Building Zero Degree Calorimeters for the High-Luminosity LHC

University researchers are developing compact, radiation-tolerant Zero Degree Calorimeters for LHC experiments – training students for careers in science, engineering, and technology.

At the Large Hadron Collider (LHC), not all of the particles produced in a collision travel into the enormous ATLAS and CMS detectors. Some neutral particles continue almost directly along the accelerator beamline, escaping through openings around the beam pipes. About 140m from each collision point, compact instruments called Zero Degree Calolirimeters (ZDCs), intercept these particles. Their name reflects their location: they detect particles emerging at an angle close to zero relative to the incoming beams.

In collisions between lead nuclei, the ZDCs measure neutrons that did not participate directly in the collision. These ‘spectator’ neutrons provide information about how the two nuclei overlapped — information that ATLAS and CMS cannot obtain from their central detectors alone.

The next generation of these instruments must fit into a space only a few centimetres wide, tolerate years of extreme radiation, and retain a sufficiently stable energy response to determine reliably how many neutrons strike the detector. Researchers and students at the University of Illinois Nuclear Physics Laboratory, working with collaborators in the United States, Europe, Israel and China, are developing the technologies needed to meet that challenge.

Their work brings together nuclear and particle physics, materials science, precision machining, optical engineering, electronics, simulations, and machine learning. It also illustrates a broader role played by university laboratories: ambitious scientific instruments do not only produce measurements. They produce scientists and engineers.

Two ways of using the world’s largest collider

The Large Hadron Collider accelerates particles around a 27 km
underground ring near Geneva and brings them into collision inside experiments including ATLAS and CMS.

Most of the time, the LHC collides protons. The proton programme enables physicists to investigate the fundamental constituents of matter and the forces acting between them. By collecting enormous numbers of collisions, ATLAS and CMS can make increasingly precise measurements of known particles and search for rare processes that could signal previously undiscovered physics.

For shorter periods, the LHC collides heavy atomic nuclei – most often lead nuclei, each containing 82 protons and 126 neutrons. In a sufficiently central lead – lead collision, the protons and neutrons deposit an enormous amount of energy into a volume smaller than an atomic nucleus. For a fleeting moment, the collision creates a hot, dense state of matter known as the quark-gluon plasma (QGP).

Quarks and gluons are normally confined inside protons and neutrons. In the QGP, they become part of a collective medium similar to the matter believed to have filled the Universe during its first microseconds. By studying how particles are produced and move through this medium, physicists test quantum chromodynamics – the theory of the strong interaction – and explore how complex nuclear matter emerges from quarks and gluons.

Not every proton and neutron in the two lead nuclei participates in the collision. Those near the overlapping region interact strongly and help create the plasma. Others continue forward as spectators. Charged spectators are deflected by the LHC magnets, but electrically neutral spectator neutrons continue along the beam direction and can reach the ZDCs. Each spectator neutron continues with nearly the original beam energy, so the total energy measured by the ZDC is proportional to the number of neutrons that strike it.

Counting those neutrons helps determine how the two nuclei overlapped. A collision involving nearly all the nucleons produces a different plasma from a glancing collision involving only a smaller fraction. The ZDCs therefore provide essential information about the initial conditions against which measurements of the quark–gluon plasma must be interpreted.

They also play a central role in triggering on and selecting ultraperipheral collisions (UPCs). In these events, the two lead nuclei pass one another without geometrically overlapping, while photons associated with their intense electromagnetic fields interact. Such processes include light-by-light scattering. Depending on whether the nuclei remain intact or break apart, the ZDCs may detect neutrons on zero, one, or both sides of the collision.

A collider transformed

The High-Luminosity LHC programme is designed to deliver much larger proton collision samples. This will increase sensitivity to rare processes and allow measurements that are impossible with today’s datasets.

Producing and recording those samples is an enormous technological challenge. The accelerator requires new magnets, beam-control systems, and radiation-resistant equipment. ATLAS and CMS must upgrade tracking detectors, electronics, triggers, computing systems, and forward instrumentation.

The heavy-ion collision rate will not increase as dramatically as the proton collision rate. Nevertheless, the accelerator modifications associated with the High-Luminosity LHC fundamentally alter the environment in which the ZDCs operate.

Changes to the beam optics require redesigned neutral-particle absorbers – the structures in which the ZDCs are installed. At the same time, the horizontal space available for each detector decreases from approximately 10 cm to only 5 cm. The existing ATLAS and CMS ZDCs cannot simply be refurbished and reinstalled. New detectors must be built for Run 4.

