A vast physics experiment - the Large Hadron Collider (LHC) - reaches a key milestone this weekend ahead of an official start-up on 10 September.

Engineers had previously brought a beam of protons - tiny, sub-atomic particles - to the ""doorstep"" of the LHC.
Today, protons will be piped through LHC magnets for the first time.
The most powerful physics experiment ever built, the LHC will re-create the conditions present in the Universe just after the Big Bang.
For the two-day ""synchronization test"", engineers will thread a low intensity beam through the injection system and one of the LHC's eight sectors.
These two sectors have now reached a sufficient level of readiness to handle the energetic stream of particles, and this opened up the opportunity to run the test.
The purpose of the test is to help ensure that the LHC is working in step with its injector, known as the Super Proton Synchrotron (SPS) accelerator.
Before a beam can be injected into the main ring, the proton beams have to be boosted to high energies in a chain of particle accelerators called injectors.
The SPS is the last link in this chain of injectors; it is from here that protons are fed directly into the LHC ring via two ""injection lines"" - one for each beam.
""The aim is to get the timings right between the two machines and in order to do that we will take some beam into sector 2-3,"" said Roberto Saban, the LHC's head of hardware commissioning.
Cern, the organization that operates the collider, said it will attempt to circulate two proton beams all the way around the ring on 10 September. This is considered the giant lab's official ""switch-on"".
""It's been a long haul, and we're all eager to get the LHC research program underway,"" said Lyn Evans, the project leader. This full beam injection will take place at an energy of about 450 gigaelectronvolts (GeV). Over subsequent weeks, engineers will gradually boost the energy and fine tune the machine.
The proton beams are fired down pipes running through the magnets. Once engineers have two beams running, they will travel in opposite directions around the main ring at super-fast speeds. Beam collision
When the collider is commissioned, the beams will cross paths at allotted points along the tunnel, smashing into one another with cataclysmic force.
Scientists hope to see new particles in the debris of these collisions, revealing fundamental new insights into the nature of the cosmos and how it came into being.
In order to obtain high magnetic fields without consuming too much power, the LHC's magnets need to be ""superconducting"".
This is the property, exhibited by some materials at very low temperatures, to channel electrical current with zero resistance and very little power loss.
This requires cooling the magnets to a temperature of 1.9 Kelvin (-271C; -456F). Six out of eight sectors are currently at their operating temperatures; cooling of the remaining two should be completed in the next few weeks.
Over August, scientists will continue electrical testing of the LHC hardware prior to circulating beams in early September. This phase will continue into September to ensure that the entire machine is capable of accelerating and colliding beams at an energy of five teraelectronvolts (TeV).
Once stable, circulating beams have been established, they will be smashed together - in preparation for the LHC's science phase. BBC Radio 4 will broadcast live from Cern on 10 September.
(Source: BBC)