“Complete the LHC Phase 1 Upgrades….” is how the first project-specific and concrete recommendation (Recommendation 10) of the 2014 Report of the Particle Physics Project Prioritization Panel (P5), “Building for Discovery,” starts. This short phrase has been the mission statement for U.S. CMS scientists from Fermilab and around 30 U.S. institutes for the last six years. On May 1, with the successful outcome of the Department of Energy Critical Decision 4 review, the project has been completed.
During LHC Run 2, starting in 2015 and just completed at the end of 2018, the CMS data set has grown by a factor of 5, allowing the evolution from “discovery of the Higgs boson” mode (which led to a Nobel Prize in 2013) to “exploration with the Higgs boson” mode, probing exactly how the particle decays, with what kinematics and to which particles at what frequency — ultimately to understand whether nature behaves as the theory predicts or there is something more subtle at play. In short, we are “using the Higgs boson as a new tool for discovery,” per the P5 science driver.
Beyond the Higgs, with much more data, CMS has also been able extend the searches for the more rare and elusive new physics, both directly, by looking for new particles, and indirectly, by probing for small quantum interference perturbations to known physics processes. Thus, CMS is also making inroads on two other science drivers, “identify the physics of dark matter” and “explore the unknown: new particles, interactions and physical principles.” On top of the 567 publications from Run 1, in Run 2, CMS has produced 324 publications on data analysis (and counting), a clear demonstration of the breadth and depth of the scientific program of the LHC.
During this period CMS also undertook the fabrication of the $40 million strategic upgrades, officially the LHC CMS Detector Upgrade Project and unofficially “Phase 1.” Supported by both the National Science Foundation and the Department of Energy, these upgrades are motivated by the increased luminosity expected during LHC Run 3, which goes from 2020-2023, and in fact have already been experienced during Run 2. The upgrades comprised the following subprojects:
1. A completely new level-1 trigger system. The L1 trigger is responsible for sampling the detector data at the full 40 MHz collision rate and efficiently sifting interesting events from the background while reducing the output rate to 100 kHz, a factor of 400, all in about 4 microseconds. Raising the kinematic thresholds in the L1 trigger is one way to accomplish this, but this comes with a heavy penalty to the physics program, as key decay modes of W and Z bosons, for instance, get sacrificed. The upgrade uses state-of-the-art electronics (FPGAs) and high-bandwidth optical transmission, allowing for more granular input data and more complex sifting algorithms so that the rate reduction is achieved while preserving the low kinematic thresholds that ensure a vibrant physics program in Run 2 and Run 3.
2. A new forward pixel detector. Together with the barrel, made by European collaborators, the forward pixel detector comprises the pixel detector, the innermost CMS detector, providing precise space points to reconstruct the trajectories (or “tracks”) of all charged particles in an interaction. These tracks in turn are combined to form interaction vertices. Only one of typically roughly 40 in a single crossing is from an interesting process. The rest are called “pile-up,” essentially contamination of the data sample. And with the increased luminosity in Run2 and Run 3, the pile-up grows considerably. The new pixel detector adds an additional layer for the more precise track detection and vertex separation, a new fully digital readout ASIC with deeper buffers to prevent excessive data loss at high instantaneous luminosity, and less material in the tracking volume, reducing the number of misreconstructed tracks due to particles interacting with the detector substructure. The pixel detector plays a crucial role particularly in identifying physics associated with top and bottom quarks, such as the recent flagship measurements from CMS: the decay of the Higgs into two bottom quarks and the production of a Higgs boson in association with two top quarks.
3. Front-end signal processing electronics and back-end readout for the hadron calorimeter (HCAL), for the forward, endcap, and barrel regions. The HCAL measures all the energy coming from the plethora of hadron “showers” created in the event, which, in addition to being an important observable in its own right, is also used to calculate if there is any missing energy, which might have been carried off by some new particle, perhaps a dark matter candidate! It’s pretty clear the HCAL is essential to the LHC program. In CMS, this energy is measured in the form of scintillation light, whose intensity has been diminishing rapidly due to increased radiation from collisions. The HCAL front-end upgrade replaces the photodetectors exposed to this light with more sensitive devices called silicon photomultipliers, at the same time increasing the granularity of the energy measurement to allow better calibration and shower separation, and in addition to the energy measurement providing a timing measurement that helps separate signal from spurious background signals that come “too early.” The increased granularity and additional information mean increased data volume. Thus, the back-end readout upgrade provides increased throughput to pass the data to the L1 trigger and data acquisition system.
While the completion of the project is coincident with the LHC Long Shutdown 2 from 2019 to 2021, in fact, the project has been “completing” since 2015, in a piecemeal fashion, with the first components installed in the winter of 2015-2016, and portions of the upgrade installed during each year-end technical stop of the LHC since then. This staging has had the advantage of not only avoiding changing multiple parts of the detector at the same time, but also providing higher-quality physics data for the bulk of Run 2 already. So the results CMS produces today already have (most of) the benefits this upgrade brings. The installation of the HCAL front-end upgrade for the barrel region of the detector requires extended access to the inner guts of the detector and thus had to wait for the current shutdown. The full upgrade will be ready for the first data of Run 3, expected mid 2021.
With this, the CMS Phase 1 upgrade is complete, and it has been quite a journey, with the commensurate number of “bumps in the road” that accompany any project pushing the boundaries of technology in HEP experimentation. But the talented and dedicated team of people, from Fermilab, the University of Alabama, University of Minnesota, University of Notre Dame, University of Maryland, University of Iowa, Baylor University, Kansas State University, University of Wisconsin, University of Florida, Rice University, Northeastern University, University of Illinois-Chicago, University of Nebraska, Purdue University, Catholic University of America, Purdue University–Northwest, Cornell University, University of Kansas, University of California-Davis, University of California–Riverside, Colorado University, University of Mississippi, Princeton University, Ohio State University, University of Virginia, Vanderbilt University, Boston University, Rutgers, University of Buffalo, University of Puerto Rico–Mayaguez, Brown University, and the Florida Institute of Technology, overcame the obstacles to produce the excellent project deliverables (on time and on budget!), for which the project manager and the CMS collaboration are very grateful.
Still, P5 Recommendation 10 goes on: ”…and continue the strong collaboration in the LHC with the Phase 2 (HL-LHC) upgrades of the accelerator and both general-purpose experiments (ATLAS and CMS). The LHC upgrades constitute our highest-priority near-term large project.” In simple terms, “one down, one to go”! The Phase 2 upgrades are another major undertaking for Fermilab (again managing the effort) and the U.S. CMS university groups. With the Phase 1 effort officially concluded, the participants are leveraging the expertise garnered from this effort to completely focus on the next challenge, with the goal of completing the mandate of Recommendation 10 in the next seven years or so. Stay tuned!
Steve Nahn is a Fermilab scientist and the U.S. project manager for the CMS detector upgrades.