---
title: "Why a collider cannot keep every collision"
description: "Detector signals wait for decisions, processors and an output link. Independent probes show how this selection shapes the saved sample."
canonical_url: "https://sandboxphysics.com/stories/instruments/collider-trigger-daq/"
language: "en"
page_type: "story"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/stories/instruments/collider-trigger-daq/"
last_modified: "2026-11-12"
---

[How physics instruments measure](https://sandboxphysics.com/stories/instruments/)

# Why a collider cannot keep every collision

A detector can produce signals much faster than a system can examine them in detail and store them permanently. A fast trigger first selects candidates from limited information. More detailed processing follows, with buffers holding data between the steps. A complete packet must finish the output link before it joins the sample used for offline analysis.

Sandbox Physics · 2026-11-12

## Several views of the same collision

The layers around a collision point make different observations. Inner tracking detectors record positions along charged-particle paths. A magnetic field bends those paths, so calibrated positions and a measured field provide a momentum scale. Several hits must agree with one trajectory before they support that interpretation.

Farther out, calorimeters absorb energy and turn ionization or light in their materials into electrical signals. Electrons and photons often make concentrated electromagnetic showers. Hadrons produce more complicated deposits. Outer muon stations look for particles that have crossed substantial material and still leave hits. Together, the layers identify patterns worth a closer look.

The electronics initially supplies channel addresses, digital amplitudes and timestamps. A large number can reflect a stronger signal, a different gain or a shifted pedestal. Known inputs, blank signals and independent check points establish the scale. Aligning the channel times helps determine which hits belong to the same passage.

Each observation has a limit. A very slight bend can become indistinguishable from a straight path at finite position resolution. Near the noise level, a signal can remain unresolved on either side of a threshold. The available evidence sets the useful detail of the selection that follows.

Sources: [1] [2] [3]

## Data needs somewhere to wait

The first trigger level examines relatively coarse information, such as a concentration of calorimeter energy or a pattern across outer detector stations. It uses features that can be calculated promptly. Accepted candidates proceed to software processing, where finer positions, signal shapes and consistency checks become available.

A decision takes time. Front-end electronics holds the corresponding data while waiting for instructions to continue the readout. That temporary store has a finite number of places and a finite retention time. A decision can arrive after the useful data has lost its place.

The next wait is for a processor. An event with many hits or a complicated reconstruction requires more work. An accepted result can then wait at the output buffer. A large packet occupies the link longer than a small packet. The processor limits how quickly an event can be examined; the link limits how quickly its bytes can leave.

Imagine two accepted packets reaching one transmitter. The first occupies it until its last byte leaves. The second needs its own waiting place during that interval. Making the first packet smaller can let both finish, even though neither particle signal changed.

Completion therefore matters as much as acceptance. When an observation window closes, an event may still be processing or only partly transmitted. Keeping that pending state separates unfinished work from a definite loss. It also lets an operator account for every arrived event and every admitted byte.

Sources: [1] [2]

## Selection leaves a shape in the sample

A higher energy or momentum threshold tends to favor harder events. Looking only at the saved sample can confuse that preference with the underlying collision distribution. An independent control selection gives the trigger under study an opportunity to fail on known probes.

Experiments can use another selection to identify a suitable probe sample, then count how often a probe satisfies the trigger being tested. The resulting fraction depends on how those probes were chosen, which momenta and directions they cover, and which readout and calibration were used. A finite sample also gives a statistical interval. Zero observed passes still leaves a nonzero upper limit on the probability.

Resource competition adds another selection. On fixed signals, a tighter threshold admits fewer individual candidates. It can also free a waiting place, allowing a different packet that was previously blocked to finish. The final saved set depends on the full arrival sequence and the available resources.

Replaying one fixed readout through several programmes separates these effects. Small changes reveal which candidates fail a signal requirement and which lose a place while waiting. Independent probes then test the selection within their stated preparation. The sample delivered to offline analysis has a path that can be examined, measured and checked.

Sources: [2] [4] [5]

## [Follow fixed hits through a two-level trigger](https://sandboxphysics.com/labs/particle-collider/?instrument=trigger-daq)

Compare thresholds, late decisions, CPU congestion and a slow link, then inspect independent probes and completed packets.

