The experiment tests the dual-readout calorimetry principle — a technique at the frontier of particle detector design — using standard equipment already available at CERN’s T9 beamline. No custom hardware required.

The experiment is a collaboration of ten students from TED Ege College, Aydın, Turkey, the same team behind the RadiaTED Compton scattering proposal.

The Problem: Invisible Energy in Hadronic Showers

Measuring the energy of electrons and photons is well-solved in particle physics — electromagnetic calorimeters do this with excellent precision. Hadrons (pions, protons) are a different story.

When a hadron enters a calorimeter, it initiates a hadronic shower with two distinct components:

  • Electromagnetic component — neutral pions produced in the shower decay almost instantly into photon pairs, seeding small EM sub-showers.
  • Hadronic component — nuclear breakup, slow protons, neutrons, and nuclear de-excitation. These processes deposit energy in ways that don’t produce detectable light: the energy is effectively invisible to the detector.

The critical problem is that the relative size of these two components — the electromagnetic fraction, f_em — fluctuates randomly from event to event. One pion shower might be 20% EM; the next might be 60%. Because the detector responds differently to each component, this fluctuation directly broadens the measured energy distribution and sets a hard floor on hadronic energy resolution that cannot be overcome by simply adding more material or better electronics.

The Solution: Dual-Readout

The dual-readout technique exploits a key physical difference between the two shower components:

  • All particles in the EM component are relativistic (electrons, positrons, photons) and produce both scintillation light (from ionisation) and Cherenkov light (from exceeding the speed of light in the medium).
  • Slow, heavy particles in the hadronic component — protons, nuclear fragments — produce scintillation light but are below the Cherenkov threshold. They contribute to S but not C.

This means the C/S ratio is constant for pure EM showers, and lower and variable for hadronic showers depending on f_em. Measuring both signals simultaneously for each event gives two equations:

S = E × [f_em + (h/e)_S × (1 − f_em)]
C = E × [f_em + (h/e)_C × (1 − f_em)]

Since (h/e)_C ≪ (h/e)_S, the system can be solved event-by-event to extract both the true energy E and f_em. The result is a dramatic improvement in hadronic energy resolution — correcting the dominant source of smearing rather than just averaging over it.

This is the core principle of the IDEA detector (Innovative Detector for an Electron-positron Accelerator), the leading calorimeter concept for CERN’s proposed Future Circular Collider (FCC-ee). The RD52/DREAM collaboration has been developing it for over a decade. Our experiment tests the same fundamental idea in a form accessible at BL4S.

Experimental Setup

The experiment runs on the CERN PS T9 mixed secondary beam, which delivers electrons, pions, and protons at momenta between 0.5 and 10 GeV/c. The detection chain:

Trigger scintillators (S1, S2) — Two plastic scintillators bookend the setup. Their coincidence forms the trigger, ensuring only particles that traverse the full calorimeter are recorded.

Delay Wire Chambers (DWC1, DWC2) — Measure the transverse beam position with ~200 μm resolution. Used to select events where the beam hits the calorimeter centre and reject edge events with poor shower containment.

Threshold Cherenkov Counter (XCET) — A gas-filled Cherenkov detector at adjustable pressure. Different particle species at a given momentum have different velocities, hence different Cherenkov thresholds. By scanning two pressure settings, electrons, pions, and protons can be identified event-by-event. This serves as the ground truth for validating the dual-readout separation.

Dual-readout calorimeter module — The core of the experiment. Interleaved layers of:

  • Lead-glass blocks (already available at T9): produce Cherenkov light, read out by photomultiplier tubes → signal C
  • Plastic scintillator tiles (already available at T9): produce scintillation light, read out by photomultiplier tubes → signal S

Total depth is ~25 cm (≈ 1 nuclear interaction length for pions). Both signals are digitised by ADCs; for each event we record the pair (S, C).

A key strength of this proposal: all equipment is standard BL4S inventory. We bring no custom detectors.

Measurement Plan

Data is collected at six beam momenta: 1, 2, 3, 5, 7, and 10 GeV/c, using both beam polarities to access different particle compositions. At each setting, the XCET pressure is tuned for optimal particle identification. Target: at least 50,000 events per configuration, which takes under a minute at full beam rate.

Analysis pipeline:

  1. Particle ID — Tag each event as electron, pion, or proton using XCET and time-of-flight between S1 and S2.
  2. C vs S scatter plot — Plot Cherenkov signal against scintillation signal for all events at each momentum. Electrons should form a tight, high-C/S cluster; pions and protons should scatter broadly at lower C/S, with the spread encoding f_em fluctuations.
  3. EM fraction extraction — The pure electron sample (f_em = 1 by definition) calibrates the C/S scale. Each hadron event’s C/S ratio then yields its f_em.
  4. Energy correction — Apply the dual-readout formula to obtain the corrected energy per event. Compare the energy resolution before and after correction.
  5. Energy dependence — Repeat across all momenta. At higher energies the average EM fraction increases (logarithmically due to multiplicity effects), which should be directly visible as a shift in the pion C/S distribution.

Expected Results

  • Clear separation of electron and hadron populations in the C vs S plane across all momenta.
  • A constant electron C/S ratio, confirming the EM calibration anchor.
  • The pion C/S distribution shifting upward with energy, directly measuring the rise of average f_em.
  • Measurable narrowing of the hadron energy distribution after dual-readout correction — resolution approaching the intrinsic stochastic limit rather than the fluctuation-dominated uncorrected value.
  • A distinct C/S signature for protons versus pions, reflecting their different shower development.

Simulated C vs S scatter plots at 2, 5, and 10 GeV/c confirm these expectations are testable within the available statistics.

Scientific Significance

To our knowledge, no previous BL4S experiment has attempted a dual-readout measurement. The combination of lead-glass (Cherenkov) and plastic scintillator (scintillation) available at T9 is a unique opportunity to demonstrate this principle with real beam data at CERN, contributing directly — if in miniature — to the R&D programme for future collider detectors targeting sub-percent hadronic energy resolution.

Team

The Dualists — TED Ege College, Aydın, Turkey
Arda Genc · Arhan Hasan Ünsal · Atakan Korkmaz · Beren Duygu Yılmaz · Doruk Turan · Doruk Utku Tarım · Duru Sefa · Kayra Sari · Leman Ece Genclesen · Neva Yıldızlı