Clever engineering at CMS

The successful operation of the CMS hadron calorimeter relies on a team of electrical engineers at Fermilab. Front row, from left: Julie Whitmore (physicist), Jim Freeman (physicist), Johnny Green, Terri Shaw. Middle row, from left: Paula Lippert, Janina Gielata (U Rochester technician), Marcus Larwill, Albert Dyer, Nina Moibenko, Alan Baumbaugh. Back row, from left: Tom Zimmerman, Lee Scott, Sergey Los, Scott Holm, Lou Dal Monte. Photo: Reidar Hahn

Although physicists were the ones generating headlines with the discovery of a new particle last year, they aren’t the only ones who have been keeping busy at CMS.

For the last three years, while the CMS detector was dutifully gathering data from trillions of proton collisions, Fermilab electrical engineers were devising ways to make it more sensitive and more robust for the LHC’s second go-round. Now that the accelerator has begun its two-year shutdown, engineers are ready to swoop in to equip the detector’s hadron calorimeter—one of the detector’s larger sections—with nifty new devices, preparing CMS for the higher energy and luminosity of the collider’s next run.

“As technology improves, we can improve our detector. So even from the very beginning, we started an upgrade path,” said PPD’s Jim Freeman, a physicist working on the CMS experiment. “We’re going to improve things we had trouble with or now know how to make better.”

One of those improvements is advancement in the photosensor, a device that picks up signals from light particles in the detector. CMS engineers have worked with vendors to develop a more resilient and sensitive photosensor, the silicon photomultiplier—SiPM for short—for collider experiments. The SiPM is not only much smaller than the old photodetector but also has 100 times more gain, giving a better signal-to-noise ratio.

To ensure the SiPM’s success in the next CMS run, Fermilab engineer Sergey Los designed an electronics support system for the SiPM photosensor that includes low-voltage generation, leakage current measurement and temperature stabilization.

In addition, the front-end and back-end electronics will be upgraded in stages over the next five years. The microelectronics group is developing a chip that will, without sacrificing resolution, measure the charge and arrival times of signals many times larger than can be handled with the present CMS HCAL front-end electronics. To that end, engineer Tom Zimmerman is designing an upgraded readout chip based on a novel approach known in the physics community as QIE (charge-integrating encoder), which was developed at Fermilab.

“It’s a very interesting mix of all different kinds of analog and digital circuitry in one chip, which isn’t typical,” Zimmerman said. “Making everything work together presents a very diverse set of challenges, which makes it a lot of fun.”

Engineer Terri Shaw is using the QIEs in the design of the HCAL front-end electronic cards and is working to seamlessly integrate the full readout chain—a chain that goes all the way from scintillation light in the detector to computers that read out the data.

The engineers’ work is just as much about reliability as it is cutting-edge technology, since parts of the detector are inaccessible for years. If a “buried” component fails, scientists will go without data that would have otherwise come through the burnt-out part.

“We’d have lots of people pounding on our door, very upset that they have fake missing energy in their data,” said CMS physicist Julie Whitmore.

The collaboration relies on the engineering team to make sure that doesn’t happen. They understand that behind every successful physics project is a solid team of clever engineers.

Leah Hesla