Fixed surrogate readouts and empirical probe fractions. Integer queues retain losses/pending work; probability intervals and facility performance are excluded.

## Sources and further reading

1. [ATLAS · Trigger and Data Acquisition](https://atlas.cern/Discover/Detector/Trigger-DAQ)

   Main two-stage, calorimeter/muon input and buffering paragraphs; architecture only, no outreach rate or latency adopted.

2. [ATLAS Collaboration · Operation of the ATLAS trigger system in Run 2 (2020)](https://arxiv.org/abs/2007.12539)

   v2 PDF pp.3–6,13–14,19; Fig.1 on p.6 visually inspected. Coarse/fine data, CPU/output constraints, independent controls and validity only; Run-2 numbers are not current facility specifications.

3. [Particle Data Group · Particle Detectors at Accelerators (2024)](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-particle-detectors-accel.pdf)

   PDF p.109, Eqs.35.59–63, visually inspected; momentum/curvature units adopted. The three-hit surrogate does not use large-N resolution or real material/scattering performance.

4. [A. Drobac for ATLAS · The ATLAS Muon Trigger Design and Performance (2020)](https://cds.cern.ch/record/2721028/files/ATL-DAQ-PROC-2020-011.pdf)

   PDF p.2, Sections2–4: coarse/fine inputs and independently selected probes; no facility thresholds/efficiency or claim of a real Z tag-and-probe analysis.

5. [NIST/SEMATECH · Confidence intervals for proportions](https://www.itl.nist.gov/div898/handbook/prc/section2/prc241.htm)

   Wilson formulas and distinction from exact binomial intervals; The delivered slice reports fixed-cohort empirical fractions, without probability intervals or an iid binomial claim.

