Particle detection

With CERN’s Large Hadron Collider having reached a data milestone and amid reports of the possible discovery of previously unknown particles by the Tevatron, we take a look at the physics of particle detection.

The CMS detector at the Large Hadron Collider
Credit: CERN

In June, the total data accumulated by the Large Hadron Collider at CERN reached one “inverse femtobarn” of data, equivalent to 70 trillion particle collisions.

Meanwhile, the head of the Tevatron particle accelerator in the US appointed a committee to evaluate the evidence for whether a completely new particle has been discovered.

But how does particle detection work?

All particle detectors, even the early bubble chambers which used ionisation and vapour trails to detect charged particles, work by capturing data from particle collisions. But the volume of data being produced by modern high-energy experiments means that detection methods much more sophisticated than the photography used with the bubble chambers are needed.

In the LHC
The Large Hadron Collider has four main particle detectors: ALICE, ATLAS, CMS and LHCb. These detectors are showered with the particles produced when the two beams of protons circulating around the LHC collide.

Each layer of a detector has a specific function, but the main components are semiconductor detectors which measure charged particles and calorimeters which measure particle energy.

The semiconductor detectors use materials such as silicon to create diodes - components that conduct electrical current in only one direction. Charged particles passing though the large number of strips of silicon placed around the proton beam collision point create a current that can be tracked and measured.

The position where particles originate is also important – if one appears deep within the detector it is likely to have been produced by the decay of another particle produced earlier, possibly within the collision itself.

Calorimeters are calibrated to be either electromagnetic or hadronic. The former will detect particles such as electrons and photons, while the latter will pick up protons and neutrons. Particles entering a calorimeter are absorbed and a particle shower is created by cascades of interactions.

It is the energy deposited into the calorimeter from these interactions that is measured. Stacking calorimeters allows physicists to build a complete picture of the direction in which the particles travelled as well as the energy deposited, or determine the shape of the particle shower produced.
ATLAS is also designed to detect muons, particles much like electrons but 200 times more massive, which pass right through the other detection equipment. A muon spectrometer surrounds the calorimeter, and functions in a similar way to the inner silicon detector.

A representation of a detection event in the CMS detector
Credit: CERN

Left: A representation of a detection event in the CMS detector.

Analysing the data
With so many particle collisions, accelerators such as the LHC generate a huge amount of data, which take a great deal of computing power to capture and analyse. To achieve this, CERN developed the LHC Computing Grid.

The Grid is a tiered network in which data are first processed by the computers at CERN, then sent on to regional sites for further processing, and finally sent to institutions all around the world to be analysed.

The results of experiments can be compared with those predicted by current theories of particle physics to look for any differences. At the LHC, particle physicists are hoping that the collisions will produce a Higgs boson, the as-yet-theoretical particle thought to be responsible for the existence of mass.

At the Tevatron, where they collide protons and antiprotons, the data that one team believed showed an entirely new type of particle appeared as a bump in a graph of experimental data that was not present in the theoretical predictions.

However this was only seen in data from one of the accelerator’s detectors and wasn’t present in the other against which it was compared. It is now believed to have been a phantom signal.

Physicists will have to keep waiting for the first sight of the Higgs – or other new particles.

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