Wing Sheung Chan

The Large Hadron Collider and the ATLAS detector 31 Month in Year Jan '15 Jul '15 Jan '16 Jul '16 Jan '17 Jul '17 Jan '18 Jul '18 -1 fb Total Integrated Luminosity 0 20 40 60 80 100 120 140 160 ATLAS Preliminary LHC Delivered ATLAS Recorded Good for Physics = 13 TeV s -1 fb Delivered: 156 -1 fb Recorded: 147 -1 fb Physics: 139 2/19 calibration (a) 0 10 20 30 40 50 60 70 80 Mean Number of Interactions per Crossing 0 100 200 300 400 500 600 /0.1] -1 Recorded Luminosity [pb Online, 13 TeV ATLAS -1 Ldt=146.9 fb ∫ > = 13.4 µ 2015: < > = 25.1 µ 2016: < > = 37.8 µ 2017: < > = 36.1 µ 2018: < > = 33.7 µ Total: < 2/19 calibration (b) Figure 2.2.: The (a) integrated luminosities over time and (b) pile-up profile for pp collisions delivered to ATLAS during Run 2 of the LHC [66] . During bunch crossings, multiple collisions can happen at the same time. However, in physics analyses, typically only one of these collisions, usually the hardest scattering, is of interest. The other additional, usually softer, interactions are collectively referred to as (in-time) “pile-up”. Other interactions from preceding or subsequent bunch crossings can also sometimes be registered as events occurring at the same time as the collision of interest. This could happen when components of a detector have a relaxation time longer than 25 ns . These additional signals are known as out-of-time pile-up. Since pile-up events also produce particles that leave tracks and deposit energies in the detector, having a large number of them is a challenge to the detector’s trigger and readout system, as well as to the physics analyses. Pile-up depends mainly on the number of interactions per crossing, which follows a Poisson distribution with mean µ . The mean µ is proportional to the instantaneous luminosity and can be calculated as µ = L σ inel f r n b , (2.5) where σ inel is the cross section for inelastic scatterings. Therefore, the measurement of instantaneous luminosity is important to physics analyses as it determines the amount of pile-up. Pile-up changes over time as the instananeous luminosity decreases as particles in the beams are lost in collisions. The average value of µ over a period of time, denoted as µ , is also used to quantify pile-up. Figure 2.2b shows the pile-up profile for pp collisions delivered to ATLAS during Run 2. In the beginning of Run 2, the LHC operated at relatively low instantaneous luminosities with µ = 13 . 4 in 2015. The operating luminosity increased in the following years with µ = 25 . 1 in 2016, µ = 37 . 8 in 2017 and µ = 36 . 1 in 2018. The overall value of µ over the entire data-taking period of Run 2 is 33.7 [66] .