Abstract
Measuring cross-sections at the LHC requires the luminosity to be determined accurately at each centre-of-mass energy √s. In this paper results are reported from the luminosity calibrations carried out at the LHC interaction point 8 with the LHCb detector for √s = 2.76, 7 and 8 TeV (proton-proton collisions) and for √sNN = 5 TeV (proton-lead collisions). Both the "vander Meer scan" and "beam-gas imaging" luminosity calibration methods were employed. It is observed that the beam density profile cannot always be described by a function that is factorizable in the two transverse coordinates. The introduction of a two-dimensional description of the beams improves significantly the consistency of the results. For proton-proton interactions at √s = 8 TeV a relative precision of the luminosity calibration of 1.47% is obtained using van der Meer scans and 1.43% using beam-gas imaging, resulting in a combined precision of 1.12%. Applying the calibration to the full data set determines the luminosity with a precision of 1.16%. This represents the most precise luminosity measurement achieved so far at a bunched-beam hadron collider.
Highlights
The determination of the cross-section of a given subatomic process at high energy collidingbeam experiments is generally performed by the measurement of an interaction rate
The analysis described in this paper relies on two direct methods to determine the absolute luminosity calibration: the “van der Meer scan” method (VDM) [10,11,12,13,14] and the “beam-gas imaging” method (BGI) [10, 15], the latter making use of unique capabilities of the LHCb experiment
We briefly describe the principles of the two methods that are used in this paper to determine the latter
Summary
The determination of the cross-section of a given subatomic process at high energy collidingbeam experiments is generally performed by the measurement of an interaction rate. To determine such a cross-section on an absolute scale, a measurement of the colliding-beam luminosity must be performed. The uncertainty related to the beam-gas vertex position resolution has significantly larger impact than in 2012, due to the lower number of vertices in the luminous region. The con√volved bunch length σz21 + σz measurement plays a more important role compared to the s = 8 TeV data due to the larger crossing angle correction. The FBCT offset fit changes the cross-section by 0.05% and reduces the overall
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