CMS DP-2020/021

CMS ECAL Performance for Ultra Legacy re-reconstruction of 2016

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

Data collected in 2016 is used. These events are collected by CMS with a dedicated trigger at an average rate of about 5 kHz in both the Barrel and the Endcap. 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 η candidate. These triggers were prescaled by about a factor of 2 during 2016 data taking. The data sample considered corresponds to the full 35.9 fb-1 acquired by CMS in 2016.

For &eta candidates in the Endcaps, the determination of the photon position in the region with 1.7 <|η|< 2.55 is improved by the presence of the Preshower, which results in a better mass resolution.

The η mass is shown before the crystal intercalibration.

Time stability of the di-electron invariant mass distribution for the 2016 data taking period using Z→ee events. The plot shows the time stability of the median di-electron invariant mass with a refined re-calibration performed in 2019 for the full 2016 dataset. Both electrons are required 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. At the analysis level, residual drifts in the energy scale with time are corrected for in approximately 18-hour intervals corresponding to at most one LHC fill.

Stability of the 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 for the 2016 dataset. The event selection requires two electrons to be in the ECAL Barrel or in the ECAL Endcaps. 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 2016 data-taking period using Z→ee electrons.

The plot shows the di-electron invariant mass distribution for Z decay events with two calibration sets for the full 2016 dataset: the "initial" calibration performed in 2017 (RED) and a "refined" re-calibration performed in 2019 (GREEN). Both electrons are required to be in the ECAL Barrel (left) or in the ECAL Endcaps (right). The relative resolutions are quoted in the legend, defined as the ratio of σ (Gaussian standard deviation of the Gaussian that is convolved with a Breit-Wigner as the signal model (Voigtian fit)) to μ (mean). The yields are normalized to 100 events. The very good resolution already achieved after the "initial" calibration performed in 2017 has been exploited to carry out the precision measurement of the Higgs boson mass in the diphoton decay channel [1].

[1] CMS Collaboration, A measurement of the Higgs boson mass in the diphoton decay channel, Submitted to Phys. Lett. B, https://arxiv.org/abs/2002.06398v1

Di-electron invariant mass distribution for the 2016 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 2016 dataset: the "initial" calibration performed in 2017 (RED) and a "refined" re-calibration performed in 2019 (GREEN). Both electrons are required to be in the ECAL Barrel (left) or in the ECAL Endcaps (right). The relative resolutions are quoted in the legend, defined as the ratio of σ (Gaussian standard deviation of the Gaussian that is convolved with a Breit-Wigner as the signal model (Voigtian fit)) to μ (mean). The yields are normalized to 100 events. The very good resolution already achieved after the "initial" calibration performed in 2017 has been exploited to carry out the precision measurement of the Higgs boson mass in the diphoton decay channel [1].

[1] CMS Collaboration, A measurement of the Higgs boson mass in the diphoton decay channel, Submitted to Phys. Lett. B, https://arxiv.org/abs/2002.06398v1

Di-electron invariant mass distribution for the 2016 data taking period using Z→ee high-bremsstrahlung electrons.

The plot shows the di-electron invariant mass distribution for Z decay events with two calibration sets for the full 2016 dataset: the "initial" calibration performed in 2017 (RED) and a "refined" re-calibration performed in 2019 (GREEN). Both electrons are required to be in the ECAL Barrel (left) or in the ECAL Endcaps (right). The relative resolutions are quoted in the legend, defined as the ratio of σ (Gaussian standard deviation of the Gaussian that is convolved with a Breit-Wigner as the signal model (Voigtian fit)) to μ (mean). The yields are normalized to 100 events. The very good resolution already achieved after the "initial" calibration performed in 2017 has been exploited to carry out the precision measurement of the Higgs boson mass in the diphoton decay channel [1].

[1] CMS Collaboration, A measurement of the Higgs boson mass in the diphoton decay channel, Submitted to Phys. Lett. B, https://arxiv.org/abs/2002.06398v1

Relative electron (ECAL) energy resolution unfolded in bins of pseudorapidity 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 2016 data. The very good resolution already achieved after the "initial" calibration performed in 2017 has been exploited to carry out the precision measurement of the Higgs boson mass in the diphoton decay channel [1]. The relative resolution σE/E is extracted from an unbinned likelihood fit to Z→ee events, using a Voigtian (Breit-Wigner convolved with Gaussian) as the signal model. Conclusions:

• The resolution is a 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 resolution improves after a refined calibration using the full 2016 dataset with respect to an initial calibration performed during 2017. The improvement is significant in the Endcaps.

[1] CMS Collaboration, A measurement of the Higgs boson mass in the diphoton decay channel, Submitted to Phys. Lett. B, https://arxiv.org/abs/2002.06398v1