CMS ECAL status and performance with the first LHC collisions

Transcription

1 CMS ECAL status and performance with the first LHC collisions XIV International Conference on Calorimetry in High Energy Physics (Calor 2010) Konstantinos Theofilatos (ETH Zurich) on behalf of CMS ECAL ECAL crystals

2 The CMS Electromagnetic Calorimeter ECAL ECAL crystals CMS Detector ECAL Barrel ECAL Barrel supermodule ETH Zurich 2

3 . Part A ECAL Detector Pre-calibration Synchronisation ETH Zurich 3

4 ECAL Detector ECAL is inside the CMS magnet 7.9 m Lead Tungstate crystals Avalanche Photo Diodes (APDs) in EB Vacuum Photo Triodes (VPTs) in EE PbWO4 3.6m Endcaps (EE) 1.48< η <3.0 Preshower (ES) 1.65< η <2.6 optimized by design for Η γγ and H ZZ 4e Barrel (EB) η <1.48 Fast light emission: 80% of light collected in the first 25ns Outstanding energy resolution: ΔΕ/Ε < 0.5 % for unconverted γ at high energies (testbeam result) Fine granularity: Δη x Δφ=0.0175x (EB) Preshower (ES): 2 orthogonal planes of Silicon Strips (~17m 2 ) finer position resolution in the forward region (see talk photo by Chia from Ming EB KUO) ETH Zurich 4

5 Energy Scale and Precalibrations ECAL was pre-calibrated prior to LHC (EB), ~5%(EE) precalibrations: mixture of testbeams, cosmics, beam splashes and lab data Energy scale set at the testbeam Validated in-situ with an overall uncertainty <2% by measuring the cosmic muon stopping power in PbWO4 (see talk by Andrea BENAGLIA) L=0.43nb -1 First π 0 candidates were recorded within few hours from the first LHC collisions (Nov 2009) π 0 γγ is already part of the CMS online Data Quality Monitoring system and provides an important stream for in-situ calibration (see talk by Riccardo PARAMATTI) ETH Zurich 5

6 Detector Synchronisation beam splash event muons muons muons EB/EE (in red) ES (in green) Used 2009 LHC beam splashes for the online synchronisation (black) of the trigger towers (5x5 channels) Residual channel timing within a trigger tower is further improved offline (red shaded): ~ 0.3ns RMS spread ETH Zurich 6

7 . Part B ECAL Status Occupancy Maps ETH Zurich 7

8 ECAL Status We are currently taking data, we are always ON, even during LHC machine filling... Overall >99% active channels for trigger and data ECAL trigger already fully active Data Acquisition: 100% efficient we can even push DAQ above 100 khz, First Level (L1) trigger design goal ECAL takes monitoring data in the LHC abort gap: laser crystal transparency monitoring, test pulses, pedestals... A full cycle is completed every 48 minutes (see talk by Federico FERRI) ETH Zurich 8

9 DATA ECAL status (cont.) CMS relied on ECAL trigger to record the LHC splashes White regions are masked channels (0.95% of total) beam splash DATA beam splash DATA Most of them may be recovered from trigger information Square shielding structure and floor of the LHC tunnel (bottom) are visible Only for 0.15% neither data nor trigger information are available beam splash DATA Shadow of EB is visible in the external radius of EE+, through a reduction of muon flux by about 25% ETH Zurich 9

10 . Part C ECAL Signals Energy spectra Particle Resonances ETH Zurich 10

11 ECAL clusters events clusters/1 GeV 900 GeV Data 900 GeV Data number of clusters cluster ET (GeV) E.M. showers deposit their energy in several crystals in the ECAL; clusters of channels extended along φ (bending direction) are used to reconstruct their energy MC provides good description of the observed data distribution ETH Zurich 11

12 Rapidity and Azimuth Distributions 7 TeV Data 7 TeV Data ECAL Barrel Rapidity and azimuth distributions of the ECAL channel with the highest ET in minimum bias events at 7 TeV Variations as a function of η are due to the detector geometry; ECAL endcap data are prescaled by a factor six for presentation purposes Variations as a function of phi, accurately reproduced in MC, reflect modularity and the inhomogeneity of the energy-equivalent noise in ECAL ETH Zurich 12

13 ECAL anomalous signals In a small fraction of collision data we observe anomalous signals in ECAL: distinct pulse shape different timing single crystal energy deposit uniformly distributed in EB not seen in EE (VPTs readout) Origin: highly ionizing particles in the APDs pulse shape exhibits faster rising time and is inconsistent EB crystal with the signal shape from scintillation Easily identified and removed by a quality selection (e.g. an energy ratio E4/E1). Timing and pulse shape discriminants could also be deployed to tag these signals. Rate:~ 1 per 10 3 minimum-bias events on 900 GeV collision data ETH Zurich 13

14 Energy spectra per channel 7 TeV Data 7 TeV Data Energy spectra in the individual channels. Data and MC are for the same luminosity collision data; signal quality selections are applied MC describes accurately the observed Data distribution ETH Zurich 14

15 Resonances 900 GeV collisions π 0 γγ in 7 TeV data about 1461 thousands candidates for L=0.43nb -1 π 0 γγ where one of the two photons is reconstructed as a conversion ETH Zurich 15

16 More on Resonances Reconstructed η γγ peak in 7 TeV collision data, about 25.5 thousands candidates for L=0.43nb -1 ETH Zurich 16

17 Summary ECAL is fully functional and within the design specifications More than 99% of channels operational Calibration: precalibrations provided good reference at start-up, now benefit from in-situ π 0 calibration Synchronization: sub-nanosecond level Data vs MC: good agreement, further tuning in progress Currently re-discovering Standard Model particles awaiting with impatience for more LHC data ETH Zurich 17

18 BACKUP: Time Resolution ~ 1 GeV ~ 2 GeV The plot shows the time resolution as a function of the effective amplitude, derived by comparing the time in nearby crystals, in the same cluster The noise and the systematic term in the time resolution are extracted from a parametric fit to data (see CFT for a discussion of the analysis procedure) The observed noise term is consistent with expectations from test beam data and measurements during Cosmic Run at 4 Tesla (2008) The constant term in the time resolution due to local systematic effects is of about 200 ps ETH Zurich 18

19 BACKUP: Pulse Shape & Timing Anomalous signal Anomalous signals (red) have faster rising time consistent with instantaneous signal from the APDs, and different from the typical scintillation pulse shape (green) in the crystal Offline channel s timing distribution of e.m. shower signals (blue) and anomalous ECAL signals (red), tagged with E4/E1 Normal signal timing is at 0 ns and with a resolution better than 1ns for E>1 GeV Anomalous signals have much wider spread and have predominantly an apparent timing ~10 ns earlier, due to reconstruction bias (different pulse shape) Further suppression is achieved by requiring time of flight compatibility from IP (not yet deployed) ETH Zurich 19

20 BACKUP: Calibration from Cosmics Distribution of the relative difference between the calibration coefficients measured with test beam electrons and with cosmic ray muons Precision of intercalibration constants from cosmic ray muons as a function of the η index, as obtained from RMS spread of the relative difference between the inter-calibration coefficients from test beam and cosmic ray data averaged over ranges of η ETH Zurich 20