After two-week extension to a two-year upgrade, Europe's Large Hadron Collider particle accelerator is up and running again.
LHC physicists used the mammoth machine to discover the Higgs boson in 2012 but had shut it down for improvements that will let it. That should extend their ability to search for different varieties of Higgs boson-- sometimes referred to as the "God particle" for its role as one of the basic cogs in the machinery of the universe -- as well as dark matter, supersymmetry and other mysteries of how the universe works.
CERN, the European Center for Nuclear Research, operates the facility built into an underground ring 27 km in circumference. "Today, CERN's heart beats once more to the rhythm of the LHC," CERN Director General Rolf Heuer said in a statement on Sunday.
CERN had hoped to restart the LHC about two weeks ago, but an intermittent short-circuit problem put those plans on hold. Making repairs on the machine can be difficult: its super-powerful electromagnets need extremely cold temperatures to work, but warming up a section of the ring to send in personnel takes a long time. Cooling the machine back down -- to its operating temperature of 1.9 degrees Celsius above absolute zero (minus 456 Fahrenheit) also takes time.
The LHC's physics phenomena are hard for a lot of people to grasp, but the motivation to understand how the universe works is widespread. Twenty-one member states oversee CERN, but citizens from many other countries also help design and run its experiments. In addition, about 3,000 students are involved in the present operations. CERN is based near Geneva, Switzerland, though the ring itself extends underneath the border with France.
Universities in the United States are heavily involved, though the US is only an observer state. US researchers had hoped for an accelerator called the Superconducting Super Collider even more powerful than the LHC. But the US Congress canceled the project in 1993 after the US already had spent $2 billion planning and building the facility in Waxahachie, Texas.
LHC researchers hope to find more Higgs bosons -- siblings to the single type discovered in 2012. Higgs bosons are particles stemming from the Higgs field, which imbues other more ordinary particles like quarks and electrons with mass.
Researchers also hope to find evidence of "supersymmetry," an idea that posits an entirely new set of particles that so far haven't been directly observed. Supersymmetry holds one explanation for the mysterious dark matter that amounts for mass in the galaxy than the conventional matter of which we're made. Dark matter interacts through gravitational forces, keeping stars within spinning galaxies from being flung apart, but not through the chemical and electromagnetic interactions that make up humans' daily lives.
The LHC uses two beams of protons circulating in opposite directions at nearly the speed of light. Once facility operators are satisfied things are working smoothly, they steer the beams to intersect at particular experiments. There, complicated detectors and data-capturing equipment register the sprays of short-lived, high-energy particles that result from such collisions.
Analyzing those sprays shows evidence of what happens at the super-high energy conditions that prevailed in the universe only moments after the Big Bang began the universe.
It'll take some weeks of testing and tuning before the LHC is working well enough to begin physics experiments again. Once it begins, CERN plans to turn up the intensity of the beams, stuff more protons into each of the bundles that circulate, and stuff more bundles per second into the accelerator. All that has the effect of increasing the number of collisions, which means it's more likely physicists can spot unlikely events that show what's going on.
But all that begins by just getting the beams running.
"Beam 1, the second to circulate, is successfully through all sectors of the LHC! Startup complete! #RestartLHC," CERN said in a tweet at 12:28 p.m. local time, or 3:28 a.m. PT.