Chromatic Calorimetry -- A Novel Approach to Validate Energy Resolution and Particle Discrimination

To understand the work presented in this thesis, one must first grasp how physicists study the universe's smallest building blocks. They do not look at these particles directly; instead, they smash them together at incredible speeds and watch what happens when they collide. These collisions create a cascade of new particles, a shower of debris that spreads out like a ripple in a pond. To make sense of this chaos, scientists use massive detectors called calorimeters. These devices act like giant, three-dimensional nets, catching the energy of every particle that flies through them. By measuring how much energy is deposited and where it stops, researchers can identify what the original particle was. However, in the future of particle physics, these collisions will happen so frequently that the showers from different particles will overlap, creating a dense fog of data that is difficult to untangle. The challenge is to find a way to see through this fog, distinguishing a single electron from a pion (a type of heavy particle) even when they arrive at the exact same moment.

This is the problem that Devanshi Arora tackled in her doctoral research at Shizuoka University and CERN. She explored a new way to build these energy-measuring nets, a method called chromatic calorimetry. The core idea is simple yet elegant: instead of using a single type of material to catch the energy, she stacked layers of different materials, each designed to glow in a specific color of light when struck by a particle. Just as a prism splits white light into a rainbow, this stack splits the energy of a particle shower into different colors based on how deep the particle travels. Electrons, which are light and fast, tend to dump their energy near the surface, lighting up the top layers. Heavier particles like pions penetrate deeper, lighting up the lower layers. By separating the light by color, the detector can reconstruct the shape of the shower in three dimensions, allowing scientists to tell particles apart even in crowded conditions.

Arora tested this concept in two major experiments at CERN's Super Proton Synchrotron, a particle accelerator in Switzerland. In 2023, she built a prototype using four different types of crystal and plastic materials, each chosen because it emits light at a unique wavelength. She fired beams of electrons and pions at this stack, ranging in energy from 25 to 100 billion electron volts. The results were promising. By analyzing the specific mix of colors detected, the team could separate electrons from pions with a purity of 95 percent. This means that out of every 100 particles identified as electrons, only five were actually pions. The system also measured the energy of the particles with a precision of 2.5 percent at the highest energies tested. However, the 2023 design had a flaw: one of the crystals produced very little light, making it hard to get a clear signal from the middle of the stack.

Undeterred, the team refined the design for a second experiment in 2024. They replaced the dim crystal with a material that produces a different kind of light, known as Cherenkov radiation, which is emitted almost instantly when a particle moves faster than light does in that material. They also swapped out other crystals for faster, brighter plastics. This new stack, tested with beams from 10 to 100 billion electron volts, performed even better. The energy resolution improved to 1.6 percent at an energy of 91.51 billion electron volts, a level of precision comparable to the best detectors currently in use. The particle identification remained at that high 95 percent purity. The success of these tests proved that the concept of separating particle showers by color works in the real world, not just in theory.

Looking beyond these immediate results, the thesis also explored the future potential of this technology using computer simulations. The researchers modeled a next-generation version of the detector that would use tiny, man-made particles called quantum dots embedded in plastic layers. These quantum dots can be tuned to emit very narrow bands of color, allowing for even finer separation of the light. The simulations suggested that this advanced design could achieve an energy resolution with a constant error of just 0.35 percent, a level of accuracy that would be essential for the Future Circular Collider, a proposed machine that will smash particles together with ten times the energy of today's most powerful accelerators. While these quantum dot designs have not yet been built or tested with real beams, the computer models indicate they could handle the extreme crowding of particles expected in future experiments.

The journey from the 2023 prototype to the 2024 upgrade highlights the iterative nature of scientific discovery. The initial tests confirmed that the basic idea was sound, but they also revealed the limitations of the materials available at the time. By swapping out the components and optimizing the filters that separate the light, the team turned a working concept into a high-performance tool. The research also identified specific hurdles that remain, particularly at lower energies where the particle showers are not deep enough to create distinct color patterns, and where the light from different materials can still bleed into one another. To solve this, the team plans to return to the accelerator in 2025 to test prototypes using cadmium selenide and perovskite quantum dots, materials that promise even sharper color separation and brighter signals.

Ultimately, this work demonstrates that by changing how we look at particle showers—not just as a total amount of energy, but as a spectrum of colors—we can see the invisible details of the subatomic world. The ability to distinguish between different types of particles with such high precision is not just a technical achievement; it is a necessary step for the next generation of physics experiments. As the field moves toward colliders that will produce thousands of overlapping events in a single instant, the ability to slice through that noise with spectral clarity will be the key to discovering new laws of nature. The path forward is clear: refine the materials, narrow the color bands, and build detectors that can see the universe with a resolution never before possible.