The ATLAS collaboration achieved a milestone in February when it applied the finishing touches to the measurement of the luminosity for proton–proton (pp) data recorded in 2011 at 7 TeV in the centre of mass. With a relative uncertainty of ±1.8%, the understanding of the luminosity delivered to ATLAS exceeds the accuracy expected before running at the LHC began and opens up exciting possibilities for precision measurements.

The absolute scale for luminosity – a measure of how many particles pass through a given area in a given time – is calibrated by combining simultaneous precision measurements of the bunch currents in the LHC and of the convolved transverse size of the colliding bunches. Using a technique pioneered by Simon van der Meer nearly 50 years ago at CERN’s Intersecting Storage Rings, the inelastic pp collision rate is monitored as the beams are separated first in the horizontal and then in the vertical direction. This "vdM scan" provides a measurement of the beam-overlap area, which when combined with the numbers of protons in each bunch, determines the absolute luminosity produced in head-on collisions.

The success of the procedure for vdM scans at the LHC resulted from close co-operation between the LHC accelerator team and the four large experimental collaborations. The scans are performed in special fills with carefully tailored beam conditions. These fills are optimized for the accuracy of the luminosity measurement while remaining within acceptable operational parameters for the accelerator complex. One key input, the understanding of the number of protons per bunch in the LHC, is determined from several different beam instrumentation measurements, as well as from additional supporting measurements by each of the four LHC collaborations. This effort, led by the LHC Bunch Current Normalization Working Group, has reduced the uncertainty on this key component of the luminosity calibration from around 10% in early 2010 to 0.5% for the final 2011 result.

ATLAS uses two main detectors to monitor the luminosity delivered during physics collisions. LUCID is a segmented Cherenkov detector wrapped around the forward beam pipe; it has been designed specifically for luminosity measurements. The beam conditions monitor (BCM) is a set of small sensors made from synthetic (CVD) diamonds, which also provide fast-abort signals to protect the inner tracking-detectors from radiation damage. LUCID and the BCM both deliver individual luminosity measurements for each of the 3564 possible colliding-bunch slots in the LHC’s fill pattern.

The vdM scans provide a direct calibration of these detectors at a single point in time. The accuracy of that calibration in 2011 was determined to be ±1.5%. The dominant uncertainties in this calibration are linked to the reproducibility of the result from one scan to the next and among different colliding bunches in the same scan, as well as to the understanding of the numbers of protons mentioned above.

To verify that the luminosity calibration determined during vdM scans is stable over an entire year of LHC operation, ATLAS relies on the consistency between several different detectors and algorithms. In addition to LUCID and the BCM, the electrical current flowing through the liquid argon gaps of the forward calorimeter, as well as the photomultiplier currents in selected cells of the hadronic calorimeter, have proved to be remarkably good luminosity monitors. Additional measurements, such as the rate of primary collision vertices reconstructed by the ATLAS tracking system, provide additional cross-checks. Altogether, the agreement among the different luminosity methods has limited any possible variation of the luminosity scale to less than ±1% over the entire year.

The story of the 2011 luminosity measurement has come to a close with the submission of an ATLAS paper on the topic. However, each year brings new challenges and past performance does not guarantee future returns. Considerable machine time was devoted to vdM scans in 2012 to provide the data necessary for a successful luminosity calibration, this time at 8 TeV. This analysis is ongoing, but the accuracy established in 2011 has set a high standard for future luminosity measurements at the LHC.