The CMS experiment (exploded view)
CMS is the detector of one of the 2 out of 4 multi-purpose experiments located on the LHC at CERN. It should start to take data in 2008.

Main goals :

During the last decades, research in particle physics has made tremendous progress and succeeded in validating the theoretical framework called “Standard Model”. Nevertheless, lot of questions remain opened: Why matter is in excess in our universe while it was produced the same amount of matter and antimatter at the Big Bang ? How elementary particles acquire their mass? Would we have to modify our understanding of matter and energy if we observe new processes or new particles? In particular, the field associated to the “Higgs boson” could have generated the mass of all other particles. Other new particles are expected in the framework of Standard Model extensions such as Super Symmetry. Would we, one day, observe exotic phenomena related to these extensions such as extra-dimensions or mini black holes?


LHC, interaction Point No 5 at Cessy


 Dapnia Contributions:


Study, production follow-up , installation and validation  of the solenoid (see the solenoid  page)


Optical fibres system developed to uniformly distribute laser light to crystals during the follow-up of the crystal transparency of CMS-ECAL.

Electromagnetic calorimeter:

* R&D on photo-detectors APD (Avalanche Photo-Diodes) (Sédi/SPP)

* R&D on crystals (Crystal-Clear collaboration)

* Study, development, installation, test and data analysis of the crystal transparency monitoring system (Sédi/SPP):

Even if crystals used in CMS-ECAL are radiation hard, they suffer for loss of light transmission under irradiation due to colour centres creation in the crystal structure.

We have developed a system to measure in real time the transparency of each crystal which subsequently, permit to correct the detector response and thus stabilize it at the level of few per mille in order to be able to perform a physics calibration during a period of about one month.

This measurement is done by injecting laser light in front of each crystal and by reading the associated photo detector response. The amount of laser light send to the crystal is measured with a reference PN diode.

In order to reach the desired performances, we have developed a light distribution system with optical fibres associated to multilevel randomisation splitters to avoid propagation of laser speckles.

A specific electronics has been developed to read the reference PN diodes: a low noise preamplifier in DMILL technology (FEM) and a digitisation module with an interface with the CMS DAQ (MEM).

 The SRP consists of 12 6U-VME cards, grouped in triplets. Each triplet processes the data coming from one ECAL partition.


Triplet of VME cards of the selective readout processor developed for the CMS ECAL acquisition system.

* Study, development, installation, test and data analysis of the Selective Readout Processor (Sédi/SPP):

In the CMS experiment, a complete readout of the electromagnetic calorimeter represents a data block of 1.5 MB, which is bigger than the allowed size for the full CMS event.

In order to decrease the ECAL size by a factor of about 20 without introducing biases in physics analysis, we have developed the Selective readout Processor (SPR), which allow to readout only the relevant regions for each event.

The SRP receives the information from the trigger system through “trigger primitives” and send back to the data concentrator the basic unit numbers which will be fully read out. On the other regions, we perform a zero suppression.

The main difficulties in such a system are located at the level of the communications with the other ECAL parts. A protocol based on very high speed serial transfers with optical fibres has been implemented in this project. This offered the opportunity for the SEDI to extend its know-how in this strategic domain. This communication architecture has been later reuse in CMS by the Global Trigger team to develop a new system.

* "Squirrel":

Study, construction and commissionning of the facility (dubbed "squirrel") to introduce and install the EM calorimeter supermodules inside the CMS Barrel.



Last update : 01/16 2017 (2292)

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