Physicists and science enthusiasts were excited last month when the Large Hadron Collider, the most ambitious particle accelerator ever built, went online. Nine days later, the accelerator was shut down because of a helium leak. (The superconducting magnets that steer particles on their 27-kilometer collision course are cooled with large volumes of liquid helium.)

Yesterday, CERN, the European Organization for Nuclear Research, released a report detailing what went wrong. Steven Nahn, an MIT physics professor currently working from CERN, says that the analysis took some time because the area had to be warmed up from near absolute zero before it could be accessed for investigation. The problem, the report concludes, arose because of a faulty electrical connection between two magnets, which led to mechanical problems.

“The fact that this happened surprised no one in this business,” says Nahn. “You’re just starting up a machine that’s taken you 20 years to build, you’re gonna run into some problems–you can’t possibly foresee everything.” Over the next several years, Nahn and his thousands of collaborators hope to use the accelerator to solve long-standing physics problems, such as why fundamental particles have mass.

The collider is slated to go online again in early 2009.