Advancements in the knowledge of fundamental constituents of matter and their interactions are usually driven by the development of experimental techniques and facilities, with a significant role of particle accelerators. The Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN) is the world's largest and most powerful particle accelerator colliding opposite beams of protons (p) and lead ions (Pb), allowing for unprecedentedly high centre-of-mass energies of up to 14 TeV and 5.5 TeV, respectively. Its research potential can be extended by implementing a fixed-target experiment. One proposed idea is to perform a fixed-target experiment at ALICE (A Large Ion Collider Experiment), one of four main detectors at the LHC being optimised to register products of heavy-ion collisions, which provides appropriate apparatus to host such an experiment.
The general research objective of the project is to perform a feasibility study of the dedicated machine layout that would allow for the fixed-target programme at ALICE (ALICE-FT). The concept is based on steering onto a solid internal target a fraction of the beam halo split by bent crystals, similar to crystals being developed for beam collimation in the LHC. Splitting the beam is performed by exploiting the channelling process occurring inside a bent crystal, resulting in a trajectory deflection equivalent to the geometric bending angle of a crystal body. The problem to address was to design the machine layout that provides a number of protons on target high enough to exploit the full capabilities of the ALICE detector acquisition system without affecting the LHC availability for regular beam-beam collisions.
A significant innovation of the project is to bring particles of high energy collider to collisions with the fixed-target by deflecting them using the bent crystal. This technique allows profiting from the circulating beam halo particles that are usually considered useless, or even harmful, from the collider perspective. Another important novelty is to operate the LHC both in the collider mode and in the fixed-target mode simultaneously, substantially extending its scientific potential without significant investments into the new equipment. Moreover, due to collisions geometry and ALICE detector configuration, such a setup allows reaching far backward regions of pseudo-rapidity, uncharted with head-on collisions and not accessible with other similar experiments, like LHCb (also in a potential fixed-target mode), PHENIX, STAR.
The successful implementation of the ALICE-FT programme would also be a significant step forward in crystal applications for accelerators as crystals so far were rather used to remove or extract particles from the beam. Experience collected may lead to further crystal-based applications with a big potential in, e.g. crystal-based collimation systems, considered a possible upgrade in the LHC.
