Abstract
The measurement of the luminosity recorded by the CMS detector installed at LHC interaction point 5, using proton–proton collisions at sqrt{s}=13,{text {TeV}} in 2015 and 2016, is reported. The absolute luminosity scale is measured for individual bunch crossings using beam-separation scans (the van der Meer method), with a relative precision of 1.3 and 1.0% in 2015 and 2016, respectively. The dominant sources of uncertainty are related to residual differences between the measured beam positions and the ones provided by the operational settings of the LHC magnets, the factorizability of the proton bunch spatial density functions in the coordinates transverse to the beam direction, and the modeling of the effect of electromagnetic interactions among protons in the colliding bunches. When applying the van der Meer calibration to the entire run periods, the integrated luminosities when CMS was fully operational are 2.27 and 36.3 ,text {fb}^{-1} in 2015 and 2016, with a relative precision of 1.6 and 1.2%, respectively. These are among the most precise luminosity measurements at bunched-beam hadron colliders.
Highlights
We report the precise determination of the absolute luminosity at the CERN LHC interaction point (IP) 5 with the CMS collisions adte√tecsto=r
The Pixel Luminosity Telescope (PLT) measures the rate of triple coincidences, where a hit is observed in all three planes, typically corresponding to a track from a particle originating at the IP
For pixel cluster counting (PCC), the single-bunch response (SBR) is determined by optimizing αT1, which is time dependent and measured in intervals of about 20 min, and the parameters of the exponential used for T2 corrections, such that there is minimal residual rate in the noncolliding bunch slots
Summary
A central component of the physics program at the LHC consists of measurements that can precisely test the validity of standard model (SM) predictions, e.g., cross sections for the production of electroweak gauge bosons [4,5] or top quark pairs [6,7]. The ability to measure the luminosity of individual bunch crossings (bunchby-bunch luminosity) is necessary so that the distribution of number of collisions per crossings is known to the experiments This information is important when preparing simulations as well as optimization of thresholds to keep event-recording rates near data acquisition design targets. A relative normalization method is needed to transfer the absolute luminosity calibration to the complete data-taking period To this end, for a given subdetector, the cross section σvis in the “visible” phase space region, defined by its acceptance, is measured for several observables. Changes in the detector response over time can result in variations in σvis, which could appear as nonlinearity and/or long-term instability in the measured luminosity To address these challenges, CMS employs a multifaceted approach, in which measurements from various individual subsystems are used to produce a final luminosity value with high precision, good linearity, and stability.
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