Luminosity measurement at CMS

Luminosity is a key quantity of any collider, since it allows for the determinationof the absolute cross sections from the observed rates in a detector. Since theHiggs boson discovery in 2012, the highest priority at the Large Hadron Collider(LHC) has been given to an accurate understanding of the electroweak scale anda search for new physics. Precise luminosity measurements in such conditions areof crucial importance, as they determine the precision of any physics cross sectionmeasurement.To increase the production of particles of interest, usually of low cross section,the LHC is running at the highest possible luminosity. After the first Long Shutdown (LS1) the original performance goal for the luminosity of 1 × 1034 cm−2 s−1was reached with 1011 protons per bunch and a bunch spacing of 25 ns. In suchconditions radiation hard detectors with extremely fast response time are required,especially for instrumentation near the beam.The Compact Muon Solenoid experiment is equipped with three online luminometers, which fulfill the listed requirements: the Fast Beam Conditions Monitor(BCM1F), the Pixel Luminosity Telescope (PLT) and the Forward Hadron calorimeter (HF).The BCM1F was upgraded during LS1 from 8 to 24 diamond sensors and isread out by a dedicated fast ASIC. The back-end comprises a deadtime-less histogramming unit, with 6.25 ns bin width and analog-to-digital converters with 2 nssampling time in the VME standard. A microTCA system with better time resolution is in development.Particles originating from collisions and machine induced background arrive with12 ns time difference. Because of its excellent time resolution BCM1F measuresseparately both luminosity and machine induced background particles. The performance of the detector in the first running period and radiation damage monitoringof the sensors and electronics chain form the first part of this thesis.Calibration of the luminometers at the LHC is done using van der Meer (VdM)scans. The proton beams are scanned against each other separately in the x andy directions. The effective width of the beams is measured and the visible crosssection, the key quantity for the luminosity measurement, is determined. The im-