It is widely expected that the growth of parton densities at low x predicted from linear QCD evolution cannot continue indefinitely, and that non-linear effects will lead to gluon saturation. We propose the measurement of forward(3.5 < y < 5) direct photons in a new region of low x(∼ 10 −6 ) in proton-nucleus collisions at the LHC as a decisive probe of gluon saturation. In order to discriminate decay photons with very small opening angle from neutral pions, which is the dominant background, an extremely high-granularity electromagnetic calorimeter is required, which we propose as a detector upgrade to the ALICE experiment, the Forward Calorimeter (FoCal).
To facilitate the design of the upgrade and to perform generic R&D necessary for such a novel calorimeter, a compact Si/W sampling electromagnetic calorimeter prototype using CMOS pixel sensors with a granularity of 30×30 μm has been built and used for measurements in test beams. This digital calorimeter has shown to have good energy linearity and a very small Moliere radius(∼11mm). We will discuss new results of the R&D with electromagnetic showers, in particular a position resolution of better than 30μm, and first studies of hadron identification performance. This precise position determination and the detailed knowledge of the electromagnetic shower shape obtained will provide the crucial capability for two-photon separation down to a separation of a few mm. The results show the extremely high potential of this technology for future calorimeter development
|Preferred Track||Future Experimental Facilities, Upgrades, and Instrumentation|