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.