Does the Higgs boson have a cousin?
Two teams of physicists working independently at the Large Hadron Collider at CERN, the European Organization for Nuclear Research,reported on Tuesday that they had seen traces of what could be a new fundamental particle of nature.
One possibility, out of a gaggle of wild and not-so-wild ideas springing to life as the day went on, is that the particle — assuming it is real — is a heavier version of the Higgs boson, a particle that explains why other particles have mass. Another is that it is a graviton, the supposed quantum carrier of gravity, whose discovery could imply the existence of extra dimensions of space-time.
At the end of a long chain of “ifs” could be a revolution, the first clues to a theory of nature that goes beyond the so-called Standard Model, which has ruled physics for the last quarter-century.
It is, however, far too soon to shout “whale ahoy,” physicists both inside and outside CERN said, noting that the history of particle physics is rife with statistical flukes and anomalies that disappeared when more data was compiled.
A coincidence is the most probable explanation for the surprising bumps in data from the collider, physicists from the experiments cautioned, saying that a lot more data was needed and would in fact soon be available.
“I don’t think there is anyone around who thinks this is conclusive,” said Kyle Cranmer, a physicist from New York University who works on one of the CERN teams, known as Atlas. “But it would be huge if true,” he said, noting that many theorists had put their other work aside to study the new result.
When all the statistical effects are taken into consideration, Dr. Cranmer said, the bump in the Atlas data had about a 1-in-93 chance of being a fluke — far stronger than the 1-in-3.5-million odds of mere chance, known as five-sigma, considered the gold standard for a discovery. That might not be enough to bother presenting in a talk except for the fact that the competing CERN team, named C.M.S., found a bump in the same place.
“What is nice is that it is not a particularly crazy signal, in a quite clean channel,” said Nima Arkani-Hamed, a particle theorist at the Institute for Advanced Study in Princeton, N.J., speaking before the announcement. “So, while we are nowhere near moving champagne even vaguely close to the fridge, it is intriguing.”
Physicists could not help wondering if history was about to repeat itself. It was four years ago this week that the same two teams’ detection of matching bumps in Large Hadron Collider data set the clock ticking for thediscovery of the Higgs boson six months later. And so the auditorium at CERN, outside Geneva, was so packed on Tuesday that some officials had to sit on the floor for a two-hour presentation about the center’s recent work that began with the entire crowd singing “Happy Birthday” to Claire Lee, one of the experimenters, from Brookhaven National Laboratory on Long Island.
At one point, Rolf Heuer, the departing director-general of CERN, tried to get people to move off the steps, declaring they were a fire hazard. When they did not move, he joked that he now knew he was a lame duck.
When physicists announced in 2012 that they had indeed discovered the Higgs boson, it was not the end of physics. It was not even, to paraphrase Winston Churchill, the beginning of the end.
It might, they hoped, be the end of the beginning.
The Higgs boson was the last missing piece of the Standard Model, which explains all we know about subatomic particles and forces. But there are questions this model does not answer, such as what happens at the bottom of a black hole, the identity of the dark matter and dark energy that rule the cosmos, or why the universe is matter and not antimatter.
The Large Hadron Collider was built at a cost of some $10 billion, to speed protons around an 18-mile underground track at more than 99 percent of the speed of light and smash them together in search of new particles and forces of nature. By virtue of Einstein’s equivalence of mass and energy, the more energy poured into these collisions, the more massive particles can come out of them. And by the logic of quantum microscopy, the more energy they have to spend, the smaller and more intimate details of nature physicists can see.
Parked along the underground racetrack are a pair of mammoth six-story conglomerations of computers, crystals, wires and magnets: Atlas and C.M.S., each operated by 3,000 physicists who aim to catch and classify everything that comes out of those microscopic samples of primordial fire.
During its first two years of running, the collider fired protons, the building blocks of ordinary matter, to energies of about four trillion electron volts, in the interchangeable units of mass and energy that physicists prefer. By way of comparison, the naked proton weighs in at about one billion electron volts and the Higgs boson is about 125 billion electron volts.
Since June, after a two-year shutdown, CERN physicists have been running their collider at nearly twice the energy with which they discovered the Higgs, firing twin beams of protons with 6.5 trillion electron volts of energy at each other in search of new particles to help point them to deeper laws.
The main news since then has been mainly that there is no news yet, only tantalizing hints, bumps in the data, that might be new particles and signposts of new theories, or statistical demons.
The most intriguing result so far, reported on Tuesday, is an excess of pairs of gamma rays corresponding to an energy of about 750 billion electron volts. The gamma rays, the physicists said, could be produced by the radioactive decay of a new particle, in this case perhaps a cousin of the Higgs boson, which itself was first noticed because it decayed into an abundance of gamma rays.
Or it could be a more massive particle that has decayed in steps down to a pair of photons. Nobody knows. No model predicted this, which is how some scientists like it.
“The more nonstandard the better,” said Joe Lykken, the director of research at the Fermi National Accelerator Laboratory and a member of one of the CERN teams. “It will give people a lot to think about. We get paid to speculate.”
Maria Spiropulu, a professor at Caltech and member of one of the detector teams, said, “As experimentalists, we see a 750-billion-electron-volt beast decaying to two photons.” Explaining it, she added, is up to the theorists.
The new results are based on the analysis of some 400 trillion proton-proton collisions.
If the particle is real, Dr. Lykken said, physicists should know by this summer, when they will have 10 times as much data to present to scientists from around the world who will convene in Chicago, Fermilab’s backyard.
Such a discovery would augur a fruitful future for cosmological wanderings and for the CERN collider, which will be running for the next 20 years. It could also elevate proposals now on drawing boards in China and elsewhere to build even larger, more powerful colliders.
“We are barely coming to terms with the power and the glory” of the CERN collider’s ability to operate at 13 trillion electron volts, Dr. Spiropulu said in a text message. “We are now entering the era of taking a shot in the dark!”
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