The reduced space is only one problem. During the High-Luminosity LHC heavy-ion programme, the ZDCs are expected to accumulate radiation doses reaching 5 megagray. No practical detector could survive the much greater exposure from routine high-luminosity proton–proton running, so the ZDCs will be installed only for the relatively short heavy-ion periods and removed afterward.

This creates a second engineering challenge. Proton–proton collisions strongly activate the copper absorber that houses the ZDC, leaving it highly radioactive even after a cooling period. Before and after each heavy-ion run, the detector modules must therefore be installed and removed beside activated equipment. A remotely controlled crane handles the modules, while movable radiation shielding protects workers during the brief period needed to make or disconnect cables and other services. The installation infrastructure is therefore as important to the project as the detector itself.

The result is an unusually compact systems-engineering problem: the calorimeter, light-collection system, photodetectors, monitoring equipment, cables, and mechanical supports must all fit together while preserving the ability to install and operate the device safely.

Turning neutrons into light

The ZDC is built from alternating layers of dense tungsten absorber plates and fused-silica radiators. A high-energy neutron entering the detector initiates a shower of secondary particles in the tungsten. Charged particles in that shower pass through the fused silica and produce Cherenkov light – the faint optical flash emitted when a charged particle travels through a transparent material faster than light can propagate through that material.

The light is transported to photomultiplier tubes, which convert it into electrical signals. The amount of light provides a measurement of the shower energy and, therefore, of the number of incident neutrons.

Making this basic design work requires solving several difficult technical challenges. Can ultra-pure, hydrogen-loaded fused silica retain sufficient optical transmission after years of intense irradiation? Which photomultiplier windows offer the best balance between ultraviolet sensitivity and radiation tolerance? How can Cherenkov light be transported efficiently? And how can the detector response be calibrated when direct access is impossible, and radiation can alter the performance of the fused-silica radiators, the photomultiplier tubes, and even the LED-based monitoring system itself?

Earlier ATLAS and CMS ZDCs used fused quartz that suffered significant radiation-induced losses in optical transmission. Researchers at Illinois and Ben-Gurion University, working with CERN groups and Heraeus in Hanau, Germany, studied fused-silica samples exposed inside the LHC and compared measured radiation levels with detailed FLUKA simulations. Their work showed that material composition has a major effect on radiation response. Highly hydrogen-loaded fused silica can remain remarkably stable over a wide range of doses, while hydrogen-free material follows a different pattern of initial damage and subsequent behaviour.

The High-Luminosity detector programme has therefore required more than selecting a material from a catalogue. Students and researchers have developed techniques and procedures for cutting, grinding and polishing fused-silica rods; designed optical couplings and air light guides; tested photomultiplier tubes; built LED calibration systems; studied radiation-induced defects; and developed mechanical structures that fit within the severe dimensional constraints of the neutral-particle absorber that houses the ZDC in the LHC tunnel.

Testing materials during LHC operation

One of the project’s newest instruments is the Live Irradiation Setup (LIS). Installed in the LHC in January 2026, the LIS is designed to measure radiation damage as it develops during normal proton running rather than only examining materials after they have been removed from the accelerator.

The device contains four combinations of fused-silica radiators and photomultiplier tubes. Two silica compositions – hydrogen-loaded and hydrogen-free – are combined with photomultipliers using different window materials. Signals produced by particles from the LHC beams are compared with signals from a controlled LED pulser. This allows the researchers to distinguish changes in the fused-silica radiator from changes in the photomultiplier or its window.

The LIS was designed and constructed at Illinois. The University of Kansas developed its LED pulser system. In January 2026, students and technicians from Illinois, Ben-Gurion University, and the University of Turin installed the device in the LHC. Students and postdoctoral researchers from Ben-Gurion, Turin and Illinois supported its operation during the 2026 proton run with significant help from BNL and Columbia collaborators and are now analysing the data.

The work is supported through a joint NSF–BSF project led by Matthias Grosse Perdekamp at Illinois, Zvi Citron at Ben-Gurion University, and Riccardo Longo, who was at Illinois when the proposal was submitted and is now at Turin. The LIS makes it possible to investigate a question central not only to the ZDCs but also to many future instruments: how can an optical detector distinguish and monitor the separate effects of radiation on its radiator, light-collection system, and photodetector?

