Hard Times for Theorists in a Post-Higgs World 

 

The Large Hadron Collider’s big success leaves no clear avenue for new physics

In 1964, Peter Higgs (left) proposed the existence of a particle that is now named for him. Now young theorists like Flip Tanedo (right) wonder what’s next.

From left: Murdo Macleod; courtesy of Xiaoyue Guo

At 5 a.m. last Fourth of July, Flip Tanedo rolled out of bed after an hour of repeatedly smacking his alarm clock’s snooze button. Rousting himself at dawn would be worth it, he hoped, because what he was about to hear was likely to have a huge bearing on the course of his career.

Tanedo, a fifth-year theoretical physics Ph.D. candidate at Cornell University, tuned in to a live video feed from Geneva and listened intently as physicists working with the world’s largest particle accelerator discussed a momentous discovery. Data from the Large Hadron Collider revealed what looked very much like the long-sought Higgs boson. The product of a decades-long effort by thousands of physicists, the discovery solidified the leading theory of particle physics, the standard model. The Higgs particle confirmed the existence of a field that permeates the universe, imparting certain subatomic particles with mass while letting photons and other massless particles pass unimpeded.

Even from 4,000 miles away, the excitement was palpable. Two hours earlier, when the discovery was formally announced, hundreds of experimentalists who had sifted through the noise of more than a thousand trillion particle collisions to identify the Higgs entered into sustained applause, about as raucous as particle physicists get. British physicist Peter Higgs, who in 1964 proposed the particle that now bears his name, removed his glasses and wiped away tears.
While Tanedo shared the enthusiasm of his colleagues on the screen, he also had an unsettled feeling. As a theorist his job is to speculate on the inner workings of the universe. Theorists love proposing the existence of new particles and forces, but their theories must be consistent with the findings of past experiments. That makes deviations from the expected like catnip to theorists — opportunities to come up with novel explanations.
But with every new speaker in Geneva, it gradually became clear that there was nothing particularly surprising about this newest addition to the particle zoo. The experimental work seemed to fit perfectly with existing theory. “It wasn’t until a few hours after the talk that I started thinking, ‘OK, what’s next for us?’ ” Tanedo says.
That is the question many theoretical physicists are asking themselves right now. A year after the announcement, the latest analyses confirm a Higgs boson that is as vanilla as Tanedo initially feared.

 

MEASURE OF EFFORT

 

Scientists proposed the Higgs’ existence nearly five decades ago, but the search intensified when proton collisions began at the LHC. The numbers above refer to the LHC’s operation from November 2009 to December 2012.

Source: Fermilab/DOE; Icons: M. Atarod

In confirming the crowning theoretical achievement of 20th century physics, the LHC did exactly what it was designed to do. But Tanedo and other theorists had clung to loftier goals. Although the standard model explains extraordinarily well the particles and forces that dictate much of the world around us, it ignores gravity, and it doesn’t meld with Einstein’s theory of general relativity. And there’s much that the standard model can’t address at all: the dark matter that clumps around, within and between galaxies, for example, and the dark energy that is increasingly stretching the universe apart. In essence, even a complete standard model is incomplete, describing stuff that collectively composes a mere 5 percent of the mass-energy content of the universe. The rest is a mystery.
Scientists had hoped that clues to that mystery — or at least hints about how to start solving it — might emerge from the debris of smashed protons at the LHC. Some expected the machine to detect particles of dark matter; others thought it might find evidence of extra dimensions or of supersymmetry, a popular theory that predicts a menagerie of heavy particles. Ideally, discovering the unexpected within the subatomic shrapnel would allow theoretical physicists to expand the standard model into a stronger theory that more fully explains how the universe works.
Yet as the LHC shuts down for two years of repairs after three years of collisions, it has yet to reveal a single surprise. Adding insult to injury, other intensive physics experiments over the last year have also failed to reveal anything truly exotic. Nature’s secrets, at least for the time being, are frustratingly out of reach. Physicists are now banking on revamped theories and a few peculiar clues that have popped up in a handful of experiments to advance the standard model. “It’s gradually become more and more sobering,” Tanedo says.