasts tout in their bestsellers.
But the Higgs doesn’t take us any closer to a unified theory than climbing a tree would take me to the Moon. As I’ve pointed out previously, string theory, loop-space theory and other popular candidates for a unified theory postulate phenomena far too minuscule to be detected by any existing or even conceivable (except in a sci-fi way) experiment. Obtaining the kind of evidence of a string or loop that we have for, say, the top quark would require building an accelerator as big as the Milky Way.
Kaku asserted that finding the Higgs “is not enough. What is needed is a genuine theory of everything, which can simply and beautifully unify all the forces of the universe into a single coherent whole—a goal sought by Einstein for the last 30 years of his life.” He insisted that we are at “the beginning, not the end of physics. The adventure continues.” Maybe. But I’m not hopeful. Whether or not physicists find the Goddamn Particle, the quest for unification, which has given physics its glitter over the past half century, looks increasingly like a dead end.
Almost 10 years ago, I put my money where my mouth is. The Long Now Foundation, a nonprofit that encourages long-term thinking, asked a bunch of people to make bets about trends in science, technology and other realms of culture. I bet Kaku $1,000 that by the year 2020, “no one will have won a Nobel Prize for work on superstring theory, membrane theory or some other unified theory describing all the forces of nature.” (Lee “loop space” Smolin was my original counter-bettor but backed out at the last minute, the big chicken.)
Kaku and I each put up $1,000 in advance, which the Long Now Foundation keeps in escrow. If civilization–or more importantly, the Long Now Foundation–still exists in 2020, it will give $2,000 to a charity designated by me (the Nature Conservancy) or Kaku (National Peace Action). In defending my bet, I stated:
“The dream of a unified theory, which some evangelists call a ‘theory of everything,’ will never be entirely abandoned. But I predict that over the next twenty years, fewer smart young physicists will be attracted to an endeavor that has vanishingly little hope of an empirical payoff. Most physicists will come to accept that nature might not share our passion for unity. Physicists have already produced theories–Newtonian mechanics, quantum mechanics, general relativity, nonlinear dynamics–that work extraordinarily well in certain domains, and there is no reason why there should be a single theory that accounts for all the forces of nature. The quest for a unified theory will come to be seen not as a branch of science, which tells us about the real world, but as a kind of mathematical theology.”
I added, however—and this is both mawkish tripe and the truth–that “I would be delighted to lose this bet.”
-Originally published: Scientific American online July 4, 2012.