Run 3: Proving the technology in the LHC

The recently completed Run-3 programme provided the most important validation so far.

For Run 3, the ATLAS ZDCs were rebuilt with hydrogen-loaded fused-silica radiators from Heraeus and equipped with new cables, electronics and an LED monitoring system. Illinois designed and constructed a pair of new reaction-plane detectors, or RPDs, installed immediately behind the electromagnetic modules of the ZDCs, close to the maximum of the developing neutron showers.

The RPD is a distinctive Illinois innovation. Rather than measuring only the total energy of a neutron shower, it divides the shower into spatial channels and reconstructs its transverse shape and centre. Spectator neutrons can receive a small, correlated sideways deflection during a lead–lead collision. Measuring that deflection on both sides of the interaction gives physicists access to the orientation of the collision, making it possible to study how particle production varies with angle relative to the reaction plane.

Two Run-3 results provide especially clear demonstrations of the system’s performance.

The calibrated ZDC energy spectrum shows well-separated peaks corresponding to showers produced by one, two, three, and four neutrons. Each neutron carried approximately 2.68 TeV, or 2.68 trillion electronvolts, of energy. The spectra recorded on opposite sides of ATLAS agree closely, showing that the calorimeters could count small numbers of spectator neutrons with a consistent energy response.

The RPD produced an equally direct result. During a machine-development study, LHC operators deliberately varied the vertical beam-crossing angle at the centre of the ATLAS detector. The centroid of the neutron shower reconstructed by the RPD changed linearly with the crossing angle. The measurement demonstrated that the detector could resolve millimetre-scale changes in the transverse shower position using real LHC data. A related analysis was used to diagnose a residual horizontal beam tilt and determine a correction for the 2024 heavy-ion run.

Together, these results show that the Run-3 system did more than survive. The calorimeters retained the energy resolution needed to distinguish different neutron multiplicities, while the RPD added new spatial information about the forward showers. The instruments served simultaneously as working physics detectors and as prototypes for the High-Luminosity era of the LHC.

From components to a full-size detector

The project reached another milestone in summer 2026 with beam tests of the first full-size preproduction detector (PPD), using beams from CERN’s Super Proton Synchrotron. It is the first prototype to incorporate the final mechanical dimensions and the principal design features envisioned for the High-Luminosity ZDC.

The detector was designed by Illinois in close collaboration with the University of Kansas, with component production shared between the two institutions. Initial preassembly took place at Kansas, where Kansas technicians worked alongside an Illinois graduate student. Kansas also provided the LED monitoring system. Columbia and Turin, working with ATLAS technical staff based at CERN, procured and integrated the photomultiplier tubes, cables, and connectors. Teams from Illinois, Columbia University, Ben-Gurion University, the University of Turin and a new CMS group at Rice University then completed the detector assembly at CERN.

South China Normal University, a new collaborator in the CMS programme, supplied the tungsten absorber material. Scientists and students from across the participating institutions operated the detector during the beam campaign, which was led by Brian Cole of Columbia University.

In July, the PPD was exposed to electron, proton, and pion beams at CERN’s Super Proton Synchrotron. A further campaign with lead beams took place in August. Detailed data analysis is only beginning, so quantitative results are not yet available for public release. The detector was delivered on schedule, commissioned successfully, and performed consistently with expectations during the initial tests.

The campaign marks the transition from component-level research to full-system validation. It allows the project team to study energy response, shower development, optical performance, calibration, and agreement with simulation in a detector having the dimensions and architecture intended for Run 4.

Building instruments – and careers

At Illinois, this project continues a tradition that reaches back to the beginning of modern accelerator physics.

In 1940, Illinois physicist Donald Kerst built the first successful betatron, using magnetic induction to accelerate electrons. The instrument was later used in research related to thorium, uranium, and plutonium for the Manhattan Project. Nineteen members of the Illinois Physics Department worked at Los Alamos during the Second World War.

Rosalyn Sussman Yalow earned her doctorate in nuclear physics at Illinois in 1945, becoming only the second woman to receive a PhD from Illinois Physics. She later helped develop radioimmunoassay, a technique that transformed medical diagnostics, and in 1977 became the first American-born woman to receive the Nobel Prize in Physiology or Medicine.

The scientific questions have changed, but an important part of the educational model remains: students learn by helping to create instruments whose performance is not guaranteed in advance.

