The Large Hadron Collider has begun smashing protons together for the first time since it was shut down in 2013 for a 2-year upgrade intended to push the field of particle physics to new frontiers.

The first runs of particle collisions were at relatively low energy levels, part of testing and preparation for the next round of experiments at CERN, the European nuclear research facility near Geneva, Switzerland.

Beams of protons were sent in opposite directions around the 17-mile circumference of the LHC 450 then smashed together with gigaelectronvolts (GeV) of energy.

The current round of experiments is expected to eventually reach energies of 6.5 teraelectronvolts (TeV) per beam to yield collisions at 13 TeV.

On Tuesday, the first collisions sent subatomic particles cascading into the machine's giant detectors.

"It's a nice milestone today," said Dave Charlton, spokesperson for the multipurpose Atlas detector. "There were a lot of smiling faces in the control room today."

In addition to Atlas, the LHC has three other detectors - CMS, Alice and LHCb - all of which recorded collisions in the new tests.

The low power collisions are allowing physicists to tune and calibrate their experiments in preparation for the higher-energy runs.

"So just as the LHC team tests each component, system, and algorithm one after the other, the experiments go through checklists that confirm that everything is fully functional and no mistakes, bugs or failures are present when collisions are delivered at 13 TeV," CERN said in a release.

The scientists at the LHC made news in 2012 with their discovery of the Higgs boson, the particle that underlies the Standard Model of physics by conferring mass on other fundamental particles.

The new, high-power experiments may yield evidence of the existence of dark matter particles or clues to why there is more matter in the universe than antimatter, they say.

LHC operations are halfway through a scheduled 8-week period of testing of all systems to ensure they are working as expected in preparation for high-energy collisions set to begin in June.

Scientists and engineers from more than 100 countries have been involved in the design, construction and operation of the LHC, as have hundreds of laboratories and universities.

Construction took 10 years, from 1998 until the first beams were sent through it in 2008.