One of the building blocks of the CMS Silicon Tracker: a part of the detector that reconstructs the trajectories of charge particles emerging from the proton-proton collisions. A lightweight structure, made mostly of carbon fibre, supports silicon detectors and their readout electronics. These detectors generate an electrical pulse when they are traversed by a charged particle, and they are segmented into fine strips (in this case the strips are 180 microns wide, about the size of a human hair) that collect those pulses, such that the position of the strip provides a coordinate on the particle trajectory.
42 modules like this one surround the collision point inside the LHCb detector. Their role is to measure the tracks of short-lived particles spraying out from the collision and to pinpoint the exact spots where they decay into secondary particles. Some exist for just trillionths of a second before decaying! The silicon modules operate so close to the collision point, they can only be moved into position once the circling particle beams are at their most focused. Otherwise, peripheral particles on the outside of the finer-than-a-hair beam would bore a hole right through them.
LHCb measures muons using gold plated tungsten wires stretched over read-out pads. A high voltage is applied across the wires and pads and the set-up is bathed in a gas mixture. Passing muons interact with the gas, knocking out electrons from its atoms in a process called ionization. Both the ionized atoms and the electrons then drift in the electric field. This movement creates an electric signal in the wires and pads that is used to identify where the muon has passed. In total, the LHCb muon detectors contain about 2 million wires and are capable of making measurements 40 million times a second – every time the particle beams collide.
The innermost layers of all four LHC detectors are made of silicon. This piece comes from the ATLAS detector where its job is to record the paths of the particles close to the collision. Here, hundreds of particles spray outwards and the silicon detectors must identify the exact points from which the particles originate and make an accurate measurement of the curvature of every particle track. Inside ATLAS, the first layer is made of 80 million silicon pixels, each smaller than a grain of sand. Surrounding the pixels are six million silicon strips, each about the thickness of a hair. The object on display here contains 1536 such silicon strips. Together, the layers of tracking detectors are like a giant 92 mega pixel camera taking a photo 40 million times every second.
The ATLAS transition radiation tracker is made of 300'000 straw tubes, up to 144cm long. Filled with a gas mixture and threaded with a wire, each straw is a complete mini-detector in its own right. An electric field is applied between the wire and the outside wall of the straw. As particles pass through, they collide with atoms in the gas, knocking out electrons. The avalanche of electrons is detected as an electrical signal on the wire in the centre. The tracker plays two important roles. Firstly, it makes more position measurements, giving more dots for the computers to join up to recreate the particle tracks. Also, together with the ATLAS calorimeters, it distinguishes between different types of particles depending on whether they emit radiation as they make the transition from the surrounding foil into the straws.
Particles coming from the universe are crossing the earth all the time – they are harmless but invisible to us. Cloud Chambers are detectors which make the tracks of the particles visible. Some decades ago these detectors were used at CERN in the first particle physics experiments.