The world's largest physics experiment is on hold until spring while scientists and engineers try to figure out what caused a helium leak into the tunnel deep beneath the Large Hadron Collider, its operator says.

Making the tunnel warm enough for humans, then giving them the time to inspect the magnets blamed for the Sept. 19 leak, will take three to four weeks, the European Organization for Nuclear Research (CERN) says in a statement. CERN believes a faulty electrical connection between two magnets that will guide protons in planned collision studies is behind the leak.

After the magnets are inspected and fixed, the collider must then undergo a scheduled winter maintenance, CERN says.

CERN Director General Robert Aymar admits that the delay "is undoubtedly a psychological blow."

“Nevertheless … I have no doubt that we will overcome this setback with the same degree of rigor and application,” Aymar says.

When we spoke to Judy Jackson of Fermilab yesterday, she told us that "there inevitably are going to be setbacks along the way — it's part of the process" of starting up a particle accelerator. You can read more about what she and theoretical physicist Sean Carroll had to say about the unexpected delays here.

(Image of LHC beam pipe by CERN)