Image copyright Jonathan Webb/BBC Image caption Experiments like CMS, pictured during the shutdown, are now sealed up and colliding protons

The Large Hadron Collider has smashed protons together for the first time since early 2013.

The low-energy collisions, part of preparations for the next round of experiments, began on Tuesday morning.

Proton beams circled the LHC and collided at an energy of 450 gigaelectronvolts (GeV) per beam.

The aim for this second run of the LHC, following its planned two-year shutdown for repairs and improvements, is to stage collisions at 7,000 GeV per beam.

A key milestone was reached on Easter Sunday when proton beams once again did full circuits of the LHC's 27km subterranean circle.

Now the two beams have been steered into each other - another important step in the gradual restart of the world's biggest particle accelerator.

'Shopping list' of checks

According to the current schedule, collisions will first take place at those new, historic energies in the week commencing June 1st. Even those will only be used for calibration; "physics collisions" with usable results will start some time after that.

Image copyright ATLAS/CERN Image caption The Atlas experiment released this snapshot of tracks produced by fallout from one of the first collisions

Image copyright CMS/CERN Image caption A similar plot was released by the CMS experiment

Excitement is building for physicists working at Cern - the European nuclear research campus near Geneva, Switzerland, which houses the LHC.

What is an electronvolt? Image copyright SPL Image caption The LHC's experiments create showers of particles Particle accelerators use strong electric fields to speed up tiny pieces of matter

An electronvolt (eV) is the energy gained by one electron as it accelerates through a potential of one volt

The LHC reaches particle energies measured in trillions of eV: teraelectronvolts (TeV)

This is only the energy in the motion of a flying mosquito - per particle

All together, the hundreds of trillions of particles in each LHC beam have the energy of a TGV high-speed train travelling at 150 km/h

"These are the first colliding beams in the machine for over two years, and it brings home that physics collisions are close," said Prof Tara Shears from the University of Liverpool, who works on LHCb - one of four big experiments, spaced around the LHC's ring, where the collisions occur.

She said these early collisions are very valuable, even though they are only happening at injection energy, which means the LHC itself isn't adding any acceleration to the protons. They simply circulate and collide with the energy already delivered by the accelerators that feed protons into the main ring.

Instead of getting physics results, Prof Shears and her team use this early data to fine-tune their experiments. "This time, we used the data to make sure subdetectors are time-aligned with each other," she told BBC News.

"We have a shopping list of checks to do; checking the trigger, our luminosity calculation, the performance of each part of our experiment, and we'll work through this with these collision runs."

Some of the experiment teams released images illustrating Tuesday's first collisions. They show the various paths taken by debris from smashed-together protons, pieced together using data from some of the layers of detectors inside the experiments.

The LHC was the scene for the famous discovery of the Higgs boson in 2012, towards the end of its first run. In its second tilt, researchers hope that the almost doubled energy levels will yield new insights beyond the Standard Model of particle physics.

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