[How a vernier reads between millimetres](https://sandboxphysics.com/stories/instruments/length-metrology/) [Air helps hold up the object on a balance](https://sandboxphysics.com/stories/instruments/precision-balance/) [How Cavendish Measured the Attraction Between Spheres](https://sandboxphysics.com/stories/instruments/cavendish-balance/) [Which pressure does a liquid column measure?](https://sandboxphysics.com/stories/instruments/manometer/) [The cup absorbs heat too](https://sandboxphysics.com/stories/instruments/calorimeter/) [Rotational rheometers need both torque and speed](https://sandboxphysics.com/stories/instruments/rotational-rheometer/) [Why the air beyond a pipe changes its resonance](https://sandboxphysics.com/stories/instruments/resonance-tube/) [How a seismometer reads the ground through motion inside its case](https://sandboxphysics.com/stories/instruments/seismometer/) [Why does a meter change the circuit?](https://sandboxphysics.com/stories/instruments/electrical-meters/) [How a null detector reveals a resistance](https://sandboxphysics.com/stories/instruments/resistance-bridges/) [Why a capacitor can behave like an inductor](https://sandboxphysics.com/stories/instruments/lcr-impedance/) [Where does the slow wave on an oscilloscope come from?](https://sandboxphysics.com/stories/instruments/digital-oscilloscope/) [How a lock-in amplifier finds a signal buried in noise](https://sandboxphysics.com/stories/instruments/lock-in-amplifier/) [Where a small peak in a spectrum can come from](https://sandboxphysics.com/stories/instruments/spectrum-analyzer/) [How a network analyzer separates the cable from the device](https://sandboxphysics.com/stories/instruments/vector-network-analyzer/) [When a slower quartz crystal can measure added mass](https://sandboxphysics.com/stories/instruments/quartz-microbalance/) [How an echelle spectrograph gives overlapping colors a second direction](https://sandboxphysics.com/stories/instruments/echelle-spectrograph/) [How total brightness reveals many infrared colors](https://sandboxphysics.com/stories/instruments/ftir-spectrometer/) [Measuring a Solution’s Concentration with Light](https://sandboxphysics.com/stories/instruments/uv-vis-spectrophotometer/) [How an ellipsometer reads thin-film thickness from reflected light](https://sandboxphysics.com/stories/instruments/spectroscopic-ellipsometer/) [How a polarimeter reveals mixed polarization](https://sandboxphysics.com/stories/instruments/mueller-polarimeter/) [How individual photons reveal a fluorescence lifetime](https://sandboxphysics.com/stories/instruments/tcspc-fluorescence-lifetime/) [Brillouin Microscopy Listens to Sound with Light](https://sandboxphysics.com/stories/instruments/brillouin-microscope/) [How a white sphere compares a lamp’s total output](https://sandboxphysics.com/stories/instruments/integrating-sphere-radiometer/) [How powder rings reveal atomic spacing](https://sandboxphysics.com/stories/instruments/powder-xray-diffractometer/) [How Small-Angle X-ray Scattering Reads a Particle Halo](https://sandboxphysics.com/stories/instruments/small-angle-xray-scattering/) [Why X-ray fluorescence depends on sample thickness](https://sandboxphysics.com/stories/instruments/xray-fluorescence-spectrometer/) [Why an AFM makes a bump look wider](https://sandboxphysics.com/stories/instruments/atomic-force-microscope/) [How a small indentation separates hardness from elasticity](https://sandboxphysics.com/stories/instruments/nanoindenter/) [How Differential Scanning Calorimetry Reads a Melting Peak](https://sandboxphysics.com/stories/instruments/differential-scanning-calorimeter/) [How thermogravimetry separates mass loss from the gas around it](https://sandboxphysics.com/stories/instruments/thermogravimetric-analyzer/) [How a diamond anvil cell reads pressure in ruby light](https://sandboxphysics.com/stories/instruments/diamond-anvil-cell/) [How diamond fluorescence reveals a magnetic-field direction](https://sandboxphysics.com/stories/instruments/nv-center-magnetometer/) [Reading a magnetic field in the rhythm of atoms](https://sandboxphysics.com/stories/instruments/optically-pumped-magnetometer/) [Keeping a laser in time with atoms](https://sandboxphysics.com/stories/instruments/optical-lattice-clock/) [Why a Kibble balance needs two experiments](https://sandboxphysics.com/stories/instruments/kibble-balance/) [How a dilution refrigerator keeps carrying heat away](https://sandboxphysics.com/stories/instruments/dilution-refrigerator/) [Why a bigger vacuum pump may barely help the sample](https://sandboxphysics.com/stories/instruments/ultra-high-vacuum-system/) [How microwaves read a superconducting qubit](https://sandboxphysics.com/stories/instruments/superconducting-qubit-readout/) [How current leaves a voltage across a thin sheet](https://sandboxphysics.com/stories/instruments/four-probe-hall/) [How an ion flight becomes a mass spectrum](https://sandboxphysics.com/stories/instruments/time-of-flight-mass-spectrometer/) [How four straight rods select ion mass](https://sandboxphysics.com/stories/instruments/quadrupole-mass-filter/) [How a rapidly reversing field can hold ions](https://sandboxphysics.com/stories/instruments/paul-ion-trap-mass-spectrometer/) [How a cyclotron keeps a proton on the accelerating beat](https://sandboxphysics.com/stories/instruments/cyclotron-synchrocyclotron/) [Why the drift tubes in an RF linac get longer](https://sandboxphysics.com/stories/instruments/radio-frequency-linac/) [The directions hidden inside a small beam spot](https://sandboxphysics.com/stories/instruments/beam-diagnostics/) [How defocusing magnets help a beam stay in a ring](https://sandboxphysics.com/stories/instruments/synchrotron-storage-ring/) [Getting synchrotron light onto a sample](https://sandboxphysics.com/stories/instruments/synchrotron-beamline/) [How a free-electron laser gathers electrons into a brighter beam](https://sandboxphysics.com/stories/instruments/xray-free-electron-laser/) [Following energy exchange with neutron flight times](https://sandboxphysics.com/stories/instruments/pulsed-neutron-time-of-flight/)

