physics

MilliQan Experiment

MilliQan Experiment is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand MilliQan Experiment rather than just read about it. In short: The MilliQan experiment is a small-scale detector experiment at CERN's Large Hadron Collider (LHC). MilliQan is not a separate CERN experiment but is handled as a CMS sub-detector, with a dedicated memorandum of understanding to define authorship and responsibilities.

Key takeaways

MilliQan Experiment belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect MilliQan Experiment to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of MilliQan Experiment from memory before moving on to harder problems.

Reference excerpt

The MilliQan experiment is a small-scale detector experiment at CERN's Large Hadron Collider (LHC). MilliQan is not a separate CERN experiment but is handled as a CMS sub-detector, with a dedicated memorandum of understanding to define authorship and responsibilities. The goal of the MilliQan experiment is to detect millicharged particles: particles with charges much smaller than that of the electron. These particles are motivated by the existence of a dark photon, and discovery of millicharged particles would provide a first probe into the dark sector. The MilliQan prototype detector collected data during LHC Run 2 in 2018 and set competitive constraints on millicharged particle charges and masses. The Run 3 milliQan detectors are currently collecting data, following the completion of the prototype detector upgrade in 2023 and the installation of a second detector apparatus in 2024.

Collaboration The MilliQan collaboration has 29 members over 10 institutions in 5 countries. It was proposed in 2016 by Christopher Hill and Andy Haas. Its name is in reference to the Millikan Experiment, which measured the charge of the electron; milli-, the metric prefix for a thousandth; and Q, a common scientific unit for electric charge.

MilliQan detector The MilliQan experiment is located in an underground access tunnel 33m from CMS. It is shielded from CMS by 17m of rock, protecting it from hadronic backgrounds while allowing millicharged particles to pass through to the detector. Located 70m underground, it is also shielded from cosmic ray backgrounds penetrating through the surface of the earth. The detector consists of a set of plastic scintillator bars and slabs which are mounted to photo-multiplier tubes (PMTs). The basic concept of the MilliQan detector is to exploit the relationship between ionization and charge in scintillator: ionization in scintillator due to a charged particle is proportional to the charge squared. This means that small charges will deposit amounts of energy that are small enough to evade the general purpose LHC detectors but large enough to be seen in a dedicated detector. Additionally, a charged particle will deposit a consistent amount of energy as it passes through a scintillator, so aligning a series of layers—consisting of scintillator coupled to PMTs—with the LHC P5 interaction point allows the experiment to be sensitive to millicharged particles.

MilliQan demonstrator

The first operation of the milliQan experiment came in the form of a ~1% prototype detector called the MilliQan demonstrator. It consisted of three layers of plastic scintillator bars mounted to PMTs in a 3x2 layout pointed towards the LHC interaction point near CMS, as well as additional material for shielding. The demonstrator collected 1106 hours of beam-on data and 1042 hours of beam-off data during LHC Run 2. This set leading constraints on the possible masses and charges for millicharged particles between masses from 20 to 4700 MeV and charges from 0.006e to 0.3e.

Run 3 detectors An upgrade to the milliQan detector for LHC Run 3 includes an expansion of the number of bars in the detector, from 3x2 to 4x4 bars per layer and from 3 to 4 layers. It also includes the addition of a new "slab detector", a detector which sacrifices charge sensitivity for acceptance in an effort to search for millicharged particles of higher mass. The bar detector upgrade was completed in June 2023, and the slab detector upgrade in July 2024. These upgrades are expected to improve sensitivity to millicharged particles with charges as low as 0.003e and masses up to 45 GeV.

References

Illustrations

Worked examples

Example 1 — a first encounter with MilliQan Experiment

Start with the simplest possible case. Write down what MilliQan Experiment claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to MilliQan Experiment before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about MilliQan Experiment ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of MilliQan Experiment

In research: MilliQan Experiment appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses MilliQan Experiment in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: MilliQan Experiment is common in secondary-school and first-year university syllabi. It links to neighbouring topics CERN experiments, Particle experiments, so understanding it makes those chapters shorter.
In everyday life: Look for MilliQan Experiment outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study MilliQan Experiment in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what MilliQan Experiment means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain MilliQan Experiment out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is MilliQan Experiment in simple terms?

Why does MilliQan Experiment matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study MilliQan Experiment?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on MilliQan Experiment.