High-energy physics · collision event display

Particle Collider · Event Reconstruction Room

Generate dijet, dimuon, photon-recoil, and invisible-recoil events inside a three-dimensional cylindrical detector. Tune the solenoidal field, hard-scatter scale, and pileup; isolate the tracker, calorimeters, and muon system; then select any charged track to inspect its reconstruction.

Event topologyBack-to-back dijet

Reconstructed tracks0 / 0

Visible energy

E_{\mathrm{vis}}

$0\,\mathrm{GeV}$

Missing momentum

p_T^{\mathrm{miss}}

$0\,\mathrm{GeV}/c$

Selected candidateClick a reconstructed track

Selected

p_T

$\text{—}$

Curvature radius

R

$\text{—}$

This parametric toy generator explains how event displays are read; it is neither real collision data nor a full particle-transport simulation. Track curvature follows $p_T\simeq0.3\,|q|BR$ , while $\vec p_T^{\,\mathrm{miss}}=-\sum_i\vec p_{T,i}$ reconstructs the visible momentum imbalance.

Physics tutorial

From luminous tracks to a collision event

BackgroundA collider detector does not photograph particles. Silicon trackers record discrete hits left by charged particles, electromagnetic and hadronic calorimeters absorb particles and measure showers, and outer systems identify candidates able to penetrate the calorimeters.

Why it mattersReal high-energy physics starts from these incomplete, noisy local signals and reconstructs particle candidates, jets, and event-wide momentum balance. Reading an event display connects theoretical particles to detector evidence.

Start with the essentials

Focus question: Given only curved tracks, luminous energy clusters, and a few penetrating signals, how can you distinguish dijet, dimuon, photon-recoil, and invisible-recoil events?
One-sentence intuition: Each subsystem supplies only part of the answer. Track curvature constrains charge and $p_T$ ; calorimeters give energy and jet direction; outer chambers identify penetrating candidates. The full transverse vector sum then tests for $\vec p_T^{\,\mathrm{miss}}$ .

Core mathematical model

Transverse curvature in a solenoid

p_T[\mathrm{GeV}/c]\simeq0.3\,\left|\frac{q}{e}\right|B[\mathrm T]R[\mathrm m]

In a uniform axial field, a larger curvature radius $R$ means greater transverse momentum, while bending direction gives the charge sign. Set the field to zero and the trajectory approaches a straight line.

Pseudorapidity coordinate

\eta=-\ln\tan\frac{\theta}{2}

$\theta$ is the polar angle relative to the beam axis. Cylindrical detectors commonly describe directions with $(\eta,\phi)$ because this naturally unfolds the forward region.

Missing transverse momentum

\vec p_T^{\,\mathrm{miss}}=-\sum_{\mathrm{visible}}\vec p_{T,i}

The initial state is approximately balanced in the transverse plane. A nonzero vector sum of visible reconstructed objects therefore requires an opposing balance vector. Invisible particles can cause it, but so can mismeasurement or missed reconstruction.

Common difficulties

An event display is not a collision photograph

Typical misconceptionEach luminous curve is a continuously visible trail left by one particle.

Better mental modelThe curve is a fitted reconstruction of discrete hits. Neutral particles do not bend through the tracker, some particles appear only as calorimeter showers, and candidate identity combines several subsystems.

Large missing momentum is not a dark-matter discovery

Typical misconceptionA long yellow imbalance arrow proves that a new invisible particle was produced.

Better mental modelMissing transverse momentum is an event-level observable. Jet mismeasurement, detector gaps, noise, and missed objects can all create imbalance, so real analyses require control samples and background models.

A toy generator is not a full simulation chain

Typical misconceptionParticle counts and shapes after changing the hard-scatter scale can be read directly as experimental predictions.

Better mental modelThis lab keeps the core structures of curvature, jet cones, calorimeter clusters, and momentum balance. It does not model parton distributions, hadronization, material interactions, electronics, or experiment-grade reconstruction.

Run the experiment

01
Read momentum from the field
Choose a dijet event, select tracks with different bending, then raise the solenoidal field from zero.
What to observe: Low- $p_T$ trajectories bend more strongly. For one fixed candidate, increasing $B$ reduces the displayed curvature radius.
02
Remove detector layers
Toggle the silicon tracker, calorimeter showers, and muon system while comparing dimuon with photon-plus-jet events.
What to observe: Dimuon candidates penetrate to the outer system while depositing little calorimeter energy. A photon has no charged track but creates a compact, high-energy electromagnetic cluster.
03
Create invisible recoil
Choose invisible recoil, raise the hard-scatter scale, then toggle the missing-momentum vector.
What to observe: No visible object balances the jet side, so $\vec p_T^{\,\mathrm{miss}}$ points approximately opposite the visible recoil.
04
Make a clean event crowded
Return to dimuon, increase pileup interactions, and generate several new events.
What to observe: The two hard penetrating tracks remain recognizable, but many soft tracks from displaced vertices complicate combinations and momentum balance.