The CMS electromagnetic calorimeter (ECAL) is made of about 75000 scintillating lead tungstate crystals arranged in a barrel and two endcaps. The scintillation light is read out by avalanche photodiodes in the barrel and vacuum phototdiodes in the endcaps, at which point the scintillation pulse is amplified and sampled at 40 MHz by the on-detector electronics. The fast signal from the crystal scintillation enables energy as well as timing measurements from the data collected in proton-proton collisions with high energy electrons and photons. The stability of the timing measurement required to maintain the energy resolution is on the order of 1ns. The single-channel time resolution of ECAL measured at beam tests for high energy showers is better than 100 ps. The timing resolution achieved with the data collected in proton-proton collisions at the LHC during Run 2 is presented. The timing precision achieved is used in important physics measurements and also allows the study of subtle calorimetric effects, such as the timing response of different crystals belonging to the same electromagnetic shower.
The time resolution of each crystal is given by:
where Axtal is the crystal amplitude, sigma_{n} is the pedestal noise of that crystal extracted from
the event record, and N and C are constants derived from the ultimate time performance of the
ECAL. C sets the intrinsic time resolution of the ECAL. The crystal
amplitude is computed with:
where Extal is the recHit energy and C^{Inter},
C^Laser, and C^{A2G are the intercalibration, laser, and ADC to GeV constants for that crystal,
extracted from the event record, respectively.
To derive the constants N and C, we performed an extensive study on the ECAL time performance. The method for extracting the ultimate performance uses like-energy neighboring crystals and measuring the time difference between them to determine the time resolution of the ECAL, following a similar procedure used in Run1. The resolution is measured as a function of the effective crystal amplitude normalized to the pedestal noise, given by
We used GED photons with above 20 GeV that have a seed time greater than -25 ns. We focused only on objects arriving in the ECAL barrel. As we wanted to use real electromagnetic deposits (to veto jets) we also required that the photons have an smaj < 0.5 and smin < 0.3.
We compare the timing of neighboring crystals of an ECAL cluster which have a very similar energy. This is to minimize shower propagation effects. We require:
* E1,E2 < 120GeV (to avoid gain switch effects)
* |E1/E2| < 1.2
The resolution is estimated from a gaussian fit, taking the core of the distribution. The fit is in the range mean±2RMS.
A study based on Z reconstruction has been also performed. For electrons we apply
* simple isolation and cluster shape requirements.
* E1,E2 > 10GeV
* E1,E2 < 120GeV (to avoid gain switch effects)
* 60GeV<m_inv(e1,e2)<150GeV
The time of the electron corresponds to the time of the cluster seed crystal. When comparing the time of the two electrons we correct for time of flight differences due to primary vtx position.
Link to GitLab repo: DN-17-043
Figure | Caption |
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2017 pdf version 2016 pdf version |
Resolution of time difference between the two most energetic neighboring crystals of an ECAL cluster as a function of the effective amplitude, normalized to the noise in the ECAL Barrel for 2016 and 2017 data. The selection applied and the resolution are the ones specified in the introduction. The effective amplitude, Aeff, corresponds to A1A2 /sqrt(A1^2+A^2), where A1 and A2 are the amplitude of the two crystals. |
2017 pdf version 2016 pdf version |
Resolution of time difference between the two electrons from Z->ee decays, as a function of the effective amplitude, normalized to the noise in the ECAL Barrel for 2016 and 2017 data. The selection applied, the method and the resolution are the ones specified in the introduction. The effective amplitude, Aeff, corresponds to A1A2 /sqrt(A1^2+A^2), where A1 and A2 are the amplitude of the two crystals. The noise corresponds to 60 (80) MeV for 2016 (2017). |
2017 pdf version 2016 pdf version |
Resolution of time difference between the two most energetic neighboring crystals of an ECAL cluster as a function of the effective amplitude, normalized to the noise in the ECAL Barrel for 2016 and 2017 data, for crystals belonging to the same readout unit (trigger tower). The selection applied and the resolution are the ones specified in the introduction. The effective amplitude, Aeff, corresponds to A1A2 /sqrt(A1^2+A^2), where A1 and A2 are the amplitude of the two crystals. The noise corresponds to 60 (80) MeV for 2016 (2017). |
2017 pdf version 2016 pdf version |
Resolution of time difference between the two most energetic neighboring crystals of an ECAL cluster as a function of the effective amplitude, normalized to the noise in the ECAL Barrel for 2016 and 2017 data, for crystals belonging to different readout units (trigger tower). The selection applied and the resolution are the ones specified in the introduction. The effective amplitude, Aeff, corresponds to A1A2 /sqrt(A1^2+A^2), where A1 and A2 are the amplitude of the two crystals. The noise corresponds to 60 (80) MeV for 2016 (2017). |
pdf version |
(Plot Needed for EXO-19-005) The time resolution as measured by the time difference between two neighboring ECAL crystals with similar energy in the same readout electronics as a function of the effective amplitude, normalized to the pedestal noise, in the ECAL Barrel is shown for the 2016 (red) and 2017 (blue) data sets. The average pedestal noise corresponds to 60 (80) MeV for 2016 (2017). |
-- LiviaSoffi - 2019-06-19