Physics with Photons Using the ATLAS Run 2 Data by Stefano Manzoni

Author:Stefano Manzoni
Language: eng
Format: epub
ISBN: 9783030243708
Publisher: Springer International Publishing

7.3 Uniformity Correction

A set of corrections is implemented in data to account for response variations not included in the simulation due to the non nominal setup of the LAr high voltage system, geometric effects such as the intermodule widening induced by gravity (i.e. the gap between the barrel EM modules are widened due to a gravity-induced sagging effect that is not included in the simulation), and bias introduced by the LAr hardware electronic calibration [2, 3].

In particular, a new study of the impact of the non linearity of the gain of the read-out electronics has been performed using special runs in 2015 and in 2017. As described in Sect. 5.​3.​2.​1, in order to adapt to the wide range of expected energies the electronic signals of the EM calorimeter cells are reconstructed with three different gain settings. The electrons coming from a Z boson usually have their cell energies recorded using the High Gain (HG), while about the 30% of the H events have a photon reconstructed with at least one cell in Medium Gain (MG). In order to estimate the relative intercalibration of the MG and HG, a sample of data have been recorded in 2015 and 2017 under special condition, corresponding to an integrated luminosity of 12 and 160 pb, requesting a threshold lowered by a factor 5 to switch between HG and MG, so that almost all the electrons from Z boson decays have at least one L2 cell recorded using MG. In the standard configuration, where the threshold at low in the barrel is GeV per L2 cell, the electrons decayed from the Z boson are recorded using HG only. The relative energy scale between the MG and HG configurations is derived using events from standard and special runs. The relative energy scale difference between standard and special runs is consistent with 0 except in the region , where a small but significant difference is observed, as shown by Fig. 7.18. This difference is not completely understood at the moment, thus no correction is applied to data and the deviation from 1 is taken as systematic uncertainty. Propagating the uncertainty of the gain intercalibration to the reconstructed particle energies results in a typical uncertainty of 0.05–0.1% depending on for photons with GeV.

Fig. 7.18Difference of energy scales, , extracted from events, as a function of between data recorded with the standard thresholds for the transition between HG and MG in the readout of the layer-two cells and data with lowered thresholds. Only statistical uncertainties are shown [3]

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