The activity in the field of diffractive physics at the Large Hadron Collider has been constantly increasing. This includes the planning for additional dedicated apparatus -horizontal forward proton detectors. This work focuses on the problems related to the alignment of such devices. The effects of the misalignment of the detectors on their geometric acceptance and on the reconstruction of the proton kinematics are studied. The requirements for the alignment precision are inferred for different types of possible measurements. .pl (J. Chwastowski), Krzysztof.Korcyl@ifj.edu.pl (K. Korcyl), Maciej.Trzebinski@ifj.edu.pl (M. Trzebiński) 1 Although diffractive interactions are possible in any hadron-hadron collision, in this work we focus on proton-proton interactions at the LHC.
1The size of such a gap has a steeply falling exponential distribution. The size of the gap in diffractive events drops more slowly, which makes the rapidity gap selection method appropriate for events with large gaps, i.e. small diffractive masses. For diffractive processes with large masses in the final state the use of this method is problematic. In addition, the high pile-up environment of the LHC makes it impossible for calorimeters to be used in the rapidity gap reconstruction.The second method used to measure diffraction is based on the fact that the exchange of a colour singlet may leave the proton intact. Such a proton is characterised by a very steep distribution of the scattering angle. In fact, at the LHC, the scattered diffractive protons remain inside the beam pipe and traverse the magnetic structures of the accelerator accompanying the proton bunch. However, the kinematics of the diffractive protons is slightly different from that of the protons of the beam (greater transverse momentum or lower longitudinal momentum). Because of this the trajectories of such protons recede from the beam orbit. At a far enough distance from the interaction point (more than hundred meter), the diffractive protons may depart far enough from the beam core to be detected.Measurements of diffractively scattered protons in the vicinity of the beam require dedicated detectors. To access small scattering angles, the detectors have to be placed inside the accelerator beam pipe. Moreover, it must be possible to adjust the distance between the detector and the beam during the operation. This is because the minimal distance depends on the actual condition of the beam. Typically, during beam injection and acceleration, when the beams are unstable, the detectors must be retracted into the parking positions. This is possible by using a dedicated system -the Roman pot [1] or the Hamburg movable beam pipe [2,3]. Depending on the nature of the studied process, the detectors can be designed to move in the machine plane (horizontal detectors) or along the normal to this plane (vertical detectors).The procedures of inserting and retracting the detectors imply that the positions of the detectors will vary from one data taking period to another. This makes the apparat...
