The CMS experiment is installed at the LHC proton collider at Cern and has begun its data taking campaign in 2009.

The electromagnetic calorimeter was designed to initially allow for low luminosity discovery of the Higgs boson decaying into two photons. The CMS has a very compact electromagnetic calorimeter with a very good energy resolution

The contributions of Irfu

Scientific and technical responsibilities of Irfu

  • Design, manufacture and commissioning of the calibration system, by laser light injection, of the electromagnetic calorimeter with lead tungstate crystals.
  • R & D on APD photodetectors (avalanche photodiodes)
  • R & D on crystals (Crystal-Clear collaboration)
  • Study, development, installation, testing and exploitation of the crystal transparency monitoring system data.
  • Study, development, installation, test and exploitation of selective read-out processor data
  • Study, fabrication and implementation of the supermodule loading system of the electromagnetic calorimeter in the CMS experiment.

Experimental means

The electromagnetic calorimeter consists of more than 75,000 lead tungstate crystals. The central barrel consists of 18 supermodules arranged in phi in each half plane. Each supermodule contains 4 modules of 400 crystals each. The two endcaps contain 14648 crystals. Each crystal of the central barrel is read by 2 APDs (Avalanche Photo Diodes). Each crystal of the endcap is read by a VPT (Vacuum Photo Triodes).

Technical description

Laser light injection system to follow crystal transparency.
Optic fibers distribution system.
PN diodes calibrate the laser light received by group of 200 crystals.


PN diodes must withstand radiation doses equivalent to 10 years ofLHC at high luminosity. At high luminosity, the crystal response is assumed to decrease by 3-5% in the first few hours and then saturate. In the absence of irradiation, the crystals recover. The loss of transparency is corrected in the short term (20 to 30 minutes) with an accuracy of 0.4%.



Collaboration CMS (Compact Muon Solenoid): more than 2800 physicists from 200 institutes from 46 countries.

The main laboratories that share the design and construction of the central barrel of the electromagnetic calorimeter are Caltech, Cern (Geneva), CEA/DRF/Irfu, INFN Milan, INFN Rome, IPNL Lyon, LLR Palaiseau, Zurich-ETH




Status and perspectives

Important Dates

  • 1998: approval of the Technical Design Report of the electromagnetic calorimeter (Ecal) by the CERN LHCC.
  • 1999: fabrication and characterization of pre-assembly elements of the calibration system of the electromagnetic crystal calorimeter.
  • 1999 - 2002: manufacture and characterization of the standard elements of the calibration system.
  • 2002 - 2005: preliminary calibration of the crystal calorimeter at Cern from baem tests.
  • End 2003: The 25 micron technology was chosen by Ecal as the basis of the front-end electronics. 10 assembled supermodules.
  • October 2004: Realization, calibration and tests of the supermodule 0
  • Mi-septembre 2005: half of the barrels made
  • December 2007: barrel in the cave
  • June 2008: End of assembly of the endcaps
  • 10 September 2008: Circulation of the first beam in the LHC


Scientific and technical assessment

Scientific evaluation: Scientific Council of the SPP on 24-26 / 09/2001, 28/04/2002, 03/07/2003, 04/11/2011, 13/11/2013, 30/11/2015.


2002: Precalibration of a module of the calorimeter (400 crystals); Beam validation of the calorimeter monitoring system.
2003: start of assembly of the light distribution system on the series modules; Realization of a prototype of the calorimeter read-out system.



#3880 - Last update : 02/08 2017


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