The pixel detector contains 124 million pixels, allowing it to track the paths of particles emerging from the collision with extreme precision. It is also the closest detector to the beam pipe, with cylindrical layers roughly at 3cm, 7cm, 11cm, and 16cm and disks at either end, and so is vital in reconstructing the tracks of very short-lived particles.

Each of the four layers is composed of individual silicon modules, split into little silicon sensors, like tiny kitchen tiles: the pixels.  Each of these silicon pixels is 100µm by 150µm, about two hairs' width. When a charged particle passes through a pixel, it disturbs the electrons in the silicon pixel, which results in an electric pulse, or as we call it, a 'hit'. These hits are used to draw the track or trajectory of the charged particle.

By applying a voltage to the sensor, these pulses collect into a small electric signal, which is then amplified by a readout chip for a total of 16 chips per module.

CMS silicon pixel detector

The detector is made of 2D tiles and has four layers, meaning that we can create a three-dimensional picture of the particle track through the layers.

However, being so close to the collision means that the number of particles passing through is huge: the rate of particles received at 3cm from the beamline is around 600 million particles per square centimetre per second! The pixel detector is able to disentangle and reconstruct all the tracks particles leave behind, and withstand such a pummeling over the duration of the experiment. 

Because there are 124 million pixels, the power for each must be kept to a minimum. Even with  only around 50 microwatts per pixel, the total power output is 7.5kW- around the same as the energy produced by two electric ovens. So as not to overheat the detector, the pixels are mounted on cooling tubes and kept at -20°C degrees. Operating at this temperature also helps to reduce the effects of the damage to the silicon modules inflicted by the continuous stream of particles. 

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