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Hadronic Calorimeter for Forward Physics



The ALICE Forward Calorimeter will address, for the first time and with unprecedented high sensitivity, the question of whether the Color Glass Condensate and gluon saturation is the correct fundamental description of nucleons. A high-energy nucleus travelling at near the speed of light appears compressed, along its direction of motion, causing the gluons inside the nucleus to appear as a "gluonic wall". At very high energies, the density of the gluons in this wall is seen to increase greatly. Unlike the quark gluon plasma produced in collisions between such walls, the color-glass condensate describes the walls themselves, and thus intrinsic properties of the interacting particles. We will test, experimentally, the predictions of the Color Glass Condensate theory.


Measurements of the gluonic content of nucleons show that the number of low momentum gluons increases dramatically, which would lead to an infinite number of very low momentum gluons. This unphysical 'infra-red catastrophe' must be prevented by another mechanism to limit the density of gluons in a nucleon. Simply put, there must be a density at which the number of gluons stops growing: a saturation density. The theory proposed to explain this is the Color Glass Condensate and models based on this theoretical approach are widely used and many consider it the correct fundamental description. Our measurements will test this theory and shed new light on the fundamental structure of matter under extreme conditions.


We will build a new calorimeter to measure particles emitted at small angles, the ALICE Forward Calorimeter (FoCal). The Danish contribution will be the hadronic component(FoCal-H) of this calorimeter, which is currently in the prototype phase. The next two years will be used to finalise the FoCal-H design, with construction of the final detector in 2024-26. The detector will then be installed in ALICE during the next "long shutdown" of the Large Hadron Collider in 2026-2027. Our first data will arrive with LHC Run 4. These data will be analyzed to determine whether or not they agree with theoretical predictions based on the Color Glass Condensate description of matter or whether another description must be found.


The true long term impact of this project is that humankind will learn more about the universe we inhabit. The physics goals of this project are about fundamental aspects of the interplay between quarks and gluons in extreme states of matter. On a more practical level, this project will allow for many students, at the bachelor, masters, and PhD level to hone skills relevant to industry. We also aim to help build capacity in Danish industry to contribute to large scale scientific projects at CERN and other international research facilities.