--- title: "IceCube: From Sensor Hits to a Track Interactive Lab | Sandbox Physics" description: "Can the light recorded by IceCube’s sensors reveal which way a particle was going?" canonical_url: "https://sandboxphysics.com/labs/icecube-reconstruction/" language: "en" page_type: "lab" alternate_languages: zh-CN: "https://sandboxphysics.com/zh/labs/icecube-reconstruction/" last_modified: "2026-10-27" --- Neutrinos · D12 · The inverse problem # IceCube: From Sensor Hits to a Track **Can the light recorded by IceCube’s sensors reveal which way a particle was going?** Yes: fit the times at which light reaches many sensors spread through the ice. Scattering and absorption in the ice smear those times. Collecting more light does not, on its own, make the direction more accurate or free of bias. Generate a simulated track in an ice array, hide its true direction and fit it from the times at which the sensors see light. Change scattering and absorption to see when even a bright event becomes harder to reconstruct. > Interactive simulation: open this page in a web browser to change the parameters and run the model. This teaching array is inspired by IceCube: 81 sensors on nine strings, not the real detector geometry or data. A relativistic charged particle travels on an infinite straight line through a known reference point. The ice is uniform, with a refractive index of 1.32 at every wavelength. Light collection follows a simple empirical rule, scattering adds exponential delays and each clock has 2 ns of Gaussian noise. The fit searches over direction only, with a free start time. It does not measure neutrino energy, fit a vertex, classify cascades or give a calibrated uncertainty. Recorded data from a teaching simulation ## Inspect the measurement Preparing a reproducible teaching record… Read a sample of the record [Scientific reference ↗](https://arxiv.org/abs/astro-ph/0407044) [Read the worldwide history →](https://sandboxphysics.com/stories/neutrinos/) [2026 Nobel Prize in Physics →](https://sandboxphysics.com/nobel/2026/physics/) Physics tutorial ## Fit the recorded times, then inspect the residuals **Background** Positions are in metres, with z pointing up and x and y horizontal. The polar angle is measured from straight up, and the azimuth turns from x toward y. The track’s reference point is assumed known, as in a calibration exercise with a fixed anchor. **Why it matters** How much of the direction can still be recovered after absorption and scattering? Start with the essentials - **Focus question**: How much of the direction can still be recovered after absorption and scattering? - **One-sentence intuition**: The fitted direction is inferred from the recorded hits. Collecting more light does not by itself make the fit better or unbiased. ### Core mathematical model #### Arrival time of the earliest direct light in uniform ice $$ t_i=t_0+\frac{l_i+d_i\sqrt{n^2-1}}{c} $$ The track is infinite and moves at the speed of light in vacuum. l is the sensor’s position along the track and d its perpendicular distance from it. The ice has index 1.32 at every wavelength, so the phase and group speeds of light are equal. #### Fitting the direction with an unknown start time $$ \widehat{\mathbf u}=\arg\min_{\mathbf u}\sum_i(t_i-t_{\mathrm{geo},i}(\mathbf u)-\widehat t_0)^2 $$ For each trial direction, the start time is set to the mean difference between measured and predicted times. A coarse search over all directions is followed by a local refinement. This deliberately simple fit assumes direct light, so late scattered photons can bias it. The RMS residual shows how well the times fit; it is not the angular uncertainty. ### Common difficulties #### Fit the recorded times, then inspect the residuals **Typical misconception** The fitted charged-particle track points exactly along the neutrino’s direction. **Better mental model** The model leaves out the kinematics of the interaction. It is a timing fit for a charged track through a known point. It does not classify events, measure neutrino energy or perform a full IceCube reconstruction. ### Run the experiment 1. 01 #### Reconstruct blind Generate an event with the true direction hidden. Fit the recorded hits, then reveal the generated direction. **What to observe:** The fitted arrow and the angular error appear only after the fit, and the fit never reads the generated direction. 2. 02 #### Break the propagation model Choose Strong scattering. Fit the new event and compare timing residuals. **What to observe:** The first unscattered photons can still point the way, but sensors whose first hit arrives late can pull the fit off. 3. 03 #### Lose photons Choose Strong absorption and compare hit count and residuals across several generated events. **What to observe:** Fewer sensors see light, so the fit has less to go on. The display gives no calibrated confidence interval, and the error need not grow in every single random event. Physical conclusion The fitted direction is inferred from the recorded hits. Collecting more light does not by itself make the fit better or unbiased. ## Related experiments and articles Continue exploring ### How detectors see a neutrino A neutrino leaves only light, charge or a pair of signals close in time. How does each detector turn that into a direction, an energy or a source? 