Fermilab hosts international workshop on cutting-edge detector technology

Figure caption: The recent CMS high-granularity calorimetry workshop included site visits, including this one at SiDet, where attendees saw a prototype detector built by a team led by Fermilab scientist Zoltan Gecse. Photo: Leticia Shaddix

An exciting detector workshop was recently held at Fermilab to help plan an important upgrade to the Compact Muon Solenoid (CMS), one of the two large particle detectors located at the CERN Large Hadron Collider (LHC). After nearly a decade of operations and a planned upgrade in the collision rate at the LHC, both the LHC and the CMS detector will be upgraded to prepare itself for the future.

Fermilab has a long history of being the location of the highest-energy particle accelerator on the planet. That changed a few years ago, first when the LHC ran at even higher energies, and even more finally when the Tevatron shut down for the last time in 2011. However, that bittersweet day was not a surprise. It was always clear that there would be a time after the Tevatron.

Indeed, Fermilab has turned much of its attention to CMS and the LHC, the world’s highest-energy accelerator. Fermilab built magnets used in the LHC accelerator and made very important contributions to the component detectors in CMS. These detectors include those that tracked the particles’ passage through the apparatus after collisions, as well as others that measured the energy of the collision.

Indeed, over the years, the assemblage of Fermilab scientists and technical staff has grown to be the largest group working at CERN (besides CERN itself, of course). Fermilab scientists have played crucial roles in all aspects of the CMS experiment, including leading operations, computing, designing and building equipment, performing analyses, and — most recently — actually leading the experiment, with Fermilab’s Joel Butler being the experiment’s spokesperson from 2016–18, and now Patricia McBride being the deputy co-spokesperson, working with a newly elected leader after Butler stepped down.

After years of operation, the CMS experiment discovered the Higgs boson, leading to the Nobel Prize in 2013, and made many measurements, leading to over 800 scientific publications. Now it is time to make serious preparations for the future.

The LHC accelerator complex will be upgraded to increase the number of collisions tenfold. This exciting prospect, called era of the High-Luminosity LHC (HL-LHC), will come with a significant set of improvements in the apparatus. CMS will be an almost new detector based on cutting-edge technologies. Fermilab plays crucial roles in upgrading the detector hardware, specifically the equipment that handles the tracking, time of arrival of particles, calorimetry (energy-measuring) and triggering.

The recent workshop focused on the upgrade of the calorimetry. Part of the existing CMS calorimeter will be removed and replaced with a new one based on predominantly silicon detectors, with some plastic scintillator where the radiation environment permits. This new detector is called the HGCal, or high-granularity calorimeter. The HGCal will be the largest silicon detector ever built in the world, covering 600 square meters of area and comprising 6 million readout channels. Fermilab’s responsibility is to lead the assembly of the forward calorimetry. In particle physics lingo, “forward” means “near the beams.” If you imagine CMS looking like a soup can, with the beam running along the axis of the can, the Fermilab group will be responsible for assembling the flat ends of the can. This work is done in collaboration with other U.S. institutions.

During the workshop, which brought HGCal project managers and world experts to Fermilab. All facets of the project were discussed, ranging from the technologies that will be employed to build the detector, to assembly techniques, to such seeming minutiae as the location of equipment, with a precision of fractions of a millimeter. The workshop also involved tours of a number of Fermilab research facilities: SiDet (where the silicon detectors will be assembled), the Test Beam Facility (which is home to what will be the only high-energy hadron test beam in the world for the next few years), and Lab 6 (where the scintillator portions of the detector are likely to be assembled).

Fermilab’s facilities and experience building particle detectors were on full display. And Fermilab will be the heart of the upgrade, with almost 400 “cassettes” like the one in the photo to be assembled starting in 2020 (half of the entire end-can).

The workshop was a great success. When discussions get to the point where the conversations focus on engineering details and exactly which sites will lead the detector assembly, then a project is well on its way. The next few years will involve improvements to the accelerator and detector. Installation of HGCal will begin in 2024, with upgraded operations likely in 2026.