Just after I finished saying that there aren’t any clear “start” dates, CERN defined one. In this press release, the Conseil announced that on September 10, a 450 GeV beam will circulate the LHC storage ring. Note that “beam” is singular: colliding beams are not expected until 1 or 2 months later (that part was not in the press release).
Why 450 GeV? That’s the energy of the Super Proton Synchrotron (SPS), which feeds the LHC. Protons entering the LHC will actually get a tour of CERN’s history, as they start in the Proton Synchrotron (PS), CERN’s first collider built in the 1950′s, are injected into the SPS (15 days older than me), and finally into the LHC. At each stage, the energy increases to the capacity of that accelerator, and then the protons are passed on to the next.
The SPS accelerates protons to 450 GeV, and on September 10, they will simply be poured into the main ring without further acceleration. This is prudent: it’s wise to only test one thing at a time. This single-beam operation should test the alignment and fine field-strength control of the dipole magnets that keep the protons on a circular path, and perhaps also the focusing magnets (quadrupoles and sextapoles) that keep the positively-charged protons from diverging. Controlling a plasma is a difficult thing, even when you’re not accelerating it with strong, rapidly-oscillating electric fields!
From my point of view, even a single beam will be a great source of muons for aligning our detectors. Although the LHC beampipe is evacuated to 10-10 torr (air pressure on the moon is 10-11 torr), some protons will inevitably collide with atoms of gas, causing showers of particles that decay down to the nearly-stable muons. These muons enter our detector almost parallel to the beamline, which is normal-incidence for our “end-cap” detectors. The only muons we have seen so far have been cosmic ray muons, which fall down on us from above: a difficult angle for endcap detectors to resolve.
When the accelerating cavities of the LHC are later turned on, these 450 GeV protons will be ramped up to an energy of 5000 GeV. (Collision energy this year will be 10,000 GeV, or 10 TeV, and the single-beam energy is half that.) Thus some of the beam-gas muons will have a TeV of energy, so we may get our first look at TeV muons before the LHC operators even manage to collide two counter-rotating protons.