Characterization of new solid state particle detectors and measurement of the central exclusive production of tt ̄ pairs at CMS

The CERN Large Hadron Collider (LHC) allows particle physics to explore unprecedented regimes and make a huge step forward in the understanding of fundamental interactions. The discovery of the Higgs boson by the ATLAS [1] and CMS [2] collaborations provided an excellent test of the standard model (SM) of particle physics. On the other hand, it is long known that the SM can only be an approximation at low energies of a more general theory, yet to be discovered: therefore, in addition to precision measurements within the SM, searches for signatures of new physics (NP) models beyond the SM represent a key goal of the physics programs of the LHC experiments. Searches are performed analysing a range of experimental signatures as wide as possible. So far, no evidence for deviations from the SM has been reported. The top quark, owing to its large mass, close to the electroweak (EWK) symmetry breaking scale, has long been seen as a window on NP. Several NP models indeed foresee a privileged role for the top quark sector. The LHC experiments have a huge program of measurements of top quark properties. At the LHC, in proton-proton collisions, the top quark is predominantly produced via quantum chromodynamics (QCD) diagrams that yield top quark-antiquark pairs, or via EWK diagrams in the so called ”single top” production. Recently, at the CMS collaboration, the possibility arose to explore a new production mechanism of the top quark, the central exclusive production via gamma-gamma fusion. In fact, beam protons can often interact without disintegrating themselves, rather losing a small fraction of their energy and momentum, by exchanging photons, for example, and continuing their path: the lost 4-momentum can yield a variety of particles, referred to as the X system in the following, giving rise to events of the form pp → pXp; the X system can be top quark-antiquark pairs. CMS installed a new detector, the proton precision spectrometer (PPS), positioned at around 210m, along the beam line, on either side of the beam inter- action point: PPS allows to reconstruct those protons that interacted without disintegrating themselves. The measurement of the proton lost momentum, together with the reconstruction of the decay products of the X system by the central CMS detector, allows the study of events of the form pp → pttp. The cross section for this process has never been measured before. From a theoretical point of view, in the context of the standard model, the cross section is foreseen to be very small, generally below 1fb: calculations using the Monte Carlo generator FPMC [3] combined with MadGraph5 [4] yield a value of around 0.3fb. However NP scenarios can enhance it to values that can be tested with the data already collected by the LHC. In my PhD thesis, I participated in the ongoing efforts to measure the cross section of the pp → pttp process, selecting the so-called semi-leptonic channel, that is events where one of the two top quarks decays to a fully hadronic final state and the other to a final state containing a charged lepton-neutrino pair. While in the first run of its operation PPS comprised silicon-strip tracking detectors, a new silicon-pixel-based detector has been designed and built: during my PhD, I participated in all phases of the construction, commissioning and installation of the new pixel detectors. The system is made of several layers of sensitive material arranged in a mechanical structure, called ”roman pot”, that allows a positioning very close to the beam line: in fact, protons undergo only a tiny deviation after the interaction and, exploiting the LHC optics system, at 210mathrmm from the interaction point, they are still very close to the beam line. For these reasons, PPS silicon detectors operate under extreme conditions, in a very high radiation environment. Characterising and optimising the behaviour of the new detectors under various levels of radiation exposure played a role of paramount importance during the commissioning phase. The LHC is in operation since 2008. With the aging of some parts and the evolution of the operating conditions, the experiments have constantly updated and improved all systems along the years, taking advantage of the continuous advancements of the technologies for particle detectors. In 2020, the LHC and the experiments were in a shutdown phase for repairs and upgrades. In 2022, the operations will resume with a higher proton center-of-mass energy and with a larger luminosity. Very high luminosities yield extreme pile-up conditions, that is a large number of multiple interactions during the same beam bunch crossing, a phenomenon that can make event reconstruction problematic for the experiments. To cope with such extreme operating conditions, and at the same time maintain excellent performances, efforts have been devoted to design new generation timing detectors: in addition to spatial information, a precise tim- ing information in fact can help correctly assigning the reconstructed tracks to the interaction that produced them. In my PhD, I joined the TimeSpot collaboration, a team aimed at conceiving new solid-state timing detectors that implement novel configurations of p-n junctions to achieve unprecedented reso- lutions on the timing measurements.