The Department of Particle Physics at the CEA-Saclay/IRFU is currently seeking a talented and highly qualified postdoctoral researcher to work in the CMS experiment with a two years contract, renewable for one additional year.

 

The successful candidate will join the CMS group of the Department which effort is focused on, but not limited to, the exploration of the scalar sector of the Standard Model via the study of Higgs boson decays in the diphoton channel. The group was involved in recent years in the discovery of the ttH production [1], the constraints on the CP structure of the H-tt coupling [2], and the search for non-resonant double Higgs production in the channel with two b quarks and two photons [3], with two convenerships of the subgroup dedicated to the diphoton decay channel of the Higgs boson during Run 2. The CMS-Saclay group is also consistently strongly involved in the electromagnetic calorimeter (ECAL): its operation, its calibration and its upgrades, with several major responsibilities over the past years (for instance ECAL DPG convener and ECAL System Manager). Regarding the ECAL barrel phase 2 upgrades, the group is leading the design of the new front-end readout ASIC “CATIA” and the upgrade of the Laser Monitoring (LM) system. The LM system is designed to measure in quasi-real time the crystal transparency losses due to irradiation and their recovery, in order to correct the variation of the ECAL response in time and preserve the excellent intrinsic energy resolution of the ECAL at the percent level. This system is crucial in particular for the diphoton decay channel of the Higgs boson, where the resolution of the mass peak is of primary importance. The transparency measurement is performed by sending laser light, during abort gaps, through the ECAL crystals read by their photodetectors and to reference PN diodes, in order to infer a transparency correction from the response ratio, with a requirement on the precision at the per mil level (with more than one measurement per crystal per hour). For phase 2, a new charge pre-amplifier ASIC was developed in Saclay (MONACAL), together with new modules (called HL-MEM) to read the generated signals and integrate them in the DAQ system. Several tasks will have to be conducted in parallel by the successful candidate to define the integration of the new developments for the LM at phase 2 and to define the ECAL timing calibration strategy using LM data.

 

 The first task, which should be short, will be the definition of the integration chain of these new LM components in the existing ECAL super-modules (SM) and LM system. The installation procedure of the new elements will have to be defined on the spare SM “SM36” (SM36 will be used also to test the integration and performance of other developments like CATIA for example with the upcoming November 2022 test-beam at CERN). This work should take a few months only and will require to interact with the ECAL technical coordination, the ETH group for the installation, the ECAL management, and the various actors in Saclay. The candidate will acquire a deep knowledge of the ECAL and its LM system as well as the ECAL community, with several trips at CERN.

 

The second task is the definition of the new LM data acquisition. The full data acquisition chain will have to be revisited for phase 2: from the data format to the laser monitoring software. This will require software development on one side, and interacting with people responsible of the firmware to define the needs on the other side. Interacting with many groups will thus again be needed, including with the DAQ group at CERN, but also the HLT and the Tier0. It will be possible to test the definition of this processing chain at test-beams on the spare SM 36, with a system closer and closer to the nominal system for phase 2.

 

The third and main task will be the definition of the procedure to (inter-)calibrate the ECAL timing with LM data and its automatization. For phase 2, the timing resolution will be significantly improved (from about 120 ps currently to less than 50 ps) and the timing information from ECAL and from other sub-detectors will allow to reduce the impact of increased PU drastically. The laser data will be used to monitor the timing in the ECAL and to intercalibrate channels. This timing calibration will have to take into account the transparency losses and pulse shape changes. Once again, the spare SM36 could be used to take laser data to validate the procedure, once defined, and assess its precision.

 

The candidate will thus have a very wide variety of activities and interact with many people from different teams. She/he will thus acquire a deep knowledge of ECAL. The contract is expected to start in 2022. The successful candidate will be based at CEA-Saclay with opportunities to visit CERN regularly.

 

Interested candidates should send their scientific curriculum vitae, a cover letter, a research statement and arrange for at least two letters of recommendation to be sent to the contact below. The review of applications will start immediately and continue until the position is filled.

  

Julie Malcles

CEA Paris-Saclay

DRF/Irfu/DPhP, Bât 141, p. 44 A

91191 Gif-sur-Yvette

France

julie.malcles@cea.fr , +33 1 69 08 86 83

 

[1] Phys. Rev. Lett. 120 (2018) 231801

[2] Phys. Rev. Lett. 125 (2020) 061801

[3] JHEP 03 (2021) 257