Any perturbation, however, such as a bad soldering job on a splice, can cause resistance and heat the cable and cause it to lose its superconductivity in what physicists call a “quench.” Which is what happened on Sept. 19, when the junction between two magnets vaporized in a shower of sparks, soot and liberated helium.

Technicians have spent most of the last year cleaning up and inspecting thousands of splices in the collider. About 5,000 will have to be redone, Steve Myers, head of CERN’s accelerator division, said in an interview.

The exploding splices have diverted engineers’ attention from the mystery of the underperforming magnets. Before the superconducting magnets are installed, engineers “train” each one by ramping up its electrical current until the magnet fails, or “quenches.” Thus the magnet gradually grows comfortable with higher and higher current.

All of the magnets for the collider were trained to an energy above seven trillion electron volts before being installed, Dr. Myers said, but when engineers tried to take one of the rings’ eight sectors to a higher energy last year, some magnets unexpectedly failed.

In an e-mail exchange, Lucio Rossi, head of magnets for CERN, said that 49 magnets had lost their training in the sectors tested and that it was impossible to estimate how many in the entire collider had gone bad. He said the magnets in question had all met specifications and that the problem might stem from having sat outside for a year before they could be installed.

Retraining magnets is costly and time consuming, experts say, and it might not be worth the wait to get all the way to the original target energy. “It looks like we can get to 6.5 relatively easily,” Dr. Myers said, but seven trillion electron volts would require “a lot of training.”

Many physicists say they would be perfectly happy if the collider never got above five trillion electron volts. If that were the case, said Joe Lykken, a Fermilab theorist who is on one of the CERN collider teams, “It’s not the end of the world. I am not pessimistic at all.”