As of August 2026, eight doctoral students and 56 undergraduate researchers have contributed to the ZDC programme at Illinois. Five undergraduates came from other institutions through the National Science Foundation’s Research Experiences for Undergraduates programme. Their projects have included optical-transmission measurements, radiation-damage studies, Raman spectroscopy, mechanical design, fused-silica cutting and polishing, light-guide development, photomultiplier testing, LED calibration, detector simulations, machine learning, software, assembly and test-beam operation.

These are not observational roles. Students have been responsible for identifiable parts of detector systems that must operate at CERN. Farah Rafee contributed to the assembly of the RPD before entering a doctoral programme in nuclear engineering at the University of Michigan. Sam Lund worked on air light guides for the High-Luminosity ZDC before entering a physics PhD programme at the University of Texas at Austin. Nivedha Vasanth tested and integrated components of the LED monitoring system before continuing her studies in electrical and computer engineering at Georgia Tech. More recently, students have contributed directly to the design, construction, and integration of the LIS and PPD, as well as to radiation shielding and detector calibration.

Illinois undergraduates have made 23 ZDC-related presentations at annual meetings of the American Physical Society’s Division of Nuclear Physics, including eight in 2025. Former students have continued into graduate programmes in physics, nuclear engineering, electrical engineering, applied mathematics, and related fields. Others have moved directly into technology and engineering positions at Fermilab, Los Alamos National Laboratory, Intel, IBM, Samsung, Accenture, and Northrop Grumman.

The destinations of former doctoral students are similarly broad. Graduates from the programme now work as a senior software engineer at Intuitive, a lead artificial-intelligence engineer at the Gates Foundation, a machine-learning software engineer at Meta and a principal physicist at Northrop Grumman.

Nuclear-physics instrumentation does not train students for only one narrow occupation. It teaches them to integrate physics, computation, optics, electronics, materials, mechanical systems and collaborative problem-solving. Those capabilities are needed across national laboratories, medicine, advanced manufacturing, computing, and private industry.

A necessary detector with an uncertain path

The original US construction programme brought together ATLAS groups at Brookhaven National Laboratory, Columbia, Illinois, Ben-Gurion and Turin, and CMS groups at Kansas and the University of Maryland. It included both ZDCs and RPDs for the two experiments.

In October 2025, the U.S. Department of Energy cancelled the approximately $3.4m grant intended to support that construction effort. The scientific programme at CERN will continue: ATLAS and CMS still require Zero Degree Calorimeters for Run 4. The participating institutions therefore had to restructure the project around smaller federal funding opportunities, university commitments, new collaborators, and in-kind technical contributions.

The University of Kansas has received a $1m Department of Energy EPSCoR award supporting construction of the CMS High-Luminosity ZDCs. South China Normal University’s contribution of tungsten to the PPD shows how new international partners can provide essential materials. Other institutions are contributing machining, personnel, assembly effort, and specialised technical capabilities.

This distributed approach has preserved momentum, but it cannot automatically replace stable project funding. Long-term engineering, technical coordination, quality assurance and student support are difficult to sustain through isolated contributions.

The future of the reaction-plane detector illustrates the consequences. The Run-3 RPDs were designed, constructed and successfully operated. The original plan included next-generation RPDs for both ATLAS and CMS. Under current funding constraints, however, both experiments may have to prioritise the essential calorimeters and omit the additional reaction-plane capability.

The issue is no longer whether the RPD technology works. Run 3 showed that it does. The question is whether sufficient resources will exist to deploy it.

At Illinois, broader reductions in support for the Nuclear Physics Laboratory have resulted in the loss of five technical positions and reduced funding for graduate and undergraduate researchers. The consequences extend beyond a single detector or measurement. When a university instrumentation programme contracts, the country also loses part of the system through which future scientists and engineers gain practical experience.

The High-Luminosity ZDC project demonstrates what sustained research investment can produce: new radiation-tolerant materials, compact detector designs, successful instruments operating at the LHC, international partnerships and dozens of technically trained students.

The PPD test campaign shows that the international team developing the HL-ZDC remains capable of delivering the next generation of detectors. The remaining challenge is ensuring that the full scientific capability – and the workforce-development system that created it – can be carried through to Run 4 and sustained for the projects that follow.

At Illinois, this project was supported with funding from the U.S. National Science Foundation (NSF).

Please note, this article will also appear in the 27th edition of our quarterly publication.