Central exclusive production of top quarks

Abstract

This summer student project at CERN was dedicated to exclusive production of top quarks. For proton colliders exclusive production means that colliding protons after the collision do not get destroyed but through photons (25% of times) or gluons (75% of times) loose some of their momentum and create in our case top and anti top quark pair (see Feynman diagram, figure 1). Due to the momentum loss, these protons are more bent in the magnetic field of the accelerator and escape the beam. There is no detector in the CMS to detect protons but as these protons are declined from the beam only slightly, it is possible to detect them in special detectors called Roman pots (RP) owned by the TOTEM experiment [1]. RP are aligned on both sides of the interaction point. From the position in the RP where the proton was detected and knowing how protons interact with the magnetic field bending the proton beam, it is possi-ble to calculate the fractional momentum loss ξ of a proton - how much momentum it lost after the collision. The proton interaction with the magnetic field can be described by the dispersion value Dx so that xRP = Dx(ξ)ξ If the energy loss of both protons is known, it is also possible to reconstruct the invariant mass of the tt¯system by a formula: mtt¯ = √sξ1ξ2, (1) where s is the centre of mass energy. The exclusive production is a new research field at CERN because it uses data from detectors owned by two different experiments - CMS and TOTEM. Only in a recent years a joint experiment called CMS-TOTEM Precision Proton Spectrometer (CT-PPS) was created and so far only one paper on the exclu-sive production has been published. It provided an evidence for the semi exclusive µµ− pair production -an event where only one of the protons was detected [2]. One of the problems for obtaining a good results for the exclusive top quark production is a large number of the background protons in the RP which could be solved by using the low pile up (low PU) data. The goal of this project was to make the first examination of data from the low PU run of 2018. At the time of doing this project the dispersion value Dx for the low pile up run of 2018 was not known but was to be calculated soon. This work would ensure the usage of these data for the search of central exclusive top quark production as soon as all of the information about the dispersion of the protons for this run would be received. The main tasks was to calculate the resolution of tt¯system for the 2018 data, improve the selection criteria, look for a shift of the PT for the central system at low PT scale and look if there are any problems with the data