## Keep reading

How physics instruments measure · Reading path

### How physics instruments measure

1. [Article **How a vernier reads between millimetres**](https://sandboxphysics.com/stories/instruments/length-metrology/)
2. [Article **Air helps hold up the object on a balance**](https://sandboxphysics.com/stories/instruments/precision-balance/)
3. [Article **How Cavendish Measured the Attraction Between Spheres**](https://sandboxphysics.com/stories/instruments/cavendish-balance/)
4. [Article **Which pressure does a liquid column measure?**](https://sandboxphysics.com/stories/instruments/manometer/)
5. [Article **The cup absorbs heat too**](https://sandboxphysics.com/stories/instruments/calorimeter/)
6. [Article **Rotational rheometers need both torque and speed**](https://sandboxphysics.com/stories/instruments/rotational-rheometer/)
7. [Article **Why the air beyond a pipe changes its resonance**](https://sandboxphysics.com/stories/instruments/resonance-tube/)
8. [Article **How a seismometer reads the ground through motion inside its case**](https://sandboxphysics.com/stories/instruments/seismometer/)
9. [Article **Why does a meter change the circuit?**](https://sandboxphysics.com/stories/instruments/electrical-meters/)
10. [Article **How a null detector reveals a resistance**](https://sandboxphysics.com/stories/instruments/resistance-bridges/)
11. [Article **Why a capacitor can behave like an inductor**](https://sandboxphysics.com/stories/instruments/lcr-impedance/)
12. [Article **Where does the slow wave on an oscilloscope come from?**](https://sandboxphysics.com/stories/instruments/digital-oscilloscope/)
13. [Article **How a lock-in amplifier finds a signal buried in noise**](https://sandboxphysics.com/stories/instruments/lock-in-amplifier/)
14. [Article **Where a small peak in a spectrum can come from**](https://sandboxphysics.com/stories/instruments/spectrum-analyzer/)
15. [Article **How a network analyzer separates the cable from the device**](https://sandboxphysics.com/stories/instruments/vector-network-analyzer/)
16. [Article **When a slower quartz crystal can measure added mass**](https://sandboxphysics.com/stories/instruments/quartz-microbalance/)
17. [Article **How an echelle spectrograph gives overlapping colors a second direction**](https://sandboxphysics.com/stories/instruments/echelle-spectrograph/)
18. [Article **How total brightness reveals many infrared colors**](https://sandboxphysics.com/stories/instruments/ftir-spectrometer/)
19. [Article **Measuring a Solution’s Concentration with Light**](https://sandboxphysics.com/stories/instruments/uv-vis-spectrophotometer/)
20. [Article **How an ellipsometer reads thin-film thickness from reflected light**](https://sandboxphysics.com/stories/instruments/spectroscopic-ellipsometer/)
21. [Article **How a polarimeter reveals mixed polarization**](https://sandboxphysics.com/stories/instruments/mueller-polarimeter/)
22. [Article **How individual photons reveal a fluorescence lifetime**](https://sandboxphysics.com/stories/instruments/tcspc-fluorescence-lifetime/)
23. [Article **Brillouin Microscopy Listens to Sound with Light**](https://sandboxphysics.com/stories/instruments/brillouin-microscope/)
24. [Article **How a white sphere compares a lamp’s total output**](https://sandboxphysics.com/stories/instruments/integrating-sphere-radiometer/)
25. [Article **How powder rings reveal atomic spacing**](https://sandboxphysics.com/stories/instruments/powder-xray-diffractometer/)
26. [Article **How Small-Angle X-ray Scattering Reads a Particle Halo**](https://sandboxphysics.com/stories/instruments/small-angle-xray-scattering/)
27. [Article **Why X-ray fluorescence depends on sample thickness**](https://sandboxphysics.com/stories/instruments/xray-fluorescence-spectrometer/)
28. [Article **Why an AFM makes a bump look wider**](https://sandboxphysics.com/stories/instruments/atomic-force-microscope/)
29. [Article **How a small indentation separates hardness from elasticity**](https://sandboxphysics.com/stories/instruments/nanoindenter/)
30. [Article **How Differential Scanning Calorimetry Reads a Melting Peak**](https://sandboxphysics.com/stories/instruments/differential-scanning-calorimeter/)
31. [Article **How thermogravimetry separates mass loss from the gas around it**](https://sandboxphysics.com/stories/instruments/thermogravimetric-analyzer/)
32. [Article **How a diamond anvil cell reads pressure in ruby light**](https://sandboxphysics.com/stories/instruments/diamond-anvil-cell/)
33. [Article **How diamond fluorescence reveals a magnetic-field direction**](https://sandboxphysics.com/stories/instruments/nv-center-magnetometer/)
34. [Article **Reading a magnetic field in the rhythm of atoms**](https://sandboxphysics.com/stories/instruments/optically-pumped-magnetometer/)
35. [Article **Keeping a laser in time with atoms**](https://sandboxphysics.com/stories/instruments/optical-lattice-clock/)
36. [Article **Why a Kibble balance needs two experiments**](https://sandboxphysics.com/stories/instruments/kibble-balance/)
37. [Article **How a dilution refrigerator keeps carrying heat away**](https://sandboxphysics.com/stories/instruments/dilution-refrigerator/)
38. [Article **Why a bigger vacuum pump may barely help the sample**](https://sandboxphysics.com/stories/instruments/ultra-high-vacuum-system/)
39. [Article **How microwaves read a superconducting qubit**](https://sandboxphysics.com/stories/instruments/superconducting-qubit-readout/)
40. [Article **How current leaves a voltage across a thin sheet**](https://sandboxphysics.com/stories/instruments/four-probe-hall/)
41. [Article **How an ion flight becomes a mass spectrum**](https://sandboxphysics.com/stories/instruments/time-of-flight-mass-spectrometer/)
42. [Article **How four straight rods select ion mass**](https://sandboxphysics.com/stories/instruments/quadrupole-mass-filter/)
43. [Article **How a rapidly reversing field can hold ions**](https://sandboxphysics.com/stories/instruments/paul-ion-trap-mass-spectrometer/)
44. [Article **How a cyclotron keeps a proton on the accelerating beat**](https://sandboxphysics.com/stories/instruments/cyclotron-synchrocyclotron/)
45. [Article **Why the drift tubes in an RF linac get longer**](https://sandboxphysics.com/stories/instruments/radio-frequency-linac/)
46. [Article **The directions hidden inside a small beam spot**](https://sandboxphysics.com/stories/instruments/beam-diagnostics/)
47. [Article **How defocusing magnets help a beam stay in a ring**](https://sandboxphysics.com/stories/instruments/synchrotron-storage-ring/)
48. [You are here **Why a collider cannot keep every collision**](https://sandboxphysics.com/stories/instruments/collider-trigger-daq/)
49. [Article **Getting synchrotron light onto a sample**](https://sandboxphysics.com/stories/instruments/synchrotron-beamline/)
50. [Article **How a free-electron laser gathers electrons into a brighter beam**](https://sandboxphysics.com/stories/instruments/xray-free-electron-laser/)
51. [Article **Following energy exchange with neutron flight times**](https://sandboxphysics.com/stories/instruments/pulsed-neutron-time-of-flight/)

