在ATLAS双喷注加光子末态和FASER探测器上寻找新粒子

Since ancient times, human beings have been constantly questioning the origin of the world, and often believe that the deeper mysteries of the universe are hidden on a smaller scale. The Standard Model describes the physics of known elementary particles with unprecedented precision, but it has some obvious shortcomings, which promotes physicists pursue the theory beyond Standard Model. Some efforts to search for new physics are presented in this thesis. First of all, this thesis presents an analysis to search for new resonance on the invariant mass spectrum of the dijet associated with a high transverse energy photon in the final state with ATLAS detector. Such final state can be used to search for new particles in the low-mass range generally. The data used in this analysis was LHC proton–proton collisions collected by the ATLAS experiment at a centre-of-mass energy of 13 TeV from 2015 to 2017, corresponding to an integrated luminosity up to $79.8\ \text{fb}^{-1}$. The single-photon trigger is used in low-mass range, and combined trigger used in high-mass range. Two variants are presented: one which makes no jet flavour requirements and one which requires both jets to be tagged as $b$-jets. In total 4 channels, the observed mass distributions are all consistent with multi-jet processes in the Standard Model. Thus, the data are used to set upper limits on the production cross section for a benchmark Z′ model and, separately, on generic Gaussian-shape contributions to the mass distributions, extending the current ATLAS constraints on dijet resonances to the mass range between 225 and 1100 GeV. Secondly, for the upcoming LHC Run-3, this thesis introduces the FASER experiment dedicated to searching for new long-lived particles with a mass of $\sim$MeV that are weakly coupled to the standard model. This new small experiment is complementary to the existing experiment on the LHC. Some new BSM predict these long-lived particles are produced abundantly along the beam axis, so FASER detector is located 480 m downstream of the ATLAS IP in the unused service tunnel TI12. Track reconstruction is quite important for identifying the signal of new particles. FASER uses ACTS in track reconstruction where tracks are fit by Kalman filter algorithm. The development and preliminary validation of FASER offline track reconstruction software will be presented in this thesis. Finally, HL-LHC that will collect more than $3000\ \text{fb}^{-1}$ data in the future will bring greater possibilities to new physics. To cope with higher luminosity, the track detectors in ATLAS and CMS need to be upgraded. The new generation pixel detector module RD53A, which uses the 65 nm CMOS technology, has been tested comprehensively. The vital spatial resolution of RD53A module is measured by SLAC test beam (11 GeV electron) in November, 2018. Tracks of electrons are reconstructed from hits on telescope planes using the EUTelescope package. The spatial resolution is extracted by comparing the extrapolated track and the hit position on the RD53A modules, correcting for the tracking resolution. Two design RD53A modules with pitch $50\times50\ \upmu$m$^2$ and $100\times25\ \upmu$m$^2$ are measured. 14.6 $\upmu$m (10.9 $\upmu$m) and 7.9 $\upmu$m (6.8 $\upmu$m) spatial resolution can be achieved for the 50 and 25 $\upmu$m directions, respectively, with non-tilt (13 degree tilt).