Pixel Chamber: an active target with monolithic pixel sensors for precision measurements of charm and beauty

Modern vertex detectors are based on cylindrical or planar layers of silicon sensors. These detectors are used for precision measurements of tracks and vertices of particles produced in the interactions. Since the tracking layers are always few to tens of cm from the interaction point, this poses an ultimate limitation in the achievable resolution of the vertex position. A silicon-based active target detector capable to image particles produced inside the detector volume in 3D, similarly to a bubble chamber, does not exist. Ideas for a silicon active target providing continuous tracking were put forward already 40 years ago but the required technology just did not exist until recently. In my thesis, I will describe the idea for the first silicon active target based on silicon pixel sensors, called Pixel Chamber, capable to perform continuous, high resolution (order of microns) 3D tracking, including open charm and beauty particles. The aim is to create a bubble chamber-like high-granularity stack of hundreds of very thin monolithic active pixel sensors (MAPS) glued together. To do this, the ALPIDE sensor chip, designed for the ALICE experiment at the CERN LHC, will be used. R&D for prototypes developments is ongoing. The power consumption of a stack consisting of hundreds of ALPIDE is such as to bring the sensor to very high temperatures. For this reason, simulations were carried out to evaluate different cooling options. A tracking and vertexing algorithm has been specifically developed to reconstruct tracks and vertices inside Pixel Chamber. It has been tested on Monte Carlo simulations of proton-silicon interactions occurring inside the sensor. According to those simulations, it is possible to obtain a high efficiency for the reconstruction of hadronic tracks, and for the primary and secondary vertices inside the detector. The vertex resolution can be up to one order of magnitude better than state-of-the-art detectors like those of LHC experiments. The tracking algorithm has been also tested to reconstruct tracks produced in a single ALPIDE sensor exposed to electrons and hadrons beams. Results show that it is possible to obtain very good performances in long track reconstruction on a single ALPIDE.