1 more group

### Other articles that use the same experiments

Each experiment from this article that another series also uses, followed by the articles there.

1. [Lab **Particle Collider · Event Reconstruction Room**](https://sandboxphysics.com/labs/particle-collider/)
2. [Article **Blackett and Occhialini let a particle take its own photograph**](https://sandboxphysics.com/stories/particle-evidence/blackett-counter-controlled-cloud-chamber/)
3. [Article **The pion leaves a second track in photographic emulsion**](https://sandboxphysics.com/stories/particle-evidence/powell-pion-emulsion/)
4. [Article **Alvarez’s group finds short-lived particles in combinations of tracks**](https://sandboxphysics.com/stories/particle-evidence/alvarez-resonance-events/)
5. [Article **Charpak turns a particle’s position into an electrical signal**](https://sandboxphysics.com/stories/particle-evidence/charpak-multiwire-chamber/)
6. [Article **Two experiments met at the same narrow particle peak**](https://sandboxphysics.com/stories/particle-evidence/november-revolution-jpsi/)
7. [Article **The tau appeared through an electron, a muon and missing energy**](https://sandboxphysics.com/stories/particle-evidence/perl-tau-missing-energy/)
8. [Article **W and Z particles emerged from the collider debris**](https://sandboxphysics.com/stories/particle-evidence/ua1-ua2-w-z/)
9. [Article **Two mass peaks brought the Higgs field within reach**](https://sandboxphysics.com/stories/particle-evidence/higgs-discovery-mass/)
10. [Article **Reconstruct a collision and count its rate**](https://sandboxphysics.com/stories/collider-qcd/four-momentum-and-rates/)
11. [Article **Why a proton beam has many collision energies**](https://sandboxphysics.com/stories/collider-qcd/partons-and-factorization/)
12. [Article **Four-momentum and the mass every frame agrees on**](https://sandboxphysics.com/stories/relativity-measurement/four-momentum-and-invariant-mass/)