CMS-DP-2019/038

CMS ECAL Performance for Ultra Legacy re-reconstruction of 2018 and run 2 Preshower plots

Examples of the invariant mass of photon pairs with one photon depositing a fraction of its energy in a crystal of the ECAL Barrel at η = -0.03 (top), and of the ECAL Endcap at η = 1.83 (bottom), in the mass range of the π0.

A subset of the data collected in 2018 is used, corresponding to half of the total dataset. These events are collected by CMS with a dedicated trigger at a rate of 7 (2) kHz in the Barrel (Endcaps). The high trigger rate is made possible by a special clustering algorithm that saves only a minimal amount of information of the events, in particular, energy deposits in the ECAL crystals surrounding a possible π0 candidate. For candidates in the Endcaps, the determination of the photon position in the region with 1.7 <abs(η)< 2.55 is improved by the presence of the Preshower, which results in a better mass resolution.

These events are used as prompt feedback to monitor the effectiveness of the laser monitoring calibration and to inter-calibrate the energy of ECAL crystals. The π0 mass is shown before the crystal inter-calibration.

Stability of the shower shape of the electromagnetic deposits in the ECAL for leading

electrons from Z decays.

The plot shows the time stability of the shower shape of the leading electron from Z decays

with a refined re-calibration performed in 2019. The event selection requires two electrons to be

in the ECAL Barrel (left) or in the ECAL Endcaps (right). Each time bin has around 10,000 events.

The error bar on the points denotes the statistical uncertainty on the median, which is

evaluated as the central 95% interval of medians obtained from 200 "bootstrap" re-samplings.

The right panel shows the distribution of the medians. The shower shape is measured

by the variable R9, defined as the ratio of the energy deposit in the 3x3 crystal matrix around

the seed crystal to that in the supercluster. R9 is responsive to changes in pedestal and noise.

Di-electron invariant mass distribution for the 2018 data taking period using

Z→ee low-bremsstrahlung electrons.

The plot shows the di-electron invariant mass distribution for Z decay events with two calibration sets for the full 2018 dataset: the “preliminary” autumn 2018 calibration (RED) and a “refined” re-calibration performed in 2019 (GREEN). While for the refined calibration a complete re-calibration of the crystals was performed, for the preliminary calibration only time-dependent effects for the first part of the dataset were accounted for.

Both electrons are required to be in the ECAL Barrel (left) or in the ECAL Endcaps (right) and to have low bremsstrahlung. The relative resolutions are quoted in the legend, defined as the ratio of σ (Gaussian standard deviation of the Gaussian that is convoluted with a Breit-Wigner as the signal model (Voigtian fit)) to μ (mean). Events used in the calibration are excluded from the fit.

Refined calibration fitted resolution values (1.4% and 2.7% for EB and EE, respectively)are consistent with those obtained in the final SuperCluster unfolded resolution results (~2%(/sqrt(2)) and ~4%(/sqrt(2)) for EB and EE, respectively.)

ECAL energy resolution with Zee in 2018 data

Relative electron (ECAL) energy resolution unfolded in bins of pseudo-rapidity η for the ECAL Barrel and the ECAL Endcaps. Electrons from Z→ee decays are used. The resolution is shown separately for low bremsstrahlung electrons and for all electrons (“inclusive”). The resolution is measured on 2018 data. The relative resolution σE/E is extracted from an unbinned likelihood fit to Z→ee events, using a Voigtian (Breit-Wigner convoluted with Gaussian) as the signal model.

Conclusions:

• The resolution is affected by the amount of material in front of the ECAL and is degraded in the vicinity of the eta cracks between ECAL modules (indicated by the vertical lines in the plot)

• The last point of the preliminary calibration is out of the y-axis scale of the plot

• The resolution improves significantly after a refined calibration using the full 2018 dataset with respect to a preliminary calibration for which only time dependent effects in the first part of the dataset were corrected for

The two planes of the Preshower (ES), coupled with the EE crystals, form a sampling calorimeter. The ES essentially counts the number of charged particles passing through the layers of silicon, which is an estimate of the amount of energy deposited in the ES lead absorbers. We use charged particles with momentum nearly close to minimum ionizing to calibrate the ES, so for simplicity we refer to them as “MIPs”. They are collected with the Preshower operated in high-gain mode. The design-goal accuracy of the channel-by-channel calibration is set to 5%. This corresponds to a contribution of about 0.25% to the overall EE+ES energy resolution for high-energy electrons since only a few percent of electron/photon energy is deposited in ES. The sources of response variation (sensor-to-sensor and channel-to-channel) are the sensor thickness seen by the incident particles (depends on angle of incidence), gain of the front-end electronics chain, and charge collection efficiency which varies with the radiation damage.

The plot shows the decreasing of MIP response with regard to the 2015 calibration (from 2015B to the end of 2018D) in 5 eta regions (the interval in eta is 0.2) of the front plane. The x-coordinate of each data point represents the integrated luminosity at which the MIP calibration was done. The calibration was performed every 10-20 fb-1.

The decreasing rate on ES+ is similar to one on ES-, thus the averages of those rates are used.

In general, the MIP responses decrease as a function of luminosity. The only exception is in the beginning of 2017 data taking, when the bias voltage was increased by 80 Volts

The MIP response decreases faster in the higher eta region. This result implies that the ES sensors in high eta regions are more affected by the radiation damage.