High-energy physics · collision event display

Particle Collider · Event Reconstruction Room

How do physicists reconstruct a collision they cannot see from what the detector records? Combine the detector layers. How much a track curves in the magnetic field gives its charge and transverse momentum. Calorimeters measure energy and jet direction, and muon chambers pick out particles that punch through everything inside them. If the total transverse momentum does not balance, something escaped unseen, such as a neutrino.

Inspect collision tracks in a three-dimensional detector, then build a mass spectrum from frozen muon-pair candidates. Change the selection, compare opposite-charge and same-charge pairs, and click the spectrum to inspect a recorded pair.

Event topologyOpposite-sign dimuon

Reconstructed tracks0 / 0

Visible energy

E_{\mathrm{vis}}

$0\,\mathrm{GeV}$

Missing momentum

p_T^{\mathrm{miss}}

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

Model assumptions

This parametric toy generator teaches how to read event displays. It is neither real collision data nor a full particle-transport simulation. Track curvature follows $p_T\simeq0.3\,|q|BR$ , and $\vec p_T^{\,\mathrm{miss}}=-\sum_i\vec p_{T,i}$ reconstructs the visible momentum imbalance. Jet examples treat track energies as massless; frozen muon-pair four-vectors retain the muon rest mass.

Opposite chargeSame-charge control

The simulated sample loads automatically.

· $91.2\,\mathrm{GeV}/c^2$

Selection and model

Minimum momentum across the beam

12\,\mathrm{GeV}/c

Maximum absolute pseudorapidity

2.4

The generator mixes a fixed-mass two-muon teaching resonance with a smooth continuum. The blurred preset changes the momentum response. It omits the natural width, radiation and a full detector simulation. Same-charge pairs are a control sample, with no background normalization implied.

M^2c^4=(E_1+E_2)^2-c^2\lvert\boldsymbol p_1+\boldsymbol p_2\rvert^2

CERN Open Data: two-lepton analysis

Start with the essentials

Focus question: How can two muon tracks reveal the mass of their common source?
One-sentence intuition: Add the two energies and add the momenta with their directions. Their invariant mass groups candidates from a common parent around the same place in a spectrum.

Core mathematical model

Combined energy and momentum

M^2c^4=(E_1+E_2)^2-c^2\lvert\boldsymbol p_1+\boldsymbol p_2\rvert^2

$M$ is the pair mass, $E_1,E_2$ are the particle energies, and $\boldsymbol p_1,\boldsymbol p_2$ are their momenta. Opposing momenta can cancel even though both energies are positive.

Bending across a magnetic field

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

At a fixed field, higher-momentum tracks bend less. Raising the field bends a selected pair more tightly while its recorded momenta and mass stay fixed.

Common difficulties

Continuum events also enter the peak region

Typical misconceptionEvery pair near the peak must have the same parent.

Better mental modelA smooth continuum can contribute events at the same mass. Opposite-charge and same-charge histograms show the peak and a control sample separately. The control is not normalized into a background prediction.

Cuts change which events remain

Typical misconceptionA higher momentum threshold simply shrinks the whole histogram by one factor.

Better mental modelPairs with different masses and directions pass a cut at different rates. Watch both the number of accepted records and the shape as the selection changes.

Run the experiment

01
Go from a mass bin to its tracks
Click near the peak, then click far from it.
What to observe: Each click selects a frozen candidate pair. Its record number, mass and tracks update together.
02
Blur the peak
Compare Mass peak, Blurred peak and Continuum only.
What to observe: A poorer momentum resolution broadens the mass peak. Removing the resonance leaves a smooth continuum. All three presets use labelled simulated records.
03
Select from the same records
Open Selection and model. Raise the transverse-momentum cut and narrow the pseudorapidity range.
What to observe: Fewer records pass, and each keeps its recorded momenta. Pseudorapidity describes direction relative to the beam; a smaller allowed absolute value keeps more transverse tracks.
04
Compare other event patterns
Choose Dijet or Photon + jet and open Detector layers and event generation.
What to observe: These presets generate separate teaching events for comparing tracks, calorimeters and penetrating signals. Return to Mass peak to resume the pair analysis.

Particle Collider · Event Reconstruction Room

Find a peak in the same set of events

Combine two tracks into a mass

Core mathematical model

Combined energy and momentum

Bending across a magnetic field

Common difficulties

Continuum events also enter the peak region

Cuts change which events remain

Run the experiment

Go from a mass bin to its tracks

Blur the peak

Select from the same records

Compare other event patterns

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