The passage of a cosmic muon (yellow) recorded in the barrel of the ATLAS ‘tilecal’ calorimeter (red).

At last week's Lepton–Photon Symposium1, held at Uppsala University, Sweden, the ATLAS collaboration proudly reported the first 'data' taken at CERN’s Large Hadron Collider (LHC). Any sign of the long-sought Higgs boson will not appear before the proton collider springs to life in mid 2007. But as the monster detectors are assembled — ATLAS will stand five storeys high — signals generated by cosmic-ray particles are the first sign that all is working as it should.

The particle track shown here is that of a muon, produced in a cosmic-ray shower and penetrating solid rock to reach ATLAS, 90m underground. Its passing was recorded in a part of the ATLAS detector called the 'tilecal' calorimeter. Calorimeters are standard features in large particle detectors, intended to absorb particles and measure their energy. The ATLAS tilecal is made of alternate layers of steel and scintillating tiles — steel to absorb and scintillator to sample the energy loss.

Muons, however, are the most penetrating of all particles (except the rarely detectable neutrinos), and, as can be seen here, tend to stream straight through the mass of the calorimeter without being absorbed. Under normal running conditions, when protons collide at high energy inside the LHC ring at the centre of the ATLAS detector, ‘cosmics’ like this are a nuisance and must be stripped out of the data. But at the beginning of a data-taking run, the bold straight tracks through the entire body of the detector are useful for checking the alignment of different detector components — for instance, of the tilecal and the semiconductor tracker that will eventually sit inside it.

ATLAS is one of two general-purpose detectors that will stand in the LHC ring and is being assembled in its underground cavern. But its more compact counterpart, CMS, is being assembled mostly above ground. It will be lowered, in a feat of engineering, into its cavern in March 2006. Other detectors — ALICE and LHCb, to study quark–gluon plasma and the physics of the bottom quark, respectively — are also under construction. The LHC pilot run is set to start in June 2007.