1. [Lab **Reactor Antineutrino Coincidence** Select prompt and delayed signals against accidental pairs.](https://sandboxphysics.com/labs/reactor-coincidence/) 2. [Lab **Cherenkov Light & Detector Rings** Turn charged-particle geometry into a recorded light pattern.](https://sandboxphysics.com/labs/cherenkov-rings/) 3. [Lab **Liquid Argon Time Projection Chamber** Turn electron arrival times and wire projections into calibrated slice centroids.](https://sandboxphysics.com/labs/liquid-argon-tpc/) 4. [You are here **IceCube: From Sensor Hits to a Track** Infer a track direction from the recorded sensor times.](https://sandboxphysics.com/labs/icecube-reconstruction/) 5. [Lab **Supernova Neutrino Burst Observatory** Scan a frozen burst record, then inspect one interaction at the optical timescale.](https://sandboxphysics.com/labs/supernova-neutrino-burst/) 6. [Lab **Neutrino Sky Search Observatory** Search frozen neutrino directions and compare a local hotspot with paired finite-grid background trials.](https://sandboxphysics.com/labs/neutrino-sky-search/) Neutrinos · Reading path ### Start with the prize Articles on this path that use this experiment, followed by the other experiments they suggest. 1. [Article **2026 Nobel Prize in Physics: why look for neutrinos in Antarctic ice?**](https://sandboxphysics.com/stories/neutrinos/nobel-physics-2026/) 2. [Lab **Cherenkov Light & Detector Rings**](https://sandboxphysics.com/labs/cherenkov-rings/) 3. [Article **What can a neutrino tell us that light cannot?**](https://sandboxphysics.com/stories/neutrinos/high-energy-neutrinos-astronomy-window/) 4. [Article **A huge detector, a few events: reading neutrino scales**](https://sandboxphysics.com/stories/neutrinos/neutrino-energy-distance-and-counts/) 5. [Article **How does a faint signal become a neutrino discovery?**](https://sandboxphysics.com/stories/neutrinos/neutrino-discovery-evidence-chain/) 6. [Lab **Reactor Antineutrino Coincidence**](https://sandboxphysics.com/labs/reactor-coincidence/) 7. [Article **Five ways a neutrino headline can lead us astray**](https://sandboxphysics.com/stories/neutrinos/neutrino-nobel-common-misconceptions/) 8. [Article **Francis Halzen and the team that turned Antarctic ice into a telescope**](https://sandboxphysics.com/stories/neutrinos/halzen-and-collaboration/) 9. [Article **Why do neutrinos keep appearing in the Nobel Prize?**](https://sandboxphysics.com/stories/neutrinos/neutrinos-and-nobel-prizes/) Neutrinos · Reading path ### Build a new way to observe the sky Articles on this path that use this experiment, followed by the other experiments they suggest. 1. [Article **Before IceCube: learning to build telescopes in water and ice**](https://sandboxphysics.com/stories/neutrinos/before-icecube-underwater-telescopes/) 2. [Lab **Cherenkov Light & Detector Rings**](https://sandboxphysics.com/labs/cherenkov-rings/) 3. [Article **IceCube: how do you turn a cubic kilometre of ice into an instrument?**](https://sandboxphysics.com/stories/neutrinos/icecube-ice-sensors-and-calibration/) 4. [Article **IceCube’s 28 events: finding high-energy neutrinos from space**](https://sandboxphysics.com/stories/neutrinos/icecube-2013-evidence/) 5. [Article **TXS 0506+056: one alert, two separate lines of evidence**](https://sandboxphysics.com/stories/neutrinos/txs-0506-056-multimessenger/) 6. [Lab **Neutrino Sky Search Observatory**](https://sandboxphysics.com/labs/neutrino-sky-search/) 7. [Article **NGC 1068: what does a neutrino excess establish?**](https://sandboxphysics.com/stories/neutrinos/ngc-1068-neutrino-source-evidence/) 8. [Article **One neutrino sky, three different questions**](https://sandboxphysics.com/stories/neutrinos/neutrino-sky-diffuse-galactic-point-sources/) 1 more group Neutrinos · Reading path ### Follow the measurement chain Articles on this path that use this experiment, followed by the other experiments they suggest. 1. [Article **Why put a neutrino experiment under rock, sea or ice?**](https://sandboxphysics.com/stories/neutrinos/underground-underwater-neutrino-sites/) 2. [Lab **Reactor Antineutrino Coincidence**](https://sandboxphysics.com/labs/reactor-coincidence/) Fields & matter · Starter path ## From a field to an electron microscope Map an electric field and steer a charged particle. Then find the electron's wavelength and use electron beams to see a crystal and a surface. [Start here → **Make an Electric Field Visible** Move the probe around a dipole and check that the field always points downhill on the potential landscape. Observe · 8 min](https://sandboxphysics.com/labs/electric-field/) 1. [1 Make an Electric Field Visible](https://sandboxphysics.com/labs/electric-field/) 2. [2 A Charge Bending through Fields](https://sandboxphysics.com/labs/charged-particle/) 3. [3 Electron Wavelength & Voltage](https://sandboxphysics.com/labs/electron-wavelength/) 4. [4 Electron Diffraction & Reciprocal Space](https://sandboxphysics.com/labs/electron-diffraction/) 5. [5 TEM Bright Field, Dark Field & SAED](https://sandboxphysics.com/labs/tem-mode-selector/) 6. [6 SEM Secondary-Electron Topography](https://sandboxphysics.com/labs/sem-topography/) [View the full path →](https://sandboxphysics.com/topics/fields-and-matter/#starter-path)