Speaker
Description
Modern vertex detectors are based on cylindrical or planar layers of silicon sensors, generally immersed in a magnetic field. These detectors are used for precision measurements of the particles produced in the interactions and, in particular, of the dacay products of those with a long mean life, such as open charm and beauty. 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 (1).
In this talk, I will describe the idea for the first silicon active target based on silicon pixel sensors, called Pixel Chamber (2), capable to perform continuous, high resolution (O($\mu m$)) 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 (3). This sensor is a matrix of 1024x512 monolithic active pixels (size $\sim$ 29x27 $\mu m^2$) with an area of $\sim$1.5x3 $cm^2$ and thickness $\sim$ 50 $\mu m$. Pixel Chamber is conceived as a stack of 216 ALPIDE chips, having a volume of $\sim$1x1.5x3 $cm^3$ and forming a 3D matrix of almost $10^8$ pixels.
Figure 1 shows a comparison between a bubble chamber image for the strange barion $\Omega^{-}$ (4) and a Geant4 simulation of a $D^{+}$ meson decaying to $K \pi \pi$ in a proton-silicon interaction with Pixel Chamber.
A tracking and vertexing algorithm developed specifically for reconstructing the interactions inside Pixel Chamber will be discussed. Monte Carlo simulations of proton-silicon interactions at 400 GeV, based on Geant4, were used to test tracking and vertexing performances. According to those simulations, it is possible to obtain a high efficiency for the reconstruction of hadronic tracks, and for the primary interaction vertex and secondary decay 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.
(1) G. Bellini et al., Miniaturization of High-Energy Physics Detectors, pp41-55, Springer, 1983
(2) G.Usai et. al, “Pixel Chamber: A universal silicon heavy-avor imager for fixed-target measurements of charm and beauty with unprecedented precision”, R&D project funded by the Regional Government of Sardinia - Italy
(3) Gianluca Aglieri Rinella, on behalf of the ALICE Collaboration, Nucl. Instrum. Meth. A, volume 845 (2017)
(4) V. E. Barnes et al., Observation of a Hyperon with Strangeness Minus Three, Phys. Rev. Lett. 12, 204, 24 February 1964
